Formation of the nanocrystalline structure in the Ti50Ni25Cu25 shape-memory alloy under severe thermomechanical treatment

Abstract

Results of comparative studies of the structure of the cast martensitic Ti50Ni25Cu25 alloy in the initial state, after severe plastic deformation by high-pressure torsion (HPT), and after subsequent annealing are presented. The studies have been performed by X-ray diffraction, transmission and scanning electron microscopy, and measurements of electrical properties. It has been established that the alloy undergoes almost complete amorphization after torsion using 5 and 10 rev of anvils under a pressure of 7 GPa. This result can be explained by the large value of shear deformation (true strain from 6 to 7 units) and the retention of an extremely large quantity of highly dispersed (less than 3–4 nm in size) nanocrystals with a distorted B2 lattice in the amorphous matrix even at room temperature. Their determining role as nuclei of crystallization ensures the total process of low-temperature nanocrystallization upon subsequent annealing, beginning from 250–300°C. It is shown that the annealing of the alloy amorphized during HPT makes it possible to produce extremely uniform nanocrystalline (NC), submicrocrystalline (SMC), or bimodal (NC + SMC) structures of B2 austenite. For the first time, a complete diagram of thermoelastic martensitic transformations in the field of B2-austenite states, from nanostructured to usual polycrystalline, has been constructed for the Ti50Ni25Cu25 alloy. The size effect of stabilization of the martensite transformation has been found in the nanocrystalline B2 alloy.