NiTi形状记忆合金形变机制的应变率相关性研究

利用高速拉伸实验机在宽的应变率范围内(0.001–1200 s^-1), 研究了NiTi形状记忆合金的宏观力学性能随应变率的变化规律, 并借助透射电子显微镜深入研究了微观结构在不同应变率下的演变机制. 研究发现: NiTi合金马氏体(B19’相)孪晶的解孪晶应力随应变率的升高而近乎线性增大, 表明NiTi 合金解孪晶应力具有正向应变率相关性. 在拉伸应变率为10 s^-1的样品微观结构中发现了大量的解孪晶区域, 而当应变率进一步增大到100 s^-1和 1200 s^-1时, 在样品中没有发现解孪晶区域的存在, 样品微观组织以孪晶形式存在. 该结果表明, NiTi合金的马氏体解孪晶速率应在 10–100 s^-1范围内. 在高应变率下(≥qslant10 s^-1)均发现了热引发奥氏体相(B2)的存在, 表明随应变率的增加, 拉伸过程由等温过程逐渐变为绝热过程. 此外, 在1200 s^-1 的样品差示扫描热量曲线中还发现了一个小肩峰, 表明相变过程由一步相变变为两步相变. 关键词： NiTi形状记忆合金 高速拉伸 应变率相关性 透射电子显微镜

Rate-dependences of deformation mechanisms in NiTi shape memory alloys

In the NiTi shape memory alloys (SMAs), the macro-mechanical deformations and the microstructural evolutions at different strain-rates (0.001-1200 s^-1) are investigated. It is found that the detwinning stress of martensitic twin increases with strain-rate increasing, which indicates that the detwinning stress has the positive strain-rate dependence. A large number of detwinning regions are found in the NiTi specimen which is deformed at the strain-rate of 10 s^-1 under tension. However, with the strain-rate further increasing up to 100 s^-1 and 1200 s^-1, no detwinning region is observed and many twins still exist. It is shown that the detwinning rates of martensitic twin in NiTi SMAs are in a range of 10-100 s^-1. Simultaneously, thermally-induced austenite is detected in the NiTi specimens deformed at high strain-rates (≥qslant10 s^-1). It is ascribed to the fact that there is a change from the isothermal process to the adiabatic process when the tensile strain-rate goes up to a critical value. Additionally, a small shoulder peak is detected in differential scanning calorimeter peak of 1200 s^-1 strain-rate specimen, indicating that the two-stage phase transformation occurs.
Keywords： NiTi shape memory alloys high-speed tensile strain rate dependence transmission electron microscopy