室温下压致铁钴镍合金的bcc→hcp相变

 本文采用高压X光衍射方法在金刚石对顶压砧中在位地（in situ）研究了Fe₆₈Co₂₄Ni₈（wt%）合金在室温下的压致bcc→hcp结构相变和直到40.5 GPa的等温压缩行为。实验结果表明该合金在常压下为bcc结构，晶格常数a₀=(0.287 0±0.000 1) nm，体积V₀=(7.119±0.007) cm³/mol，密度ρ₀=(7.981±0.008) g/cm³；在20.9 GPa附近出现bcc→hcp结构相变，两相共存压力区约10 GPa，在此区域内有晶面间距d(002)_hcp=d(110)_bcc，且原子平面(002)_hcp//(110)_bcc，hcp相比bcc相体积减小(0.33±0.02) cm³/mol；高压相hcp结构的晶格参数比值c/a=1.608±0.004；相变后原子配位数的增加使得hcp相(002)平面内及(002)平面间的最近邻原子间距比bcc相最近邻原子间距分别增大约1.6%和0.5%；用Murnaghan状态方程对实验数据进行最小二乘法拟合，得到bcc相B₀=(130±13) GPa，B₀'=12.6±0.5；hcp相V₀=(6.62±0.04) cm³/mol，B₀=(243±21) GPa，B₀'=6.8±0.3；对于该合金的bcc→fcp相变时的结构转变机制做了详细的讨论。

PRESSURE INDUCED bcc-hcp PHASE TRANSITION OF Fe-Co-Ni ALLOY AT ROOM TEMPERATURE

The pressure induced bcc→hcp phase transition and the equation of state for Fe₆₈Co₂₄Ni₈ (wt%) alloy have been determined by X-ray powder diffraction in a diamond anvil cell (DAC) at room temperature and high pressure up to 40.5 GPa using Zr-filtered Mo K_α radiation. The experimental results show that the alloy has bcc structure at normal pressure, the lattice parameter, molar volume and density are (0.287 0±0.000 1) nm, (7.119±0.007) cm³/mol and (7.981±0.008) g/cm³, respectively. The bcc→hcp transition was observed at 20.9 GPa, the pressure region of coexisting phases is about 10 GPa. Within this region, d(002)_hcp=d(110)_bcc and (002)_hcp//(110)_bcc are obtained, and the molar volume of the hcp is (0.33±0.02) cm³/mol smaller than that of the bcc. The c/a ratio for the hcp is 1.608±0.004 between 20.9 and 40.5 GPa. As a results of the phase transitions, the interatomic distances in the (002)_hcp layer and between (002)_hcp layer increase by 1.6% and 0.5%, respectively, due to the increase in coordination number. The Murnaghan equation of state was employed to fit the experimental data with the least square method, B₀=(130±13) GPa, B₀'=12.6±0.5 for the bcc, and V₀=(6.62±0.04) cm³/mol, B₀=(243±21) GPa, B₀'=6.8±0.3 for the hcp. The mechanism of crystal structure transition from bcc phase to hcp phase for Fe₆₈Co₂₄Ni₈ alloy was discussed in detail.