Phase Transformation Stability Research Articles

Giant caloric effect of low-hysteresis metamagnetic shape memory alloys with exceptional cyclic functionality

Solid-state caloric cooling is currently under extensive study owing to its great potential to replace the conventional vapor-compression technique. The search for refrigeration materials displaying a unique combination of pronounced caloric effect, low hysteresis and high reversibility on phase transformation, as well as multiferroic behavior is nowadays very active. Here we report a singular Ni50Mn31.5In16Cu2.5 metamagnetic shape memory alloy exhibiting giant adiabatic temperature changes of 13 K upon loading and −10 K upon unloading. This value significantly exceeds any previously reported data of ferroic materials. Simultaneously, a small thermal hysteresis of 3 K and an exceptional phase transformation stability over 105 magnetic field cycles have been achieved by ensuring the compatible kinematic conditions of specific lattice interface. Moreover, we propose a strategy to further reduce hysteretic losses and improve the reversibility of magnetocaloric effect by manipulating transformation paths evoked by magnetic field and stress, and therefore such a multicaloric approach is attractively beneficial for reaching high energetic utilization efficiency.

Evolution of phase transformations after multiple steps of marforming in Ti-rich Ni-Ti SMA

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-51 at % Ti) when subjected to two distinct thermal treatments (500°C for 30 minutes in air and 800°C for 300 minutes in vacuum) and subsequently multiple steps of marforming thermomechanical treatments intercalated with thermal treatments (500°C for 30 minutes in air) and subsequent four distinct final thermal treatments (400, 450, 500 or 600°C 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 X-Ray Diffraction (XRD) were used to identify the transformation temperatures and the phases that are present after the thermomechanical treatments.