Investigation of oxidation behavior at ni-rich niti shape memory alloy

The isothermal oxidation behavior of Ni-45.16 % Ti (composition in atomic percent) alloy was investigated by thermo gravimetric analysis (TG), differential scanning calorimeter (DSC) and methods. The alloys were oxidated a hour between 400-800?C in air and oxygen atmosphere. The oxidation products were identified by X-ray diffraction (XRD). It was found that Ni rich NiTi alloy exhibits a different oxidation behavior at temperatures above 400 °C in air and oxygen atmosphere. The alloys were exposed to air and oxygen atmosphere isothermally, i.e., between 400-800 °C, for 1 h. A gravimetric method was used to determine the oxidation kinetics and it was seen that the oxidation constant increases significantly with isothermal temperature. According to air and oxygen atmosphere, the activation energy of oxidation reaction for NiTi was calculated and activation values were found 65.47kJ/mol and 111,64kJ/mol, respectively. DSC measurements the transformation temperature of alloy (Ms, Mf, As and Af) was increased and also R phase disappeared above 500 °C. The formal oxides were determined by means of SEM-EDX measurements and obtained oxides are TiO and TiO2 oxides. The transformation characteristics to including phases determined through XRD are compared with those obtained from DSC in air atmosphere.