Mechanical properties and deformation behavior of gradient-structured Cu–Al–Zn alloys processed by SMAT at room and liquid-nitrogen temperatures

This study examines the evolution of surface-height variation in gradient-structured Cu-Al-Zn alloys with a stacking fault energy of 7 mJ/m 2 processed by surface mechanical attrition treatment (SMAT) at room and liquid-nitrogen temperatures. Surface topographies before and after tensile loading were analyzed using three-dimensional white-light interferometry. A pronounced mechanical incompatibility develops between the hard gradient layer and the soft coarse-grained core, and this property mismatch substantially affects the overall mechanical response. The associated strain-gradient accumulation produces hetero-deformation-induced (HDI) hardening and strain hardening. In addition, processing at liquid-nitrogen temperature induces high-density twinning, which plays a key role in strengthening the initial strain-hardening response and achieving a favorable strength-ductility combination. Cryogenic SMAT introduces a higher density of twin boundaries and promotes geometrically necessary dislocations (GNDs) accumulation during subsequent tensile deformation. Under quasi-static uniaxial tensile loading, differences in surface height between homogeneous and heterogeneous samples before and after deformation were visualized by three-dimensional white-light interferometry. The results show that mechanical incompatibility in the gradient material drives continuous evolution of the surface-height profile during deformation, thereby generating microscopic surface-height differences. By contrast, the homogeneous samples exhibit only limited roughness differences. This study provides fundamental insights into the mechanical behavior of heterogeneous copper-alloy structures.