Study on the differences in response of GNDs in gradient heterogeneous structures and heterogeneous deformation-induced (HDI) strengthening

Gradient heterogeneous structures significantly enhance the mechanical properties of magnesium alloys via heterogeneous deformation induction (HDI), yet the continuous strengthening mechanisms of gradient structures remain understood inadequately. In this study, a continuous strain gradient field was introduced along the rolling direction during rolling to fabricate a Mg-1Bi-0.7Zr alloy exhibiting varied characteristics of microstructure. With increasing of gradient, grain size and the proportion of low-angle grain boundaries gradually enlarged in the alloy. Concurrently, dynamic recrystallization (DRX) was induced by the high shear stress in the surface region, resulting in a fine-grained region (FR) with an average grain size of 2.67 μm. In contrast, the core region predominantly was comprised by deformed grains from the rolling process, forming a coarse-grained region (CR) with an average grain size of 4.21 μm. The surface FR exhibited superior hardness (0.71 GPa) and an elastic modulus of 47.59 GPa, which was attributable to the combined effects of Hall–Petch strengthening, Mg 3 Bi 2 precipitate formation and Zr precipitation. The enhanced plasticity of CR primarily due to back stress generated by HDI strengthening from geometrically necessary dislocations (GNDs). Moreover, nano-scratch experiments quantitatively characterized a transition in the deformation mode from elastic-dominated to plastic-dominated behavior. Specifically, the recovery index (η) progressively decreased with increasing gradient, while both the penetration depth (Pd) and residual depth (Rd) increased. Additionally, the elastic modulus (EIT) exhibited a negative correlation with the gradient. This study thereby provided theoretical insights into dislocation behavior and HDI hardening mechanisms in heterogeneous magnesium alloy.