TEM/HREM observations of nanostructured superplastic Ni3Al

Nanostructured Ni3Al intermetallic (Ni-8.5 Al-7.8 Cr-0.6 Zr-0.02 B; in wt%) with an average grain size of 50 nm was produced using the technique of severe plastic deformation (SPD). Previous studies of Ni3Al in the as-processed state demonstrated non-equilibrium grain boundaries, indicated by a high concentration of extrinsic defects, characteristic of material processed by SPD. Diffraction and high resolution electron microscopy (TEM/HREM) have been used here to examine specimens of the material annealed at 650 degreesC and 750 degreesC, plus material taken from the gauge section of a specimen superplastically deformed to about 300% elongation at a temperature of 650 degreesC. Examination of the sample annealed at 650 degreesC showed that grain boundaries remained in high energy, nonequilibrium configurations. The sample annealed at 750 degreesC contained grain boundaries with a high density of dislocations. By contrast, the superplastically deformed sample showed no visible distortions of the crystal lattice near the grain boundary. Additionally, TEM/HREM observations of twin boundaries in the deformed specimen showed no indication of trapped lattice dislocations, suggesting that grain boundary sliding was accommodated by diffusion.