Özet:

Phase transformation temperatures, certain thermodynamic parameters and microstructural features of Ni30Ti50Cu20 shape memory alloy produced by arc-melting process were examined. Owing to DSC results obtained at 10°C/min heating-cooling rate, it was determined that austenite start (As) temperature is 23.5 oC, austenite finish (Af) temperature is 50.6 oC, martensite start (Ms) temperature is 26.7 oC and martensite finish (Mf) temperature is -0.10 oC. It was observed that according to DSC measurements obtained at different heating rate, while transformation temperature of the alloy from martensite phase to austenite phase (As and Af) varied, from austenite phase to martensite phase (Ms and Mf) did not. Thermal activation energy of the alloy measured by Kissinger method was found as Ea=63.208 kJ/mol. Moreover, Gibbs free energy was observed to display slight increases with the heating-cooling rates. Also, from the results of DSC curves, it was seen that at one-step B2 ↔B19 occurred and crystalline structures of these phases were determined at room temperature with the help of XRD analysis. Besides, precipitations of Ti2(Ni, Cu) as well as matrices in the form of TiNi0,8Cu0,2 of element Cu dissolved in interphases of NiTi were encountered. With the results of SEM-EDX, atomic percentage of precipitations (at.%) of Ti2(Ni, Cu) formed in the alloy were determined. The microhardness value of the alloy was measured as 219 HV. It was concluded that this value makes increased Cu amount to be softer metal than traditional binary NiTi alloys.