On the evolution of a deformation induced nanostructure in a Ni3Al alloy

The evolution of a nanocrystalline structure of a polycrystalline L12 ordered Ni3Al alloy was investigated by transmission electron microscopy methods after severe plastic deformation by high pressure torsion. As a precursor of the nanocrystalline structure, blocks and bands are formed containing misoriented fragments in the 100 nm range separated by a three-dimensional array of low angle boundaries. The inhomogeneous deformation leads to localized disorder and a high density of dislocations, especially geometrically necessary ones. Dynamic recovery causes a heterogeneous formation of the nanocrystalline structure. The interplay of dynamic recovery and disordering yields nanograins in the disordered state. At strains of about 8000% a duplex structure is formed consisting of veins of disordered nanograins that are embedded in the ordered coarse-grained structure. The nanocrystalline veins showing a weak texture grow by the formation of nanograins generated autocatalytically at their interface. Finally, at strains above 50,000% the whole volume transforms to the nanocrystalline structure.