Deformation characteristics of a superplastic aluminium alloy produced from amorphous powders

Abstract

The tensile behaviours of an Al-14wt.%Ni-14wt.%Mm (Mm ≡ misch metal) crystalline alloy consolidated from its amorphous powders was characterized at strain rates between 10 −3 and 100 s −1 at temperatures from 773 to 898 K. This alloy exhibited superplasticity at unusually high strain rates of nearly 1 s −1 in a temperature range from 848 to 885 K. A maximum elongation of about 650% was obtained at a constant strain rate of 1 s −1 at a very high temperature of 885 K, which was close to or lower than the melting point. The microstructure of the AlNiMm crystalline alloy annealed in the optimum superplastic temperature range consists of grains with a mean size of about 1–2 μm and fine particulates a size of about 1.2 μm and a high volume fraction of about 40%. By incorporation of the temperature dependence of grain size and shear modulus into the constitutive equation, the stress exponent and the activation energy were found to be 2 and 421 kJ mol −1 respectively. It is postulated that superplastic flow in the AlNiMn crystalline alloy is controlled by a grain boundary sliding mechanism, but the accommodation process remained unclear.