To shed light on the origin of the loss of functional properties of NiTi with temperature increasing above 100 °C, we have investigated stress induced martensitic transformations in nanocrystalline NiTi shape memory wire by thermomechanical tensile testing supplemented with post-mortem reconstruction of martensite variant microstructures in grains by nanoscale orientation mapping in TEM. The stress induced martensitic transformation generating recoverable transformation strain as well as plastic strain is not completed at the end of the upper stress plateau. The higher is the test temperature, the larger is the volume fraction of retained austenite as well as the plastic strain. The martensite variant microstructures in NiTi wire deformed up to the end of the stress plateau at 120 °C contain partially detwinned single domains of (001) compound twin laminate filling entire grains. It is proposed that the stress induced martensitic transformation proceeds via habit plane interface between austenite and second order laminate of (001) compound twins and that the martensite promptly reorients and deforms plastically by dislocation glide in the [100](001) slip system. When the wire is loaded further beyond the end of the stress plateau, the stress induced martensitic transformation continues and the martensite deforms plastically. It is concluded that the observed gradual loss of superelastic functionality of NiTi with increasing temperature does not originate from the plastic deformation of austenite, as widely assumed in the literature, but that it derives from the loss of resistance of the stress induced martensite to the plastic deformation under increasing stress.

中文翻译：

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高温下镍钛合金中应力诱发的马氏体转变：马氏体变体微观结构、可恢复应变和塑性应变


为了阐明随着温度升高超过 100 °C，NiTi 功能特性丧失的根源，我们通过热机械拉伸测试，并辅以马氏体变体微观结构的事后重建，研究了纳米晶 NiTi 形状记忆线中的应力诱导马氏体转变。通过 TEM 中的纳米级取向映射来观察晶粒。产生可恢复转变应变以及塑性应变的应力诱发马氏体转变在上应力平台结束时并未完成。试验温度越高，残余奥氏体的体积分数越大，塑性应变也越大。 NiTi 丝中的马氏体变体微观结构在 120 °C 下变形直至应力平台结束，包含填充整个晶粒的 (001) 化合物孪晶层合体的部分解孪单域。提出应力引起的马氏体相变通过奥氏体和(001)复合孪晶的二阶层状之间的习惯平面界面进行，并且马氏体通过[100](001)滑移系中的位错滑移迅速重新取向和塑性变形。当线材的负载进一步超过应力平台末端时，应力诱发的马氏体转变继续并且马氏体发生塑性变形。结论是，所观察到的 NiTi 超弹性功能随温度升高而逐渐丧失的现象并非如文献中广泛假设的那样源于奥氏体的塑性变形，而是源于应力诱发马氏体对塑性的抵抗力的丧失。在增加的应力下变形。