Fig.1. Schematic illustration of the gradient grain structures of the AZ31 Mg alloy sample upon surface mechanical attrition treatment (SMAT) and the corresponding micro-hardness test results for the different thickness layers

Fig.2. (a) Set-up of quantitative in situ SEM tensile testing; (b) magnified image of the mounted ‘dog-bone’ shape Mg alloy sample corresponding to the red rectangular area in (a); the red rectangular area in (b) is for the high resolution in situ SEM imaging; (c) SEM image of ‘coarse-grain’ features in the deep center of a SMATed sample before test; (d) SEM image of “fine grain” features on the top surface of a SMATed sample before test.

Fig.3. (a)-(f) In situ SEM images of grain evolution and crack propagation at different strains of the SMATed ‘coarse-grain’ sample; (g) microstructure morphology after fracture; (h) corresponding engineering stress-strain curve of the tensile tested sample; (i) comparison of the localized strain (measured from local SEM images) and the global engineering strain of the whole specimen. The dash lines in (a)-(f) show the grain outlines of two adjacent grains, and the red and blue arrows show the transgranular and intergranular cracks, respectively. The letters in (h) correspond to the SEM images of (a)-(f). The white arrow beside the SEM images shows the loading/tensile direction.

Fig.4. (a)-(g) In situ SEM images of grains evolution and cracks propagation at different strains of the SMATed ‘fine-grain’ sample; (h) corresponding engineering stress-strain curve of the tensile tested sample; (i) comparison of the localized strain (measured from local SEM images) and the global engineering strain of the whole specimen. The dash lines in (a)-(f) show the grain outlines of two adjacent grains and the blue arrows show the intergranular cracks. The letters in (h) correspond to the SEM images of (a)-(f). The white arrow beside the SEM images shows the loading/tensile direction.

Fig.5. Engineering stress-strain curve of a raw AZ31 Mg alloy tensile test (center) with: (a) the corresponding SEM image of grain features before test; (b) SEM image of morphology at the strain of 9.0%, in which both intergranular (blue) and transgranular (red) cracks were initiated; (c) SEM image of the features at the strain of 10.8%, with more cracks being activated; (d) SEM image of grain morphology at peak stress and strain of 11.9%; (e) SEM image of grain morphology after peak stress and strain of 13.2%, during which some of the intergranular and transgranular cracks started to align and nucleate into one single crack; (f) SEM image showing the final nucleated major crack (red dash line) which led to the fracture failure of the whole specimen. The white arrow on the top of the SEM images shows the loading/tensile direction.