Landau理论研究TiNi顺磁合金热/强磁场耦合下的马氏体相变

利用改进后的Landau理论模型研究了顺磁TiNi合金在热/强磁场耦合作用下的马氏体相变行为.利用第一原理计算了TiNi合金在不同相变剪切应变(序参量)下的Flermi面态密度,得到相变过程中磁化率和磁自由能的变化,把磁场效应引入到Landau模型中.结果表明,稳恒强磁场可使顺磁材料的相变温度(M6和T0)出现突变性的升高,这主要是因为相变驱动力随磁场增强而呈二次曲线上升规律.另外,因强磁场下马氏体变体之间的自由能差急剧增大,导致磁场对变体的促进和抑制作用变得明显,出现取向生长现象.在5 T稳恒强磁场下的TiNi合金相变实验中,利用TEM观察到了一些变体以相互垂直的二变体形式出现(即取向生长),证实了模型的结果.

MARTENSITE TRANSFORMATION IN TiNi ALLOY UNDER COUPLING TEMPERATURE AND HIGH MAGNETIC FIELD USING LANDAU THEORY MODEL

The martensitic transformation behavior under coupling temperature and high magnetic field in TiNi paramagnetic shape memory alloy was studied by using the modified Landau model. In order to introduce the effect of high magnetic field, the Fermi surface total density of state (DOS) of TiNi alloy under different phase--transformed shear strain (order parameter) was calculated by using the first--principles calculations, then the relationship between magnetic susceptibility and shear strain was carried out. The calculation results from the modified Landau model involving the effect of magnetic free energy indicated that the martensitic transformation temperatures (M_s and T₀) suddenly increase. It can be mainly contributed to the parabola increasing of martensitic transformation driving force under coupling temperature and high magnetic field. Also, it is found that the oriented growth of martensitic variants occurs under coupling temperature and high magnetic field due to the increased difference of free energy between variants, which is consistent with the TEM observation that some vertical two--variants are present in TiNi alloy under 5 T magnetic field.