High strain rate superplasticity in fine-grained (γ′ + β) two phase nickel aluminides produced by Wet MA process

Grain refinement of Ni₃Al (γ′) and NiAl (β) two-phase nickel aluminides (34Al), together with nominally single γ′ phase (30Al) or β phase (43Al) nickel aluminides has been attained by a wet mechanical alloying and vacuum hot pressing process. The average grain sizes of the three as-compacted intermetallics were very close and about 1.0 μm. High temperature compression tests were carried out at a temperature range of 1073 K - 1273 K and at an initial strain rate range of 1.4 × 10^-4s^-1 - 5.6 × 10^-2s^-1. Tensile tests were also performed at selected strain rates at 1273 K. The strain rate sensitivity exponent m for 34Al evaluated by compression test at 1273 K was as high as 0.38 at an initial strain rate range from 1.4 × 10^-4s^-1 to 5.6 × 10^-2s^-1. The maximum elongation of about 250% was obtained at a strain rate of 4.2 × 10^-3s^-1 for 34Al. Even at high strain rate of 4.2 × 10^-2s^-1 and at 1273 K, 188% elongation was obtained for 34Al. These results suggested the possibility of high strain rate superplasticity in the two phase intermetallics. By the microstructure observations using SEM and TEM, grain boundary sliding and dislocation activity within grains were found in the two phase intermetallics deformed at the strain rates higher than 10^-2s^-1. The deformation behavior and mechanism for the two phase nickel aluminides were discussed in the comparison of the results of γ′ and β nominally single phase intermetallics.