Order parameter evolution and Landau free energy coefficients for the B2↔R-phase transitionin a NiTi shape memory alloy

Abstract

The thermal evolution of the rhombohedral order parameter of cold-rolledNi50.7Ti49.3 alloy was measured by x-ray diffraction. The slightly first-order paraelastic to improperferroelastic B2↔R phase transition occurs on cooling at 305 K; on cooling below 240 K the R-phase and residual austenitetransform to B19′-martensite. From the rhombohedral splitting of the (030)/(112)reflections of the R-phase the spontaneous strain (self-strain)ε was determinedexperimentally as ε = 2(d030−d112)/(d030+d112). It is proportional to the square of the order parameter. The strain evolution agrees wellwith a 2–4–6 Landau model. The values of the Landau free energy coefficients weredetermined by normalization with DSC data of the latent heat of the B2↔R transformation and free-energy isotherms as a function of order parameter arepresented.