Transformation-start/finish conditions in an Fe-based shape memory alloy under multiaxial stresses

通过对一种铁基形状记忆合金薄壁管状试样进行拉伸/压缩-扭转试验，确定了变形的开始和结束条件。等温比例加载确定的马氏体起始条件在轴向应力-剪切应力平面上表现为椭圆曲线，与塑性上的von Mises屈服条件不同。温度效应是各向同性的。因此，在构造相变热力学理论时，应考虑到应力的第三个不变量。奥氏体启动条件（反向转换启动条件）用应力-温度空间中的平面表示。其单位法向的方向取决于预应力的方向，即预应力过程中诱发的马氏体变异的类型。当预应力过程中产生多种类型的变量时，形成一个多边形锥体来表示应力-温度空间中的奥氏体起始条件。平面在应力-温度空间中的位置由变量的数量决定。

The transformation-start and -finish conditions are identified in an Fe-based shape memory alloy by carrying out the tension/compression-torsion tests of thin-walled tubular specimens.The martensite-start condition determined in the isothermal proporional loading is represented by the ovalshape curves on the axial stress-shear stress plane, thus being different from the von Mises yield condition in plasticity. The temperature effect is isotropic. The third invariant of the stress should, therefore, be taken into account when constructing a transformation thermomechanical theory.The austenite-start condition (the reverse transformation-start condition) is represented by a plane in the stress-temperature space. The direction of its unit normal depends on the direction of the pre-stressing, meaning on the type of the martensite variants induced in the pre-stressing process. When several types of variants are induced during pre-stressing, a polygonal cone is formed to represent the austenite-start condition in the stress-temperature space. The position of the plane in the stress-temperature space is governed by the amount of the variants.