PSI - Issue 21

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

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at room temperature. More so, unlike face centred cubic structures which are dominated with plenty of slip systems, grain refinement in Mg and its alloys is influenced by thermally activated recovery methods at elevated temperatures. Micrographs and EBSD images were observed at cross-section surface parallel to extrusion direction before and after NTE processing. as highlighted in Fig. 3 and 4 respectively. The outcome of EBSD after electropolishing and subsequent ion beam cross polishing had clearer grains revealed. The as-received specimen in Fig. 3, exhibited a heterogeneous structure with alternating bands of coarse grains and colonies of fine grains. Processing at 373 K and 473 K was less slightly effective in grain size refinement than at 523 K. The grain sizes calculated as received, after NTE processing at 373 K, 473 K and 523 K were 10.64 µm, 7.71 µm, 6.86 µm and 3.11 µm respectively as indicated in Fig. 6(a). Temperature in NTE affected both grain sizes and micro-textures as shown in Fig. 4 & 5. However, at 473 K it was observed that the structure was homogeneous and the twinning randomly distributed in the micrographs. The twinning also observed at 373 K is as a result of structural shear in plastic deformation driven by shear stress on extruded AZ31 alloy. During NTE processing, twinning allows the AZ31 specimen to accommodate the imparted energy in the form of mechanical stress by finding a higher energy alignment of atoms in the crystal lattice [Knauer et al. (2013)]. At 523 K, the micrographs revealed that the structure of AZ31 was bimodal due to dynamic recrystallization (DRX). The elevated temperature triggered abnormal growth that led to increase of largest grains at a higher rate than normal grain growth. This conventional DRX is characterized by nucleation and growth attributed to limited active slip. In other materials conventional DRX may occur due to low to medium stacking fault energy (SFE) [Fatemi-Varzaneh et al. (2015)]. The new grains formed are as a result of strain-induced boundary migration during deformation at elevated temperatures. Galiyev et al. (2001) states that during the mechanism of plastic deformation of Mg alloys, new boundaries characterized by low-medium angle misorientation are created within the grain structure. Studies by Orlov et al. (2009) demonstrated that twist extrusion process exerts strains transforming the low angle boundaries (LABs) which are transverse to the grains and with approximately 5-15° misorientation values into high angle boundaries (HABs). According to T. Sakai et al. (2014) the formation of grains occurs by CDRX (low temperatures), DDRX (intermediate temperatures), subgrains rotation and twinning. The convectional DRX is characterized by nucleation via bulging of pre-existing high angle grain boundaries. Plastic deformation at range of 473K-523K is controlled by cross slip of a dislocation. These cross slips are activated near the original grain boundaries when high shear stresses are imposed. Dislocation due to cross slip and climb generates LABs within the original boundaries. Therefore, the continuous absorption of dislocations in LABs during deformation at 373 K, 423 K and 523 K shows that CDRX occurred. At high temperatures where strains are relatively low, the formation of bulges of grain boundaries is common. This is partly influenced by strain localization at slip planes. The bulges eventually lead to the nucleation of the RX grains. NTE die geometry minimizes the strain localization at the entry and outlet points of the twisting channel thus impeding nucleation. The textures of as received samples and after NTE at 373 K, 473 K and 523 K analyzed using pole figures as illustrated in Fig. 5 were insightful. The results indicate that the basal slip plane {0001} in the extruded condition lies parallel to the extrusion axis. It is demonstrated that the c-axis is in the (RD, TD-image; ED, ND-die) also ( r, θ ) plane of the extruded rod and thus parallel to the NTE extrusion axis. It is clearly demonstrated that the pole figures on the basal, prismatic and pyramidal planes were distinct at all the temperatures. Examination of the {0001} pole figures shows a strong basal character to the primarily basal texture. The initial texture is radially symmetrical at basal plane but transversely symmetrical on the prismatic and pyramidal planes. At 373 K, texture develops off-basal on basal planes {0001}. There was slight significant change at {10-10} and {11-20}. It clearly shows that strong basal textures on basal slip plane produced by NTE deformation allowed taking advantage of grain refinement in terms of improving the mechanical properties. At 473 K, there was a slight tendency for {0001} to align preferentially with RD while both the {10-10} and {11-20} has weak preference for RD but strong preference for the TD. The slip enhances the texture character along RD while prismatic and pyramidal slip modes maintains the initial radially symmetric texture along the TD. The resultant texture evolution at 473 K clearly supports the significant improvement of mechanical properties upon deformation. The tendency at 523 K corresponds to as received specimen but with a weaker preference for {0001} to align with the RD than TD. At 523 K, there is a higher tendency to maintain off-basal texture associated with basal slip modes whilst less preference to maintain off-basal texture orientation at prismatic and pyramidal modes. This clearly explains the slight increase in mechanical properties of AZ31 alloy after NTE at 523 K compared to deformation at 373 K and 423 K. Results by Song et al. (2010) reveal that the control of texture influences material mechanical properties.