Transformation behavior in NiTi–20Ta and NiTi–20Nb SMAs: Transformation temperatures, microstructure and micro-hardness

Shape memory alloys Ni 30 Ti 50 X 20 (at.% X = Nb, niobium, and Ta, tantalum) were produced through arc-melting method. Phase transformation temperatures were studied by using differential scanning calorimetry (DSC), and microstructures were examined with the aid of optical microscopy, scanning electron microscopy-energy dispersive X-ray (SEM–EDX) and X-ray diffraction (XRD). According to DSC results obtained at heating rate of 10 C min −1 , transformation temperature of Ni 30 Ti 50 Nb 20 alloy was determined as A s = 24.8 °C, A f = 62.1 °C, M s = 27.9 °C, M f = − 10.7 °C and that of Ni 30 Ti 50 Ta 20 alloy was determined as A s = 84.0 °C, A f = 110.4 °C, M s = 56.0 °C, M f = 32.1 °C. These results have shown that NiTi alloys which were added Nb and Ta at the rate of 20% demonstrate shape memory properties. Also, it has been identified that Ni 30 Ti 50 Ta 20 and Ni 30 Ti 50 Nb 20 alloys demonstrate B19 ı ↔ B2 phase transformation. Thermal activation energy calculated by Kissinger method was found: E a = 114.208 kJ mol −1 for NiTiNb and E a = 105.819 kJ mol −1 for NiTiTa. It was observed that as a result of SEM–EDX analysis, there exist Nb 2 Ti phase (Nb rich) in NiTiNb alloy and Ta 2 Ti phase (Ta rich) in NiTiTa alloy. These phases were supported with XRD results. In micro-hardness measurements, it was determined that NiTiNb shape memory alloy has the value of 361HV 0.1 micro-hardness and NiTiTa shape memory alloy has the value of 420 HV 0.1 micro-hardness.