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Abstract

A rapidly solidified and thermomechanically processed fine-grained eutectic NiAl-Cr alloy of the composition Ni₃₃Al₃₃Cr₃₄ (at %) exhibits structural superplasticity in the temperature regime from 900 °C to 1000 °C at strain rates ranging from 10^-5 to 10^-3 s^-1. The material consists of a B2-ordered intermetallic NiAl(Cr) solid solution matrix containing a fine dispersion of bcc chromium. A high strain-rate-sensitivity exponent of m = 0.55 was achieved in strain-rate-change tests at strain rates of about 10^-4 s^-1. Maximum uniform elongations up to 350 % engineering strain were recorded in superplastic strain to failure tests. Activation energy analysis of superplastic flow was performed in order to establish the diffusion-controlled dislocation accommodation process of grain boundary sliding. An activation energy of Q_c = 288 ± 15 kJ/mole was determined. This value is comparable with the activation energy of 290 kJ/mole for lattice diffusion of nickel and for ⁶³Ni tracer self-diffusion in B2-ordered NiAl. The principal deformation mechanism of superplastic flow in this material is grain-boundary sliding accommodated by dislocation climb controlled by lattice diffusion, which is typical for class II solid-solution alloys. Failure in superplastically strained tensile samples of the fine-grained eutectic alloy occurred by cavitation formations along NiAl || Cr interfaces.