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

最近，铁磁形状记忆合金引起了人们的极大关注，因为一些铁磁形状记忆合金表现出与马氏体变种重排相关的高达几个百分点的巨磁感应应变。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%的大应变。由于巨场致应变只能在FCT马氏体态下获得，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相变温度的升高，BCT相变温度也随之升高。BCT马氏体相变不适合磁致应变，应避免相变以获得良好的性能。在此基础上，研究了FCT相变温度…的最适成分在Fe-Pd二元系中，e值最高。结果表明，当Pd含量低于30at%时，除FCT马氏体外，还出现了BCT马氏体，当Pd含量大于或等于30at%时，获得了单一的FCT相条件。我们尝试生长Fe-30at%Pd合金的单晶，发现样品的成分变化很大，这是因为在该成分下，固、液两相线相差很大。为了减少生长过程中引入的偏析，我们将样品保持在略低于熔点的温度几天，成功地获得了FCT转变温度接近室温的均匀单晶。利用这一单晶，我们证实了在磁场作用下发生了马氏体异构体的重排。此外，我们还测量了Fe-31.2Pd合金在较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的磁场感应应变机制。较少

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