Mechanisms of High Strain-Rate Superplasticity of Al-14 mass%Ni-14 mass%Mm (Misch Metal) Alloy Produced from Amorphous Powder

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Abstract
The deformation mechanisms of a fine grained Al-14 mass%Ni-14 mass%Mm [Mm=misch metal] crystalline alloy consolidated from its amorphous powders have been discussed from the mechanical data examined previously at strain-rates between 10−3 and 100 s−1 at temperatures from 773 to 898 K. This alloy exhibits superplasticity at unusually high strain rates of nearly 1 s−1 in the temperature range from 848 to 885 K, which is close to the measured melting point. By incorporation of the temperature dependence of grain size, threshold stress and shear modulus into the constitutive equation, the activation energy is found to be 158 kJ mol−1 at temperatures below the melting point. In the temperature range above the melting point, however, the activation energy is very large at more than 350 kJ mol−1. This activation energy in the temperature range below the melting point is similar to that for lattice self-diffusion of aluminum, and all mechanical data in these temperatures ranges can be represented by a single equation. It is postulated that superplastic flow in the Al–Ni–Mm crystalline alloy is controlled by a grain boundary sliding mechanism accommodated by dislocation climb controlled by lattice self-diffusion at the solid state, and in the temperature range higher than the melting point is accommodated by the liquid phase at grain boundaries and/or interfaces. The further deformation mechanisms in high strain rate superplasticity should take the phase state of liquid and solid into account.

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