Grain Refinement and Superplasticity in Ni-30Cr-5Al Alloy

Abstract

The Ni-30Cr-5Al alloy which is used for an ornamental material is relatively soft in an as-solid-solution state, and the alloy can be strengthened to about 600 in Vickers hardness by precipitation hardening. In the present study, a possibility of superplasticity in the alloy is examined by refining its microstructure using a thermomechanical treatment. The thermomecahnical treatment is composed of a cold rolling of at least 80% in rolling reduction followed by a precipitation and recrystallization at temperatures below 1225 K. A fine-grained dual phase microstructure in which fine round α-Cr phase particles are uniformly dispersing in γ-Ni phase matrix can be obtained by the treatment. An alloy which was undergone such treatments shows the total elongation of more than 500% and the strain-rate sensitivity index of about 0.4 when deformed at 1225 K. The deformation at temperatures below 1225 K obeys thermally activated process and the activation energy found is 251 kJ mol^-1. It can therefore be concluded that the Ni-30Cr-5Al alloy which has undergone the above-stated thermomechanical treatment shows superplasticity.