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

TiNi形状记忆合金已被应用于不同的应用领域。近年来，随着微创外科技术的发展，在导丝、支架等医疗器械领域的应用日益扩大。人们还知道，通过剧烈的塑性变形，TiNi经历了结晶到非晶的转变。在本研究中，通过透射电镜观察了冷轧、高压扭转和喷丸强化等多种剧烈塑性变形方法下的非晶化/纳米晶化过程。结果表明，B19′马氏体在塑性变形后恢复为B2相，进一步变形后转变为非晶态。由非晶态和B2晶相交替层组成的特殊片层结构。同时发现，非晶层两侧的B2层具有几乎相同的晶体取向。这表明在可变边界处的缺陷积累是造成非定形的原因。为了比较，采用高温高温pt对不同成分的Zr-Cu合金进行了变形。只有Zr_<50>Cu_<50>表现出非晶化。变形前合金为B19′相，具有良好的马氏体孪晶组织。由此可以推断，精细的孪晶结构促进了位错的积累，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.