Abstract
Magnetic field-induced, reversible martensitic transformations in NiCoMnIn meta-magnetic shape memory alloys were studied under constant and varying mechanical loads to understand the role of coupled magneto-mechanical loading on the transformation characteristics and the magnetic field levels required for reversible phase transformations. The samples with two distinct microstructures were tested along the [001] austenite crystallographic direction using a custom designed magneto-thermo-mechanical characterization device while carefully controlling their thermodynamic states through isothermal constant stress and stress-varying magnetic field ramping. Measurements revealed that these meta-magnetic shape memory alloys were capable of generating entropy changes of 14 J kg−1 K−1 or 22 J kg −1 K−1, and corresponding magnetocaloric cooling with reversible shape changes as high as 5.6% under only 1.3 T, or 3 T applied magnetic fields, respectively. Thus, we demonstrate that this alloy is suitable as an active component in near room temperature devices, such as magnetocaloric regenerators, and that the field levels generated by permanent magnets can be sufficient to completely transform the alloy between its martensitic and austenitic states if the loading sequence developed, herein, is employed.
Highlights
In our previous work[26], a custom Magneto-Thermo-Mechanical Characterization (MaTMeCh) device was developed lending us the ability to carefully control the MMSMAs’ thermodynamic state and drive martensitic transformations under both mechanical stress and magnetic fields across a wide temperature window
Dark microstructural regions correspond to B2 order and the bright regions to L21 order
The micrograph shows that the L21 regions that form on quenching from 1173 K are, on average, no larger than 30 nm
Summary
Magnetic field-induced, reversible martensitic transformations in NiCoMnIn meta-magnetic shape memory alloys were studied under constant and varying mechanical loads to understand the role of coupled magneto-mechanical loading on the transformation characteristics and the magnetic field levels required for reversible phase transformations. The heat treatment and change in long range order increased some of the martensitic transformation temperatures in the single crystals by nearly 25 K and decreased the thermal hysteresis, defined here as Af −Ms47, from 19 K to 16 K As mentioned above, these transformation characteristics should influence the magnetic field levels needed to complete the martensitic transformation. Thermodynamic state of the MMSMA was numbered corresponding to the VS-FR loading sequence in Fig. 4a and b, the stress and field levels that were applied to the SHT alloy were different than those on the SHT + 873 K 30 min sample due to their microstructural differences. More work is needed to link transformation hardening to crystallographic order and martensite crystal structure and variant size scales
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