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

通过薄壁管拉伸/压扭试验，确定了铁基形状记忆合金的相变开始和结束条件，等温比例加载下确定的马氏体相变开始条件在轴向应力-剪应力平面上以椭圆形曲线表示，不同于塑性力学中的von Mises屈服条件。温度效应是各向同性的。因此，在建立相变热力学理论时，应考虑应力的第三个不变量：相变开始条件（逆相变开始条件）在应力-温度空间中用一个平面表示。其单位法线的方向取决于预加应力的方向，意味着在预加应力过程中诱导的马氏体变体的类型。当在预应力过程中诱发几种类型的变量时，在应力-温度空间中形成一个多边形锥来表示方解石开始条件。平面在应力-温度空间中的位置取决于变量的数量。

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.