Ni_2MnGa铁磁形状记忆材料

Ni_2MnGa Ferromagnetic Shape Memory Alloys

铁磁形状记忆合金 (FSMA)是在一定温度范围马氏体相稳定同时又具铁磁性的一类特殊的形状记忆合金。Ni2MnGa铁磁形状记忆合金近年来成为呈现磁场驱动大应变的新型驱动材料 ,这些应变来自磁场诱发马氏体孪晶的重排 ,而不是磁场对奥氏体至马氏体相变的作用。孪晶变体的重排在宏观上呈现为正或切应变 ,一非化学计量比Ni2 MnGa单晶于室温加 0 .4T磁场能产生6 %的应变 ,Ni Mn Ga单晶在高至 15 0Hz的交变磁场仍可得到 2 .5 %的应变。本文阐述了与这种磁控形状记忆效应相关的孪晶界迁动的磁学和晶体学理论。马氏体相的大磁晶各向异性能使磁化沿c轴方向有利 ,穿过孪晶界c轴刚好转动 90度 ,同时 ,这个孪晶界也构成了约 90度的畴界。在各向异性的情况下 ,孪晶界的迁动仅有相邻孪晶变体的Zeeman能差驱动 ,μ0 ΔMis·Hi。磁场和外应力对应变的影响通过对一简单的自由能表达式取极小值来表示 ,自由能表达式包括Zeeman能、磁晶各向异性能和外应力以及在某些情况下需考虑的内部弹性能 ,模型的所有参数可通过应力 应变曲线和磁化曲线测量得到。铁磁形状记忆合金的磁场诱发应变可类比传统热弹性形状记忆效应 ,与更为人们所熟知的磁致伸缩现象不同。

Ni 2MnGa ferromagnetic shape memory alloys (FSMAs) have recently become a new class of active materials showing magnetic-field-induced strains (MFIS) of several percent. These strains arise not from any effect of the magnetic field on the austenite-martensite transformation, but rather from the field-induced motion of twin boundaries in the martensite state. The consequent twin variant rearrangement can be shown macroscopically as either an extensional or shear strain. Applied magnetic fields of 0.4 T (4 kOe) have produced 6 % strains at room temperature in an off-stoichiometry Ni 2MnGa single crystal. Alternating-field actuation of FSMA crystals produces strains of 2.5 % up to the actuation frequencies of 150 Hz. The description of magnetic and crystallographic aspects of the twin boundary motion responsible for this effect is presented. The large magnetocrystalline anisotropy of Martensitic phase favors magnetization along the c axis. Since the c axis orientation changes by nearly 90( across a twin boundary, this boundary also constitutes a nearly 90( domain wall. The magnetic field dependence and external stress dependence of the strain are reasonably well accounted for by minimization of a simple free energy expression including Zeeman energy, magnetic anisotropy energy, external stress, and in some cases, internal elastic energy. The field-induced strain in ferromagnetic shape memory alloys is compared to the conventional thermoelastic shape memory effect and contrasted with the more familiar phenomenon of magnetostriction.