Effect of ECAP strain path on deformation behaviour, microstructure, texture evolution, and mechanical properties in magnesium

This work investigates the effect of different strain paths related to equal channel angular pressing (ECAP) routes A, BC, and C on the deformation behaviour, microstructure and texture evolution in magnesium (Mg). ECAP was done using a 90° die with 0° arc-angle at ≥200°C up to eight passes. In routes A and C, strong B and shifted C2 fiber evolve after every pass. In BC, strong and shifted B-fibre evolves in the ideal P-fiber position. The area (number) fraction average grain size reduces from ∼51.4(∼12)μm to ∼4(∼2.6)μm, ∼7.5(∼3.1)μm, and ∼8.7(∼4.7)μm, for A, BC, and C after eight passes. The input basal planes were almost normal, ∼45° shifted, and parallel to the ECAP shear plane for routes A, BC, and C, respectively. Thus, lattice rotation in route A was significantly larger with a ∼90° shift in texture per pass than ∼20° in BC and was least for route C. Therefore, even though the strain path changes are highest in BC, followed by A and C, the largest grain refinement was obtained in A, comparatively less in BC, and least in C. In route C, the B-fiber position meagrely varied in each pass except in the 2nd pass, and only the C2 component position changed. Crystal plasticity indicates that after basal, pyramidal (pyr) ⟨c+a⟩-II for route A, prismatic (prism) and pyr ⟨c+a⟩-I and II slips for route BC were active. For route C, the activity of the pyr ⟨c+a⟩-II slip decreases, while basal activity increases with strain for all passes. Due to the strain path variation, the changes in the critical shear strength ratio of the basal, prism, pyr ⟨c+a⟩-I and II slip systems differed for the ECAP passes by A, BC, and C. The texture evolution and grain refinement affect the compressive strain hardening behavior. The sigmoidal flow behavior for the initial material was due to geometric hardening associated with extension twining (ET), which reduces after the ECAP for all routes due to grain refinement. The presence of C2 is favourable for ET activation and leads to slight sigmoidal behavior in grain-refined Mg, prominent after route A. Micro-hardness and compressive yield strength increase with ECAP pass due to grain refinement.