Martensitic transformation and premartensitic phenomena in Ti-Ni-based alloys

摘要

Martensitic transformation and premartensitic phenomena in Ti-Ni and Ti-Ni-based alloys are critically reviewed with emphasis on recent progress and highlight on unsolved issues. For these alloys, the B2 parent phase transforms into a unique monoclinic B19' martensite via one of three possible paths; (1) B2-B19', (2) B2-R-B19' and (3) B2-B19-B19'. The actual transformation path adopted is strongly dependent on both alloying addition and thermo-mechanical treatment. The formation of B19' martensite is found to be due to the softening in elastic constant c_(44) in addition to c'; the three transformation paths are closely related to the multi-mode phonon anomalies in both TA_2 and TA_1 phonon branches. Landau theory has been able to connect these anomalies with the competing martensite phase. First-principle calculations have demonstrated encouraging possibility of explaining some of the phonon softening and the stability of the martensite phases. The incommensurate 1/3<110>~* diffuse scattering indicates the existence of static displacement waves (or embryos of the incipient R phase) prior to the R formation. These static waves exist in the parent phase up to 100K above the Rs temperature and gradually fade away. There is no " clear boundary" between ideal parent phase and parent phase with the incommensurate diffuse scattering.