Mechanical Consideration on the Stress-Strain-Strain Rate Diagram of Zn-22Al Superplastic Alloy

Abstract

A study of the constitutive equation σ=K\dotεm (K and m are constants) and the mechanical behavior of Zn-22Al superplastic alloy on the stress-strain curve was made with improvement of the usual method of the strain measurement (mean strain; εm) by using a short gage length. That is, the wire strain gage was used to obtain the strains smaller than 2% (wire strain; εw), and the elongation of the grid (0.5 mm) printed on the tensile specimen was measured to obtain the strains more than 1% (grid strain; εg). The compressive strains more than 2% were obtained by the displacement of the crosshead. Furthermore, the replicas from the specimen surface before and after deformation were observed, and the following results were obtained:\ oindent(1) On the equation.(i) The strain rate \dotεw increases with time in tension and compression at a constant crosshead speed at 20°C in the range of the strain hardening, and the equation holds for all cases.(ii) The mean strain rate \dotεm decreases with time at a constant crosshead speed in tension at 20°C, while the grid strain rate \dotεg increases. However, the experimental equation, which has been obtained with \dotεm, can be also described by \dotεg in tension and compression in the range of the steady state plastic flow.(iii) The value \dotεg is almost constant and equal to the initial value of \dotεm in tension at 250°C.\ oindent(2) On the stress-strain curve at a constant strain rate.(iv) Zn-22Al alloy does not harden with strain in the range of the steady state plastic flow at 20 and 250°C.(v) With increasing tensile velocity, the value of the stress in the transition range more rapidly decreases from the strain hardening to the steady state plastic flow. It may be qualitatively ascribed to the stress relaxation and the generation of the necking.(vi) The compressive stress is about 15% larger than the tensile stress at 20°C. This may be mainly due to the difference of the amount of cavities or voids which generate along the grain boundaries in tension and compression.