The fine grained Ni3Al (γ′)-NiAl (β) two-phase nickel aluminides reinforced with or without TiC particles were fabricated by a wet mechanical alloying and vacuum hot pressing process. The average grain sizes of the as-compacted intermetallics decreased from about 1.2 μm to about 0.4 μm with increasing in TiC volume fraction from 0% to 30%. High temperature compression tests for these intermetallics were carried out at a temperature in a range of 1073 K-1273 K and at an initial strain rate in a range of 1.4×10−4 s−1-5.6×10−2 s−1. Tensile tests were also performed at selected strain rates at 1273 K.
In the log true stress-log strain rate curves for the nickel aluminides containing TiC particles, two strain rate regions showing low and high strain rate sensitivity exponents, m, (region I and II, respectively) were recognized, while the intermetallic without TiC showed only region II in this compression condition. At strain rates higher than about 10−2 s−1 (region II) and at 1273 K, the value of m was larger than 0.32 for the nickel aluminides containing TiC particles. About 180% and 100% elongations were obtained for the nickel aluminides containing 0 and 10 vol.%TiC, respectively, at a condition of 4.2×10−2 s−1 and 1273 K. By the microstructure examinations using SEM and TEM, the occurrence of grain boundary sliding was observed in these nickel aluminides compressed at the strain rate of 1.4×10−2 s−1 and 1273 K.
The threshold stress, σth, estimated by using an extrapolation method increased with increasing in the TiC content. Each double logarithmic plot of the strain rate and the effective stress, σe(=σ-σth), can be approximated by a straight line with a slope of about 0.5 for the nickel aluminides containing TiC. This suggests that the superplasticity of the composites at the high strain rates can be explained by the grain boundary sliding model accommodated by dislocation slip.
