Khalil-Allafi, Jafar; Schmahl, Wolfgang W.; Reinecke, T.
Order parameter evolution and Landau free energy coefficients for the B2↔R-phase transition in a NiTi shape memory alloy.
In: Smart Materials and Structures, Vol. 14, Nr. 5:
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The thermal evolution of the rhombohedral order parameter of cold-rolled Ni50.7Ti49.3 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 B19′-martensite. From the rhombohedral splitting of the (030)/(112) reflections of the R-phase the spontaneous strain (self-strain) ε was determined experimentally as ε ≤ 2(d 030-d112)/(d030+d112). 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.