Amorphization/Nanocrystallization of Shape Memory Alloys and Mechanism of Phase Transformation

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TiNi shape memory alloy has been used in diverse areas of applications. Recently the application is expanding in the field of medical devices such as guide wires and stents due to the development of minimally invasive surgery. It is also know that the TiNi undergoes crystalline-to-amorphous transformation by severe plastic deformations. In the present investigation, the process of amorphization/nanocrystallization by various methods of severe plastic deformation, including cold rolling, high pressure torsion and shot peening, was investigated by TEM observations.It was revealed that the B19' martensite reverts to B2 phase by plastic deformation and it transformed to amorphous by further deformation. Peculiar lamellar structures composed of alternative layers of amorphous and B2 crystalline phase. It was also found that the B2 layers on both sides of amorphous layers possess the almost identical crystallographic orientation. This indicates that the defect accumulation at the variant boundaries is responsible for the amorphization.For comparison, Zr-Cu alloys with various compositions were deformed by HPT. Only Zr_<50>Cu_<50> exhibited amorphization. The alloy was in B19' phase and has the fine martensitic twin structure before deformation. Thus it can be inferred that the fine twin structure promotes the accumulation of dislocations and invoke amorphization.