Widening the Martensitic Hysteresis in Ni47Ti44Nb9 Shape Memory Alloy by Grain Refinement

Widening the martensitic hysteresis (\({T}_{\text{Dhys}}\)) in NiTiNb shape memory alloys (SMAs) holds potential for broadening their working temperature range while enabling the room-temperature storage. In this study, the \({T}_{\text{Dhys}}\) is divided into two parts: the thermal-induced hysteresis (\({T}_{\text{hys}}\)) and the deformation-induced hysteresis (\({T}_{\text{hys}}{{^{\prime}}}\)). In addition to decreasing the martensitic transformation start temperature (\({M}_{\text{S}}\)), it is found that grain refinement is an effective method for widening both \({T}_{\text{hys}}\) and \({T}_{\text{hys}}{{^{\prime}}}\) of the Ni₄₇Ti₄₄Nb₉ alloy, a commercial SMA widely used for shape memory couplings. According to thermodynamic analysis, grain refinement increases the dissipation energy (\(\Delta {E}_{\text{dis}}\)) (caused by thermal friction at martensite/austenite interface), thereby widening \({T}_{\text{hys}}\). Moreover, the Ni₄₇Ti₄₄Nb₉ alloy with finer grains has the potential to release more elastic strain energy (\(\Delta {E}_{\text{el}}\)) after deformation, thereby exhibiting a wider \({T}_{\text{hys}}{{^{\prime}}}\). When deforming a large strain, the alloy with finer grains generates more dislocations which stabilize the martensitic phase, thus further widening \({T}_{\text{hys}}{{^{\prime}}}\).

Graphical Abstract