  Reversible martensitic transformations (MTs) are the origin of many
fascinating phenomena, including the famous shape memory effect. In this work,
we present a fully ab initio procedure to characterize MTs in alloys and to
assess their reversibility. Specifically, we employ ab initio molecular
dynamics data to parametrize a Landau expansion for the free energy of the MT.
This analytical expansion makes it possible to determine the stability of the
high- and low-temperature phases, to obtain the Ehrenfest order of the MT, and
to quantify its free energy barrier and latent heat. We apply our model to the
high-temperature shape memory alloy Ti-Ta, for which we observe remarkably
small values for the metastability region (the interval of temperatures in
which the high-and low-temperature phases are metastable) and for the barrier:
these small values are necessary conditions for the reversibility of MTs and
distinguish shape memory alloys from other materials.
