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°C, and 750°C, plus material taken from the gauge section of a specimen superplastically deformed to about 300% elongation at a temperature of 650°C. Examination of the sample annealed at 650°C showed that grain boundaries remained in high energy, nonequilibrium configurations. The sample annealed at 750°C 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