Order parameter evolution and Landau free energy coefficients for the B2 <-> R-phase transition in a NiTi shape memory alloy
Smart Materials & Structures 14(5): S192-S196
The thermal evolution of the rhombohedral order parameter of cold-rolled Nk(50.7)Ti(49.3) alloy was measured by x-ray diffraction. The slightly first-order paraelastic to improper ferroelastic B2 <-> R phase transition occurs on cooling at 305 K; on cooling below 240 K the R-phase and residual austenite transform to B 19'-martensite. From the rhombohedral splitting of the (030)/(112) reflections of the R-phase the spontaneous strain (self-strain) 8 was determined experimentally as epsilon = 2(d(030) - d(112))/(d(030) + d(112)). It is proportional to the square of the order parameter. The strain evolution agrees well with a 2-4-6 Landau model. The values of the Landau free energy coefficients were determined by normalization with DSC data of the latent heat of the B2 <-> R transformation and free-energy isotherms as a function of order parameter are presented.