Microstructural evolution of Mg-Nd-Zr alloy during multi-directional reverse shear processing

This study investigates the microstructural evolution and hardness response of a Mg–Nd–Zr alloy subjected to a novel hot multi-directional reverse shear (MDRS) process developed by the authors. The MDRS technique represents a severe plastic deformation route designed to enhance shear deformation. This method was applied to deform the magnesium alloy under near-isothermal conditions. Finite element modeling and experimental data show that MDRS provides a more uniform distribution of equivalent strain than traditional MDF/CCDF processing routes. After four consecutive cycles, the ratio of accumulated strain in the central region to that in the peripheral region (inhomogeneity coefficient) is 2.5 for MDF and 1.62 for MDRS, respectively. The as-cast microstructure consists of equiaxed α-Mg grains (~47 μm) and grain-boundary Mg₁₂Nd-type and Zr-rich particles, as confirmed by combined XRD and SEM/EDS analysis. MDRS processing leads to a non-monotonic grain size evolution: refinement to ~32 μm after one cycle, partial coarsening to ~39.7 μm after three cycles, and subsequent stabilisation at ~41 μm after five cycles. The formation of a near-normal grain size distribution and ~120° triple junctions after five cycles indicates a thermodynamically stabilised grain boundary network. The grain size and grain boundaries are controlled by the competition between deformation-induced grain refinement, accompanied by Zener pinning of the boundaries by dispersed particles, and thermally activated grain growth driven by continuous dynamic recrystallization. Microhardness increases by 6–16% compared with the initial state. Higher hardness values are associated with regions of higher accumulated strain and refined grains. In contrast, the slight decrease in central hardness after five cycles correlates with partial grain coarsening, which remains above the initial level. The established relationships between strain distribution, microstructure and hardness demonstrate that MDRS is an efficient route for producing Mg–Nd–Zr alloy workpieces with controlled gradient properties and improved performance, promising for Mg-based bioresorbable implants and lightweight structural applications.