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

Torsion straining under a high quasi-hydrostatic pressure (TQHP) has been s uccessfully applied to intermetallic compounds which are bridle under usual conditions. The processing of pure Ni3Al by TQHP yielded a totally disorde red nanometric equiaxed structure, with high internal stresses. The mechani sms by which this structure is formed From the initial ordered millimetric grains were investigated for increasing amounts of torsion. At the microstr uctural level, the crystals were first fragmented by the propagation of she ar bands, then nanosized equiaxed crystallites with high misorientations we re formed. At the macroscopic level, the cold-worked structure appears to f irst forme at the sample surface and then to propagate into the volume. In stoichiometric TiAl, TQHP also led to samples with a nanocrystalline struct ure. Different structures, lath-type or granular, were produced at differen t levels of cold-work. Furthermore, increasing the applied pressure and the amount of deformation led to a weak disordering of the compound, and to it s 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.