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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3.2. Microstructural and mechanical characterization Mechanical grinding and polishing for all specimen were performed on Struers tegraforce-5. The ion etching was conducted on the JEOL SM-09010DM cross polisher. The experimental characterization plan was followed sequentially as follows; micrographs, hardness and tensile testing. The Optical Micrographs (OM) and electron backscatter diffraction (EBSD) analysis were obtained using Keyence laser scanning confocal microscope VK-X210 and JEOL JSM7001FD scanning electron microscope (SEM). For EBSD observation, the specimen size 6 mm*6 mm*1 mm was prepared by slow polish up to SiC grid number #2000. The specimen was then subjected to electropolishing to achieve a mirror-like surface. The electrolyte chemical solutions composed of 15% HNO 3 in ethanol. AZ31 and stainless steel were connected to the anode and cathode respectively in the electrolytic cell. Processing parameters were 12 V, 15 s and 255 K. Ion etching was carried out on the specimen with specific conditions since magnesium is soft and susceptible to surface damages. For improved EBSD results, the ion polishing was conducted at 5.9 kV for a duration of 4 hours. Mechanical characterization consisted of microhardness and tensile testing. Hardness was measured using the Shimadzu HMV microhardness tester. The hardness processing parameters were HV0.1, 980.7 mN load and 15 s dwell time. Tensile tests of as received and NTE specimen were obtained at room temperatures using Shimadzu tensile-compressive test machine; autograph AGS-X 10 kN. A constant testing speed was set at 0.2 mm/min to achieve a strain rate of 10 -3 s -1 . The specification of each tensile specimen had a gauge length of 3 mm and thickness of 2 mm as illustrated in Fig. 2(a). Each billet before and after NTE measuring 20 mm by 9.5 mm was machined as shown in Fig. 1c(III) to produce 3 tensile specimens which were all tested to validate the results obtained. The tensile axis for all machined test pieces was set parallel to the extrusion direction (ED) of the specimen [See Fig. 2(b)]. True stress strain data was plotted. Fig. 2. Schematic illustration of (a) tensile specimen; (b) AZ31 extruded rod: ED extrusion direction, ND-normal directions, TD- transverse direction. 4. Results and Discussion 4.1. Microstructures and textures The AZ31 alloy was successfully deformed through NTE processing for 1 pass at temperatures as low as 373K. Mg alloy often crack during SPD as demonstrated by previous studies [Kim et al. (2003)]. The NTE geometry eliminates strain localization at the inlet and outlet points of the twisting part allowing shear strain to be imposed down the floor channel without rigid body rotation. The punch force exerted in NTE is higher due to high contact surface between the die channel and the specimen. This in turn contributes to a moderate back pressure within the channel floor that established a relatively low hydrostatic pressure. This plays a vital role in suppressing the nucleation process witnessed in AZ31 processing at elevated temperatures and eliminates shear failures on the billet surface. Studies by T. Sakai et al. (2014) show that DRX recrystallization proceeds by nucleation and nucleus growth. Initially grain refinement at room temperature was associated with cracking and hence subjected to elevated temperatures. According to Leiva et al. (2009) deformation of Magnesium and its alloys through ECAP at room temperature is strongly dependent on the basal slip systems, 〈 〉 {0001} 〈112̅ 0〉 and the extension twinning system {101̅ 2} 〈101̅̅̅1̅ 〉 . The lack of slip systems that would facilitate dislocation with Mg and its alloys poses a challenge to successfully conduct NTE