Handbook Properties and Selection Nonferrous Alloys and Spl Purpose Mtls (1992) WW Part 11 docx

1 Coefficient of linear expansion at 20 °C versus Ni content for Fe-Ni alloys containing 0.4% Mn and 0.1% C After the discovery of Invar, an intensive study was made of the thermal and

total of less than 1%) has a coefficient of expansion so low that its length is almost invariable for ordinary changes in temperature This alloy is known as Invar, which is a trade name (of Imphy, S.A.) meaning invariable

Fig 1 Coefficient of linear expansion at 20 °C versus Ni content for Fe-Ni alloys containing 0.4% Mn and 0.1% C

After the discovery of Invar, an intensive study was made of the thermal and elastic properties of several similar alloys Iron-nickel alloys that have nickel contents higher than that of Invar retain to some extent the expansion characteristics of Invar Alloys that contain less than 36% nickel have much higher coefficients of expansion than alloys containing 36% or more nickel Further information on iron-nickel alloys besides Invar is given in the section "Iron-Nickel Alloys Other Than Invar" in this article

Invar

Invar (UNS number K93601) and related alloys have low coefficients of expansion over only a rather narrow range of temperature (see Fig 2) At low temperatures in the region from A to B, the coefficient of expansion is high In the interval between B and C, the coefficient decreases, reaching a minimum in the region from C to D With increasing temperature, the coefficient begins again to increase from D to E, and thereafter (from E to F), the expansion curve follows a trend similar to that of the nickel or iron of which the alloy is composed The minimum expansivity prevails only in the range from C to D

Fig 2 Change in length of a typical Invar over different ranges of temperature

In the region between D and E in Fig 2, the coefficient is changing rapidly to a higher value The temperature limits for a well-annealed 36% Ni iron are 162 and 271 °C (324 and 520 °F) These temperatures correspond to the initial and final losses of magnetism in the material (that is, the Curie temperature) The slope of the curve between C and D is then a measure of the coefficient of expansion over a limited range of temperature

Table 1 gives coefficients of linear expansion of iron-nickel alloys between 0 and 38 °C (32 and 100 °F) The expansion behavior of several iron-nickel alloys over wider ranges of temperature is represented by curves 1 to 5 in Fig 3 For comparison, Fig 3 also includes the similar expansion obtained for ordinary steel

Table 1 Thermal expansion of Fe-Ni alloys between 0 and 38 °C

Ni, % Mean coefficient, μm/m · K

Fig 3 Thermal expansion of Fe-Ni alloys Curve 1, 64Fe-31Ni-5Co; curve 2, 64Fe-36Ni (Invar); curve 3, 58Fe-42Ni; curve 4, 53Fe-47Ni;

curve 5, 48Fe-52Ni; curve 6, carbon steel (0.25% C)

Effects of Composition on Expansion Coefficient. The effect of variation in nickel content on linear expansivity is shown in Fig 1 Minimum expansivity occurs at about 36% Ni, and small additions of other metals have considerable influences on the position of this minimum Because further additions of nickel raise the temperature at which the inherent magnetism

of the alloy disappears, the inflection temperature in the expansion curve (Fig 2) also rises with increasing nickel content

The addition of third and fourth elements to Fe-Ni provides useful changes of desired properties (mechanical and physical), but significantly changes thermal expansion characteristics Minimum expansivity shifts toward higher nickel contents when manganese or chromium is added, and toward lower nickel contents when copper, cobalt, or carbon is added Except for the ternary alloys with nickel-iron-cobalt compositions (Super-Invars), the value of the minimum expansivity for any of these ternary alloys is, in general, greater than that of a typical Invar alloy

The effects of additions of manganese, chromium, copper, and carbon are shown in Fig 4 Additions of silicon, tungsten, and molybdenum produce effects similar to those caused by additions of manganese and chromium; the composition of minimum expansivity shifts towards higher contents of nickel Addition of carbon is said to produce instability in Invar, which is attributed to the changing solubility of carbon in the austenitic matrix during heat treatment

Fig 4 Effect of alloying elements on expansion characteristics of Fe-Ni alloys (a) Displacement of nickel content caused by additions of

manganese, chromium, copper, and carbon to alloy of minimum expansivity (b) Change in value of minimum coefficient of expansion caused by additions of manganese, chromium, copper, and carbon

Effects of Processing. Heat treatment and cold work change the expansivity of Invar alloys considerably The effect of heat treatment for a 36% Ni Invar alloy is shown in Table 2 The expansivity is greatest in well-annealed material and least in quenched material

Table 2 Effect of heat treatment on coefficient of thermal expansion of Invar

Heat Treatment. The iron-nickel binary alloys are not hardenable by heat treatment Annealing practice should be adjusted

to be consistent with requirements of the intended application Exposure to temperatures and times that promote excessive grain growth will limit further fabricating steps that require extreme bending, forming, deep drawing, chemical etching, and so forth

Annealing is done at 750 to 850 °C (1380 to 1560 °F) When the alloy is quenched in water from these temperatures, expansivity is decreased, but instability is induced both in actual length and in coefficient of expansion To overcome these deficiencies and to stabilize the material, it is common practice to stress relieve approximately at 315 to 425 °C (600

to 800 °F) and to age at a low temperature 90 °C (200 °F) for 24 to 48 hours

Cold drawing also decreases the thermal expansion coefficient of Invar alloys The values for the coefficients in the following table are from experiments on two heats of Invar:

1.4 (heat 2)

0.5 (heat 1) Annealed and quenched

0.8 (heat 2)

0.14 (heat 1) Quenched and cold drawn (>70% reduction with a diameter of 3.2 to 6.4 mm, or 0.125 to 0.250 in.)

0.3 (heat 2)

By cold working after quenching, it is possible to produce material with a zero, or even a negative, coefficient of expansion A negative coefficient may be increased to zero by careful annealing at a low temperature However, these artificial methods of securing an exceptionally low coefficient may produce instability in the material With lapse of time and variation in temperature, exceptionally low coefficients usually revert to normal values For special applications (geodetic tapes, for example), it is essential to stabilize the material by cooling it slowly from 100 to 20 °C (212 to 68 °F) over a period of many months, followed by prolonged aging at room temperature However, unless the material is to be used within the limits of normal atmospheric variation in temperature, such stabilization is of no value Although these variations in heat-treating practice are important in special applications, they are of little significance for ordinary uses

Magnetic Properties. Invar and all similar iron-nickel alloys are ferromagnetic at room temperature and become paramagnetic at higher temperatures Because additions in nickel contents raise the temperature at which the inherent magnetism of the alloy disappears, the inflection temperature in the expansion curve rises with increasing nickel content The loss of magnetism in a well-annealed sample of a true Invar begins at 162 °C (324 °F) and ends at 271 °C (520 °F)

In a quenched sample, the loss begins at 205 °C (400 °F) and ends at 271 °C (520 °F) Figure 5 shows how the Curie temperature changes with nickel content in iron

Fig 5 Effect of nickel content on the Curie temperature of iron-nickel alloys

The thermoelastic coefficient, which describes the changes in the modulus of elasticity as a function of temperature, varies according to the nickel content of iron-nickel low-expansion alloys Invar has the highest thermoelastic coefficient of all low-expansion iron-nickel alloys, while two alloys with 29 and 45% nickel have a zero thermoelastic coefficient (that is, the modulus of elasticity does not change with temperature) However, because small variations in nickel content produce large variations in the thermoelastic coefficient, commercial application of these two iron-nickel alloys with a zero thermoelastic coefficient is not practical Instead, the iron-nickel-chromium Elinvar alloy provides a practical way of achieving a zero thermoelastic coefficient

Electrical Properties. The electrical resistivity of 36Ni-Fe Invar is between 750 and 850 nΩ · m at ordinary temperatures The temperature coefficient of electrical resistivity is about 1.2 mΩ/Ω · K over the range of low expansivity As nickel

content increases above 36%, the electrical resistivity decreases to ~165 nΩ · M at ~80% NiFe This is illustrated in Fig

6

Fig 6 Effect of nickel content on electrical resistivityof nickel-iron alloys

Other Physical and Mechanical Properties. Table 3 presents data on miscellaneous properties of Invar in the hot-rolled and forged conditions The effects of temperature on mechanical properties of forged 66Fe-34Ni are illustrated in Fig 7

Table 3 Physical and mechanical properties of Invar

Specific heat, at 25-100 °C (78-212 °F), J/kg · °C (Btu/lb · °F) 515 (0.123)

Thermal conductivity, at 20-100 °C (68-212 °F), W/m · K (Btu/ft · h · °F) 11 (6.4)

Thermoelectric potential (against copper), at -96 °C (-140 °F), μV/K 9.8

Fig 7 Mechanical properties of a forged 34% Ni alloy Alloy composition: 0.25 C, 0.55 Mn, 0.27 Si, 33.9 Ni, balance Fe Heat treatment:

annealed at 800 °C (1475 °F) and furnace cooled

The binary iron-nickel alloys are not hardenable by heat treatment Significant increases in strength can be obtained by cold working some product forms such as wire, strip, and small-diameter bar Table 4 shows tensile and hardness data for both 36% and 50% nickel-iron alloys after cold working various percent cross-section reduction

Table 4 Mechanical properties of Invar and a 52% Ni-48% Fe glass-sealing alloy

UNS number (alloy name) 0.2%

yield strength

Ultimate tensile strength

Elongation,

%

Approximate equivalent hardness, HRB

MPa ksi MPa ksi

0.2% yield strength

Ultimate tensile strength UNS number (alloy name)

°C °F MPa ksi MPa ksi

315 600 95 14 420 61 50 73

480 900 90 13 275 40 63 73

24 75 295 43 550 80 43.7 73.5

150 300 225 32.5 510 74 45.6 67.1

315 600 188 27.3 495 71.8 52.8 67.1 42% Ni low-expansion alloy (K94100)

480 900 157 22.8 370 54 43 58.4

24 75 300 43.3 538 78 46.2 79.3

150 300 243 35.3 483 70 43.2 75.6

315 600 223 32.3 462 67 42 73.5 49% Ni low expansion alloy

480 900 217 31.5 385 55.8 35.5 51.9

Corrosion Resistance. The iron-nickel low-expansion alloys are not corrosion resistant, and applications in even relatively mild corrosive environments must consider their propensity to corrode A comparison to corrosion of iron, in both high humidity and salt spray environments, is shown in Fig 8 and Table 6 Rust initiation occurs in approximately 24 hours for nickel contents less than ~40% in high-humidity tests Severe corrosion occurs after 200 hours exposure to a neutral salt spray at 35 °C (95 °F)

Table 6 Effects of relative humidity on selected nickel-iron low-expansion alloys

Specimens exposed to 95% relative humidity for 200 h at 35 °C (95 °F)

Alloy type (UNS number) Condition Portion of surface

rusted (average), % (a)

Cold rolled Few rust spots 96, 96, 96

42% Ni low-expansion alloy (K94100) Annealed 0

(a) Visual estimate of the percentage of surface rusted

(b) Provided under the tradename of "Temperature Compensation 30" by Carpenter Technology Corporation

(c) Provided under the tradenames of "MolyPermalloy" by Allegheny Ludlum and "HyMu80" by Carpenter Technology Corporation

Fig 8 Rust versus nickel content from 200 h neutral salt spray at 35 °C (95 °F)

Machinability. The iron-nickel alloys can be machined using speeds or feeds that are modified to accommodate their gummy and stringy characteristics As a general comparison to other austenitic alloys, they are similar to 316 stainless steel Using single point turning as a measure of machinability, the iron-nickel low-expansion alloys exhibit a 25% machinability rating compared to resulfurized carbon steel (such as B 1112) Some general machining parameters for

iron-nickel alloys are shown in Table 7 There are "free-cut" varieties of Invar-type alloys available These require minor additions of other elements (such as selenium) which when combined with moderate adjustments to other residual elements (such as manganese) will produce a twofold improvement in the machinability characteristic of these alloys Some increase in thermal expansion characteristics results from the modified compositions

Table 7 Examples of various machining parameters for iron-nickel low-expansion alloys

Turning (single-point and box tools)

Roughing

Finishing

Turning (cutoff and form tools)

Feed, mm/rev (in./rev) with a tool width of:

Speed, m/min (ft/min) 10 (35)

Feed, mm/rev (in./rev) for a drill diameter of:

Tapping speed, m/min (ft/min)

End milling parameters

With 0.5 mm (0.020 in.) radial depth of cut:

Feed, mm/tooth (in./tooth), with 13 mm (1

2 in.) cutter diam

0.05 (0.002)

Feed, mm/tooth (in./tooth), with 25-50 mm (1-2 in.) cutter diam 0.10 (0.004)

With 1.5 mm (0.06 in.) radial depth of cut:

Feed, mm/tooth (in./tooth), with 13 mm (1

2 in.) cutter diam

0.075 (0.003)

Feed, mm/tooth (in./tooth), with 25-50 mm (1-2 in.) cutter diam 0.125 (0.005)

Welding. Invar can be successfully welded using most standard arc-welding processes In general, preparation for welding should be similar to stainless steels and should include proper cleaning and handling Joint designs should allow easy access to the weld because of poor weld pool fluidity but also should limit total weld volume to reduce shrinkage problems Preheating and postheating are not required and should be avoided A low interpass temperature (150 °C, or

300 °F max) should be maintained

Welding is most commonly performed using the gas-tungsten-arc or gas-metal-arc processes Gas-tungsten-arc welding can be accomplished with argon and/or helium shielding gases Welding is best performed with a freshly ground thoriated tungsten electrode Gas-metal-arc welding can be successfully performed in all metal transfer modes, depending primarily

on base metal thickness Shielding gases should be argon or argon-helium mixtures Other nonarc welding processes (such as resistance welding) may also be used

When a filler metal is needed, a matching composition will provide the best match in thermal expansion properties Invarod weld filler metal (a 36Ni-Fe alloy containing ~1% Ti and 2.5% Mn) has been successfully used for matching expansion characteristics If a matching composition is not available, a high-nickel filler metal conforming to AWS A5.14 ERNi-1 or ER-NiCrFe-5 can be used These materials will result in a weld with different thermal expansion properties

Iron-Nickel Alloys Other Than Invar

Although iron-nickel alloys other than Invar have higher coefficients of thermal expansion, there are applications where it

is advantageous to have nickel contents above or below the 36% level of Invar The alloy containing 39% Ni, for example, has a coefficient of expansion corresponding to that of low-expansion glasses

Alloys that contain less than 36% Ni have much higher coefficients of expansion than alloys with a higher percentage Alloys containing less than 36% Ni include temperature-compensator alloys (30 to 34% Ni) These exhibit linear changes

in magnetic characteristics with temperature change They are used as compensating shunts in metering devices and speedometers

Iron-nickel alloys that have nickel contents higher than that of Invar retain to some extent the expansion characteristics of Invar Because further additions of nickel raise the temperature at which the inherent magnetism of the alloy disappears, the inflection temperature in the expansion curve (Fig 2) rises with increasing nickel content Although this increase in range is an advantage in some circumstances, it is accompanied by an increase in coefficient of expansion Table 8 and Fig 9 present additional information on the coefficients of expansion of nickel-iron alloys at temperatures up to the inflection temperature They also give data on alloys with up to 68% Ni

Table 8 Expansion characteristics of Fe-Ni alloys

Fig 9 Effect of nickel content on expansion of Fe-Ni alloys (a) Variation of inflection temperature (b) Variation of average coefficient of

expansion between room temperature and inflection temperature

Of significant commercial interest are those alloys containing approximately 40% to 50% nickel-iron alloys Typical compositions and thermal expansions for some of these alloys are given in Table 9

Table 9 Composition and typical thermal expansion coefficients for common iron-nickel low-expansion alloys

(a) Balance of iron with residual impurity limits of 0.25% max Si, 0.015% max P, 0.01% max S, 0.25% max Cr, and 0.5% max Co

(b) From room temperature to 90 °C (200 °F)

(c) From room temperature to 150 °C (300 °F)

(d) From room temperature to 370 °C (700 °F)

The 42% Ni-irons are widely used in applications for their low-expansion characteristics These include semiconductor packaging components, thermostat bimetals, incandescent light bulb glass seal leads (copper clad), and seal beam lamps

Dumet wire is an alloy containing 42% Ni It is clad with copper to provide improved electrical conductivity and to prevent gassing at the seal It can replace platinum as the seal-in wire in incandescent lamps and vacuum tubes

The 43 to 47% Ni-iron alloys are commonly used for glass seal leads, grommets, and filament supports This group of alloys includes Platinate (36% Ni to 64% Fe), which has a coefficient of thermal expansion equivalent to that of platinum (9.0 ppm/°C)

Iron-Nickel-Chromium Alloys

Elinvar is a low-expansion iron-nickel-chromium alloy with a thermoelastic coefficient of zero over a wide temperature range It is more practical than the straight iron-nickel alloys with a zero thermoelastic coefficient, because its thermoelastics coefficient is less susceptible to variations in nickel content expected in commercial melting

Elinvar is used for such articles as hair-springs and balance wheels for clocks and watches and for tuning forks used in radio synchronization Particularly beneficial where an invariable modulus of elasticity is required, it has the further advantage of being comparatively rustproof

The composition of Elinvar has been modified somewhat from its original specification of 36% Ni and 12% Cr The limits now used are 33 to 35 Ni, 61 to 53 Fe, 4 to 5 Cr, 1 to 3 W, 0.5 to 2 Mn, 0.5 to 2 Si, and 0.5 to 2 C Elinvar, as created by Guillaume and Chevenard, contains 32% Ni, 10% Cr, 3.5% W, and 0.7% C

Other iron-nickel-chromium alloys with 40 to 48% Ni and 2 to 8% Cr are useful as glass-sealing alloys because the chromium promotes improved glass-to-metal bonding as a result of its oxide-forming characteristics The most common

of these contain approximately 42 to 48% nickel with chromium of 4 to 6% Although chromium additions increase the minimum thermal expansion and lower inflection points (Curie temperature), they have a beneficial effect on the glass-sealing behavior of these alloys The chromium promotes formation of a surface chromium oxide that improves wetting at the metal/glass interface Some of this metal oxide is absorbed by the glass during the actual glass seal and promotes a higher-strength metal/glass bond (graded seals) Compositions and thermal expansions for some Fe-Ni-Cr alloys are shown in Table 10

Table 10 Type, composition, and typical thermal expansion for some iron-nickel-chromium glass-seal alloys

Composition (a) , % Alloy type ASTM

Super-Invar. Substitution of ~5% Co for some of the nickel content in the 36% Ni (Invar) alloy provides an alloy with an expansion coefficient even lower than that Invar A Super-Invar alloy with a nominal 32% Ni and 4 to 5% Co will exhibit

a thermal expansion coefficient close to zero, over a relatively narrow temperature range Figure 10 compares thermal expansion for 32% Ni-5% Co Super-Invar with that of an Invar alloy

Fig 10 Comparison of thermal expansion for Super-Invar (63% Fe, 32% Ni, 5% Co) and Invar (64% Fe, 36% Ni) alloys

Cobalt has been added to other Fe-Ni alloys in amounts as high as 40% Such additions increase the coefficient of expansion at room temperature However, because they also raise the inflection temperature, they produce an alloy with a moderately low coefficient of expansion over a wider range of temperature If Θis inflection temperature in °C, X is

nickel content, Y is cobalt content, and Z is manganese content The inflection temperature of any low-expansion

Fe-Ni-Co alloy is approximated by θ= 19.5 (X + Y) - 22Z - 465 Carbon content does not significantly affect the inflection

temperature

For practical applications, these Fe-Ni-Co alloys require that Ni+Co content be sufficient to lower the martensite start temperature (Ms) to well below room temperature Nickel-cobalt contents for Ms temperatures of about -100 °C (-150 °F) can be approximated by:

Y = 0.0795 θ+ 4.82 + 19W - 18.1

X = 41.9 - 0.0282 θ- 37Z - 19W

where W is carbon content

Kovar is a nominal 29%Ni-17%Co-54%Fe alloy that is a well-known glass-sealing alloy suitable for sealing to hard (borosilicate) glasses Kovar has a nominal expansion coefficient of approximately 5 ppm/°C and inflection temperature

of ~450 °C (840 °F) with an Ms temperature less than -80 °C (-110 °F) The Dilver-P alloy produced by Imphy, S.A., is a competitive grade with the Kovar alloy of Carpenter Steel

Special Alloys

Iron-Cobalt-Chromium Low-Expansion Alloys. An alloy containing 36.5 to 37%Fe, 53 to 54.5% Co, and 9 to 10% Cr has an exceedingly low, and at times, negative (over the range from 0 to 100 °C, or 32 to 212 °F) coefficient of expansion This alloy has good corrosion resistance compared to low-expansion alloys without chromium Consequently, it has been referred to as "Stainless Invar." Fernichrome, a similar alloy containing 37% Fe, 30% Ni, 25% Co, and 8% Cr, has been used for seal-in wires for electronic components sealed in special glasses

Hardenable Low-Expansion Alloys. Alloys that have low coefficients of expansion, and alloys with constant modulus of elasticity, can be made age hardenable by adding titanium In low-expansion alloys, nickel content must be increased when titanium is added The higher nickel content is required because any titanium that has not combined with the carbon

in the alloy will neutralize more than twice its own weight in nickel by forming an intermetallic compound during the hardening operation

As shown in Table 11, addition of titanium raises the lowest attainable rate of expansion and raises the nickel content at which the minimum expansion occurs Titanium also lowers the inflection temperature Mechanical properties of alloys containing 2.4% titanium and 0.06% carbon are given in Table 12

Table 11 Minimum coefficient of expansion in low-expansion Fe-Ni alloys containing titanium

Ti, % Optimum Ni, % Minimum coefficient of

expansion, μm/m · K

Table 12 Mechanical properties of low-expansion Fe-Ni alloys containing 2.4 Ti and 0.06 C

Tensile strength Yield strength Condition

MPa ksi MPa ksi

Elongation (a) ,

%

Hardness,

HB

42Ni-55.5Fe-2.4Ti-0.06C

Solution treated and age hardened 1140 165 825 120 14 330

Solution treated, cold rolled 50% and age hardened 1345 195 1140 165 5 385

52Ni-45.5Fe-2.4Ti-0.06C (c)

(a) In 50 mm (2 in.)

(b) Inflection temperature, 220 °C (430 °F); minimum coefficient of expansion, 3.2 μm/m · K

(c) Inflection temperature, 440 °C (824 °F); minimum coefficient of expansion, 9.5 μm/m · K

In alloys of the constant-modulus type containing chromium, addition of titanium allows the thermoelastic coefficients to

be varied by adjustment of heat-training schedules The alloys in Table 13 are the three most widely used compositions The recommended solution treatment for the alloys that contain 2.4% Ti is 950 to 1000 °C (1740 to 1830 °F) for 20 to 90 min., depending on section size Recommended duration of aging varies from 48 h at 600 °C (1110 °F) to 3 h at 730 °C (1345 °F) for solution-treated material

Table 13 Thermoelastic coefficients of constant modulus Fe-Ni-Cr-Ti alloys

Composition, %

Ni Cr C Ti

Thermoelastic coefficient, annealed condition, μm/m · K

Range of possible coefficients (a) , μm/m · K

(a) Any value in this range can be obtained by varying the heat treatment

For material that has been solution treated and subsequently cold worked 50% aging time varies from 4 h at 600 °C (1100

°F) to 1 h at 730 °C (1350 °F) Table 14 gives mechanical properties of a constant-modulus alloy containing 42% Ni, 5.4% Cr, and 2.4% Ti Heat treatment and cold work markedly affect these properties

Table 14 Mechanical properties of constant-modulus alloy 50Fe-42Ni-5.4Cr-2.4Ti

Tensile strength

Yield strength

Modulus of elasticity Condition

MPa ksi MPa ksi

Solution treated and aged 3 h at 730 °C (1345 °F) 1240 180 795 115 18 345 185 26.5

Solution treated, cold worked 50% and aged 1 h at 730 °C

Table 15 Composition and thermal expansion coefficients of high-strength controlled-expansion alloys

Coefficient of thermal expansion, from room temperature to:

260 °C (500 °F) 370 °C (700 °F) 415 °C (780 °F)

Inflection temperature

7.51 4.17 7.47 4.15 7.45 4.14 440 820

Incoloy 907 and

Pyromet CTX-3

0.06 C max, 0.5 Si, 38.0 Ni, 13.0

Co, 1.5 Ti, 4.8 (Nb + Ta), 0.35 Al max, 0.012 B max, bal Fe

7.65 4.25 7.50 4.15 7.55 4.20 415 780

Incoloy 909 and

Pyromet

CTX-909

0.06 C max, 0.40 Si, 38.0 Ni, 14.0

Co, 1.6 Ti, 4.9 (Nb + Ta), 0.15 Al max, 0.012 B max, bal Fe

7.75 4.30 7.55 4.20 7.75 4.30 415 780

Table 16 Typical tensile properties of high-strength, controlled-expansion alloys

Test Temperature Ultimate

tensile strength

0.2% yield strength Alloy designation

°C °F MPa ksi MPa ksi

540 1000 1310 190 1035 150 15 45

Room temperature 1170 170 825 120 15 25 Incoloy 907 and Pyromet CTX-3

Table 17 Tradenames of various low-expansion alloys

Nominal composition,

%

UNS number

Tradename and producing company

Iron-nickel alloys

36% Ni, bal Fe K93601 Invar (INCO and Imphy, S.A.) Invar M63 (Imphy, S.A.) AL-36 (Allegheny Ludlum) Invar

"36" (Carpenter Steel)

39% Ni, bal Fe Low expansion "39" (Carpenter Steel)

42% Ni, bal Fe K94100 Low expansion "42" (Carpenter Steel) AL-42 (Allegheny Ludlum) N42 (Imphy, S.A.)

46% Ni, bal Fe Platinate (same expansion coefficient as platinum) Glass Sealing "46" (Carpenter Steel)

47-48% Ni, bal Fe N47, N48 (Imphy, S.A.)

49% Ni, bal Fe AL-4750 (Allegheny Ludlum) Low expansion "49" (Carpenter Steel)

52% Ni, bal Fe K14042 Glass Sealing "52" (Carpenter Steel) AL-52 (Allegheny Ludlum) N52 (Imphy, S.A.)

There is increasing use of Invar-type alloys for shadow masks in color television picture tubes The low thermal expansion of Invar prevents excessive distortion of this shadow mask as internal temperatures increase during operation

of the picture tube

Shape Memory Alloys

Darel E Hodgson, Shape Memory Applications, Inc.; Ming H Wu, Memry Corporation; and Robert J Biermann, Harrison Alloys, Inc

Introduction

THE TERM SHAPE MEMORY ALLOYS (SMA) is applied to that group of metallic materials that demonstrate the ability to return to some previously defined shape or size when subjected to the appropriate thermal procedure Generally, these materials can be plastically deformed at some relatively low temperature, and upon exposure to some higher temperature will return to their shape prior to the deformation Materials that exhibit shape memory only upon heating are referred to as having a one-way shape memory Some materials also undergo a change in shape upon recooling These materials have a two-way shape memory

Although a relatively wide variety of alloys are known to exhibit the shape memory effect, only those that can recover substantial amounts of strain or that generate significant force upon changing shape are of commercial interest To date, this has been the nickel-titanium alloys and copper-base alloys such as Cu-Zn-Al and Cu-Al-Ni

A shape memory alloy may be further defined as one that yields a thermoelastic martensite In this case, the alloy undergoes a martensitic transformation of a type that allows the alloy to be deformed by a twinning mechanism below the transformation temperature The deformation is then reversed when the twinned structure reverts upon heating to the parent phase

History

The first recorded observation of the shape memory transformation was by Chang and Read in 1932 (Ref 1) They noted the reversibility of the transformation in AuCd by metallographic observations and resistivity changes, and in 1951 the shape memory effect (SME) was observed in a bent bar of AuCd In 1938, the transformation was seen in brass (copper-zinc) However, it was not until 1962, when Buehler and co-workers (Ref 2) discovered the effect in equiatomic nickel-titanium (Ni-Ti), that research into both the metallurgy and potential practical uses began in earnest Within 10 years, a number of commercial products were on the market, and understanding of the effect was much advanced Study of shape memory alloys has continued at an increasing pace since then, and more products using these materials are coming to the market each year (Ref 3, 4)

As the shape memory effect became better understood, a number of other alloy systems that exhibited shape memory were investigated Table 1 lists a number of these systems (Ref 5) with some details of each system Of all these systems, the Ni-Ti alloys and a few of the copper-base alloys have received the most development effort and commercial exploitation These will be the focus of the balance of this article

Table 1 Alloys having a shape memory effect

Transformation-temperature range Transformation

Fe-Mn-Si 32 wt% Mn, 6 wt% Si -200 to 150 -330 to 300 ≈100 ≈180

References cited in this section

1 L.C Chang and T.A Read, Trans AIME, Vol 191, 1951, p 47

2 W.J Buehler, J.V Gilfrich, and R.C Wiley, J Appl Phys., Vol 34, 1963, p 1475

3 Proceedings of Engineering Aspects of Shape Memory Alloys (Lansing, MI), 1988

4 D.E Hodgson, Using Shape Memory Alloys, Shape Memory Applications, 1988

5 K Shimizu and T Tadaki, Shape Memory Alloys, H Funakubo, Ed., Gordon and Breach Science Publishers,

General Characteristics

The martensitic transformation that occurs in the shape memory alloys yields a thermoelastic martensite and develops from a high-temperature austenite phase with long-range order The martensite typically occurs as alternately sheared platelets, which are seen as a herringbone structure when viewed metallographically The transformation, although a first-order phase change, does not occur at a single temperature but over a range of temperatures that varies with each alloy system The usual way of characterizing the transformation and naming each point in the cycle is shown in Fig 1 Most of the transformation occurs over a relatively narrow temperature range, although the beginning and end of the transformation during heating or cooling actually extends over a much larger temperature range The transformation also exhibits hysteresis in that the transformation on heating and on cooling does not overlap (Fig 1) This transformation

hysteresis (shown as T in Fig 1) varies with the alloy system (Table 1)

Fig 1 Typical transformation versus temperature curve for a specimen under constant load (stress) as it is cooled and heated T,

transformation hysteresis M s , martensite start; M f , martensite finish; A s , austenite start; A f , austenite finish

Crystallography of Shape Memory Alloys

Thermoelastic martensites are characterized by their low energy and glissile interfaces, which can be driven by small temperature or stress changes As a consequence of this, and of the constraint due to the loss of symmetry during transformation, thermoelastic martensites are crystallographically reversible

The herringbone structure of athermal martensites essentially consists of twin-related, self-accommodating variants (Fig 2b) The shape change among the variants tends to cause them to eliminate each other As a result, little macroscopic strain is generated In the case of stress-induced martensites, or when stressing a self-accommodating structure, the variant that can transform and yield the greatest shape change in the direction of the applied stress is stabilized and becomes dominant in the configuration (Fig 2c) This process creates a macroscopic strain, which is recoverable as the crystal structure reverts to austenite during reverse transformation

Fig 2 (a) A β phase crystal (b) Self-accommodating, twin-related variants A, B, C, and D, after cooling and transformation to martensite (c)

Variant A becomes dominant when stress is applied

Thermomechanical Behavior

The mechanical properties of shape memory alloys vary greatly over the temperature range spanning their transformation This is seen in Fig 3, where simple stress-strain curves are shown for a nickel-titanium alloy that was tested in tension below, in the middle of, and above its transformation-temperature range The martensite is easily deformed to several percent strain at quite a low stress, whereas the austenite (high-temperature phase) has much higher yield and flow stresses The dashed line on the martensite curve indicates that upon heating after removing the stress, the sample

"remembered" its unstrained shape and reverted to it as the material transformed to austenite No such shape recovery is found in the austenite phase upon straining and heating, because no phase change occurs

Fig 3 Typical stress-strain curves at different temperatures relative to the transformation, showing (a) Austenite (b) Martensite (c)

Pseudoelastic behavior

An interesting feature of the stress-strain behavior is seen in Fig 3(c), where the material is tested slightly above its transformation temperature At this temperature, martensite can be stress induced It then immediately strains and exhibits the increasing strain at constant stress behavior, seen in AB Upon unloading, though, the material reverts to austenite at a lower stress, as seen in line CD, and shape recovery occurs, not upon the application of heat but upon a reduction of stress This effect, which causes the material to be extremely elastic, is known as pseudoelasticity Pseudoelasticity is nonlinear The Young's modulus is therefore difficult to define in this temperature range as it exhibits both temperature and strain dependence

In most cases, the memory effect is one way That is, upon cooling, a shape memory alloy does not undergo any shape change, even though the structure changes to martensite When the martensite is strained up to several percent, however, that strain is retained until the material is heated, at which time shape recovery occurs Upon recooling, the material does not spontaneously change shape, but must be deliberately strained if shape recovery is again desired

It is possible in some of the shape memory alloys to cause two-way shape memory That is, shape change occurs upon both heating and cooling The amount of this shape change is always significantly less than obtained with one-way memory, and very little stress can be exerted by the alloy as it tries to assume its low-temperature shape The heating shape change can still exert very high forces, as with the one-way memory

A number of heat-treatment and mechanical training methods have been proposed to create the two-way shape memory effect (Ref 6, 7) All rely on the introduction of microstructural stress concentrations, which cause the martensite plates to initiate in particular directions when they form upon cooling, resulting in an overall net-shape change in the desired direction

References cited in this section

6 J.R Willson, et al., U.S Patent 3,625,969, 1972

7 A.D Johnson, U.S Patent 4,435,229, 1972

The second method often used is to measure the resistivity of the sample as it is heated and cooled The alloys exhibit interesting changes and peaks in the resistivity (by up to 20%) over the transformation-temperature range; however, correlating these changes with measured phase changes or mechanical properties has not always been very successful Also, there are often large changes in the resistivity curves after cycling samples through the transformation a number to times Thus, resistivity is often measured as a phenomenon in its own right, but is rarely used to definitely characterize one alloy versus another

The most direct method of characterizing an alloy mechanically is to prepare an appropriate sample, then apply a constant stress to the sample and cycle it through the transformation while measuring the strain that occurs during the transformation in both directions The curve shown in Fig 1 is the direct information one obtains from this test The values obtained for the transformation points, such as Ms and Af, from this method are offset to slightly higher temperatures from the values obtained from DSC testing This happens because the DSC test occurs at no applied stress, and the transformation is not stress induced; therefore, increasing test stress will lead to increasing transformation-temperature results This test is directly indicative of the property one can expect in a mechanical device used to perform some function using shape memory Its disadvantages are that specimens are often difficult to make, and results are quite susceptible to the way the test is conducted

Finally, the stress-strain properties can be measured in a standard tensile test at a number of temperatures across the transformation-temperature range, and from the change in properties the approximate transformation-temperature values can be interpolated This is very imprecise, through, and is much better applied as a measure of the change in properties

of each phase, due to such things as work hardening or different heat treatments

Commercial SME Alloys

The only two alloy systems that have achieved any level of commercial exploitation are the Ni-Ti alloys and the base alloys Properties of the two systems are quite different The Ni-Ti alloys have greater shape memory strain (up to 8% versus 4 to 5% for the copper-base alloys), tend to be much more thermally stable, have excellent corrosion resistance compared to the copper-base alloys' medium corrosion resistance and susceptibility to stress-corrosion cracking, and have much higher ductility On the other hand, the copper-base alloys are much less expensive, can be melted and extruded in air with ease, and have a wider range of potential transformation temperatures The two alloy systems thus have advantages and disadvantages that must be considered in a particular application

copper-Nickel-Titanium Alloys. The basis of the nickel-titanium system of alloys is the binary, equiatomic intermetallic compound

of Ni-Ti This intermetallic compound is extraordinary because it has a moderate solubility range for excess nickel or titanium, as well as most other metallic elements, and it also exhibits a ductility comparable to most ordinary alloys This solubility allows alloying with many of the elements to modify both the mechanical properties and the transformation properties of the system Excess nickel, in amounts up to about 1%, is the most common alloying addition Excess nickel strongly depresses the transformation temperature and increases the yield strength of the austenite Other frequently used elements are iron and chromium (to lower the transformation temperature), and copper (to decrease the hysteresis and lower the deformation stress of the martensite) Because common contaminants such as oxygen and carbon can also shift the transformation temperature and degrade the mechanical properties, it is also desirable to minimize the amount of these elements

The major physical properties of the basic binary Ni-Ti system and some of the mechanical properties of the alloy in the annealed condition are shown in Table 2 Note that this is for the equiatomic alloy with an Af value of about 110 °C (230

°F) Selective work hardening, which can exceed 50% reduction in some cases, and proper heat treatment can greatly improve the ease with which the martensite is deformed, give an austenite with much greater strength, and create material that spontaneously moves itself both in heating and on cooling (two-way shape memory) One of the biggest challenges in using this family of alloys is in developing the proper processing procedures to yield the properties desired

Table 2 Properties of binary Ni-Ti shape memory alloys.

Corrosion resistance Similar to 300 series stainless steel or titanium alloys

Latent heat of transformation, kJ/kg · atom (cal/g · atom) 167 (40)

Because of the reactivity of the titanium in these alloys, all melting of them must be done in a vacuum or an inert atmosphere Methods such as plasma-arc melting, electron-beam melting, and vacuum-induction melting are all used commercially After ingots are melted, standard hot-forming processes such as forging, bar rolling, and extrusion can be used for initial breakdown The alloys react slowly with air, so hot working in air is quite successful Most cold-working processes can also be applied to these alloys, but they work harden extremely rapidly, and frequent annealing is required Wire drawing is probably the most widely used of the techniques, and excellent surface properties and sizes as small as 0.05 mm (0.002 in.) are made routinely

Fabrication of articles from the Ni-Ti alloys can usually be done with care, but some of the normal processes are difficult Machining by turning or milling is very difficult except with special tools and practices Welding, brazing, or soldering the alloys is generally difficult The materials do respond well to abrasive removal, such as grinding, and shearing or punching can be done if thicknesses are kept small

Heat treating to impart the desired memory shape is often done at 500 to 800 °C (950 to 1450 °F), but it can be done as low as 300 to 350 °C (600 to 650 °F) if sufficient time is allowed The SMA component may need to be restrained in the desired memory shape during the heat treatment, otherwise, it may not remain there

Commercial copper-base shape memory alloys are available in ternary Cu-Zn-Al and Cu-Al-Ni alloys, or in their quaternary modifications containing manganese Elements such as boron, cerium, cobalt, iron, titanium, vanadium, and zirconium are also added for grain refinement

The major alloy properties are listed in Table 3 The martensite-start (Ms) temperatures and the compositions of Cu-Zn-Al alloys are plotted in Fig 4 Compositions of Cu-Al-Ni alloys usually fall in the range of 11 to 14.5 wt% Al and 3 to 5 wt% Ni The martensitic transformation temperatures can be adjusted by varying chemical composition Figure 4 and the following empirical relationships are useful in obtaining a first estimate:

• Cu-Zn-Al: Ms(°C) = 2212 - 66.9 (at.% Zn) - 90.65 (at.% Al) (Ref 8)

• Cu-Al-Ni: Ms(°C) = 2020 - 134 (wt% Al) - 45 (wt% Ni) (Ref 9)

Table 3 Properties of copper-base shape memory alloys

Property value Property

Heat capacity, J/kg · °C (Btu/lb · °F) 400 (0.096) 373-574 (0.089-0.138)

Mechanical properties

Ultimate tensile strength, MPa (ksi) 600 (87) 500-800 (73-116)

Shape memory properties

Fig 4 Ms temperatures and compositions of Cu-Zn-Al shape memory alloys

The melting of Cu-base shape memory alloys in similar to that of aluminum bronzes Most commercial alloys are induction melted Protective flux on the melt and the use of nitrogen or inert-gas shielding during pouring are necessary to

prevent zinc evaporation and aluminum oxidation Powder metallurgy and rapid solidification processing are also used to produce fine-grain alloys without grain-refining additives

Copper-base alloys can be readily hot worked in air With low aluminum content (<6 wt%), Cu-Zn-Al alloys can be cold finished with interpass annealing Alloys with higher aluminum content are not as easily cold workable Cu-Al-Ni alloys,

on the other hand, are quite brittle at low temperatures and can only be hot finished

Manganese depresses transformation temperatures of both Cu-Zn-Al and Cu-Al-Ni alloys and shifts the eutectoid to higher aluminum content (Ref 10) It often replaces aluminum for better ductility

Because copper-base shape memory alloys are metastable in nature, solution heat treatment in the parent β-phase region and subsequent controlled cooling are necessary to retain β-phase for shape memory effects Prolonged solution heat treatment causes zinc evaporation and grain growth and should be avoided Water quench is widely used as a quenching process, but air cooling may be sufficient for some high-aluminum content Cu-Zn-Al alloys and Cu-Al-Ni alloys The as-quenched transformation temperature is usually unstable Postquench aging at temperatures above the nominal Af

temperature is generally needed to establish stable information temperatures

Cu-Zn-Al alloys, when quenched rapidly and directly into the martensitic phase, are susceptible to the martensite stabilization effect (Ref 11) This effect causes the reverse transformation to shift toward higher temperatures It therefore delays and may completely inhibit the shape recovery For alloys with Ms temperatures above the ambient, slow cooling

or step quenching with intermediate aging in the parent β-phase state should be adopted

The thermal stability of copper-base alloys is ultimately limited by the decomposition kinetics For this reason, prolonged exposure of Cu-Zn-Al and Cu-Al-Ni alloys at temperatures above 150 °C (300 °F) and 200 °C (390 °F) respectively, should be avoided Aging at lower temperatures may also shift the transformation temperatures In case of aging in the β phase, this results from the change in long-range order (Ref 12) When aged in the martensitic state, the alloys exhibit an aging-induced martensite stabilization effect (Ref 11) For high-temperature stability, Cu-Al-Ni is generally a better alloy system than Cu-Zn-Al However, even for moderate temperature applications, which demand tight control of transformation temperatures, these effects need to be evaluated

References cited in this section

8 L Delaey, M Chandrasekaran, W De-Jonghe, W Rapacioli, and A Deruytere, INCRA Research Report

238, International Copper Research Association

9 K Sugimoto, Bull Jpn Inst Met., Vol 24, 1985, p 45

10 P.L Brook, U.S Patent 4,166,739, Sept 1979

11 M Ahlers, Proceedings of International Conference on Martensitic Transformations (Nara, Japan), 1986, p

Constrained Recovery. The most successful example of this type of product is undoubtedly the Cryofit hydraulic couplings made by Raychem Corporation (Ref 14) These fittings are manufactured as cylindrical sleeves slightly smaller than the metal tubing they are to join Their diameters are then expanded while martensitic, and, upon warming to austenite, they shrink in diameter and strongly hold the tube ends The tubes prevent the coupling from fully recovering its manufactured

shape, and the stresses created as the coupling attempts to do so are great enough to create a joint that, in many ways, is superior to a weld

Similar to the Cryofit coupling, the Betalloy coupling (Ref 15) is a Cu-Zn-Al coupling also designed and marketed by Raychem Corporation for copper and aluminum tubing In this application, the Cu-Zn-Al shape memory cylinder shrinks

on heating and acts as a driver to squeeze a tubular liner onto the tubes being joined The joint strength is enhanced by a sealant coating on the liner

Force Actuators. In some applications the shape memory component is designed to exert force over a considerable range of motion, often for many cycles Such an application is the circuit-board edge connector made by Beta Phase Inc (Ref 16)

In this electrical connector system, the SMA component is used to force open a spring when the connector is heated This allows force-free insertion or withdrawal of a circuit board in the connector Upon cooling, the Ni-Ti actuator becomes weaker and the spring easily deforms the actuator while it closes tightly on the circuit board and forms the connections

Based on the same principle, Cu-Zn-Al shape memory alloys have found several applications in this area One such example is a fire safety valve, which incorporates a Cu-Zn-Al actuator designed to shut off toxic or flammable gas flow when fire occurs (Ref 17)

Proportional Control. It is possible to use only a part of the shape recovery to accurately position a mechanism by using only a selected portion of the recovery because the transformation occurs over a range of temperatures rather than at a single temperature A device has been developed by Beta Phase Inc (Ref 18) in which a valve controls the rate of fluid flow by carefully heating a shape0-memory-alloy component just enough to close the valve the desired amount Repeatable positioning within 0.25 μm (10-5 in.) is possible with this technique

Superelastic Applications. A number of products have been brought to market that use the pseudoelastic (or superelastic) property of these alloys Eyeglass frames that use superelastic Ni-Ti to absorb large deformations without damaging the frames are now marketed, and guide wires for steering catheters into vessels in the body have been developed using Ni-Ti wire, which resists permanent deformation if bent severely Arch wires for orthodontic correction using Ni-Ti have been used for many years to give large rapid movement of teeth

The properties of the Ni-Ti alloys, particularly, indicate their probable greater use in biomedical applications The materials is extremely corrosion resistant, demonstrates excellent biocompatibility, can be fabricated into the very small sizes often required, and has properties of elasticity and force delivery that allow uses not possible any other way

References cited in this section

13 M Simon, et al., Radiology, Vol 172, 1989, p 99-103

14 J.D Harrison and D.E Hodgson, Shape Memory Effects in Alloys, J Perkins, Ed., Plenum Press, 1975, p

517

15 Product Brochure, Raychem Corporation, Menlo Park, CA

16 J.F Krumme, Connect Technol., Vol 3 (No 4), April 1987, p 41

17 E Waldbusser, Semicond Saf Assoc J., Aug 1987, p 34

18 D.E Hodgson, Proceedings of Engineering Aspects of Shape Memory Alloys (Lansing, MI), 1988

Future Prospects

Although specific products that might use the Ni-Ti alloys in the future cannot be foretold, some directions are obvious The cost of these alloys has slowly decreased as use has increased, so uses that require lower-cost alloys to be viable are being explored Alloy development has yielded several ternary compositions with properties improved over those obtained with binary material, and alloys tailored to specific product needs are likely to multiply The medical industry has developed a number of products using Ni-Ti alloys because of their excellent biocompatibility and large pseudoelasticity, and many more of these applications are likely Finally, the availability of small wire that is stable, is easily heated by a small electrical current, and gives a large repeatable stroke should lead to a new family of actuator devices (Ref 19) These devices can be inexpensive, are reliable for thousands of cycles, and are expected to move Ni-Ti into the high-volume consumer marketplace

Recent interest in the development of iron-base shape memory alloys has challenged the concept that long-range order and thermoelastic martensitic transformation are necessary conditions for shape memory effect Among the alloys, Fe-Pt (Ref 20), Fe-Pd (Ref 21), and Fe-Ni-Co-Ti (Ref 22) can be heat treated to exhibit thermoelastic martensitic transformation, and, therefore, shape memory effect However, alloys such as Fe-Ni-C (Ref 23), Fe-Mn-Si (Ref 24), and Fe-Mn-Si-Cr-Ni (Ref 25) are not ordered and undergo nonthermoelastic transformation, and yet exhibit good shape memory effect These alloys are characteristically different from conventional shape memory alloys in that they rely on stress-induced martensite for shape memory effect, exhibit fairly large transformation hysteresis, and, in general, have less than 4% recoverable strain The commercial potential of these alloys has yet to be determined, but the effort has opened up new classes of alloys for exploration as shape memory alloys These new classes included β-Ti alloys and iron-base alloys

References cited in this section

19 Product brochure, Dynalloy Inc., Irvine, CA

20 M Foos, C Frantz, and M Gantois, Shape Memory Effects in Alloys, J Perkins, Ed., Plenum Press, 1975, p

407

21 T Sohmura, R Oshima, and F.E Fujita, Scr Metall., Vol 14, 1980, p 855

22 T Maki, K Kobayashi, M Minato, and I Tamura, Scr Metall., Vol 18, 1984, p 1105

23 S Kajiwara, Trans Jpn Inst Met., Vol 26, 1985, p 595

24 A Sato, K Soma, E Chishima, and T Mori, in Proceedings, International Conference on Martensitic

Transformations (Louvain, Belgium), 1982, p C4-797

25 H Otsuka, H Yamada, H Tanahashi, and T Maruyama, in Proceedings, International Conference on

Martensitic Transformations (Sydney, Australia), 1989

The principle of incorporating a high-performance second phase into a conventional engineering material to produce a combination with features not obtainable from the individual constituents is well known In a MMC, the continuous, or matrix, phase is a monolithic alloy, and the reinforcement consists of high-performance carbon, metallic, or ceramic additions Reinforced intermetallic compounds such as the aluminides of titanium, nickel, and iron are also discussed in this article (for more information on intermetallic compounds, see the article "Ordered Intermetallics" in this Volume)

Reinforcements, characterized as either continuous or discontinuous, may constitute from 10 to 60 vol% of the composite Continuous fiber or filament reinforcements include graphite (Gr), silicon carbide (SiC), boron, aluminum oxide (Al2O3), and refractory metals Discontinuous reinforcements consist mainly of SiC in whisker (w) form, particulate (p) types of SiC, Al2O3, or titanium diboride (TiB2), and short or chopped fibers of Al2O3 or graphite Figure 1 shows cross sections of typical continuous and discontinuous reinforcement MMCs

Fig 1 Cross sections of typical fiber-reinforced MMCs (a) Continuous-fiber-reinforced boron/aluminum composite Shown here are 142 μm

diam boron filaments coated with B 4 C in a 6061 aluminum alloy matrix (b) Discontinuous graphite/aluminum composite Cross section shows 10 μm diam chopped graphite fibers (40 vol%) in a 2014 aluminum alloy matrix (c) A 6061 aluminum alloy matrix reinforced with

40 vol% SiC particles (d) Whisker-reinforced (20 vol% SiC) aluminum MMC (e) and (f) MMCs manufactured using the PRIMEXTMpressureless metal infiltration process (e) An Al 2 O 3 -reinforced (60 vol%) aluminum MMC (f) A highly reinforced (81 vol%) MMC consisting of SiC particles in an aluminum alloy matrix The black specks in the matrix are particles of an inorganic preform binder and do not indicate porosity (a) and (b) Courtesy of DWA Composite Specialties, Inc (c) and (d) Courtesy of Advanced Composite Materials Corporation (e) and (f) Courtesy of Lanxide Corporation

The salient characteristics of metals as matrices are manifested in a variety of ways; in particular, a metal matrix imparts a metallic nature to the composite in terms of thermal and electrical conductivity, manufacturing operations, and interaction with the environment Matrix-dominated mechanical properties, such as the transverse elastic modulus and strength of unidirectionally reinforced composites, are sufficiently high in some MMCs to permit use of the unidirectional lay-up in engineering structures

This article will give an overview of the current status of MMCs, including information on physical and mechanical properties, processing methods, distinctive features, and the various types of continuously and discontinuously reinforced MMCs More information on the processing and properties of MMCs is available in the Section "Metal, Carbon/Graphite,

and Ceramic Matrix Composites" in Composites, Volume 1 of the Engineered Materials Handbook published by ASTM

INTERNATIONAL

Property Prediction

Property predictions of MMCs can be obtained from mathematical models, which require as input a knowledge of the properties and geometry of the constituents For metals reinforced by straight, parallel continuous fibers, three properties that are frequently of interest are the elastic modulus, the coefficient of thermal expansion, and the thermal conductivity

in the fiber direction Reasonable values can be obtained from rule-of-mixture expressions for Young's modulus (Ref 1):

coefficient of thermal expansion (Ref 2):

f f f m m m c

where v is volume fraction, and E, α, and k are the modulus, coefficient of thermal expansion, and thermal conductivity in

the fiber direction, respectively The subscripts c, f, and m refer to composite, fiber, and matrix, respectively

References cited in this section

1 Z Hashin and B.W Rosen, The Elastic Moduli of Fiber-Reinforced Materials, J Appl Mech (Trans

ASME), June 1964, p 223

2 D.E Bowles and S.S Tompkins, Prediction of Coefficients of Thermal Expansion for Unidirectional

Composites, J Compos Mater., Vol 23, April 1989, p 370

3 G.S Springer and S.W Tsai, Thermal Conductivities of Unidirectional Materials, J Compos Mater., Vol 1,

1967, p 166

Processing Methods

Processing methods for MMCs are divided into primary and secondary categories Primary processing is the operation by which the composite is synthesized from its raw materials It involves introducing the reinforcement into the matrix in the appropriate amount and location, and achieving proper bonding of the constituents Secondary processing consists of all the additional steps needed to make the primary composite into a finished hardware component

Many reinforcement and matrix materials are not inherently compatible, and such materials cannot be processed into a composite without tailoring the properties of an interface between them In some composites the coupling between the reinforcing agent and the metal is poor and must be enhanced For MMCs made from reactive constituents, the challenge

is to avoid excessive chemical activity at the interface, which would degrade the properties of the material These problems are usually resolved either by applying a surface treatment or coating to the reinforcement or by modifying the composition of the matrix alloy

Solidification processing (Ref 4, 5), solid-state bonding, and matrix deposition techniques have been used to fabricate MMCs Solidification processing offers a near-net-shape manufacturing capability, which is economically attractive Developers have explored various liquid metal techniques that use multifilament yarns, chopped fibers, or particulates as the reinforcement A castable ceramic/aluminum MMC is now commercially available (Ref 6); cast components of this composite are shown in Fig 2 Solid-state methods use lower fabrication temperatures with potentially better control of the interface thermodynamics and kinetics The two principal categories of solid-state fabrication are diffusion bonding of materials in thin sheet form (Ref 7) and powder metallurgy techniques (Ref 8) Matrix deposition processes, in which the matrix is deposited on the fiber, include electrochemical plating, plasma spraying, and physical vapor decomposition (Ref 7) A new method, metal spray deposition, is currently being investigated (Ref 9) After deposition processing, a secondary consolidation step such as diffusion bonding often is needed to produce a component

Fig 2 Discontinuous silicon carbide/aluminum castings Pictured are a sand cast automotive disk brake rotor and upper control arm, a

permanent mold cast piston, a high-pressure die cast bicycle sprocket, an investment cast aircraft hydraulic manifold, and three investment cast engine cylinder inserts Courtesy of Dural Aluminum Composites Corporation

Which secondary processes are appropriate for a given MMC depends largely on whether the reinforcement is continuous

or discontinuous Discontinuously reinforced MMCs are amenable to many common metal forming operations, including extrusion, forging, and rolling Because a high percentage of the materials used to reinforce discontinuous MMCs are hard oxides or carbides, machining can be difficult, and methods such as diamond sawing, electrical discharge machining

(Ref 10), and abrasive waterjet cutting (Ref 11) are sometimes utilized (see Machining, Volume 16 of ASM Handbook, formerly 9th Edition Metals Handbook for more information about these machining methods)

References cited in this section

4 A Mortensen, J.A Cornie, and M.C Flemings, Solidification Processing of Metal-Matrix Composites, J

Met., Feb 1988, p 12

5 P.K Rohatgi, R Asthana, and S Das, Solidification, Structures, and Properties of Cast Metal-Ceramic

Particle Composites, Int Met Rev., Vol 31 (No 3), 1986, p 115

6 D.E Hammond, Foundry Practice for the First Castable Aluminum/Ceramic Composite Material, Mod

Cast., Aug 1989, p 29

7 T.W Chou, A Kelly, and A Okura, Fibre-Reinforced Metal-Matrix Composites, Composites, Vol 16 (No

3), July 1985, p 187

8 D.L Erich, Metal-Matrix Composites: Problems, Applications, and Potential in the P/M Industry, Int J

Powder Metall., Vol 23 (No 1), 1987, p 45

9 J White, T.C Willis, I.R Hughes, and R.M Jordan, Metal Matrix Composites Produced by Spray

Deposition, in Dispersion Strengthened Aluminum Alloys, Y.W Kim and W.M Griffith, Ed., The Minerals,

Metals and Materials Society, 1988, p 693

10 M Ramula and M Taya, EDM Machinability of SiCw/Al Composites, J Mater Sci., Vol 24, 1989, p 1103

11 P.K Rohatgi, N.B Dahotre, S.C Gopinathan, D Alberts, and K.F Neusen, Micromechanism of High

Speed Abrasive Waterjet Cutting of Cast Metal Matrix Composites, in Cast Reinforced Metal Composites,

S.G Fishman and A.K Dhingra, Ed., ASM INTERNATIONAL, 1988, p 391

Aluminum-Matrix Composites

Most of the commercial work on MMCs has focused on aluminum as the matrix metal The combination of light weight, environmental resistance, and useful mechanical properties has made aluminum alloys very popular; these properties also make aluminum well suited for use as a matrix metal The melting point of aluminum is high enough to satisfy many application requirements, yet low enough to render composite processing reasonably convenient Also, aluminum can

accommodate a variety of reinforcing agents, including continuous boron, Al2O3, SiC, and graphite fibers, and various particles, short fibers, and whiskers (Ref 12) The microstructures of various aluminum matrix MMCs are shown in Fig

1

Continuous Fiber Aluminum MMC. Boron/aluminum is a technologically mature continuous fiber MMC (Fig 1a) Applications for this composite include tubular truss members in the midfuselage structure of the Space Shuttle orbiter and cold plates in electronic microchip carrier multilayer boards Fabrication processes for B/Al composites are based on hot-press diffusion bonding or plasma spraying methods (Ref 13) Selected properties of a B/Al composite are given in Table 1

Table 1 Room-temperature properties of unidirectional continuous fiber aluminum-matrix composites

Property B/6061 Al SCS-2/6061 Al P100 Gr/6061 Al FP/Al-2Li (a)

Longitudinal modulus, GPa (106 psi) 214 (31) 204 (29.6) 301 (43.6) 207 (30)

Transverse modulus, GPa (106 psi) 118 (17.1) 48 (7.0) 144 (20.9)

Longitudinal strength, MPa (ksi) 1520 (220) 1462 (212) 543 (79) 552 (80)

Transverse strength, MPa (ksi) 86 (12.5) 13 (2) 172 (25)

Source: Ref 14, 15, 16

(a) FP is the proprietary designation for an alpha alumina (α-Al 2 O 3 ) fiber developed by E.I Du Pont de Nemours & Company, Inc

Continuous SiC fibers (SiCc) are now commercially available; these fibers are candidate replacements for boron fibers because they have similar properties and offer a potential cost advantage One such SiC fiber is SCS, which can be manufactured with any of several surface chemistries to enhance bonding with a particular matrix, such as aluminum or titanium (Ref 14) The SCS-2 fiber, tailored for aluminum, has a 1 μm (0.04 mil) thick carbon rich coating that increases

in silicon content toward its outer surface

Silicon carbide/aluminum MMCs exhibit increased strength and stiffness as compared with unreinforced aluminum, and with no weight penalty Selected properties of SCS-2/Al are given in Table 1 In contrast to the base metal, the composite retains its room-temperature tensile strength at temperatures up to 260 °C (500 °F) (Fig 3) This material is the focus of development programs for a variety of applications; an example of an advanced aerospace application for an SCS/Al MMC is shown in Fig 4