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

TiNi形状记忆合金已在不同领域得到应用。近年来，由于微创外科的发展，其在医疗器械领域的应用不断扩大，如导丝和支架。我们还知道，TiNi通过严重的塑性变形经历了从晶态到非晶态的转变。用电子显微镜研究了冷轧、高压扭转和喷丸等多种严重塑性变形方法的非晶化/纳米化过程，发现B19‘马氏体通过塑性变形转变为B2相，进一步变形后转变为非晶态。由非晶相和B2晶相的交替层组成的特殊的片层结构。研究还发现，非晶层两侧的B2层具有几乎相同的结晶取向。作为对比，对不同成分的合金进行了HPT形变。只有Zr_&lt；50&gt；Cu_&lt；50&gt；出现非晶化。变形前合金为B19‘相，具有细小的马氏体孪晶组织。因此，可以推断，细小的孪晶组织促进了位错的积累并引发了非晶化。

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.