Recent Developments in Ni-Mn-Ga Foam Research

Abstract

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 magneticshape 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.

Microstructural evolution and magnetic properties of binder jet additive manufactured Ni-Mn-Ga magnetic shape memory alloy foam

“…Another alternative to reach high MFIS is to manufacture a porous structure to reduce internal constraints. Chmielus et alpresented a Ni-Mn-Ga open-pore foam with a monomodal [22] and bimodal [23]pore size of 76% volume fraction and an MFIS of 0.12%.Müllneret al [25] proposed that the main mechanism of reducing internal and external constraints via porosity and foam architecture is based on the reduction in twin-twin and twingrain boundary interactions.It is expected that high saturation magnetization is favorable to the MFIS [9]. The saturation magnetization of Ni-Mn-Ga single crystals may vary between 50 and 70 Am 2 /kg [26,27] and after ball milling, it may drop to 15-25 Am 2 /kg depending on the time and milling energy [28].…”

Long-Term Cyclic Loading of 10M Ni-Mn-Ga Alloys

“…The polymer composites produced by using 10M Ni-Mn-Ga [16][17][18][19] alloy particles are of special interest in this respect. It has been shown that the polycrystalline 14M Ni-Mn-Ga foam materials can be used in the cyclic loading [20,21], and it would be interesting to know, if this was applicable for the 10M martensite alloys also. In all these applications, twin boundaries between the twin variants are driven back and forth at each operating cycle, such as one actuation stroke.…”

Magnetic-field-induced recovery strain in polycrystalline Ni–Mn–Ga foam

“…The mechanism reducing internal and external constraints via porosity and foam architecture is based on the reduction in twin-twin and twin-grain boundary interactions and is further discussed in Ref. 17.…”