Effects of Ta Content on Thermodynamic Properties and Transformation Temperatures of Shape Memory NiTi Alloy

Effects of Ta Content on Thermodynamic Properties and Transformation Temperatures of Shape Memory NiTi Alloy

In the present work, Ni32-x-Ti-Ta18+x (x = 0, 2, 3, 4, 5, 6, 7, 8) shape memory alloys, produced by arc-melting method. Then,differential scanning calorimetry (DSC), optical microscopy (OM), x-ray diffraction (XRD), and Vickers micro-hardnessmeasurements were carried out to investigate thermodynamic parameters, microstructure, crystal structure, and mechanicalproperties of the alloys, respectively. The DSC results showed that, as the amount of Ta increased, the phase transformationtemperatures of the specimens significantly changed. In addition, increasing of Ta composition raised the mass density andelectron participation of NiTi alloy, and thus, the vibrational term of entropy overcomes the electron participation; consequently,the total entropy declined in the alloys. It is found that OM images possess a dendritic microstructure, where byincreasing the amount of Ta, the dendrites length increase while random orientations decrease. Moreover, XRD patternsexhibited the existence of each austenite phase (B2), martensite phase (B19ʹ), and β-Ta riched phase in all samples.

In the present work, Ni32-x-Ti-Ta18+x (x = 0, 2, 3, 4, 5, 6, 7, 8) shape memory alloys, produced by arc-melting method. Then,differential scanning calorimetry (DSC), optical microscopy (OM), x-ray diffraction (XRD), and Vickers micro-hardnessmeasurements were carried out to investigate thermodynamic parameters, microstructure, crystal structure, and mechanicalproperties of the alloys, respectively. The DSC results showed that, as the amount of Ta increased, the phase transformationtemperatures of the specimens significantly changed. In addition, increasing of Ta composition raised the mass density andelectron participation of NiTi alloy, and thus, the vibrational term of entropy overcomes the electron participation; consequently,the total entropy declined in the alloys. It is found that OM images possess a dendritic microstructure, where byincreasing the amount of Ta, the dendrites length increase while random orientations decrease. Moreover, XRD patternsexhibited the existence of each austenite phase (B2), martensite phase (B19ʹ), and β-Ta riched phase in all samples.