Speaker
Description
Ni-Mn-based metamagnetic shape-memory alloys exhibit outstanding caloric effects in the vicinity of martensitic transformations driven by magnetic fields, uniaxial load or hydrostatic pressure. However, the application in refrigeration devices is hindered by energy losses and/or irreversibilities during cyclic operation which originate from a large inherent thermal hysteresis of the non-diffusional phase transformation. A promising way to improve cyclic performance is a simultaneous or sequential combination of more than one external stimulus. We have recently presented that a sequential application of magnetic field and uniaxial load can even make use of thermal hysteresis. In this so called “exploiting-hysteresis cycle”, the shape-memory alloy initially undergoes a magnetic-field-induced reverse martensitic transformation from low-magnetization martensite to high-magnetization austenite [1]. In consequence of the large thermal hysteresis, the material is trapped in the high-magnetization austenite even after the magnetic field is removed. To ensure cyclability, the back transformation to the original martensite state is driven by uniaxial load.
We studied the correlation of microstructure and the caloric response to magnetic fields, uniaxial load and their combination in an "exploiting-hysteresis cycle" exemplarily for the metamagnetic shape-memory alloy Ni-Mn-In. We reveal a significant effect of grain orientation on the stress-induced martensitic transformation by correlating temperature-dependent XRD and SEM-EBSD data. By strain measurements in static and pulsed magnetic fields a crucial role of microstructure on the field-induced transformation dynamics is demonstrated [2]. We show that for a tailored microstructure the maximum cyclic effect in 1.9 T can be raised by more than 200 % when a moderate sequential stress of 59 MPa is applied.
[1] Gottschall et al.; Nature Materials 17, 929-934 (2018)
[2] Pfeuffer et al.; Physical Review Materials 4, 111401 (2020)
| Speaker Country | Germany |
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