Role of vibrational and configurational excitations in stabilizing the L12 structure in Co-rich Co-Al-W alloys

A first-principles study of the L1(2) Co-3(Al,W) phase was performed to assess phase stability at elevated temperature. Configurational degrees of freedom were treated with a cluster expansion and Monte Carlo simulations, while contributions to the free energy from vibrational excitations were determined within the quasiharmonic approximation accounting for effects of thermal expansion. It was found that while contributions from configurational degrees of freedom are important, vibrational entropy is crucial in making L1(2) Co-3(Al,W) stable at high temperature in the Co-rich portion of the Co-Al-W ternary. The emergence of L1(2) at high temperature can be attributed to a large difference in vibrational entropy between the close packed fcc-based L1(2) phases and the energetically stable, yet vibrationally stiff B2 CoAl compound.