Structure and elasticity of cubic Fe-Si alloys at high pressures

Using a combination of picosecond acoustics and synchrotron x-ray diffraction, the sound velocities and equations of state of Fe-Si alloys have been determined over a wide range of compositions (10--29 at. % Si) and pressures (up to 65 GPa) under quasihydrostatic conditions. We observe marked variation in the elastic properties of the alloys depending on synthesis method and degree of Si ordering. In particular, it is observed that there is a sharp change in the density dependence of the sound velocities which coincides with the observation of long-range ordering by diffraction methods. This change in elasticity is probably due to a change in bond character of the Fe-Si alloy. Furthermore, the bcc-hcp transition in these alloys has been mapped to high pressures and Si contents. We observed that the onset of the transition changes by more than 6 GPa per at. % for alloys containing more than $\ensuremath{\sim}15$ at. % Si.