Handbook Properties and Selection Nonferrous Alloys and Spl Purpose Mtls (1992) WW Part 8 pdf

Additional information on the properties of undoped tungsten is available in the section "Tungsten" in the article "Properties of Pure Metals" in this Volume.. Table 25 Typical mechanica

Stress-relief temperature. 1 h at 1095 to 1205 °C (2000 to 2200 °F)

Tensile properties. Stress relieved: tensile strength,

995 MPa (144 ksi); yield strength, 725 MPa (105 ksi); elongation, 22% in 50 mm (2 in.); reduction in area, 36% At 1095 °C (2000 °F): tensile strength, 640 MPa (93 ksi) At 1315 °C (2400 °F): tensile strength, 415 MPa (60 ksi)

TZM

Mo-0.5Ti-0.1Zr

Commercial Names

UNS number. Arc cast, R03630; P/M, R03640

ASTM designation. Arc cast: molybdenum alloy 363

Applications

Typical uses. Used in heat engines, heat exchangers,

nuclear reactors, radiation shields, extrusion dies, boring bars

Mechanical Properties

Tensile properties. See Table 21

Table 21 Typical tensile properties of TZM

Temperature Tensile strength Yield strength

at 0.2% offset

°C °F MPa ksi MPa ksi

Elongation

in 50 mm (2 in.), %

Stress-relieved condition

Thermal conductivity. See Fig 22

Fig 22 Thermal conductivities of TZM and selected other refractory metals and alloys

Fabrication Characteristics

Recrystallization temperature. 1425 to 1595 °C

(2600 to 2900 °F)

Stress-relief temperature. 1 h at 1095 to 1260 °C (2000 to 2300 °F)

Tungsten

Walter A Johnson, Institute of Materials Processing, Michigan Technological University

TUNGSTEN is consumed in four forms:

so-The high melting point of tungsten makes it an obvious choice for structural applications exposed to very high temperatures Tungsten is used at lower temperatures for applications that can use its high elastic modulus, density, or shielding characteristics to advantage

Production

Tungsten and tungsten alloys can be pressed and sintered into bars and subsequently fabricated into wrought bar, sheet, or wire Many tungsten products are intricate and require machining or molding and sintering to near-net shape and cannot

be fabricated from standard mill products

Shortly before World War II, an easily machinable, relatively ductile family of tungsten-base materials containing a relatively soft and ductile binder phase was developed These materials, commonly called tungsten heavy metals, are a classic example of the application of liquid-phase sintering to the production of P/M parts In this case, the basic metal is tungsten, nd the liquid phase in which tungsten is partly soluble is primarily nickel In the original heavy-metal alloys, it was found that the addition of copper was desirable because it lowered the melting temperature of the liquid phase, thereby lowering the sintering temperature The resulting tungsten-nickel-copper alloy had good mechanical properties, fair ductility, and good machinability Subsequently, tungsten-nickel-iron alloys that had greater ductility than the tungsten-nickel-copper materials were developed It was also found that the tungsten-nickel-iron alloys with higher percentages of tungsten could be sintered to near-theoretical density, thereby producing materials of even higher specific gravity

Tungsten

Tungsten mill products can be divided into three distinct groups on the basis of recrystallization behavior The first group consists of EB-melted, zone-refined, or arc-melted unalloyed tungsten; other very pure forms of unalloyed tungsten; or tungsten alloyed with rhenium or molybdenum These materials exhibit equiaxed grain structures upon primary recrystallization The recrystallization temperature and grain size both decrease with increasing deformation

The second group, consisting of commercial grade or undoped P/M tungsten, demonstrates the sensitivity of tungsten to purity Like the first group, these materials exhibit equiaxed grain structures (Fig 23), but their recrystallization temperatures are higher than those of the first-group materials Also, these materials do not necessarily exhibit decreases

in recrystallization temperature and grain size with increasing deformation In EB-melted tungsten wire, the

recrystallization temperature can be 900 °C (1650 °F) or lower, whereas in commercially pure (undoped) tungsten it can

be as high as 1205 to 1400 °C (2200 to 2550 °F)

Fig 23 Recrystallized microstructure of undoped tungsten wire

The third group of materials consists of AKS-doped tungsten (that is, tungsten doped with aluminum-potassium-silicon), doped tungsten alloyed with rhenium, and undoped tungsten alloyed with more than 1% ThO2 These materials are characterized by higher recrystallization temperatures (>1800 °C, or 3270 °F) and unique recrystallized grain structures (Fig 24) The structure of heavily drawn wire or rolled sheet consists of very long interlocking grains This structure is most readily found in AKS-doped tungsten or in doped tungsten alloyed with 1 to 5% Re The potassium dopant is spread out in the direction of rolling or drawing; when heated, it volatilizes into a linear array of submicron-size bubbles These bubbles pin grain boundaries in the manner of a dispersion of second-phase particles As the rows of bubbles become finer and longer with increasing deformation, the recrystallization temperature rises, and the interlocking structure becomes more pronounced A comparative impurity analysis of the three grades of tungsten is given in Table 22 Higher concentrations of rhenium (7 to 10%) destroy this effect In W-2ThO2, the occurrence of this elongated, interlocking structure depends on the thermomechanical treatment and on the fineness of the thoria dispersion Addition of 1.5% or more ThO2 raises the recrystallization temperature of tungsten in much the same way as the potassium dopant raises it, but ThO2 additions generally result in a much finer grain structure Rhenium in amounts up to about 5% inhibits recrystallization; in greater amounts, it lowers resistance to recrystallization

Table 22 Typical purity of the three commercial grades of tungsten

Fig 24 Recrystallized microstructure of doped tungsten wire

Tungsten Alloys

Three tungsten alloys are produced commercially: tungsten-ThO2, tungsten-molybdenum, and tungsten-rhenium The ThO2 alloy contains a dispersed second phase of 1 to 2% thoria The thoria dispersion enhances thermionic electron emission, which in turn improves the starting characteristics of gas tungsten arc welding electrodes It also increases the efficiency of electron discharge tubes and imparts creep strength to wire at temperatures above one-half the absolute melting point of tungsten

W-A flow diagram outlining the processing of tungsten ore concentrate into major products is shown in Fig 25 Tungsten mill products, sheet, bar, and wire are all produced via powder metallurgy These products are available in either commercially pure (undoped) tungsten or commercially doped (AKS-doped) tungsten These additives improve the recrystallization and creep properties of tungsten, which are especially important when tungsten is used for incandescent lamp filaments Wrought P/M stock can be zone refined by EB melting to produce single crystals that are higher in purity than the commercially pure product Electron beam zone-melted tungsten single crystals are of commercial interest for applications requiring single crystals with very high electrical resistance ratios

Fig 25 Processing sequence for tungsten from ore to finished products

Processes for Manufacturing Tungsten Heavy-Metal Alloys. Heavy-metal alloys usually are produced from a mixture of elemental, high-purity, fine-particle-size metal powders The tungsten powder has an average particle size of about 2 to 3 μm (80 to 120 μin.) and is 99.99% pure Fine high-purity nickel powder (such as carbonyl nickel), fine electrolytic copper powder, and fine high-purity iron powder (such as carbonyl iron) are used The powders are blended in

a powder blender or ball mill for sufficient time to produce a homogeneous mixture and to achieve an apparent density compatible with the molding operation If molding is by isostatic pressing, no binder is required If molding is by pressing

in a steel or carbide die in a hydraulic or mechanical press, the powder is coated with paraffin or another suitable organic binder Molding pressures of about 70 to 140 MPa (10 to 20 ksi) are used The molded compact must be designed to allow for considerable shrinkage during the sintering operation, usually of the order of 20% lineal or more than 50% by volume Because of the high shrinkage, most parts produced from these alloys require finish machining if close dimensional tolerances are required

Sintering. The molded parts are usually sintered in box-type electric sintering furnaces by stoking The furnaces must have molybdenum or tungsten heating elements because sintering temperatures range from about 1425 to 1650 °C (2600

to 3000 °F), depending on the exact composition of the alloy In some instances, vacuum furnaces are used for sintering these materials, but normally the operation utilizes dry hydrogen or dissociated ammonia for the sintering atmosphere Sintering times at temperature range from about 20 min for small parts to several hours for large blanks Part weights can range from a few grams to 20 kg (45 lb) or more

During sintering, rapid densification of the compact occurs as the fine tungsten particles dissolve in the liquid phase and then reprecipitate on the larger tungsten particles The compact shrinks in this process, and a very dense structure is produced with rounded tungsten-rich grains that are considerably greater in diameter than the original tungsten particles The blanks are cooled to room temperature in the cooling chamber of the furnace and then removed Tensile bars and other test blanks usually are sintered from each powder mix and tested for mechanical and physical properties before the mix is approved for production

Hot Pressing. Some vary large parts are produced by hot pressing rather than by cold pressing and sintering Hot pressing usually is done by leveling the powder mix in a graphite mold and heating the mold in an induction coil while light pressure sufficient to compact the mix to the required density at temperatures similar to those for sintering is applied to the assembly Hot-pressed compacts of this type usually are more brittle and lower in strength than the cold-pressed and sintered materials Also, the graphite mold may cause a carburized layer to form on the surface of the blank that is difficult to remove in machining

Coatings

Some promising systems for protecting tungsten from atmospheric exposure at temperatures from 1650 to 2205 °C (3000

to 4000 °F) have been developed, including:

• Roll cladding with tantalum-hafnium alloys

• Slurry-type coatings of iridium-base alloys such as Ir-30Rh

• Duplex and triplex silicide-base coating systems that combine slurry, slip, chemical vapor deposition, and pack cementation processes

Corrosion and Chemical Resistance

At room temperature, tungsten is generally resistant to most chemicals, but it can be easily dissolved with a solution of nitric and hydrofluoric acids At higher temperatures, tungsten becomes more prone to attack At about 250 °C (480 °F), it reacts rapidly with phosphoric acid and chlorine It begins to oxidize readily at 500 °C (930 °F); at 1000 °C (1830 °F), tungsten reacts with many gases, including water vapor, iodine, bromine, and carbon monoxide Above 1000 °C (1830

°F), tungsten begins to form compounds with various metals

Mechanical and Physical Properties

Undoped Tungsten and Tungsten Alloys. Tungsten has high tensile strength and good creep resistance At temperatures above 2205 °C (4000 °F), tungsten has twice the tensile strength of the strongest tantalum alloys and is only 10% denser However, its high density, poor low-temperature ductility, and strong reactivity in air limit its usefulness Maximum service temperatures for tungsten range from 1925 to 2480 °C (3500 to 4500 °F), but surface protection is required for use in air at these temperatures

Wrought tungsten (as-cold worked) has high strength, strongly directional mechanical properties, and some temperature toughness However, recrystallization occurs rapidly above 1370 °C (2500 °F) and produces a grain structure that is crack sensitive at all temperatures

room-Mechanical property data for unalloyed tungsten and tungsten-molybdenum and tungsten-rhenium alloys are shown in Fig 26, 27, 28, 29, 30, 31 Additional information on the properties of undoped tungsten is available in the section

"Tungsten" in the article "Properties of Pure Metals" in this Volume

Fig 26 Recrystallization behavior of undoped tungsten bar

Fig 27 Thermal conductivity of undoped tungsten

Fig 28 Creep curves for coiled tungsten wires at 2500 °C (4530 °F)

Fig 29 Room-temperature ductility of annealed wire for five tungsten-rhenium alloys

Fig 30 Effect of tungsten content on the room-temperature mechanical properties of tungsten-molybdenum

alloys

Fig 31 Short-time tensile strengths of five tungsten-rhenium alloys

Recrystallized tungsten undergoes a ductile-to-brittle transition above 205 °C (40 °F) Only by heavy warm or cold working is the DBTT lowered to below room temperature (Fig 32) Annealing raises the DBTT of cold-worked tungsten until it approaches that of recrystallized material

Fig 32 Variation of DBTT with annealing temperature for undoped tungsten Data are for 10-min recovery

annealing of heavily worked 0.75 mm (0.030 in.) diam wire

The exact ductile-to-brittle transition temperature is influenced by many factors, including grain size, strain rate, and impurity levels The DBTT decreases with grain size unless the grains are larger than 1 mm (0.04 in.) in diameter The DBTT also drops with increases in strain rate, but it climbs rapidly as impurity levels increase Like all brittle metals, tungsten is very notch sensitive Therefore, removal of even minute surface flaws by grinding, oxidizing, or electrolytic polishing prior to service improves ductility and lowers the DBTT

Alloying can have a beneficial effect on the DBTT; the effect of rhenium in producing a ductile alloy is the best-known example Doping with AKS dopant or alloying with a dispersion of thoria retards recrystallization, thereby improving the

ductility of annealed wire In addition, a fine dispersion of thoria causes a decrease in grain size, which in turn promotes a reduction in the DBTT

Below the DBTT, recrystallized tungsten fails by a combination of cleavage and grain-boundary fracture Near the DBTT, fracture by cleavage increases At higher temperatures, usually above 500 °C (930 °F), grain-boundary and ductile fracture predominate Generally, grain-boundary fracture predominates in commercially pure tungsten and ductile fracture

in AKS-doped tungsten

Aside from its uses in abrasive and wear-resistant tools and as an alloying element, tungsten finds its primary commercial application in filaments for incandescent lamps Thoria particles and the potassium bubble dispersion that occurs in AKS-doped tungsten impede the annealing process that progressively eliminates substructure This allows tungsten to retain hardness and tensile strength at temperatures higher than those at which commercially pure or refined tungsten is softened and weakened It also improves the creep resistance of tungsten wire at elevated temperatures Upon recrystallization, a nonsag, interlocking grain structure forms This structure gives tungsten wire added creep resistance at high temperatures, allowing tungsten filaments in incandescent lamps to burn at high temperatures without sagging

Alloying with rhenium improves the tensile strength of AKS-doped or undoped tungsten Although small additions of less than 5% Re cause softening of tungsten-rhenium alloys, hardness increases when solid-solution strengthening becomes the overriding factor Alloying with molybdenum has a softening effect that is proportional to molybdenum content

Tungsten is not as anisotropic in elastic behavior as are some other cubic metals, but its stress-strain curve does vary somewhat with crystallographic direction

Tungsten Heavy-Metal Alloys. Minimum mechanical properties of machinable heavy-metal tungsten alloys are specified at the time of purchase Three specifications are in general use: MIL-T-21014, ASTM B 459, and AMS 7725 The specifications for machinable high-density tungsten-base alloys usually divide them into four classes based on composition (Table 23) and three types based on tensile properties (Table 24) Tables 25 and 26 give typical mechanical and physical properties of tungsten heavy metal alloys according to these class and type divisions

Table 23 Classification of tungsten heavy-metal alloys by composition, density, and hardness

Density Class Tungsten content, %

g/cm 3 lb/in. 3

Hardness, HRC Type

classification (a)

1 89-91 16.85-17.25 0.609-0.633 30-36 I

1 89-91 16.85-17.25 0.609-0.623 32 max II, III

2 91-94 17.15-17.85 0.620-0.645 33 max II, III

3 94-96 17.75-18.35 0.641-0.663 34 max II, III

(a) See Table 24

Table 24 Classification of tungsten heavy-metal alloys by tensile properties

Tensile 0.2% yield Elongation,

Table 25 Typical mechanical properties of commercial machinable heavy-metal tungsten alloys

Density Tensile

strength

Yield strength

at 0.2% offset

Proportional limit

Modulus of elasticity

Coefficient of linear thermal expansion Alloy (a)

g/cm 3 lb/in. 3 MPa ksi MPa ksi

Elongation in

25 mm (1 in.),

%

Hardness, HRC

MPa ksi GPa psi ×

Class 1 17.0 0.614 895 130 615 89 16 27 260 38 275 40 5.4 3.0 Slightly magnetic

Class 3 18.0 0.650 925 134 655 95 6 29 350 51 310 45 5.3 2.9 Slightly magnetic

Class 4 18.5 0.667 795 115 690 100 3 32 450 65 345 50 5.0 2.8 Slightly magnetic

(a) For a key to the four classes of tungsten heavy-metal alloys, see Table 23

Table 26 Additional properties of machinable heavy-metal tungsten alloys

Modulus of

rupture

(flexure)

Proportional limit

Modulus of elasticity

Modulus of rigidity

Shear strength

% IACS

(a) For a key to the three type divisions of tungsten heavy-metal alloys, see Table 24

(b) This type is used almost exclusively for radiation shielding; data for properties other than modulus of rupture are not available

Class 1 alloys are basically tungsten-nickel-copper or tungsten-nickel-iron alloys The tungsten-nickel-copper alloys of this class typically contain 90% W, 6 to 7% Ni, and 3 to 4% Cu Minor additions of other metals, such as molybdenum or cobalt, can be added to modify properties such as hardness Class 1 tungsten-nickel-iron alloys usually contain 90% W, 5

to 7.5% Ni, and 3 to 5.5% Fe

Class 2, 3, and 4 alloys are usually tungsten-nickel-iron alloys with tungsten contents in the range shown in Table 23 They contain a balance of nickel-iron in a ratio of 4Ni:1Fe (class 2), 7Ni:3Fe (class 3), and 1Ni:1Fe (class 4) Sometimes

a portion of the iron may be replaced with copper

Electrical Properties

The electrical resistivity and temperature coefficient of electrical resistivity properties of tungsten are both strongly affected by purity and deformation The effects of recovery annealing on these two properties for commercially pure tungsten wire are shown in Table 27 The product of resistivity and temperature coefficient is a nearly constant value that

is independent of the degree of residual cold work The addition of rhenium, molybdenum, or thoria increases the resistivity of tungsten wire but has no appreciable effect on its temperature coefficient Some typical electrical resistivity data are shown in Fig 33, 34, 35

Table 27 Effect of annealing on the electrical resistivity and temperature coefficient of drawn tungsten wire

Annealing

temperature

Electrical resistivity, μΩ· m

Temperature coefficient

Matthiessen's rule (a)

(a) Product of specific resistance and temperature coefficient

Fig 33 Effect of tungsten content on the specific electrical resistivity of tungsten-molybdenum alloys

Fig 34 Specific electrical resistivity of tungsten-rhenium alloys as a function of rhenium content

Fig 35 Effect of temperature on the electrical resistivity of standard doped tungsten and of tungsten-rhenium

alloys

Thermocouples in which tungsten is one of the thermoelements are used extensively at very high temperatures molybdenum thermocouples, for example, can be used at temperatures up to 2205 °C (4000 °F) if maintained in a protective envelope or a reducing atmosphere

Tungsten-Rhenium

Toni Grobstein, Robert Titran, and Joseph R Stephens, NASA Lewis Research Center

PLATINUM-RHENIUM REFORMING CATALYSTS are the major rhenium end-use products and account for about 85% of rhenium consumption Rhenium catalysts are exceptionally resistant to poisoning from nitrogen, sulfur, and phosphorus They are used for the hydrogenation of fine chemicals and for hydrocracking, reforming, and the disproportionation of olefins, including increasing the octane rating in the production of lead-free petroleum products Rhenium is also used in the production of heating elements, x-ray tubes and targets, and metallic coatings Indium-coated rhenium nozzles for small chemical rockets and resistojet thrusters are used in space for satellite orientation Rhenium is a solid-solution-strengthening alloying element in superalloys; in tungsten and molybdenum-based alloys, it markedly increases room-temperature ductility (this increase is known as the rhenium effect)

Rhenium metal is widely used in filaments for mass spectrographs and ion gages because of its high electrical resistivity and low vapor pressures at high temperatures Rhenium-molybdenum alloys are superconductive at 10 K Rhenium is used as an electrical contact material because of its wear resistance and its ability to withstand arc erosion Thermocouples made of rhenium-tungsten are used for measuring temperatures up to 2200 °C (3990 °F), and rhenium wire is used in photoflash lamps for photography

Relatively little development work has been done for rhenium-base alloys as compared with that for other refractory metals The use of rhenium in aerospace applications has been restricted by its high density; in terrestrial applications, its short supply and consequent high cost have been the limiting factors For example, the addition of 3% Re to tungsten wire doubles the cost of the wire

Occurrence and Production

Most rhenium occurs in porphyry copper deposits Identified sources are estimated to to about 4.5 × 103 Mg (5.0 × 103tons) in the United States, and approximately 5.9 × 103 Mg (6.5 × 103 tons) in the rest of the world The United States relies on imports for most of its rhenium supply, with 71% coming form Chile it is estimated that the United States consumed about 6.35 Mg (7.00 tons) of rhenium in 1989 Rhenium is available as perrhenic acid (HReO4), ammonium perrhenate (NH4ReO4), and metal powder In 1988, the average price of rhenium metal was $1.05/g ($475/lb); the price was $0.66/g ($300/lb) for ammonium perrhenate

Ammonium perrhenate is converted to metal powder by hydrogen reduction The reduction is carried out at 380 °C (715

°F) and is followed by a purification and reduction cycle at 700 to 800 °C (1290 to 1470 °F) to remove any residual rhenium oxide The powder is generally consolidated by cold pressing at about 205 MPa (30 ksi) to a density of 35 to 40% using stearic acid in ether as a lubricant on the punch and the die walls Subsequent sintering at 1200 °C (2190 °F) for 2 h in vacuum results in little densification but increases the mechanical strength of the compact and burns off volatile impurities Finally, resistance heating in a vacuum or hydrogen atmosphere at 2700 to 2900 °C (4890 to 5250 °F) produces sintered compacts with densities of more than 90%

Electron beam remelting is sometimes used to reduced the impurity content of rhenium compacts Chemical vapor deposition is also a practical fabrication method

Corrosion Resistance

Rhenium oxidizes catastrophically at temperatures above 600 °C (1110 °F) Oxidation occurs as a result of the formation

of rhenium heptoxide (Re2O7), which has a melting point of 297 °C (567 °F) and a boiling point of 363 °C (685 °F) The white oxide vapor has been reported to be nonpoisonous Iridium is currently used as an oxidation-resistant coating for rhenium at high temperatures Rhenium is unique among the refractory metals in that it does not form a carbide; however,

it is similar to the other metals in the group in that it is resistant to liquid lithium metal corrosion Rhenium is resistant to water cycle corrosion in high-temperature filaments in vacuum Rhenium has good resistance to sulfuric acid and hydrochloric acid but can be dissolved by nitric acid; it is also resistant to aqua regia at room temperature In addition, rhenium is resistant to attack by molten tin, zinc, silver, copper, and aluminum

Mechanical and Physical Properties

Temperature-dependent tensile strength, elastic modulus, and physical property data for rhenium are presented in the introductory section of this article (see Table 2 and Fig 1 and 2) One of the most outstanding characteristics of rhenium

is its very high strain-hardening rate, which is about 3.5 times that of tungsten or molybdenum The general trend of existing data indicates about a twofold increase in hardness for 25% deformation This unusually rapid work hardening requires frequent intermediate annealing in inert or reducing atmospheres during fabrication, with low cold reduction levels to avoid cracking Because impurity levels are critical to fabricability, a vacuum level of 1 to 0.1 mPa (10-5 to 10-6torr) or a dry hydrogen atmosphere is used Hydrogen-nitrogen mixtures, such as dissociated ammonia or annealing hydrogen (H2 + 7N2), have also been successfully used The ultimate tensile strength of annealed rhenium sheet has been reported to increase from 1158 MPa to 2220 MPa (168 to 322 ksi) as a result of 30.7% cold reduction Detailed property data for unalloyed rhenium are given in the section "Rhenium" in the article "Properties of Pure Metals" in this Volume

Refractory Metal Fiber-Reinforced Composites

Toni Grobstein and Donald W Petrasek, NASA Lewis Research Center

REFRACTORY METAL WIRES, in spite of their poor oxidation resistance and high density, have received a great deal

of attention as fiber reinforcement materials for use in high-temperature composites Although the theoretical specific strength potential of refractory alloy fiber-reinforced composites is less than that of ceramic fiber-reinforced composites, the more ductile metal fiber systems are more tolerant of fiber-matrix reactions and thermal expansion mismatches When refractory metal fibers are used to reinforce a ductile and oxidation-resistant matrix, they are protected from oxidation, and the specific strength of the composite is much higher than that of superalloys at elevated temperatures

The majority of the studies conducted on this topic have been on refractory wire and superalloy composites that use tungsten or molybdenum wire (available as lamp filament or thermocouple wire) as the reinforcement material These refractory alloy wires were not designed for use in composites, nor were they developed to achieve optimum mechanical properties in the temperature range of interest for component application, 1000 to 1200 °C (1830 to 2190 °F) The stress-rupture properties of a tungsten lamp filament wire used in early studies were superior to those of rod and bulk forms of tungsten, and this wire showed promise for use as composite reinforcement After the need for stronger wire was recognized, high-strength tungsten, tantalum, molybdenum, and niobium alloys that were originally used for rod and/or sheet fabrication were drawn into wire

Excellent progress has been made in providing wires with increased strength Tungsten alloy wires have been fabricated that have tensile strengths 2.5 times higher than those obtained for potassium-doped tungsten lamp filament wire The strongest wire fabricated, tungsten-rhenium-hafnium-carbon, has a tensile strength of 2165 MPa (314 ksi) at 1093 °C (2000 °F), which is more than 6 times the strength of the strongest nickel-base or cobalt-base superalloy Although the ultimate tensile strength values of the tungsten alloy wires were higher than those obtained for molybdenum, tantalum, or niobium wires, their advantage is lessened when the higher density of tungsten is taken into account Nevertheless, high-strength tungsten alloy wires rank alongside molybdenum wires as offering the most promise for composite applications

Processing of Composites. The consolidation of matrix and fibers into a composite material with useful properties is one of the most difficult steps in developing composites reinforced with refractory metal wire Fabrication methods are currently in the laboratory phase of development because satisfactory techniques have not yet been developed for producing large numbers of specimens for extensive property characterization Fabrication techniques currently being developed can be classified as either liquid-phase or solid-phase methods

Liquid-phase methods consist of casting the molten matrix using investment casting techniques so that the matrix infiltrates the bundle of fibers The molten metal must wet the fibers, form a chemical bond, and yet be controlled so as not to degrade the fibers by dissolution, reaction, or recrystallization

Solid-phase methods generally use processing temperatures much lower than those reached during liquid-phase processing; diffusion rates are therefore much lower, and reaction with the fiber can be less severe The prerequisite for solid-state processing is that the matrix be in either wire, sheet, foil, or powder form Cold pressing followed by sintering

or hot pressing is used to consolidate the matrix and fiber into a composite component

Mechanical and Thermal Properties. Refractory fiber-reinforced superalloy composites have demonstrated strengths significantly above those of the strongest superalloys Tungsten fiber-reinforced superalloy composites, in particular, are potentially useful as high-temperature (1000 to 1200 °C, or 1830 to 2190 °F) materials because of their microstructural stability and superior resistance to stress-rupture and creep deformation, thermal shock, and low- and high-cycle fatigue Compared with conventional superalloys, refractory metal fiber-reinforced composites have improved ductility, impact damage resistance, and thermal conductivity

Refractory fiber-reinforced niobium alloy composites have demonstrated a potential for use at temperatures in excess of

1200 °C (2190 °F) The tensile and creep strength properties of these composites have been improved an order of magnitude by adding 50 vol% tungsten fiber to the niobium alloys

Applications. Refractory metal alloy fiber-reinforced composites are being considered for many different and demanding applications For example, tungsten fibers are being investigated for use as a reinforcement in copper for strengthening high-conductivity materials in regeneratively cooled rocket nozzles A volume fraction addition of 10% tungsten fibers in copper has a dramatic effect on the strength of the material without significantly decreasing its thermal conductivity With the addition of the tungsten fibers, copper can be used at temperatures and stresses that would normally exceed its yield strength

Tungsten fiber-reinforced superalloy composites are being developed for use in rocket engine turbine blades (Fig 36) Tungsten fiber-reinforced superalloy composites have a highly attractive combination of properties at temperatures from

870 to 1100 °C (1600 to 2010 °F); these properties make them well suited for advanced rocket engine turbopump blade applications The composites offer the potential of significantly improved operating life, higher operating temperature capability, and reduced strains induced by transient thermal conditions during engine start and shutdown

Fig 36 Location and structure of tungsten fibers in fiber-reinforced superalloy composite turbine blades for

rocket engine turbopumps Courtesy NASA Lewis Research Center

Tungsten fiber-reinforced niobium alloy systems are being investigated for potential long-term high-temperature applications in space power systems In addition, molybdenum-base fibers are being proposed for use in intermetallic-matrix composites for aerospace applications

Introduction to Titanium and Titanium Alloys

James D Destefani*, Bailey Controls Company

Introduction

TITANIUM has been recognized as an element for 200 years Only in the last 40 years or so, however, has the metal gained strategic importance In that time, commercial production of titanium and titanium alloys in the United States has increased from zero to more than 23 million kg/yr (50 million lb/yr)

The catalyst for this remarkable growth was the development by Dr Wilhelm J Kroll of a relatively safe, economical method to produce titanium metal in the late 1930s Kroll's process involved reduction of titanium tetrachloride (TiCl4), first with sodium and calcium, and later with magnesium, under an inert gas atmosphere (Ref 1) Research by Kroll and many others continued through World War II By the late 1940s, the mechanical properties, physical properties, and alloying characteristics of titanium were defined and the commercial importance of the metal was apparent

Commercial titanium production soon began in earnest in the United States, and by 1956 U.S production of titanium mill products was more than 6 million kg/yr (13 million lb/yr) (Ref 2)

Alloy development progressed rapidly The beneficial effects of aluminum additions were realized early on, and aluminum alloys were soon commercially available Two alloys that are still widely used, Ti-6Al-4V and Ti-5Al-2.5Sn, were both developed in the early 1950s The Ti-6Al-4V alloy, in fact, accounts for more than half of the current U.S titanium market (Ref 3)

titanium-General Metal Characteristics. The rapid growth of the titanium industry is testimony to the metal's high specific strength and corrosion resistance With density about 55% that of steel, titanium alloys are widely used for highly loaded aerospace components that operate at low to moderately elevated temperatures, including both airframe and jet engine components (see the section "Applications" in this article)

Titanium's corrosion resistance is based on the formation of a stable, protective oxide layer This passivating behavior makes the metal useful in applications ranging from chemical processing equipment to surgical implants and prosthetic devices The corrosion behavior of titanium is discussed in detail in the article "Corrosion of Titanium and Titanium

Alloys" in Corrosion, Volume 13 of ASM Handbook, formerly 9th Edition Metals Handbook

Current titanium technology encompasses a variety of products and processes Some of the latest developments, which are briefly reviewed in the section "New Developments" in this article, include new sponge production and melting practices, titanium-matrix composites, oxide dispersion-strengthened powder metallurgy (P/M) alloys with novel compositions and properties, superplastic forming and diffusion bonding (SPF/DB) of titanium alloy sheet and plate, and titanium-base ordered intermetallic compounds

This article is intended to provide an overview of contemporary titanium technology Detailed information on the properties, processing, and application of specific titanium alloys and product forms is available in the articles "Wrought Titanium and Titanium Alloys," "Titanium and Titanium Alloy Castings," "Titanium P/M Products," "Metal-Matrix Composites," and "Ordered Intermetallics" in this Volume

References

1.W.J Kroll, How Commercial Titanium and Zirconium Were Born, J Franklin Inst., Vol 260, Sept 1955, p 169-192

2.Titanium: The Industry, Its Future, Its Equities, F.S Smithers and Company, 1957, p 7, 33-67

3.H.B Bomberger, F.H Froes, and P.H Morton, Titanium A Historical Perspective, in Titanium Technology: Present Status and Future Trends, F.H Froes, D Eylon, and H.B Bomberger, Ed., Titanium Development Association, 1985,

p 3-17

Alpha alloys contain elements such as aluminum and tin These α-stabilizing elements work by either inhibiting change

in the phase transformation temperature or by causing it to increase (Ref 4) Alpha alloys generally have creep resistance superior to β alloys, and are preferred for high-temperature applications The absence of a ductile-to-brittle transition, a feature of β alloys, makes α alloys suitable for cryogenic applications

Alpha alloys are characterized by satisfactory strength, toughness, and weldability, but poorer forgeability than β alloys (Ref 5) This latter characteristic results in a greater tendency for forging defects Smaller reductions and frequent reheating can minimize these problems

Unlike β alloys, alpha alloys cannot be strengthened by heat treatment They most often are used in the annealed or recrystallized condition to eliminate residual stresses caused by working

Alpha + beta alloys have compositions that support a mixture of α and β phases and may contain between 10 and 50%

β phase at room temperature The most common α + β alloy is Ti-6Al-4V (Ref 4) Although this particular alloy is relatively difficult to form even in the annealed condition, α + β alloys generally have good formability

The properties of these alloys can be controlled through heat treatment, which is used to adjust the amounts and types of β phase present Solution treatment followed by aging at 480 to 650 °C (900 to 1200 °F) precipitates α, resulting in a fine mixture of α and β in a matrix of retained or transformed β phase

Beta alloys contain transition elements such as vanadium, niobium, and molybdenum, which tend to decrease the temperature of the α to β phase transition and thus promote development of the bcc β phase They have excellent forgeability over a wider range of forging temperatures than α alloys, and β alloy sheet is cold formable in the solution-treated condition

Beta alloys have excellent hardenability, and respond readily to heat treatment A common thermal treatment involves solution treatment followed by aging at temperatures of 450 to 650 °C (850 to 1200 °F) This treatment results in formation of finely dispersed α particles in the retained β

References cited in this section

4 E.W Collings, The Physical Metallurgy of Titanium Alloys, American Society for Metals, 1984, p 2

5 M.J Donachie, Jr., Titanium: A Technical Guide, ASM INTERNATIONAL, 1988, p 28

Market Development

From its inception, the titanium industry was tied very closely to the market for commercial and military jet aircraft Dependence on the aerospace industry, which is cyclical in nature, resulted in numerous setbacks Despite this, growth of the U.S titanium industry has been relatively steady Figure 1 illustrates the increase of U.S titanium ingot production since 1951

Fig 1 Growth of U.S titanium ingot production, 1951 to 1989 Introduction and growth indicate phases of

titanium product life cycle Source: Ref 6

The Product Life Cycle of Titanium (Ref 6) Product life cycle theory has been used for nearly 40 years to analyze the rise and fall of product demand A typical product life cycle begins with a product's introduction into the marketplace

As shown in Fig 2, this is followed by several more or less well-defined stages of rapid growth, maturity, and ultimate decline as replacement products enter the marketplace Using product life-cycle models originally developed for and applied to the U.S steel and aluminum industries, this theory has recently been applied to relate U.S titanium demand to industrial economic growth

Fig 2 Schematic product life cycle curve, showing position of various technologies on curve Depending on

market area, titanium ranges from rapid growth to growth/maturing stage Source: Ref 6

Reference 6 concludes that, despite continued development of new alloys and product forms, titanium has moved rapidly through its product life cycle to maturity in the aircraft industry The metal is still in the growth stage in applications

where corrosion resistance is important, such as the marine and biomedical industries Other commercial and consumer applications, such as in the automotive industry and in architecture, are only in the developmental stage

These more varied applications should strengthen and stabilize demand for titanium, making titanium producers less susceptible to fluctuations in any one application area This diversification should accelerate as the industry continues to mature and titanium makes the transition from a technology product to a commodity product

Market Trends. Reference 6 also used product life-cycle analysis to forecast future demand for titanium mill products through the end of the century Table 1 compares past and predicted average annual demands for titanium ingot, castings, and mill products

Table 1 Past and predicted average annual U.S demand for titanium and titanium alloys

Demand, kg × 1000 (lb × 1000) Product form

1984-1988 1989-1993 1994-1999

34,635 38,909 44,182 Ingot

(76,196) (85,600) (97,200)

20,630 23,273 26,545 Mill products

(45,387) (51,200) (58,400)

392 727 1,455 Castings

(862) (1,600) (3,200)

Reference cited in this section

6 O.E Nelson, The Product Life Cycle of Titanium, paper presented at the Annual Conference of the Titanium Development Association, Tucson, AZ, 13 Oct 1989

Applications

Aerospace applications including use in both structural (airframe) components and jet engines still account for the largest share of titanium alloy use Titanium, in fact, was so successful as an aerospace material that other potential applications were not fully exploited These have only more recently begun to be explored; some are in development stages, while others are using or starting to use significant quantities of metal These include:

• Applications where titanium is used for its resistance to corrosion, such as chemical processing, the pulp and paper industry, marine applications, and energy production and storage

• Biomedical applications that take advantage of the metal's inertness in the human body for use in surgical implants and prosthetic devices

• Special applications that exploit unique properties such as superconductivity (alloyed with niobium) and the shape-memory effect (alloyed with nickel)

• New application areas where the metal's high specific strength is important, such as the automotive industry

• Consumer applications ranging from cameras to jewelry, musical instruments, and sports equipment

Table 2 provides a list of many uses for titanium in all of these application areas

Table 2 Applications for titanium and titanium alloys

Applications area Typical uses

Aerospace

Airframes Fittings, bolts, landing gear beams, wing boxes, fuselage frames, flap tracks, slat tracks, brake assemblies,

fuselage panels, engine support mountings, undercarriage components, inlet guide vanes, wing pivot lugs, keels, firewalls, fairings, hydraulic tubing, deicing ductings, SPF parts

Engines Compressor disks and blades, fan disks and blades, casings, afterburner cowlings, flange rings, spacers, bolts,

hydraulic tubing, hot-air ducts, helicopter rotor hubs

Satellites, rockets Rocket engine casings, fuel tanks

Chemical processing Storage tanks, agitators, pumps, columns, frames, screens, mixers, valves, pressurized reactors, filters, piping and

tubing, heat exchangers, electrodes and anode baskets for metal and chlorine-alkali electrolysis

Shipbuilding Heat exchangers, condensers, piping and tubing, propellers, propeller and rudder shafts, data logging equipment,

gyrocompasses thruster pumps, lifeboat parts, radar components, cathodic protection anodes, hydrofoil struts

Diving equipment Deep-sea pressure hulls, submarines (Soviet Union), submarine ball valves (United States)

Deep drilling Drill pipes, riser pipes, production tubulars, casing liners, stress joints, instrument cases, wire, probes

Automotive industry Connecting rods, valves, valve springs and retainers, crankshafts, camshafts, drive shafts, torsion bars, suspension

assemblies, coil springs, clutch components, wheel hubs, exhaust systems, ball and socket joints, gears

Machine tools Flexible tube connections, protective tubing, instrumentation and control equipment

Pulp and paper Bleaching towers, pumps, piping and tubing

Food processing Tanks (dairies, beverage industry), heat exchangers, components for packaging machinery

Construction Facing and roofing, concrete reinforcement, monument refurbishment (Acropolis), anodes for cathodic protection

Superconductors Wire rod of Ti-Nb alloys for manufacture of powerful electromagnets, rotors for superconductive generators

Fine art Sculptures, fountain bases, ornaments, doorplates

Consumer products

Jewelry industry Jewelry, clocks, watches

Optics Eyeglass frames, camera shutters

Sports equipment Bicycle frames, tennis rackets, shafts and heads for golf clubs, mountain climbing equipment (ice screws, hooks)

luges, bobsled components, horse shoes, fencing blades, target pistols

Musical instruments Harmonica reeds, bells

Personal security

and safety

Armor (cars, trucks, helicopters, fighter aircraft), helmets, bulletproof vests, protective gloves

Transportation Driven wheelsets for high-speed trains, wheel tires

Cutting implements Scissors, knives, pliers

Shape-memory

alloys

Nickel-titanium alloys for springs and flanges

Miscellaneous Pens, nameplates, telephone relay mechanisms, pollution-control equipment, titanium-lined vessels for salt-bath

nitriding of steel products

Source: Ref 7

Aerospace Applications

High specific strength, good fatigue resistance and creep life, and good fracture toughness are characteristics that make titanium a preferred metal for aerospace applications Figure 3 illustrates the rapid increase in use of titanium alloys in both airframe and engine applications for commercial aircraft

Fig 3 Increase of titanium consumption on commercial aircraft for both airframe and engine applications

Source: Ref 3

Airframe Components. The earliest production application of titanium was in 1952, for the nacelles and firewalls of

the Douglas DC-7 airliner Since that time titanium and titanium alloys have been used for structural components on aircraft ranging from the Boeing 707, to the supersonic SR-71 Blackbird reconnaissance aircraft, to space satellites and missiles

Jet Engine Components (Ref 8) Titanium fan disks (Fig 4), turbine blades and vanes, and structurals are commonly used in aircraft turbine engines Titanium research is an important aspect of the drive to increase engine efficiencies, and use of titanium in jet engine hot sections is expected to increase as materials capable of withstanding higher temperatures are developed (Ref 9; see also the section "New Developments" in this article)

Fig 4 Forged Ti-6Al-4V jet engine fan disks are 890 mm (35 in.) in diameter and weigh 249 kg (548 lb)

Courtesy of Wyman-Gordon Company

Titanium-base intermetallic compounds are another class of materials that promise increased engine thrust-to-weight ratios These are discussed briefly in the section "New Developments" in this article and in the article "Ordered Intermetallics" in this Volume

Use of precision titanium castings in jet engine applications such as inlet cases and compressor frames is on the rise The article "Titanium and Titanium Alloy Castings" in this Volume contains more information on titanium casting technology

Corrosion Applications

Commercially pure titanium is more commonly used than titanium alloys for corrosion applications, especially when high strength is not a requirement Economics are often the deciding factor in selection of titanium for corrosion resistance Some of the most common applications where the corrosion resistance of titanium is important are briefly described here

A comprehensive review of the corrosion behavior of titanium materials is available in the article "Corrosion of Titanium

and Titanium Alloys" in Corrosion, Volume 13 of ASM Handbook, formerly 9th Edition Metals Handbook

Chemical and Petrochemical Processing. Titanium equipment including vessels, pumps, fractionation columns, and storage tanks is essential in the manufacture of certain chemicals (Ref 10) Figure 5 illustrates two different uses for titanium in the chemical processing industry, and the article "Corrosion in the Chemical Processing Industry" in

Corrosion, Volume 13 of ASM Handbook, formerly 9th Edition Metals Handbook contains more information on the use

of titanium in the industry

Fig 5 Two common corrosion applications for commercially pure titanium components (a) Valve body (b)

Pump body Both are used in the chemical processing industry Courtesy of Oregon Metallurgical Corporation

Marine Engineering. Titanium use in ship designs and for offshore oil platforms has increased steadily in the last few years Applications include propeller and rudder shafts, thruster pumps, lifeboat parts, deep-sea pressure hulls, and submarine components (Ref 7) More information on titanium in marine applications is available in the article "Marine

Corrosion" in Corrosion, Volume 13 of ASM Handbook, formerly 9th Edition Metals Handbook

Energy Production and Storage. Titanium plate-type heat exchangers, condensers, and piping and tubing are common in energy facilities using seawater for cooling In power generating plants, titanium steam-turbine blades and generator retaining rings are used A critical application is in the main condensers of nuclear power plants, which must remain leak-free (Ref 10) Titanium-clad steel produced by roll cladding also is used for condenser and heat-exchanger tubesheets (Ref 10)

Two relatively new uses for titanium alloys are in flue gas desulfurization (FGD) units used to scrub emissions from fired power plants, and as canisters to contain low-level radioactive waste such as spent fuels from nuclear power plants These applications are discussed in the articles "Corrosion of Emission-Control Equipment" and "Corrosion in the

coal-Nuclear Power Industry" in Corrosion, Volume 13 of ASM Handbook, formerly 9th Edition Metals Handbook

Surgical Implants and Prosthetic Devices. The value of titanium in biomedical applications lies in its inertness in the human body, that is, resistance to corrosion by body fluids Titanium alloys are used in biomedical applications ranging from implantable pumps and components for artificial hearts, to hip and knee implants Titanium implants with specially prepared porous surfaces promote ingrowth of bone, resulting in stronger and longer-lasting bonds between

bone and implant (see the article "Corrosion of Metallic Implants and Prosthetic Devices" in Corrosion, Volume 13 of ASM Handbook, formerly 9th Edition Metals Handbook.)

A recent biomedical application for titanium alloys is the use of Ti-15Mo-5Zr-3Al wire for sutures and for implant fixation Using titanium wire eliminates the galvanic corrosion that can occur when titanium implants come in contact with other implant materials such as stainless steels and cobalt-base alloys (Ref 11) Another biomedical application exploits the shape-memory effect seen in nickel-titanium alloys to create compressive stresses that promote knitting of broken bones Shape-memory Ni-Ti alloys also have been employed experimentally to dilate blood vessels, thus increasing the flow of blood to vital organs (Ref 12)

Other Applications

The unique properties of titanium make it attractive to designers in a variety of industries Titanium is still relatively expensive compared to steel and aluminum, but increasing use of the metal in the areas discussed in this section is expected to accelerate cost reductions, resulting in still more growth in application diversity (Ref 10)

Automotive Components (Ref 10) At least one automobile maker is investigating the use of titanium in valve systems and suspension springs; however, no manufacturer has yet used titanium on production models Automotive parts considered to have excellent commercial potential for use of titanium are valves and valve retainers Racing automobiles

have made extensive use of titanium alloys for engine parts (Fig 6), drive systems, and suspension components for some years, while a titanium alloy connecting rod has been used successfully by a Japanese motorcycle manufacturer Development of low-cost, durable surface treatments is considered essential to the increased automotive use of titanium

Fig 6 Forged alloy connecting rod for a racing engine is indicative of increasing automotive applications for

titanium Component, courtesy of Jet Engineering Inc.; photograph by R.T Kiepura, ASM INTERNATIONAL

Architecture. Japanese architects have used titanium as a building material for some time (Ref 10) An example is the roof of the Kobe Municipal Aquarium, which used approximately 11,000 kg (24,000 lb) of titanium Although more costly than stainless steels, titanium is considered cost-effective in structures erected in the tropics and other areas where buildings are exposed to strong, warm sea winds (Ref 10)

Consumer Goods. Interest in titanium as a material for a wide variety of consumer products is on the rise Figure 7 shows a consumer application, and Table 2 lists numerous decorative and functional consumer applications for titanium

Fig 7 Lightweight forged titanium alloy wrenches are typical of growing consumer applications for titanium

Courtesy of Jet Engineering Inc

References cited in this section

3 H.B Bomberger, F.H Froes, and P.H Morton, Titanium A Historical Perspective, in Titanium Technology: Present Status and Future Trends, F.H Froes, D Eylon, and H.B Bomberger, Ed., Titanium

Development Association, 1985, p 3-17

7 K.-H Kramer, Titanium Applications A Critical Review, in Proceedings of the Sixth World Conference on

Titanium, P Lacombe, R Tricot, and G Beranger, Ed., Societe Francaise de Metallurgie, 1988, p 521

8 Y Honnorat, Titanium Alloys Use in Turbojet Engines, in Proceedings of the Sixth World Conference on Titanium, P Lacombe, R Tricot, and G Beranger, Ed., Societe Francaise de Metallurgie, 1988, p 365

9 R Sundaresan, A.G Jackson, and F.H Froes, Dispersion Strengthened Titanium Alloys Through

Mechanical Alloying, in Proceedings of the Sixth World Conference on Titanium, P Lacombe, R Tricot,

and G Beranger, Ed., Societe Francaise de Metallurgie, 1988, p 855

10 Y Fukuhara, Nonaerospace Applications of Titanium, in Proceedings of the Sixth World Conference on Titanium, P Lacombe, R Tricot, and G Beranger, Ed, Societe Francaise de Metallurgie, 1988, p 381

11 Y Ito, Y Sasaki, and T Shinke, Beta Titanium Wire for Surgical Implant Uses, in Proceedings of the Sixth World Conference on Titanium, P Lacombe, R Tricot, and G Beranger, Ed., Societe Francaise de

Sponge Production (Ref 13) Recent work has aimed at not only improving the efficiency of the Kroll process, but also developing new production methods One of the recently developed methods involves reduction of sodium fluorotitanate (Na2TiF5) by an aluminum-zinc alloy to produce a molten titanium-zinc alloy The zinc is then removed from this by evaporation Another process uses electrolysis to reduce either TiCl4 or titanium dioxide (TiO2) to titanium metal

Melting Practice. Titanium sponge is most commonly double vacuum-arc remelted with recycled scrap material and alloying elements to produce titanium alloy ingot Electron beam and plasma cold-hearth melting are relatively new melting practices designed to minimize internal ingot defects Longer dwell times in the liquid pool, longer solution periods, and better mixing prevent nonmetallic inclusions and unmelted refractory metals from being incorporated into the ingot

Powder Metallurgy Alloys. Powder metallurgy production techniques such as rapid solidification processing and mechanical alloying are being used to produce titanium alloys with novel compositions that would be impossible to achieve through conventional processing (Ref 9, 14) Titanium alloys produced by these methods may contain rare earth elements such as cerium, or large quantities of β stabilizers, which tend to segregate under normal processing conditions Oxide dispersion-strengthening, an approach widely used to enhance the properties of nickel-base alloys, is also possible using P/M techniques to incorporate dispersion-forming elements such as silicon and boron into the titanium alloy matrix

Titanium-Base Intermetallic Compounds. Ordered intermetallics with composition near Ti3Al (actually 11Nb) have better oxidation resistance, lower density, improved creep resistance, and higher modulus than conventional titanium alloys (Ref 15) These materials have the potential to greatly increase the thrust-to-weight ratio of aircraft engines Full-scale heats of Ti3Al have been produced and fabricated using conventional equipment into billet, plate, and sheet The article "Ordered Intermetallics" in this Volume contains more information on titanium aluminide intermetallics

Ti-24Al-Titanium-Matrix Composites. Metal-matrix composites (MMCs) combine the attributes of the base (matrix) metal with those of a reinforcing phase In the case of titanium-base MMCs, this combination of properties translates to low density with increased high-temperature strength and stiffness (Fig 8) Titanium-matrix composites have been fabricated using a variety of techniques, including P/M processing (Ref 16, 17) More information on MMCs is available in the article "Metal-Matrix Composites" in this Volume

Fig 8 High-temperature strength and stiffness of a titanium MMC compared to conventional alloy Ti-6Al-4V

Produced using powder metallurgy techniques, the MMC consists of a Ti-6Al-4V matrix reinforced with 10% titanium carbide (TiC) particles Source: Ref 16

Superplastic Forming and Diffusion Bonding. Superplastic forming and concurrent diffusion bonding of titanium alloy sheet components is a technology that has moved out of the laboratory and into commercial production (Ref 18, 19) The process has the potential to drastically reduce the number of parts and fasteners needed in airframe structures and other complex components More information on SPF/DB of titanium also is available in the articles "Forming of

Titanium and Titanium Alloys" and "Superplastic Sheet Forming" in Forming and Forging, Volume 14 of ASM Handbook, formerly 9th Edition Metals Handbook

Recycling of Titanium Scrap. As the titanium industry has matured, the use of recycled material has increased In recent years even machine turnings and chips have been approved for recycling, and U.S titanium producers used nearly

18 million kg (40 million pounds) of titanium scrap in 1988 (Ref 20) More information on recycling of titanium alloys is available in the article "Recycling of Nonferrous Alloys" in this Volume

References cited in this section

9 R Sundaresan, A.G Jackson, and F.H Froes, Dispersion Strengthened Titanium Alloys Through

Mechanical Alloying, in Proceedings of the Sixth World Conference on Titanium, P Lacombe, R Tricot,

and G Beranger, Ed., Societe Francaise de Metallurgie, 1988, p 855

13 T Tanaka, New Development in Titanium Elaboration Sponge, Melting and Casting, in Proceedings of the Sixth World Conference on Titanium, P Lacombe, R Tricot, and G Beranger, Ed., Societe Francaise de

Metallurgie, 1988, p 11

14 R Sundaresan and F.H Froes, Development of the Titanium-Magnesium Alloy System Through

Mechanical Alloying, in Proceedings of the Sixth World Conference on Titanium, P Lacombe, R Tricot,

and G Beranger, Ed., Societe Francaise de Metallurgie, 1988 p 931

15 J.D Destefani, Advances in Intermetallics, Adv Mater Process., Feb 1989, p 37-41

16 S Abkowitz and P Weihrauch, Trimming the Cost of MMCs, Adv Mater Process., July 1989, p 31-34

17 C.M Cooke, D Eylon, and F.H Froes, Development of Rapidly Solidified Titanium Matrix Composites, in

Proceedings of the Sixth World Conference on Titanium, P Lacombe, R Tricot, and G Beranger, Ed.,

Societe Francaise de Metallurgie, 1988, p 913

18 P.-J Winkler, Recent Advances in Superplasticity and Superplastic Forming of Titanium Alloys, in

Proceedings of the Sixth World Conference on Titanium, P Lacombe, R Tricot, and G Beranger, Ed.,

Societe Francaise de Metallurgie, 1988, p 1135

19 E Tuegel, M.O Pruitt, and L.D Hefti, SPF/DB Takes Off, Adv Mater Process., July 1989, p 36-41

20 Titanium 1988 Statistical Review, Titanium Development Association, 1989, p 5

Wrought Titanium and Titanium Alloys

S Lampman, ASM INTERNATIONAL

Introduction

THE WROUGHT product forms of titanium and titanium-base alloys, which include forgings and the typical mill products, constitute (on a weight basis) more than 70% of the market in titanium and titanium-alloy production Various specifications for wrought titanium-base products are listed in Table 1 The wrought products are the most readily available product form of titanium-base materials, although cast and powder metallurgy (P/M) products are also available for applications that require complex shapes or the use of P/M techniques to obtain microstructures not achievable by conventional ingot metallurgy Powder metallurgy of titanium has not gained wide acceptance and is restricted to space and missile applications Cast and P/M titanium-base products are discussed in the subsequent articles in this Volume

Table 1 Various specifications for wrought products of titanium and titanium alloys

Specification or standard for:

4953, 4956; Other,

extrusions, 4933-4936

Bolts and screws, 7640; Spring wire, 4959; and rings

in listings for extrusions and bars

F 136 and F 620

SAE (see also

AMS listings)(a)

MIL-T-9047

MIL-T-24585 (rod), MIL-R-

81588 (welding rod and wire)

MIL-T-81556

(aircraft quality bar and shape extrusions)

MIL-T-40635: high-strength wrought Ti alloys for critical components

Welding wire, H3331

H4630 and H4631

TIS

7913, TIS

Welding wire, D 7030

prEN2520, prEN2522, prEN2524, prEN2531

prEN2518, prEN2519, prEN2521, prEN2530, prEN2532- prEN2534

British Standards

Institution

2TA.1, 2TA.2, 2TA.6, 2TA.10, 2TA.21,

2TA.4, 2TA.5, 2TA.8, 2TA.9, 2TA.12, TA.13, 2TA.23, TA.24, TA.39, TA.41-

Bar and section: 2TA.3, 2TA.7, 2TA.11, 2TA.22,

Wire for fasteners, TA.28

Fasteners,

TA.28; Surgical implants, BS

3531 (part 1, 2)

TA.52, TA.56-TA.59

TA.44, TA.47, TA.48, TA.50, TA.51, TA.54, TA.55

TA.38, TA.45, TA.46, TA.49, TA.53

Source: Adapted from Ref 1

(b) Except for the TIS 7607 standard for forgings, the listed TIS are for titanium-palladium alloys only

The primary reasons for using titanium-base products stem from the outstanding corrosion resistance of titanium and/or its useful combination of low density (;4.5 g/cm3, or 0.16 lb/in.3) and high strength (minimum 0.2% yield strengths vary from 480 MPa, or 70 ksi, for some grades of commercial titanium to about 1100 MPa, or 160 ksi, for structural titanium alloy products and over 1725 MPa, or 250 ksi, for special forms such as wires and springs) Some titanium alloys (especially the low-interstitial alpha alloys) are also useful in subzero and cryogenic applications because these alpha alloys do not exhibit a ductile-brittle transition

Another important characteristic of titanium-base materials is the reversible transformation (or allotropy) of the crystal structure from an alpha (α) (hexagonal close-packed) structure to a beta (β) (body-centered cubic) structure when the temperatures exceed a certain level This allotropic behavior, which depends on the type and amount of alloy contents, allows complex variations in microstructure and more diverse strengthening opportunities than those of other nonferrous alloys such as copper or aluminum This diversity of microstructure and properties depends not only on alloy additions but also on thermomechanical processing By varying thermal or mechanical processing, or both, a broad range of properties can be produced in titanium alloys

The elevated-temperature strength and creep resistance of titanium-base materials (along with the pickup of interstitial impurities due to the chemical reactivity of titanium) limits the elevated-temperature application of wrought and cast products to about 540 °C (1000 °F) or perhaps 600 °C (1100 °F) in some cases For higher temperatures, Ti-aluminide products are an active area of research and development (see the article "Ordered Intermetallics" in this Volume)

Acknowledgements

The author would like to thank Rodney R Boyer of the Boeing Commercial Aircraft Company and Stan R Seagle of RMI Titanium Company for their review of the manuscript and their meaningful suggestions for improvement

Reference

1.M.J Donachie, Jr., Titanium: A Technical Guide, ASM INTERNATIONAL, 1988

Commercially Pure Titanium

Pure titanium wrought products, which have minimum titanium contents ranging from about 98.635 to 99.5 wt% (Table 2), are used primarily for corrosion resistance Titanium products are also useful in applications requiring high ductility for fabrication but relatively low strength (Table 2) in service

Table 2 Comparison of various specifications for commercially pure titanium mill products

Tensile properties (a)

Designation Chemical composition, % max

Ultimate strength

Yield strength Minimum

elongation,

%

Source: Adapted from Ref 1

(a) Unless a range is specified, all listed values are minimums

(b) Only for sheet, plate, and coil

(c) Hydrogen limits vary according to product form as follows: 0.015H (sheet), 0.0125H (bar), and 0.0100H (billet)

Corrosion Resistance and Chemical Reactivity. Although titanium is a highly reactive metal, titanium also has an extremely high affinity for oxygen and thus forms a very stable and highly adherent protective oxide film on its surface This oxide film, which forms spontaneously and instantly when fresh metal surfaces are exposed to air and/or moisture, provides the excellent corrosion resistance of titanium However, anhydrous conditions in the absence of a source of oxygen may result in titanium corrosion, because the protective film may not be regenerated if damaged This is particularly true of crevice corrosion Titanium and titanium alloys may be subject to localized attack in tight crevices exposed to hot (>70 °C, or 160 °F) chloride, bromide, iodide, fluoride, or sulfate-containing solutions Crevices can stem from adhering process stream deposits or scales, metal-to-metal joints (for example, poor weld joint design or tube-to-tubesheet joints), and gasket-to-metal flange and other seal joints The mechanism for crevice corrosion of titanium is similar to that for stainless steels, in which oxygen-depleted reducing acid conditions develop within tight crevices

General corrosion rates for unalloyed titanium (99.2 wt% Ti with traces of oxygen) in selected media are given in Table

3 These data should be used only as a guideline for general performance Rates may vary depending on changes in medium chemistry, temperature, length of exposure, and other factors Also, total suitability of an alloy cannot be assumed from these values alone, because other forms of corrosion, such as localized attack, may be limiting These and

other factors affecting the corrosion of titanium are discussed in more detail in Corrosion, Volume 13 of ASM Handbook, formerly 9th Edition Metals Handbook

Table 3 Corrosion rates for unalloyed titanium (99.2% Ti) in selected media

Temperature Corrosion rate

°C °F m/yr mils/yr

5, 25, 75 100 212 nil nil Acetic acid

50, 99.5 100 212 0.25 0.01

Aluminum chloride, aerated

5, 10 60 140 <2.5 <0.1

More than 0.013% H 2 O 79 175 nil nil

5 + 10% NaCl 100 212 <13 <0.5

10 + 1 65 150 <25 <1

Hydrogen sulfide Saturated water 25 77 <125 <5

10-85 100 212 <125 <5 Lactic acid

10-100 Boiling <125 <5

40 200 392 <1250 <50

Saturated 25 77 nil nil

Zinc chloride

20 100 212 <125 <5

Precautions in Use. Hydrogen embrittlement of titanium can occur in pickling solutions (or other hydrogenating solutions) at room temperature and at elevated temperatures during air exposure or in exposures to reducing atmospheres Nonetheless, titanium alloys are widely used in hydrogen-containing environments and under conditions in which galvanic couples or cathodic charging (impressed current) causes hydrogen to be evolved on metal surfaces Although hydrogen embrittlement has been observed, traces of moisture or oxygen in hydrogen gas containing environments effectively form the protective oxide film, thus avoiding or limiting hydrogen uptake (Ref 2, 3, 4, 5) On the other hand, anhydrous hydrogen gas atmospheres may lead to absorption, particularly as temperatures and pressures increase Hydrogen embrittlement can also occur at relatively low hydrogen levels due to the hydrogen in the material in the presence of a stress riser under certain conditions

Elevated temperature atmospheric exposure also results in oxygen and nitrogen contamination that increases in severity with increasing temperature and time of exposure Violent oxidation reactions can occur between titanium and liquid oxygen or between titanium and red fuming nitric acid Titanium alloys exhibit good corrosion resistance to white fuming nitric acids

Crystal Structure. Pure titanium at room temperature has an alpha (hexagonal close-packed) crystal structure, which transforms to a beta (body-centered cubic) structure at a temperature of about 885 °C (1625 °F) This transformation temperature can be raised or lowered depending on the type and amount of impurities or alloying additions The addition

of alloying elements also divides the single temperature for equilibrium transformation into two temperatures the alpha transus, below which the alloy is all-alpha, and the beta transus, above which the alloy is all-beta Between these temperatures, both alpha and beta are present Depending on the level of impurities, the beta transus is about 910 ± 15 °C (1675 ± 25 °F) for commercially pure titanium with 0.25 wt% O2 max and 945 ± 15 °C (1735 ± 25 °F) with 0.40 wt% O2

max For the various ASTM grades of commercially pure titanium, typical transus temperatures (with an uncertainty of about ± 15 °C, or ±25 °F) are:

Typical β transus

Typical α transus Designation