Grain refinement and high strain rate superplasticity in (gamma '+beta) two phase nickel aluminides reinforced with TiC particles

The fine grained Ni3Al (gamma ')-NiAl (beta) two-phase nickel aluminides re inforced 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 mum to about 0.4 mum wit h increasing in TiC volume fraction from 0% to 30%. High temperature compre ssion 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 x 10(-4) s(-1)-5.6 x 10(-2) s(-1). Tensile tests were also performed at sele cted strain rates at 1273 K. In the log true stress-log strain rate curves for the nickel aluminides con taining TiC particles, two strain rate regions showing low and high strain rate sensitivity exponents, in, (region I and II, respectively) were recogn ized, while the intermetallic without TiC showed only region II in this com pression 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 nicker alumi nides containing TiC particles. About 180% and 100% elongations were obtain ed for the nickel aluminides containing 0 and 10 vol.%TiC, respectively, at a condition of 4.2 x 10(-2) s(-1) and 1273 K. By the microstructure examin ations using SEM and TEM, the occurrence of grain boundary sliding was obse rved in these nickel aluminides compressed at the strain rate of 1.4 x 10-2 s(-1) and 1273 K. The threshold stress, sigma (th), estimated by using an extrapolation metho d increased with increasing in the TiC content. Each double logarithmic plo t of the strain rate and the effective stress, sigma (e)(= sigma-sigma (th) ), 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 o f the composites at the high strain rates can be explained by the grain bou ndary sliding model accommodated by dislocation slip.