Theory of magnetostriction with applications to TbxDy1-xFe2

Abstract: "We present a new approach to magnetostriction that is formulated to describe materials with large magnetostriction. The main idea of the theory is to derive precisely from lattice considerations the potential wells of the anisotropy energy. The theory exhibits frustration in the sense explored by the authors in the rigid case (James and Kinderlehrer [1990]), with fine domains modeled by minimizing sequences. The theory is applied to the material Tb[subscript x]Dy[subscript 1-x]FeΓéé. The theory predicts accurately the domain structures observed by Lord [1990] in growth twinned crystals, and suggests a mechanism of magnetostriction involving a switch from a coarse domain structure to a different finer domain structure.

Frustration and microstructure : an example in magnetostriction

Abstract: "Microstructural properties of materials, especially crystalline solids, are implicated in many of their properties. Vice versa, there are macroscopic environments which limit microstructural configurations. Certain iron/rare earth alloys, eg, TbDyFeΓéé, display both a huge magnetostriction and frustration, i.e., minimum energy not achieved, in which microstructure plays an important, if puzzling, role. We discuss this example in the framework of continuum thermoelasticity theory, where symmetry demands energy densities which are highly degenerate. This leads to novel analytical and computational issues, many of which we have been unable to resolve.

Exceptional Resilience of Small-Scale Au₃₀Cu₂₅Zn₄₅ under Cyclic Stress-Induced Phase Transformation

Shape memory alloys that produce and recover from large deformation driven by martensitic transformation are widely exploited in biomedical devices and microactuators. Generally their actuation work degrades significantly within first a few cycles and is reduced at smaller dimensions. Further, alloys exhibiting unprecedented reversibility have relatively small superelastic strain, 0.7%. These raise the questions of whether high reversibility is necessarily accompanied by small work and strain and whether high work and strain is necessarily diminished at small scale. Here we conclusively demonstrate that these are not true by showing that Au₃₀Cu₂₅Zn₄₅ pillars exhibit 12 MJ m^–3 work and 3.5% superelastic strain even after 100 000 phase transformation cycles. Our findings confirm that the lattice compatibility dominates the mechanical behavior of phase-changing materials at nano to micron scales and points a way for smart microactuators design having the mutual benefits of high actuation work and long lifetime