Magnetic shape memory alloys (MSMAs) are promising candidates for room-temperature solid-state cooling through elastocaloric and magnetocaloric effects, offering a more efficient and environmentally friendly alternative to conventional refrigeration. Among them, Ni-Mn-based systems show excellent caloric performance [1]. However, widely studied Ni-(Co)-Mn-(In,Ga) alloys are intrinsically brittle and exhibit poor fatigue resistance, which leads to rapid degradation under cyclic use, limiting their applicability. To address these issues, alternative compositions such as Ni-(Co)-Mn-Ti alloys, composed exclusively of 3d transition metals, have been explored [2]. These alloys offer improved mechanical properties while maintaining suitable transformation behaviour and significant caloric effects. Additionally, thermal treatments can further optimize their performance by tuning transformation characteristics and enhancing mechanical strength through the formation of coherent precipitates. Related to these, several authors have reported the presence of micrometric Ni$_{3}$Ti-like precipitates in Ni-(Co)-Mn-Ti MSMAs after high-temperature heat treatments above 600 °C.

This work focuses on the effect of secondary heat treatments (SecHT) at temperatures below 600 °C, applied after solution treatments, on the martensitic transformation and microstructure of two alloys: Ni$_{50}$Mn$_{35}$Ti$_{15}$and Ni$_{50}$Mn$_{31}$Ti$_{19}$. For the alloy with lower Ti content, calorimetric measurements reveal an overall decrease in transformation temperatures, associated with the presence of Ni- and Ti-rich Ni$_{3}$Ti-type precipitates, as determined by energy-dispersive X-ray spectroscopy (EDX). The drop of transformation temperatures during the initial stages of SecHT is more pronounced at higher treatment temperatures due to increased atomic diffusivity. In contrast, the Ni$_{50}$Mn$_{31}$Ti$_{19}$ alloy treated at 600 °C presents two precipitated phases: Ni$_{3}$Ti-like and Mn-rich precipitates, a secondary phase not previously reported in the literature (figure 1.a). The apparition of Mn-rich precipitates results in an increase in the transformation temperatures for SecHT longer than 3 h, as shown in figure 1.b. Additionally, samples treated at 500 °C confirmed the epitaxial relationship between the austenite matrix and Ni$_{3}$Ti-like precipitates proposed in a previous study [3]. Heat treatments at 400 °C do not significantly affect Ni$_{50}$Mn$_{31}$Ti$_{19}$. Finally, structural characterization shows that the austenite phase has a B2 crystalline structure, while the martensite phase is the modulated 4O structure.

Joan Miquel Vaquero-Crespí (M.Sc.)