室温下Fe_(79)Ni_(19)Mn_2合金的压致bcc→hcp相变

Pressure Induced bcc→hcp Phase Transition of Fe_(79)Ni_(19)Mn_2 Alloy at Room Temperature

采用Mao-Bell型金刚石对顶砧及高压在位粉末X光衍射照相方法,研究了Fe_(79)Ni_(19)Mn_2合金在室温下,0～41.0 GPa压力范围内的压致结构相变和等温压缩行为。实验结果表明,该合金在低压时为bcc结构,在10.6 GPa压力附近出现压致bcc→hcp结构相变。二相共存压力区约12GPa。卸压到常压时,又恢复到bcc结构。用Murnaghan等温固体状态方程对合金的压缩数据进行最小二乘法拟合,得到了bcc相和hcp相的状态方程参量。对hcp相的晶格参数比小于1.633及存在宽范围的二相共存压力区进行了讨论。

By use of Mao-Bell type diamond anvil cell(DAC) and an in-situ high-pressure powder X-ray diffraction method, the pressure induced phase transition and isothermal compression of Fe_(79)Ni_(19)Mn_2 (wt％) alloy were investigated at room temperature and a high pressure up to 41.0 GPa. The crystal structure of this alloy is bcc structure below 10. 6 GPa. The pressure induced bcc→hcp phase transition is occurred at 10. 6 GPa. The mean volume change for the phase transition is 0. 30±0. 02 cm^3/mol. The ratio of the lattice parameters of the hcp phase, c/a, keeps a constant with the value 1. 605±0. 003 in the pressure range from 10. 6 GPa to 41.0 GPa. The Coexistence pressure range of the bcc phase and the hcp phase is about 12 GPa, and there is the lattice spacing relationship of d_(002)_(hcp)=d_(110)_(bcc). In the course of unloading, the hcp phase can be maintained to 2. 6 GPa, and the alloy returns to the initial bcc structure entirely after the pressure is reduced to the atmosphere. The Murnaghan isothermal equations of states of the bcc phase and the hcp phase were fitted with the least-square method, for the bcc phase V_0=7. 16±0.02 cm^3/mol, B_0=123±8 GPa, B'_0=8.7±0.6, and for the hcp phase V_0=6. 73±0. 02 cm^3/mol, B_0=176±11 GPa, B'_0=5.7±0. 4. The question for the c/a ratio of the hcp phase being smaller than 1.633 and the mechanism for having wide pressure range of the two phase coexistence are discussed.