铁基形状记忆合金及其相变特性

Fe-based Shape Memory Alloys and Their Transformation Characteristics

通过对铁基形状记忆合金从微米到亚纳米尺度的显微观察，讨论了铁基合金形状记忆效应的机制和特性。大多数有应用前景的铁基形状记忆合金与面心立方到体心四方（ｆｃｃ／ｂｃｔ）和面心立方到密排六方（ｆｃｃ／ｈｃｐ）相变有关。在这些相变体系中，对直接与形状记忆效应有关的应力诱发马氏体相变及其逆相变的模型进行了详细的研究。为取得良好的形状记忆效应，必须满足以下一些条件：（１）对ｆｃｃ／ｂｃｔ相变，ｂｃｔ马氏体的正方度必须大，这样可以降低马氏体内的孪晶界能，使非常细小的相变孪晶贯穿整个马氏体片。马氏体内低的孪晶界能是降低残留在奥氏体与马氏体（Ａ／Ｍ）界面上位错密度的一个重要因素，从而有利于界面的逆向运动。在极端情况下界面没有位错时，会发生热弹性相变。ｂｃｔ马氏体内高的滑移屈服应力是产生能可逆运动界面的另一个因素，这些结论可从加热过程中界面逆向移动后产生位借的详细分析中得出。（２）对ｆｃｃ／ｈｃｐ相变，为获得较好的形状记忆效应，在形变样品中形成均匀分布且非常薄的ｈｃｐ马氏体片是最基本的条件。为产生如此薄的马氏体片，当外力使样品形变时，奥氏体中（原始滑移系）必须存在高密度的层错。提出了一个外加应力时，形成很薄马氏体片的新机制。相?

：Shape memory mechanisms charac teristic of Fe-based alloys are discussed based on the microscopic observations with various scales ranging from micron-meter to sub-nanometer.Most of the Fe-ba sed shape memory alloys that may be technologically applicable in future are ass ociated with the fcc/bct and fcc/hcp transformations.The modes of the stress-ind uced martensitic transformation and its reverse transformation in these transfor mation systems,which are directly related to shape memory effect, are examined i n detail and it is concluded that the following conditions are the most importan t ones to achieve good shape memory effect. In the case of fcc/bct transformatio n, the tetragonality of the bct martensite must be large so that thetwin boundary energy in martensite can be lowered, which results in that very fine transformation twins are completely extended through a martensite plate. A low twin boundary energy in martensite is an important factor to reduce the density of disocations residing at the austenite-martensite interface and make easy the reverse movement of the interface. In the extreme case of no dislocations at the interface, thermoelastic transformation will be realized. A higher yield stress for slip in bct martensite is another factor to bring about the reversibly mobile interface. These are deduced from the detailed analysis of dislocations generated behind the reverse-moving interface on heating. In the case of fcc/hcptransformation, the formation of extremely thin hcp martensite plates with uniform distribut ion in a deformed sample is most required to achieve good shape memory effect. F or such thin martensite plates to be produced, high densities of stacking faults must preexist on the primary slip system in austenite when an external stress i s applied for shape Chane.A new me Chanism for the format.on of Very thin marten site Plates by applied stress is presented. A low stress for inducing martensite transformation compared with the yield stress for slip in austenite, which resu lts from the so-called “training” treatment, may be a necessary condition but not the sufficient condition for obtaining good shape memory effect. Future dire ct ions of the research on Fe-based shape memory alloys are suggested.