Abstract In this study, the effect of a stress aging process on the microstructure and martensitic phase transformation of NiTi shape memory alloy has been investigated. NiTi samples were aged at 450 °C for 1 h and 5 h under different levels of external tensile stress of 15, 60 and 150 MPa. Transmission electron microscopy (TEM) was used to characterize different variants and morphology of precipitates. The results show that application of all stress levels restricts the formation of precipitates variants in the microstructure after 1 h stress aging process. However, all variants can be detected by prolonging aging time to 5 h at 15 MPa stress level and the variants formation is again restricted by increasing the stress level. Moreover, the stress aging process resulted in changing the shape of precipitates in comparison with that of the stress-free aged samples. Coffee-bean shaped morphologies were detected for precipitates in all stress levels. According to the Differential Scanning Calorimetry (DSC) results, the martensite start temperature (Ms) on cooling shifts to higher temperatures with increasing the tensile stress during the aging process. This can be related to the change of austenite to martensite interface energy due to the different volume fractions and variants of precipitates.

中文翻译：

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应力时效过程对NiTi形状记忆合金中纳米Ni 4 Ti 3 析出物和马氏体转变温度的影响

摘要 在这项研究中，研究了应力时效过程对 NiTi 形状记忆合金组织和马氏体相变的影响。NiTi 样品在 15、60 和 150 MPa 的不同外部拉伸应力水平下，在 450 °C 下时效 1 小时和 5 小时。透射电子显微镜 (TEM) 用于表征沉淀物的不同变体和形态。结果表明，在 1 小时应力时效过程后，所有应力水平的应用都限制了微观结构中析出物变体的形成。然而，通过在 15 MPa 应力水平下将老化时间延长至 5 小时，可以检测到所有变体，并且通过增加应力水平再次限制了变体的形成。而且，与无应力时效样品相比，应力时效过程导致析出物形状发生变化。在所有压力水平下都检测到沉淀物的咖啡豆形形态。根据差示扫描量热法 (DSC) 结果，冷却时的马氏体起始温度 (Ms) 随着时效过程中拉伸应力的增加而转移到更高的温度。这可能与由于不同的体积分数和沉淀物的变体导致奥氏体到马氏体界面能的变化有关。随着时效过程中拉伸应力的增加，冷却时的马氏体起始温度 (Ms) 转移到更高的温度。这可能与由于不同的体积分数和沉淀物的变体导致奥氏体到马氏体界面能的变化有关。随着时效过程中拉伸应力的增加，冷却时的马氏体起始温度 (Ms) 转移到更高的温度。这可能与由于不同的体积分数和析出物的变体导致奥氏体到马氏体界面能的变化有关。