NDC, 47533 Westinghouse Dr, Fremont, CA 94539, United States.
J Mech Behav Biomed Mater. 2011 Oct;4(7):1431-9. doi: 10.1016/j.jmbbm.2011.05.013. Epub 2011 May 13.
The phenomenon of superelasticity in near-equiatomic NiTi, which originates from a first-order martensitic phase transition, is exploited in an increasing number of biomedical devices, most importantly endovascular stents. These stents are often manufactured from microtubing, which is shown to be highly textured crystallographically. Synchrotron X-ray microdiffraction provided microstructural, phase, and strain analysis from Nitinol tube sections that were deformed in situ along longitudinal, circumferential, and transverse orientations. We show that the large variation in the superelastic response of NiTi in these three tube directions is strongly influenced by the path that the martensitic transformation follows through the microstructure. Specifically, in severely worked NiTi, bands of [100] grains occur whose orientation deviates markedly from the surrounding matrix; these bands have an unusually large impact on the initiation and the propagation of martensite, and hence on the mechanical response. Understanding the impact of these local microstructural effects on global mechanical response, as shown here, leads to a much fuller understanding of the causes of deviation of the mechanical response from predictions and unforeseen fracture in NiTi biomedical devices.
近等原子比 NiTi 中的超弹性现象源于一级马氏体相变，这一现象在越来越多的生物医学设备中得到了应用，其中最重要的是血管内支架。这些支架通常由微管制造而成，微管在结晶学上表现出高度织构化。同步加速器 X 射线微衍射技术提供了从沿纵向、周向和横向变形的 Nitinol 管段中进行微观结构、相和应变分析。我们表明，NiTi 在这三个管方向上的超弹性响应的巨大变化强烈受到马氏体相变在微观结构中所遵循的路径的影响。具体而言，在严重加工的 NiTi 中，会出现[100]晶粒带，其取向与周围基体明显偏离；这些带对马氏体的起始和传播有异常大的影响，因此对机械响应有影响。如这里所示，了解这些局部微观结构效应对整体机械响应的影响，会使人们更充分地了解 NiTi 生物医学设备中机械响应偏离预测和意外断裂的原因。
