Nanostructure formation and phase transformation in intermetallic compounds during severe plastic deformation

Abstract
Torsion straining under a high quasi-hydrostatic pressure (TQHP) has been successfully applied to intermetallic compounds which are brittle under usual conditions. The processing of pure Ni₃Al by TQHP yielded a totally disordered nanometric equiaxed structure, with high internal stresses. The mechanisms by which this structure is formed from the initial ordered millimetric grains were investigated for increasing amounts of torsion. At the microstructural level, the crystals were first fragmented by the propagation of shear bands, then nanosized equiaxed crystallites with high misorientations were formed. At the macroscopic level, the cold-worked structure appears to first forme at the sample surface and then to propagate into the volume. In stoichiometric TiAl, TQHP also led to samples with a nanocrystalline structure. Different structures, lath-type or granular, were produced at different levels of cold-work. Furthermore, increasing the applied pressure and the amount of deformation led to a weak disordering of the compound, and to its partial transformation into a solid solution of Al in Ti. The differences between the two intermetallic compounds are discussed in terms of ordering energy, deformation mechanisms and phase stability.