The effect of the heat treatment temperature on the thermodynamic properties of the 55.89wt Ni-Ti shape memory alloy

摘要

The study of the thermomechanical behavior of the NiTi shape memory alloy (SMA) requires the determination of the transformation temperatures which govern the phase transformation process between austenite A and martensite M. These transformation temperatures indicate the beginning and the end of both forward (A-M) and reverse (M-A) phase transformations and noted M_s, M_f, A_s and A_f, respectively. Based on literature, it has been reported that these NiTi thermal properties are considerably influenced by several factors such as the chemical composition (Wang et al. 2015), heat treatment temperature (Sadiq et al. 2010), cooling/heating rate (Ben Fraj and Tourki 2016). From thermodynamic point of view, numerous previous studies have reported that the variation in NiTi transformation behavior affects both enthalpy and entropy changes, AH and AS respectively. Natalia and Sergey (2015) have proved that AS is greatly dependent on M_f. Moreover, it has been experimentally shown that both AH and AS are strongly influenced by the chemical composition where they decrease with the increase of the Ni content in the matrix (Khalil-Allafi and Amin-Ahmadi 2009). Under mechanical loading, the effect of AH is observed in the evolution of C_M and C_A which define the linear relationship between critical stresses and temperatures in the NiTi SMA diagram phase. In this work, the thermal transformation behavior of the Ni rich NiTi SMA was experimentally investigated for various heat treatment temperatures ranging from 450°C to 700°C. Based on calorimetric results, the influence of the heat treatment temperature (noted HTT in this paper) on AH and AS was studied. Finally, the effect of these changes on the critical stress-temperature diagram was discussed in the case of an isothermal compression loading.