Investigate of microstructure and thermodynamically properties of NiTiCu shapememory alloys

In this thesis, Ni50-xTi50Cux (x = %wt. 14, 17, 20) shape memory alloys were produced with arc melting method. The parts of the produced A1(Ni36Ti50Cu14), A2(Ni33Ti50Cu17) and A3(Ni30Ti50Cu20) shape memory alloys were cut in suitable sizes and examined by making the necessary measurements and analyses. Differential scanning calorimeter (DSC) measurements were made to determine the thermodynamic characteristics of the alloys. As a result of DSC measurements, phase transformation temperatures and enthalpy values of A1, A2 and A3 alloys were determined. It was observed in DSC measurements that martensite and austenite phase transformations were clear and in sharp peaks in all alloys. Besides, in order to determine the microstructure and physical characteristics of the alloys in accordance with the purpose of the study, an optical microscope was used to determine the structures that occurred on the surfaces of the alloys after all the samples were brought to certain standards. Taking the optical micrographs of the samples into consideration, there was some precipitate phase on the surface of the samples, although it was not so dense, and the grain and grain boundaries were clearly visible. In order to obtain results with more details, scanning electron microscope (SEM) with higher resolution and focal depth than an optical microscope was used. B2 (NiTi) phase and Ti2Ni phase were generally observed in A1, A2 and A3 shape memory alloys. X-ray diffractometer (XRD) measurements were made to determine crystal structures and phases of the alloys. It was observed from the XRD measurement results of the alloys that the dominant phase in A1 alloy was martensite phase while martensite and austenite phases were equal in A2 alloy and austenite phase was the dominant phase in A3 alloy. In order to determine the hardness values of the produced alloys, Vickers micro hardness measurement values of the samples taken from the alloys were obtained and the results were compared. Keywords: Shape Memory Alloys, Phase Transformation Temperatures, Microstructure, Martensite and Austenite phase