PSI - Issue 21

Nathaniel Mupe et al. / Procedia Structural Integrity 21 (2019) 73–82 Mupe et al./ Structural Integrity Procedia 00 (2019) 000 – 000

79

7

4.2. Mechanical Properties The hardness distribution values obtained by Vickers microhardness tests of AZ31 alloy along the specimen front surface are shown in Fig 6(c) for as-received, I pass at 373 K, 473 K and 523 K. It was observed that a hardness rise at centre compared to the periphery of the as received specimen. This indicated an inhomogeneous structure. However, the hardness value after NTE 1 pass at 373 K, 473 K and 523 K slightly varied along the surface due to improved grain homogeneity. This indicated that, despite the surface periphery being subjected to contact stress between the NTE die channel and the sample edges, the induced grain refinement was equally distributed to the pressed material’s core. The specimen hardness values against equivalent strain are plotted in Fig. 6(d).

Fig. 6. (a) AZ31 alloy average grain size (AGS); (b) microhardness achieved (c) microhardness distribution along the specimen surface (d) microhardness versus equivalent strains relation.

The results were consistent with the Hall-Petch relationship after 1 pass and confirms that the significant increase of hardness in Fig. 6(b) initiated by NTE processing is directly proportional to the grain refinement trend in Fig.6(a). However, further reduction of grain size after NTE at 523 K appears to have a slight impact on hardness. According to Huang et al. (2013) and Galiyev et al. (2001) this reduction of hardness is attributed to the abnormal growth that was shown in Fig. 3. Xiao-ming and Tao-tao (2009) argues that the deformation at high temperatures induces dynamic recrystallization and formation of new grains. Fatemi-Varzaneh et al. (2015) claims that during ABE and ECAP of AZ31 alloy, recrystallization resulted to sharp decrease of dislocation densities. Thus it was clearly revealed that NTE was highly effective with significant changes observed after 1 pass at 373 K and 473 K as shown in 6 (b) due to the twinning and work hardening effects. Same trends were reported by Bryla et al. (2012). The typical tensile stress-strain curves were shown in Fig. 7(a). The yield strength; and ultimate tensile strength (UTS) extracted from these curves are indicated in Fig. 7(b). The total elongation to failure of AZ31 after NTE is shown in Fig. 7(c). The ductility increased with temperature rise due to the occurrence of slip on prism planes. However, this is also alleviated by other modes such as twinning, grain boundary sliding and pyramidal 〈 + 〉 slip. The results of YS and UTS of the as-received specimen were 177 MPa and 274 MPa respectively. Both YS and UTS increased after 1 pass. However, a good hardening rate was observed at 373 K and 473 K. The results after 1 pass with back pressure at 523 K was more effective as revealed by higher YS and UTS to 235 MPa and 382 MPa.