The nature of the maximum microhardness and thickness of the gradient layer in surface-strengthened Cu-Al alloys

Abstract The presence of a gradient layer with an increased maximum microhardness and extended thickness is extremely attractive because it can enhance the service performances for surface-strengthened metallic materials. In this research, Cu-Al alloys with different Al contents and microstructures were processed by surface spinning strengthening (3S) and studies were conducted to examine the gradient microstructure and microhardness distributions of the 3S Cu-Al alloys. The experimental results show that each group of the 3S Cu-Al alloys having the same Al content has approximately the same maximum microhardness at the topmost surface layer and the maximum microhardness increases with an increase in the Al content. In addition, the thickness of the gradient layer in the 3S Cu-Al alloys increases with a decrease of yield strength and an increase in the work-hardening exponent, respectively. The relationship between the maximum microhardness and chemical composition which determines the Young's modulus and plastic deformation mode, and the relationship between the thickness of the gradient layer and the microstructure which governs the strength and work-hardening capacity, were both investigated. It is found that the maximum microhardness of the gradient layer depends mainly on the chemical composition; whereas the thickness of the gradient layer depends primarily on both the strength and work-hardening capacity which are closely related to the microstructure. By combining the compositional design and microstructure optimization of the materials, and improving the surface strengthening intensity, a gradient layer of the surface-strengthened materials with an enhanced maximum microhardness and an extended thickness may be achieved.