Processing and mechanical properties of pure mg and in situ aln reinforced mg 5al composite 5

Chapter 5 Effects of strain rate on tensile properties 5.1 Introduction The strength and ductility of nc metals are dependent upon strain rate and temperature.. Very low work hardening

Chapter 5

Effects of strain rate on tensile properties

5.1 Introduction

The strength and ductility of nc metals are dependent upon strain rate and temperature

The strain-rate sensitivity index, m, where mln/ln,T, in a  m type relationship is one of the key engineering parameters that reflects the deformation behaviors of metals A highly strain rate sensitive material is expected to resist localized deformation and hence may be ductile, and in the extreme case of very high rate sensitivity, be superplastic

Very low work hardening rates are observed with an increase in the strain rate sensitivity in Mg when the grain size is reduced to nanometric scale [1] Recent experiments on face-centered cubic (fcc) and hexagonal close packed (hcp) nc metals have reported a more than 10-fold increase in strain-rate sensitivity in contrast to their

conventional coarse-grained counterparts [2,3] In most practical applications, m is

very small and in certain cases it may even be negligible from engineering point of

view Superplastic deformation shows large m values and approaches even the value of 1.0 which corresponds to viscoplasticity Low m-value in the low strain rate range is

often observed in superplastic materials It has been reported that such values are associated with the existence of a threshold stress [4] At high strain rates of over 1 ×

10−2 s−1, the m-value is reduced to a small value where dislocation processes dominate

deformation [5, 6]

The overall strain-rate dependence of a material is influenced by dislocation activity,

GB diffusion, and lattice diffusion [7-11] Generally the contribution of lattice diffusion is negligible at room temperature Several authors [12- 16] have reported that the highly localized dislocation activity (e.g dislocation nucleation and/or dislocation de-pinning) at the GBs leads to an enhanced strain-rate sensitivity for nc metals

Besides enhanced strain-rate dependence in nc materials, a more pronounced temperature dependence arises from the thermally activated deformation mechanisms controlling the plastic flow Deformation at temperature below room temperature exhibited a rapid increase in YS in nc Ni and Cu, [17] The origin of the strong temperature dependence, as well as for the rate sensitivity, has been linked to the small activation volume of dislocation mobility observed in strain rate change tests [13-15,17] The activation volume, in turn, is a signature of the underlying deformation processes [15]

For deformation mechanism, although superplastic deformation is believed to be achieved by GBS in combination with dislocation glide, the former mechanism, being strongly dependent on diffusion, naturally leads to a high amount of strain-rate sensitivity [18] Conrad and Jung [19,20] proposed a GBS mechanism to explain the grain size dependence of the plastic deformation kinetics of Cu and Ag in the grain size range of 10−2 μm < d < 1 μm In addition, GBS has been reported as deformation

mechanism of nc metallic materials experimentally by mechanical testing and theoretically by simulation models [21-26] However, Jain and Christman [27] suggested that nc Fe alloy deformed by the ‘core-mantle’ GBS mechanism, whilst Malow et al [28] obtained strain rate sensitivity values corresponding to typical

deformation by dislocation For better understanding of deformation mechanisms of nc

Mg-5Al-1AlN composite, several deformation parameters such as strain rate

sensitivities, activation volume and activation energy have been estimated

experimentally in the present study

5.2 Experimental

7mm diameter extruded Mg-5Al-1AlN composite rods were machined to produce the

cylindrical tensile samples with a gauge diameter of 5mm and a gauge length of 25

mm according to ASTM E8M-96 standard Uniaxial tensile test was conducted using

an automated Instron 8501 servo hydraulic testing machine at controlled strain rates of

3.33x10-3 s-1, 3.33x10-4 s-1 and 3.33x10-5 s-1 as shown in Fig 5.1(a) At least three

samples were tested for reproducibility and conformity to the tensile test standard For

the purpose of comparison, pure Mg samples were synthesized with same processing

parameters and tested under the same conditions as the composite samples

(a) (b) Figure 5.1 Experimental set-up in (a) Instron 8501 for tensile test and (b) Instron 8871

with environmental chamber for creep test

Instron 8874 axial-torsional servohydraulic test system with environmental chamber

was employed to conduct constant stress test on the tensile samples at 0, 25 (room

Extensometer

Creep sample attached with external thermocouple

temperature) and 50°C according to the ASTM E139 as shown in Fig 5.1(b) The test system is equipped with 25 kN load cell with 0.005% accuracy, position control with accuracy of ±0.5% of transducer full travel, and strain controller with accuracy of

0.005% of transducer capacity or 0.25% of readingtransducer accuracy Liquid nitrogen was introduced into the chamber for low temperature (0°C) testing Type T external thermocouple attached to the sample was used to monitor its temperature The working temperature was well controlled within ±1°C After holding at the test temperature for at least 20 minutes and the sample was mechanically loaded to the target stress level At a stress of 120 MPa, the sample was held for two hours Creep strains in the elastic region of 0.002 strain and in tertiary creep region are truncated for the analysis Fractured surfaces of the tensile samples were examined under a Hitachi S4100 field emission scanning electron microscope (FESEM) at 20 kV

5.3 Results and discussion

5.3.1 Effects of stain rate at room temperature on composite samples

True stress-true strain curves of the composite samples for each milling duration tested

at different strain rates of 3.33x10-3 s-1, 3.33x10-4 s-1 and 3.33x10-5 s-1 are shown in Fig 5.2 and the detailed results are given in Table 5.1

Compared to unmilled samples, milling enhanced YS (true stress at 0.2% true strain, yield stress) with the exception of 40h-MMed samples with lower YS at strain rates of 3.33x10-4 s-1 and 3.33x10-5 s-1 0h-MMed samples were quite insensitive to strain rate Generally, at higher strain rate, enhanced YS with lower ductility is observed In terms

of ductility, the 40h-MMed samples showed an exceptional case of producing similar ductility of 351% elongation at all strain rates Except for the 10h-MMed samples, all as-milled samples showed strain softening behaviors

500 400 300

700 600

10 h

3.33x10 -5 s -1 3.33x10 -4 s -1 3.33x10 -3 s -1

500 400 300

700 600

40 h

(e) Figure 5.2 Strain rate effects on composite samples milled for durations of (a) 0h, (b)

10h, (c) 20h, (d) 30h and (e) 40h

Very distinct variation in YS and ductility with respect to strain rate was manifested in

the samples MMed for 20h and 30h Table 5.1 indicates that the highest loading rate

caused an increase of about 50% in YS in 30h- and 40h-MMed samples compared to

the lowest loading rate

Table 5.1 Yield strength and % elongation of composite samples milled for different milling durations at different strain rates

3.33x10-5 s-1 3.33x10-4 s-1 3.33x10-3 s-1Milling

At the lowest strain rate of 3.33x10-5 s-1, the YS of 0h-MMed sample was comparable

to that of 30h-MMed sample and higher than that of 40h-MMed sample However, in terms of ductility, the 30h- and 40h-MMed samples achieved 187% and 140% higher respectively compared to the 0h-MMed samples This indicates the strain rate, in other words, time dependence nature of strength in nanostructured Mg composite materials This phenomenon is one of the unique properties of nanostructured materials and it has been reported by previous studies [29] This phenomenon indicates the involvement of

a dynamic process in terms of material transport operating in the course of loading the sample [21]

Strain rate sensitivity is gauged by the strain rate sensitivity index m which is obtained from the slope of the ln(σ) versus ln( ) graph (Fig 5.3a) Strain rate sensitivity of

metal is quite low (<0.1) at room temperature but it increases with temperature up to the range 0.1≤m≤0.2 which is common in hot working conditions [30] For the present materials, the strain rate sensitivity index increases significantly with milling time from 0.0153 at 0h up to 0.0881 at 30h but it decreases slightly to 0.0873 for the 40h-

MMed sample (Fig 5.3b) The high value of m might imply the deviation of room

temperature deformation behavior from the coarse-grained counterparts

0.08 0.06

samples for different milling durations

Ductility in terms of percentage elongation at strain rates as a function of milling

duration are illustrated in Fig 5.4 Generally, gradual increase in ductility was

observed with decreasing strain rate for all milling durations and increasing milling

duration at all strain rates It is quite interesting that the loading curves for 0h-MMed

samples at all strain rates are almost identical and the slight increase in ductility with

decreasing strain rate It is noticed that ductility of 10h-MMed samples is not

significantly influenced by strain rate as compared to other as-milled samples showing

6, 5 and 8% at 3.33x10-3 s-1, 3.33x10-4 s-1 and 3.33x10-5 s-1 strain rates respectively

Another noticeable property is the strain softening of samples after 20h of MM

25 15

35 40 30

3.33x10 -5 s -1

3.33x10 -3 s -1

3.33x10 -4 s -1

Figure 5.4 Ductility (% elongation) of composite samples for different milling

durations at different strain rates

5.3.2 Effects of stain rate at room temperature on pure Mg samples

True stress-true strain curves of pure Mg samples after each milling duration tested at

different strain rates of 3.33x10-3 s-1, 3.33x10-4 s-1 and 3.33x10-5 s-1 are shown in Fig

5.5 and the detailed results are given in Table 5.2

(c) 20h, (d) 30h and (e) 40h

Table 5.2 Yield strength and % elongation of pure Mg milled for different milling durations at different strain rates

3.33x10-5 s-1 3.33x10-4 s-1 3.33x10-3 s-1Milling

It is evident that the yield stress and ductility are strain rate dependent For example, for the 40h-MMed samples, the highest strain rate of 3.33x10-3 s-1 produced the highest yield stress of 277 MPa with lowest ductility of 12% However, the lowest strain rate

of 3.33x10-5 s-1 displayed the lowest yield stress of 170 MPa with a high ductility of 38%

0.08 0.06 0.12

(b) Figure 5.6 (a) ln() versus ln( ) graph and (b) strain rate sensitivity of pure Mg

samples for different milling durations

From Fig 5.6, strain rate sensitivity increased with milling time The 20h-MMed Mg sample shows the highest strain rate sensitivity of m=0.1184 with extensive ductility of 39% followed by 40h-MMed sample with m=0.1062 From Fig 5.7, enhanced ductility is observed with decreasing strain rate and increasing milling duration However, no significant increase in ductility with milling duration at the highest strain rate is detected

25 15

35 40 30

3.33x10 -5 s -1

3.33x10 -3 s -1

3.33x10 -4 s -1

Figure 5.7 Ductility (% elongation) of pure Mg samples for different milling durations

at different strain rates

Both composite and pure Mg samples show time dependent deformation behavior at room temperature Addition of Al for solid solution strengthening and the reinforcement of AlN particles for dispersion strengthening show enhancement in YS

but does not alter the deformation behavior In nanocrystalline region, intragranular dislocation activity is expected to be rather limited so that conventional strain hardening may also be limited Although there is evidence that nc materials have higher strain rate sensitivity than those of the coarse-grained counterparts, the values of

m are still low (<0.1) to sustain large ductility

Conventional physical models for crystal plasticity and traditional approaches of solids mechanics should be revised and modified to include size effects due to the presence

of a high density of grain and phase boundaries in the nanostructured materials Interfaces and their junctions present obstacles to the deformation process and contribute to the strengthening of the material Plastic deformation in nanophase materials can take place mainly at interfaces which are softer than the bulk crystal Therefore, it is clear that the interaction of individual defects with interfaces and junctions of interfaces should be considered as main event which is responsible for the mechanical properties of nanoscaled materials [31]

This explains why during tensile testing, when the strain rate is faster than the rate of diffusion in the MMed pure Mg and composite, the sample does not yield until when the yield stress is high enough for grain boundary diffusion to be activated On the other hand, a low strain rate provides adequate time for the movement of atoms so that the yield stress to activate the grain boundary process becomes lower In a stress activated process, diffusion is essential for the continual operation of the sliding process Since diffusivity of grain boundaries is a few tenth order of magnitude higher than that of volume diffusion, it is possible for nc materials to deform by GBS accompanied with diffusional process at room temperature

5.3.3 Deformation parameters

Orowan equation depicted in equation 5.1 can be written with orientation factor M and

shear strain rate  as in equation 5.2 [32]

l b kT

where  is the dislocation density, b the Burgers vector,  the dislocation velocity, 0

the frequency of vibration of the dislocation, l the distance between dislocation

barriers, F the change in Helmholtz free energy required to overcome obstacle

without aid from external stress,  the applied resolved shear stress and V the effective activation volume

The activation barrier can be lowered by mechanical workdone V where the

activation volume V  represents the average volume of dislocation structure involved in the deformation process For thermally activated plastic flow, the apparent activation

k

where k B is the Boltzmann’s constant (1.3087x10-23 JK-1), T the absolute temperature, 

the applied shear stress and  the tensile strain rate

The activation volume in m3 is usually expressed in terms of b3 where b=3.21x10-10m

for Mg [34] Fig 5.8 shows the apparent activation volume V a of the samples with

respect to milling duration V a of composite sample decreases sharply from 241b3 to

44b3 after 10h milling An unusually small V a of 26b3, 27b3 and 41b3 are observed in the 20, 30 and 40h-MMed sample respectively For pure Mg samples, no drastic drop

occurs but gradual decrease in V a is observed as shown in Fig 5.8 The present findings agree with the fact that for truly nc metals, the activation volumes are in the

range of 3b3 to 100b3 investigated by Asarro and Suresh [15] and Wang et al [35]

Figure 5.8 Apparent activation volume of composite and pure Mg samples at different milling durations

The thermal activation energy Q for the 40h-MMed sample can be determined using

the Arrhenius relationship as in equation 5.4

)/ln( 0

T R

(5.4)

where  is the actual strain rate and  the normalized strain rate which is defined as 0

1s-1 for mathematical reasons to make the argument of the natural logarithm ln(/0)

“dimensionless”

The steady strain rate is obtained from the gradient of the creep curves of strain versus time at the various test temperatures at constant applied stress of 120MPa as shown in Fig 5.9(a) The apparent activation energy value is found to be about 50 kJmol-1 (Fig