Effects of strain rate on tensile properties Chapter 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 × 10−2 s−1, the m-value is reduced to a small value where dislocation processes dominate deformation [5, 6]. 108 Effects of strain rate on tensile properties 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 [1315,17]. 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 < 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 109 Effects of strain rate on tensile properties 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. Creep sample attached with external thermocouple Extensometer (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 110 Effects of strain rate on tensile properties 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. 111 Effects of strain rate on tensile properties 700 500 True stress (MPa) 600 True stress (MPa) 700 0h 3.33x10-3 s-1 3.33x10-4 s-1 400 300 3.33x10-5 s-1 200 100 10 h 600 3.33x10-3 s-1 500 3.33x10-4 s-1 3.33x10-5 s-1 400 300 200 100 0.1 0.2 0.3 T rue strain 0.4 0.5 0.1 (a) 3.33x10-5 s-1 300 200 100 30 h 500 3.33x10-3 s-1 400 3.33x10-4 s-1 300 200 3.33x10-5 s-1 100 0.1 0.2 0.3 T rue strain 0.4 0.5 (c) 0.1 0.2 0.3 T rue strain 0.4 0.5 (d) 700 40 h 600 True stress (MPa) 0.5 600 True stress (MPa) True stress (MPa) 3.33x10-4 s-1 400 700 20 h 3.33x10-3 s-1 500 0.4 (b) 700 600 0.2 0.3 T ru e strain 500 400 300 3.33x10-3 s-1 3.33x10-4 s-1 200 3.33x10-5 s-1 100 0.1 0.2 0.3 T ru e strain 0.4 0.5 (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. 112 Effects of strain rate on tensile properties Table 5.1 Yield strength and % elongation of composite samples milled for different milling durations at different strain rates Milling duration (h) 10 20 30 40 3.33x10-5 s-1 YS Elong(MPa) ation (%) 212 15 431 417 19 232 41 176 36 3.33x10-4 s-1 YS Elong(MPa) ation (%) 219 12 465 505 334 28 205 34 3.33x10-3 s-1 YS Elong(MPa) ation (%) 228 515 558 347 14 262 34 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 ([...]... Conference Proceedings of the Fifth International Conference on Composite Materials ICCM-V , ed by Jr WC Harrigan, J Strife, AK Dhingra, pp 843-866 San Diego, California, USA 19 85 131 Effects of strain rate on tensile properties 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 D Hull, DJ Bacon, Introduction to dislocations, Fourth edition, ButterworthHeinemann, Oxford... becomes increasingly difficult with decreasing grain size [53 ] At some critical grain size in nanophase regime, breakdown of lattice dislocation multiplication and generation can lead to the suppression of thermally activated dislocation process in the sample The following findings indicate the dominant role of grain boundary in deformation of 40h-MMed composite samples such as thermally activated unpinning... observed in the 20, 30 and 40h-MMed sample respectively For pure Mg samples, no drastic drop occurs but gradual decrease in Va 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] A ctivation v o lum e (b 3 ) 300 Mg- 5Al- 1AlN Mg 30 40 250 200 150 ... process in the nanostructured Mg composite, which is close to the activation energy for grain boundary diffusion in Mg Phenomena such as absence of dislocations in the grains, softening behaviors and high ductility in the nanostructured composite at room temperature are suggested to be due to GBS assisted by enhanced grain boundary diffusivity 130 Effects of strain rate on tensile properties 5. 5 References... values of the apparent activation volume of composite samples decreased with milling time: from 241b3 for as-blended sample to 44b3, 26b3, 27b3 and 41b3 after 10, 20, 30 and 40h MM respectively From Fig 5. 3, the strain rate sensitivity increased with milling time showing 0.0 153 , 0.0386, 0.06 35, 0.0881 and 1 25 Effects of strain rate on tensile properties 0.0873 for 0, 10, 20, 30 and 40h-MMed composite. .. stress consists of thermal component  and the athermal component G which is almost independent of temperature apart from the small variation of shear modulus G with temperature as in equation 5. 6      G (5. 6) In particular, an increase in temperature or a decrease in applied strain rate provides an increase in the probability of thermal activation and therefore results in a reduction in flow stress... thermally activated unpinning of boundaries due to enhanced diffusivity in grain boundaries [54 ] and diffusional flow [55 ]: i The apparent activation energy of 50 kJ mol-1 in the 40-MMed composite sample is close to the activation energy of 92 kJ mol-1 for grain boundary diffusion in Mg ii Unusually small activation volume of 41 b3 in the 40-MMed composite sample A value of m greater than 0.3 is usually... forming temperature Creep test will be carried out in the next chapter to verify the possibility of grain boundary deformation with grain boundary diffusion controlled process in nc Mg composite 129 Effects of strain rate on tensile properties 5. 4 Conclusions The present experimental results have clearly shown the time dependence of the mechanical properties of bulk nanostructured Mg composite and. .. 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... phenomenon of softening in these mechanically milled materials [41-44] Han et al [ 45] and Longo et al [46] reported that the annihilation of accumulated dislocations in the vicinity of the particles under high applied stress could result in softening Softening could also be due to a low energy dislocation structure reorganization or transformation, as typical for directional strain softening [47] Alternatively, . (%) 0 212 15 219 12 228 8 10 431 8 4 65 5 51 5 6 20 417 19 50 5 9 55 8 5 30 232 41 334 28 347 14 40 176 36 2 05 34 262 34 At the lowest strain rate of 3.33x10 -5 s -1 , the YS of 0h-MMed sample. are in the range of 3b 3 to 100b 3 investigated by Asarro and Suresh [ 15] and Wang et al. [ 35] . M illing duration (h) 04010 3020 Activation volume (b 3 ) 300 0 50 200 100 250 150 MgMg -5Al- 1AlN . Fig. 5. 5 and Table 5. 2, as in the composite samples, 0h-MMed samples show similar ductility of 8-10% and yield stress ranging from 122 to 138 MPa indicating insignificant dependence of strain