Investigation of thermodynamic and physical properties of NiTiCuW new quaternary shape memory alloys produced at different rates

In this study, shape-memory alloys prepared on the basis of Ni44-xTi49Cux+5W2 (x=%wt. 0, 2, 4, 6) form were produced by arc-melting method in argon gas atmosphere. From the produced A1 (Ni44Ti49Cu5W2), A2 (Ni42Ti49Cu7W2), A3 (Ni40Ti49Cu9W2) and A4 (Ni38Ti49Cu11W2) coded shape-memory alloys, samples of appropriate sizes were cut and necessary measurement and analysis procedures were carried out. Differential scanning calorimetry (DSC) measurements were taken to investigate the thermodynamic parameters of NiTiCuW quadruple shape memory alloys. As a result of DSC measurements, enthalpy values and phase transformation temperature values of A1, A2, A3 and A4 alloys were determined. When the DSC measurements were analysed, clear and sharp peaks were observed for martensite and austenite phase transformation values in all alloys. When the DSC curves were analysed, the austenite transformation temperature value decreased as the copper content in the alloy increased and decreased to values below room temperature. In addition, a two-stage phase transformation was observed. In this study, for alloy A1, the transformation occurred in a single step and the B19 martensite phase was transformed into the B2 martensite phase. Similar transformation was also observed in alloy A2, while rhombohedral (R) interphase transformation was also detected in alloys A3 and A4. X-ray diffraction (XRD) was also used for crystal structure analysis of the alloys. In NiTiCuW alloy, A1 and A2 alloys exhibited martensite phase and interphase crystal structure, while A3 and A4 alloys exhibited austenite phase crystal structure. SEM (Scanning Electron Microscopy) was used for microstructure analysis of the alloys. EDX was used for chemical structure (matrix phases, precipitate formation, segregation zones) analysis of the alloys. When the results of SEM-EDX measurements were examined, it was clearly seen that martensite phase structures and some precipitate formation were present in the sample, and the grains and grain boundaries in the alloy structure were clearly seen. In addition, the presence of the specified phases in the structure is also consistent with the X-ray results. When the images were analysed, pit and bump regions were detected. Copper (Cu) elements are concentrated in the hollow regions and tungsten (W) elements are concentrated in the bump regions. Vickers hardness measurements were taken to determine the microhardness of the alloys. When the arithmetic mean of the Vickers microhardness measurements were compared, it was observed that the phase structure of the alloys at room temperature had a significant effect on the hardness values.