Thermodynamic modeling of the recovery strains of sputter-deposited shape memory alloys Ti-Ni and Ti-Ni-Cu thin films

Ti-Ni and Ti-Ni-Cu shape memory alloy thin films were made by rf magnetron sputtering. The shape-memory behavior associated with the martensitic trans formation was measured under various constant applied stresses during therm al cycling from below Mf to above Af. Each strain vs, temperature curve rev ealed typical single stage deformation behavior. A model based on the therm odynamics of irreversible processes was used for the simulation of the shap e-memory behavior of the thin films. Phenomenological material constants th at were requisite to the modeling were obtained experimentally. The strain vs. temperature curves calculated by the thermodynamic model, and the exper imentally obtained curves, showed a reasonable correspondence in the higher applied stress region, while a large disagreement between them was observe d in the lower applied stress region; i.e. the transformation strain was hi gher in experimental data than in the corresponding calculated data in the lower applied stress region. The present phenomenological macroscopic model was efficient for mean and high stresses where one or two variants were ac tivated per grain. With lower stresses where four or more variants were act ivated per grain, our interaction energy expression was not sufficient to d escribe the multiple interactions between austenite and martensite platelet s and between martensite variants. (C) 2000 Elsevier Science S.A. All right s reserved.