Magnetic Field Induced Strain in Polycrystalline Magnetic Shape Memory Foam

Magnetic shape memory alloys (MSMA) are fascinating materials that show a recoverable shape change in a rotating magnetic field. Single crystalline MSMA’s display magnetic-field-induced strains (MFIS) up to 10%. However, single crystals have inherent drawbacks such as cost and chemical segregation during production. Polycrystalline materials are easier to produce and display chemical homogeneity but display a much smaller MFIS than single crystals. It has been shown recently that adding porosity to polycrystalline Ni-Mn-Ga (i.e. metal foam) can increase MFIS. Variables that affect the performance of polycrystalline Ni-Mn-Ga foam include phase transformation temperature, pore architecture, spatial distribution of pores, porosity, training, and magnetic anisotropy/texture. Samples were tested for MFIS and phase transformation temperatures to probe for a correlation. Single pore foam architecture with a mono-modal pore size distribution and dual pore foam architecture with a bi-modal pore size distribution were compared in terms of microstructure and magneto-mechanical behavior. Pore distributions were characterized with x-ray tomography and compared with the temperature dependent MFIS, to deduce the role of the large and small pores. Samples were systematically etched and tested for MFIS to investigate the effect of porosity on strain. Magneto-mechanical, thermo-magnetic, and thermo magneto-mechanical training effect on MFIS was also investigated. The results are discussed in terms of a concept of a network of struts (bridging metal) with hard and soft links. Where, hard links are struts that are unable to deform. It was found that increasing porosity increased strain, confirming the hypothesis that porosity is responsible for enhanced MFIS. The porosity strain relationship indicated strut thickness is a crucial factor in determining the strain, i.e. the thicker the strut the “harder the link.” The dual pore foam has much smaller struts and therefore has fewer hard links. Pore distribution affected the number and distribution of hard links. The metal is more compliant when the sample temperature approaches the phase transformation temperature. Therefore, samples with transformation temperatures close to the testing temperature contain softer links and produce more MFIS. Hard links can also be softened by training. For optimal MFIS and fatigue resistance foams of dual pore architecture with a spatially homogenous distribution of pores, high porosity ( 65-70%) and a martensitic phase transformation temperature close to testing temperature should be employed. Foams with such optimized microstructures and chemical homogeneity are expected to perform reproducibly and consistently