The phase transformations associated with Shape Memory Effect (SME) can be one step, B19’ (martensite) ↔ B2 (austenite), or two / multiple steps which include the intermediate R phase, depending on the thermal and thermomechanical history of the alloy. The transformation temperatures are generally observed above room temperature in Ti-rich Ni-Ti alloys, while those observed in Ni-rich alloys occur below room temperature.

The goal of the present work is to investigate the phase transformations evolution in Ti-Rich Ni-Ti SMA (Ni-51at%Ti) when subjected to two distinct thermal treatments (500ºC for 30 minutes in air and 800ºC for 300 minutes in vacuum) and subsequently multiple steps of marforming thermomechanical treatments intercalated with thermal treatments (500ºC for 30 minutes in air) and subsequent four distinct final thermal treatments (400, 450, 500 or 600ºC for 30 minutes in air). Further, the stability of phase transformations in the initial ten thermal cycles of these thermomechanically treated samples is also studied. Differential Scanning Calorimetry (DSC) and Electrical Resistivity (ER) and X-Ray Diffraction (XRD) were used to identify the transformation temperatures and the phases that are present after the thermomechanical treatments.

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1. In situ XRD study of the transformation characteristics of severely plastic deformed NiTi SMAs 
2. One- and Two-Step Phase Transformation in Ti-rich NiTi