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 se vere plastic deformation (SPD). Previous studies of Ni3Al in the as-process ed state demonstrated non-equilibrium grain boundaries, indicated by a high concentration of extrinsic defects, characteristic of material processed b y SPD. Diffraction and high resolution electron microscopy (TEM/HREM) have been used here to examine specimens of the material annealed at 650 degrees C and 750 degreesC, plus material taken from the gauge section of a specime n superplastically deformed to about 300% elongation at a temperature of 65 0 degreesC. Examination of the sample annealed at 650 degreesC showed that grain boundaries remained in high energy, nonequilibrium configurations. Th e sample annealed at 750 degreesC contained grain boundaries with a high de nsity of dislocations. By contrast, the superplastically deformed sample sh owed no visible distortions of the crystal lattice near the grain boundary. Additionally, TEM/HREM observations of twin boundaries in the deformed spe cimen showed no indication of trapped lattice dislocations, suggesting that grain boundary sliding was accommodated by diffusion.