Pressure Induced bcchcp Phase Transition of Fe-Co-Ni Alloy at Room Temperature

The pressure induced bcchcp 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 radiation. The experimental results show that the alloy has bcc structure at normal pressure, the lattice parameter, molar volume and density are (0.287 00.000 1) nm, (7.1190.007) cm3/mol and (7.9810.008) g/cm3, respectively. The bcchcp 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 (0.330.02) cm3/mol smaller than that of the bcc. The c/a ratio for the hcp is 1.6080.004 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=(13013) GPa, B0'=12.60.5 for the bcc, and V0=(6.620.04) cm3/mol, B0=(24321) GPa, B0'=6.80.3 for the hcp. The mechanism of crystal structure transition from bcc phase to hcp phase for Fe68Co24Ni8 alloy was discussed in detail.