Retained coarse grains in bulk nanocrystalline Ni3Al

Transmission electron microscopy methods were used to study the nanocrystalline structure achieved by severe plastic deformation of L12 long-range ordered Ni3Al. The deformation by high-pressure torsion (up to 40000 %) leads to a duplex structure consisting of a disordered nanocrystalline and an ordered coarse-grained structure carrying different shear strains. The volume fraction of the nanocrystalline structure increases with strain but varies locally. During the deformation micrometer sized coarse-grained volumes embedded in the nanocrystalline structure are retained; they show a predominance of orientations of high symmetry. This leads to the conclusion that the stability of the coarse grains results from their homogeneous slip caused by a homogeneous activation of the slip systems.