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

Widening the martensitic hysteresis ( T 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 Dhys is divided into two parts: the thermal-induced hysteresis ( T hys ) and the deformation-induced hysteresis ( T hys ′ ). In addition to decreasing the martensitic transformation start temperature ( M S ), it is found that grain refinement is an effective method for widening both T hys and T hys ′ of the Ni₄₇Ti₄₄Nb₉ alloy, a commercial SMA widely used for shape memory couplings. According to thermodynamic analysis, grain refinement increases the dissipation energy ( Δ E dis ) (caused by thermal friction at martensite/austenite interface), thereby widening T hys . Moreover, the Ni₄₇Ti₄₄Nb₉ alloy with finer grains has the potential to release more elastic strain energy ( Δ E el ) after deformation, thereby exhibiting a wider T hys ′ . When deforming a large strain, the alloy with finer grains generates more dislocations which stabilize the martensitic phase, thus further widening T hys ′ . [ABSTRACT FROM AUTHOR]