Effect of pore architecture on magnetic-field-induced strain in polycrystalline Ni-Mn-Ga

Monocrystalline Ni-Mn-Ga alloys show magnetic-field-induced strains (MFIS) of up to 10% as a result of reversible twinning; by contrast, polycrystalline Ni-Mn-Ga shows near-zero MFIS due to strain incompatibilities at grain boundaries inhibiting twinning. Recently, we showed that porous polycrystalline Ni-Mn-Ga exhibits a small, but non-zero, MFIS value of 0.12% due to reduction of these incompatibilities by the porosity. Here, we study the effect of pore architecture on MFIS for polycrystalline Ni-Mn-Ga foams. Foams with a combination of large (∼550 μm) and small (∼80 μm) pores are fabricated by the replication method and exhibit thinner nodes and struts compared to foam containing only large (∼430 μm) pores. When magnetically cycled, both types of foams exhibit repeatable MFIS of 0.24-0.28% without bias stress. As the cycle number increases from a few tens to a few thousands, the MFIS drops due to damage accumulation. The rate of MFIS decrease is lower in the dual-pore foam, as expected from reduced constraints on the twin boundary motion, since twins span the whole width of the thinner nodes and struts.