Isothermal compression and bcc→hcp phase transition of iron-cobalt alloys up to 300 kbar at room temperature

Abstract

Iron‐cobalt alloys with 10, 20, and 40 wt% cobalt have been studied at room temperature under high pressure by means of x‐ray diffraction techniques, employing the diamond‐anvil pressure cell. The crystallographic x‐ray data for the three alloys at zero pressure were obtained using the standard Debye‐Scherrer x‐ray diffraction technique. The volume‐pressure relationship was established and, using the Birch‐Murnaghan equation of state, the compressibility β for the above alloys was found to remain constant or increase slightly with increasing cobalt content. This is consistent with the electron dilution theory. The bcc→hcp phase transformation in each alloy was seen visually under the microscope in reflected light and it was found to take place when the pressure increased to 163±5, 195±6, and 290±10 kbar respectively, for the 10, 20, and 40 wt% Fe‐Co alloys. The volume change at the transition was found to decrease slightly with increasing cobalt content and the crystallographic axis ratio c/a of the hcp phase of the alloys was found to increase with increasing cobalt content.