Simulations of martensitic transformations in AuCd shape memory alloys

Two experimentally observed martensitic transformations in AuCd shape memory alloys are simulated fol-lowing the Landau theory of displacive phase transitions employing first-principles calculations. The presence of imaginary phonons in the calculated harmonic phonon dispersion relation of the fl-phase confirmed its dynamical instability at the ground state. An unstable phonon at the M wave vector has an M-5 symmetry, and another unstable phonon at the 2 ���a ��� [13, 3 1 ,0] wave vector has a 12 symmetry. In the first step of the simulated transformations, frozen calculations were used to create modulated structures and realize the lowest energy intermediate structure. In the second and last step of the simulated transformations, structure optimization of intermediate structure was performed to realize a relaxed final structure. An unstable M-5 phonon is shown to drive the Pm3m-*Pmma-*Pmma transformation, and the final Pmma structure matches closely with the relaxed structure of the experimentally observed fl ' martensite. Likewise, an unstable 12 phonon is shown to drive the Pm3m-*P31m-*P31m transformation, and the final P31m structure matches closely with the relaxed structure of the experimentally observed fl '' martensite.