Abstract
Grain boundaries hinder twin boundary motion in magnetic shape-memory alloys and suppress magnetic-field-induced deformation in randomly textured polycrystalline material. The quest for high-quality single crystals and the associated costs are a major barrier for the commercialization of magnetic shape-memory alloys. Adding porosity to polycrystalline magnetic-shape memory alloys presents solutions for (i) the elimination of grain boundaries via the separation of neighboring grains by pores, and (ii) the reduction of production cost via replacing the directional solidification crystal growth process by conventional casting. Ni-Mn-Ga foams were produced with varying pore architecture and pore fractions. Thermo-magnetic training procedures were applied to improve magnetic-field-induced strain. The cyclic strain was measured in-situ while the sample was heated and cooled through the martensitic transformation. The magnetic field-induced strain amounts to several percent in the martensite phase, decreases continuously during the transformation upon heating, and vanishes in the austenite phase. Upon cooling, cyclic strain appears below the martensite start temperature and reaches a value larger than the initial strain in the martensite phase, thereby confirming a training effect. For Ni-Mn-Ga single crystals, external constraints imposed by gripping the crystal limit lifetime and/or magnetic-field-induced deformation. These constraints are relaxed for foams.
Highlights
The very high magnetic-field-induced strain (MFIS, up to 10%) displayed by bulk monocrystalline Ni-Mn-Ga alloys [1,2,3] is a true plastic strain produced by twin-boundary motion
We have shown that introducing pores in polycrystalline Ni-Mn-Ga reduces the internal constraints of grain boundaries to the point that the material displayed a repeatable MFIS of 0.12% [14]
At the end of the second temperature cycle, the MFIS reached 2.5%, exceeding the large strain reached after the first cycle
Summary
The very high magnetic-field-induced strain (MFIS, up to 10%) displayed by bulk monocrystalline Ni-Mn-Ga alloys [1,2,3] is a true plastic strain produced by twin-boundary motion. Due to constraints imposed by grain boundaries, the MFIS is near zero in randomly textured, fine-grained, polycrystalline Ni-Mn-Ga [6]. To reduce these constraints and increase MFIS, coarse-grained, highly-textured, polycrystalline Ni-Mn-Ga was produced by directional solidification and annealing [7].
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