Collaborative Research: Size Effects on Magneto-Mechanics of Ni-Mn-Ga Fibers

TECHNICAL SUMMARY: This project lays the foundation for a new class of active materials - magnetic shape-memory fibers with tailored geometry, microstructure and magneto-mechanical properties - to be used as transducers for micro-devices and as building blocks for composites or cellular structures. Magnetic-field-induced twinning is responsible for the high magnetoplastic strains achievable in monocrystalline Ni-Mn-Ga. By contrast, polycrystalline Ni-Mn-Ga shows no magnetoplasticity because twinning is inhibited by internal incompatibility stresses developed between adjacent, misoriented grains. The PIs recently discovered that porosity, because it reduces internal stresses, allows twinning to occur in polycrystalline Ni-Mn-Ga foams, resulting in magnetoplastic strains in the foam struts. Applying this concept to individual fibers, our hypothesis is that tailored grain size (with respect to fiber size) and grain orientations will allow tuning the magnetoplastic strain from polycrystalline (0%) to monocrystalline (~10%) behavior. In this basic study, we will develop a fundamental understanding of how fiber geometry and grain microstructure enable magnetic-field-induced strains in polycrystalline Ni-Mn-Ga fibers, leading to experimentally-validated models that can quantitatively predict the magnitude of magnetoplastic strain for a given fiber structure. Fundamental experimental and theoretical studies probing the mechanisms responsible for magnetoplasticity in the fibers will be carried out. First, the fiber geometry will be varied, in terms of cross-sectional shape and diameter, by using two versatile manufacturing methods (Taylor wire drawing and melt extraction). Then, the fiber grain size and texture will be tailored: the ratio of grain to fiber diameter will be varied from