Control of martensite variants by magnetic field and giant magnetic field-induced strain in Fe-Pd ferromagnetic shape memory alloys

近年来，铁磁形状记忆合金受到了广泛的关注，因为一些铁磁形状记忆合金在马氏体变异体重排过程中表现出高达百分之几的磁场诱导应变。到目前为止，我们已经报道了Fe-31.2(at%)Pd合金在磁场作用下表现出约3%的巨大应变。由于只有在FCT马氏体状态下才能产生巨大的场致应变，因此提高马氏体相变温度是非常重要的。在本研究中，我们首先研究了第三元素对FCT马氏体相变温度的影响。结果发现，随着FCT转变温度的升高，BCT的转变温度也随之升高。BCT马氏体不适合磁场感应应变，为获得良好的性能，应避免相变。然后，我们研究了在Fe-Pd二元体系中FCT转变温度最高的最合适的成分。结果发现，当Pd含量低于30at%时，除FCT马氏体外，还出现BCT马氏体；Pd含量大于30at%时，合金呈现单一FCT相状态。我们尝试生长出Fe-30at% Pd合金的单晶，发现样品的组成变化很大，因为在这个组成中固线和液相线之间存在很大的差异。为了减少生长过程中引入的偏析，我们将样品在略低于熔点的温度下保存了几天，我们成功地获得了一个均匀的单晶，其FCT转变温度接近室温。利用该单晶，我们证实了磁场作用下马氏体变异体发生重排。此外，我们还测量了Fe-31.2Pd合金在宽温度范围内的磁晶各向异性常数和孪晶应力。利用这些结果，我们解释了铁磁形状记忆合金中磁场诱发应变的机理。少

Recently, ferromagnetic shape memory alloys have attracted considerable attention because some of them show a giant magnetic field-induced strain of several percent in association with rearrangement of martensite variants. Until now, we have reported that Fe-31.2(at%)Pd alloy exhibits a giant strain of about 3% by the application of magnetic field. Since the giant field-induced strain can be obtained only in the FCT martensite state, it is very important to raise the martensitic transformation temperature. In this study, we first investigated the effect of third elements on the FCT martensitic transformation temperature. As a result, we found that when the FCT transformation temperature is increased, the BCT transformation temperature also increases. The BCT martensite is not suitable for the magnetic field-induced strain and the transformation should be avoided for obtaining good properties. Then, we investigated the most suitable composition at which the FCT transformation temperatur … More e is highest in the Fe-Pd binary system. As a result we found that when Pd content is lower than 30at%, BCT martensite appear in addition to FCT martensite, and the single FCT phase condition is obtained for alloys with Pd content of 30at% and more. We tried to grow a single crystal of Fe-30at% Pd alloy, and found that the composition of specimen varies largely because of a large difference between solidus and liquidus lines at this composition. In order to reduce the segregation introduced during the growing process, we kept the specimen at a temperature just below the melting point for several days, and we succeeded to obtain a homogeneous single crystal whose FCT transformation temperature is close to the room temperature. By using this single crystal, we confirmed that the rearrangement of martensite variants occurs by the application of magnetic field. Furthermore, we measured magnetocrystalline anisotropy constant and twinning stress of an Fe-31.2Pd alloy in a wide temperature range. Using these results, we explained the mechanism of magnetic field-induced strain in ferromagnetic shape memory alloys. Less