The effect of mechanical incompatibility and strain delocalization on the mechanical properties of gradient structure Cu-4.5Al alloy

Gradient-structured (GS) metals have been reported to possess superior mechanical properties. In this study, the mechanical incompatibility and strain delocalization of Cu-4.5Al-SMAT specimen processed by surface mechanical attrition treatment (SMAT) during tensile was systematically investigated. The results showed that the hardness of specimen exhibited a gradient variation along the thickness after SMAT, which was related to the gradient defects (deformation twins and dislocations). EBSD observation demonstrated that the magnitude of mechanical incompatibility between gradient layers of specimen was associated to the degree of hardness gradient, which was further measured by load-unload-reload (LUR) test. The hetero-deformation induced (HDI) stress of Cu-4.5Al-SMAT specimen was more obvious compared to annealed specimen without SMAT, and the rate of HDI stress strengthening decreased with increasing tensile strain due to the reduction of degree of hardness gradient. Additional strain hardening was found for the Cu-4.5Al-SMAT specimen, which was confirmed by hardness and TEM. Furthermore, the combination of the gradient structure and the coarse-grained matrix can induce dispersed strain bands on the GS surface, which led to strain delocalization and thus obtained high yield strength and considerable ductility in the Cu-4.5Al-SMAT specimen.