Deformation Behaviours of an Al-33Cu Superplastic Material with Non-uniform Cross-sections

Abstract

Tensile tests were carried out to investigate deformation behaviours of specimens with various shapes of pre-existing non-uniformities. The results are summarized as follows: (1) The deformation behaviours of the specimens depend not only on the initial ratio of minimum/maximum cross-sectional area (initial inhomogeneity factor α₀) but also on the initial length of non-uniform region (l₀). The reduction of the inhomogeneity factor with strain increases with increasing l₀. (2) It is observed that the area-reduction rates of non-uniform region to that of uniform region (\Dot{A}_n⁄\Dot{A}_u) is less than 1 even if m<1, which is contrary to the expectation from the Backofen’s or the Hart’s analysis where \Dot{A}_n⁄\Dot{A}_u>1 for m<1. (3) It is shown that ΔA (the difference in cross-sectional area between uniform and non-uniform region) and Δ\Dot{A} cannot be connected directly with the uniformity of deformation. Therefore, it is not reasonable to use the Hart’s parameters for the evaluation of uniformity of deformation. (4) The boundary between non-uniform and uniform regions moves like Lüder’s front during deformation, which suggests strong force to act on the boundary. (5) It is pointed out that the fracture strain is controlled by the final value of α, the fracture diameter D_f and the length of necking l₀, so that the importnat is to find out the effects of m-value on α, D_f and l₀.