Quantitative Landau potentials for the martensitic transformation in Ni–Al

The martensitic phase transformation in Ni–Al (63.5at.% Ni) is described by a Landau-type Gibbs free energy G(Q)=1∕2AΘs(coth(Θs∕T)−coth(Θs∕TC))Q2+1∕4BQ4+1∕6CQ6, with A=5.6JK−1mol−1, B=−3493Jmol−1, C=4901Jmol−1, TC=86K, and Θs=257K. The dispersion (Ginzburg) energy was estimated to be g=60Jmol−1nm2. This potential was determined from high resolution diffraction data to determine the spontaneous strain as proxy for the order parameter Q and differential scanning calorimetry measurements for the calibration of the excess entropy of the martensitic phase transformation. The Gibbs free energy is compared with equivalent functions of Ni–Ti, quartz, KMnF3, BaTiO3, and SrTiO3.