Investigation of microstructure phase transformation temperatures of high temperature shape memory CuAlTa alloys

Investigation of Microstructure and Phase Transformation Temperatures of High Temperature Shape Memory CuAlTa Alloys In this study, CuAlTa alloys with different weight percentage (wt. %) were produced. The alloys were encoded such that Cu-13%Al-1%Ta (CAT1), Cu-12.5%Al-1.5%Ta (CAT2), Cu-12%Al-2%Ta (CAT3), Cu-11,5%Al-2,5%Ta (CAT4) and Cu-11%Al-3%Ta (CAT5). Some thermodynamic parameters related to the phase transformation of the alloys were investigated using differential scanning calorimetry (DSC). The differential thermal analysis (TG / DTA) device was also used to detect phase transitions, precipitates, decomposition and activation energies for surface oxidation in a high temperature. Scanning electron microscopy (SEM) and optical microscope were taken to examine the surface morphology of the alloys. X-ray analysis were performed for determining the martensite crystal structure. Also, microhardness measurements were performed to determine the resistance of materials to local plastic deformation (such as sinking or scratching). It was found that the activation energy (E0) of the alloys for surface oxidation were 51,38; 74,81; 100,13; 90,61 and 34,84 kJ/mol, respectively. According to the phase transformation temperatures, the alloys were classified as high temperature shape memory alloys (HTSMA). In addition, DSC results showed that the alloys exhibited γ_1^'(2H)→β1(DO3) transformation in heating process, while a complex phase transformations β1(DO3)→β1ꞌ(18R) + γ1ꞌ(2H) was observed in the cooling process. There are some phases were detected using x-ray analysis, including CuAl, Cu9Al4, Ta2Al3, γ_1^' (2H) and β_1^' (18R) phases. Moreover, the presence of different precipitations and how they are spread out in the alloys was determined by SEM and optical microscope images. EDX analysis was carried out to find the chemical composition of the matrix, precipitates, and decomposition regions. The Vickers microhardness of CAT1, CAT2, CAT3, CAT4 and CAT5 were respectively, 295, 305, 296, 300 and 309 HV. The results of the aforementioned measurements showed that thermodynamic, activation energies, microstructure and microhardness parameters were changed for adding Ta instead of Al in the CuAl-based alloys. Keywords: High temperature shape memory alloys, Thermodynamic parameters, Activation energy for surface oxidation, Microhardness, Homogenization.