Tailoring Twinning Modes via SMAT Temperature for Enhanced Strength–Ductility Synergy in Gradient-Structured Zr

The mechanistic basis for overcoming the strength-ductility trade-off in hexagonal close-packed (HCP) metals via cryogenic surface engineering remains unclear, particularly regarding how processing temperature tailors deformation mechanisms in gradient-structured (GS) pure Zr. Here, pure Zr with a gradient structure was fabricated via surface mechanical attrition treatment (SMAT) at room temperature (RT) and liquid nitrogen temperature (LNT) to systematically investigate the influence of processing temperature on the microstructural evolution, mechanical properties, and deformation mechanisms. The GS Zr processed at LNT achieves a superior strength-ductility synergy, with yield strength increased by 23% and uniform elongation maintained at 81%, significantly outperforming both the CG and SMAT-3-RT counterparts. This enhancement is attributed to an optimized volume fraction of gradient layers and a significantly increased density of deformation twins. Microstructural analysis combined with Schmid factor evaluation reveals that the processing temperature determines the twinning mode: the deformation during SMAT at RT is dominated by basal and prismatic dislocation slips together with {101¯2} <101¯1¯> (T1) twinning, whereas that at LNT involves the same slip modes but {112¯2} <112¯3¯> (C1) twinning, confirming the strong temperature dependence of twin variant selection. Furthermore, during subsequent tensile deformation, prismatic slip emerges as the primary deformation mode in both gradient-structured samples, demonstrating a slip-dominated response once the gradient structure has been achieved. This work elucidates the deformation mechanisms of GS Zr during both SMAT processing and tensile testing, providing guidance for tailoring gradient structures in HCP metals.