Microstructural and hardness homogeneity in an Mg–Gd−Y–Ag alloy processed by simple shear extrusion

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An extruded Mg–6Gd−3Y−1.5Ag (wt%) alloy was processed by 1, 2, 4 and 6 passes of simple shear extrusion (SSE) at 553 K. Microstructural studies through electron back-scattered diffraction (EBSD) taken from the central areas of the cross sections normal to the extrusion direction (ED) exhibited microstructures mostly comprised of large severely deformed grains having high fractions of low angle grain boundaries (LAGBs) at the early stages of processing. By increasing the number of SSE passes, the fraction of fine grains, with an average size of 1.1 μm, developed through the occurrence of dynamic recrystallization (DRX), increased continuously. The highest recrystallization fraction of 72% was obtained after 6 passes. The EBSD studies on the cross section, however, showed that some degree of heterogeneity was still persistent after 4 passes of SSE along the path from the central areas towards the periphery of the specimen. This inhomogeneity in the microstructure seems to be resolved by increasing the SSE passes to 6. The micro-hardness measurements on the cross section normal to ED of the SSE-processed alloys showed a reasonable level of homogeneity after 6 passes while at the early stages of processing the hardness distribution was in its most heterogeneous state. Furthermore, the overall hardness values increased gradually from the average of 91.3 Hv for the single-pass processed alloy to 110.1 Hv for the one processed by 6 passes of SSE. This improvement in the average hardness was attributed to the Hall-Petch effect related to grain refinement, Orowan hardening as a result of Mg5Gd-type submicron-particles formed via dynamic precipitation, and substructure strengthening caused by the residual LAGBs.