Transformation, Kinetic and Thermodynamic Behaviors of Ni₄Ti₃ Precipitated/Un-Precipitated Ni-Rich NiTi SMA

In this study, it is aimed to experimentally investigate the transformation and the thermodynamic behaviors of the Ni-rich NiTi Shape Memory Alloy (SMA) at various thermal conditions. Based on XRD and SEM analyses, Ni₄Ti₃ precipitated/un-precipitated structures were identified as function of annealing temperature (450 °C and 650 °C). Through the calorimetric study, by varying the DSC scanning rate, it was proved that transformation temperatures, transformation rate and thermal hysteresis are highly sensitive to the presence of the precipitation phase. Furthermore, thermodynamic properties and kinetic behavior were assessed highlighting the impact of microstructural alteration on changes in enthalpy, entropy, Gibbs free energy and elastic energy. Activation energies of Ni₄Ti₃ precipitated and un-precipitated samples were determined according to Kissinger and Takhor approaches revealing a notable negative activation energy for the un-precipitated sample and considerably high values for the precipitated one. This result implies that, with the presence of Ni₄Ti₃ precipitates, the NiTi SMA requires higher activation energy to conduct the martensitic transformation.