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The martensitic phase transition (MPT) from fcc to bcc is studied by a new combination of established methods. A continuous change between the two phases is possible according to the Bain transition in which the c/a ratio in a body centered tetragonal structure is varied between √2 and 1. As an example the electronic structure of Sr is determined by quantum mechanical calculations employing the full-potential linearized augmented plane wave (LAPW) method (as embodied in the WIEN code) in which exchange and correlation effects are described within the local spin density approximation. From these calculations we obtain the total energy as a function of the Bain variable c/a and the volume. The resulting energy surfaces E(c/a, V) valid at T=0K define the ground state and show e.g. the relative stability along the Bain transition. In a second step, this T=0K quantum mechanical total energy surface is fitted to a Ginzburg-Landau model to study finite temperature effects by treating fluctuations of the two order parameters c/a and V. This combined approach allows to describe the temperature induced first order phase transition between the fcc and the bcc phase as being driven by strain fluctuations rather than by phonon softening

Calculations for Martensitic Phase Transformations by Quantum Mechanics and Thermodynamics