Effect of global constraint on the mechanical behavior of gradient materials

The gradient structured (GS) samples studied here consists of gradient layer (s) with grain size gradient and a coarse-grained (CG) layer. Gradient materials have been found to have superior mechanical properties due to the hetero-deformation induced (HDI) strengthening and strain hardening. However, the effect of the interaction between the GS layer and the CG layer on the mechanical properties of the gradient materials remains to be studied. In this paper, single-sided GS Cu and double-sided GS Cu plates were prepared by surface mechanical attrition treatment (SMAT) on one side and both symmetrical sides respectively. It is found that GS samples have higher yield strength than CG samples, and the Cu samples with double-sided GS has an uniform elongation comparable to CG Cu. In addition, the global mutual constraint among different GS layers affect the mechanical behavior of gradient materials, which is revealed in the comparative study of double-sided and single-sided GS Cu. Specifically, the double-sided GS Cu exhibit higher yield strength and ductility than the single-sided GS Cu, because the former is better constrained. This study demonstrates that in addition to the local structural gradient, the symmetric constraint produce more effective HDI effect to enhance the mechanical properties. The global mutual constraints of GS layers played a positive role in producing superior mechanical properties.