QUASI-HARMONIC AND MOLECULAR-DYNAMICS STUDY OF THE MARTENSITIC-TRANSFORMATION IN NI-AL ALLOYS

We study the martensitic phase transformation in NixAl1-x in the composition range 0.60 < x < 0.66 with the quasiharmonic approximation and molecular dynamics. These alloys present a high-temperature bcc phase (austenite) transforming at low temperature into a compact 3M structure (martensite). In our computation the potential energy of the system is approximated by an embedded-atom potential calibrated to reproduce selected properties of the pure elements in the fcc structure and of the Ni-Al intermetallic compound. We determine the transition temperature T0 as a function of composition, pressure, and uniaxial stress. At zero pressure the computed T0 is in qualitative agreement with the experimental results, and reproduces the strong composition dependence of the transition temperature. Again in agreement with experiment, the computation of thermodynamic properties shows that the transition is weakly first order, entropy driven, and closely matches the Zener picture. These conclusions are supported by the analysis of several microscopic quantities, including the mean-square displacement, the structure factor, and the phonon modes. Computations performed with applied uniaxial stress show that it always favors the 3M structure. The effect of pressure is more subtle: the 3M phase is favored by a moderate pressure, while higher pressures stabilize the bcc structure and decrease T0. We analyze the effect of disorder on the transition.