Explicit relations for determining the effective magnetostriction of polycrystals with cubic magnetostrictive crystallites and pores are derived based on the Green’s function technique. For comparison, two extreme approximations-the Reuss and Voigt-type approximations-are also given. To illustrate the technique, the influence of porosity on the effective saturation magnetostriction of common magnetostrictive metals (Fe and Ni), giant magnetostrictive alloy terfenol-D, and Fe–Co spinel are illustrated. The theoretical estimates show an interesting nonmonotonic concentration dependence of magnetostrictive behavior of the polycrystals with such magnetostrictive microcrystallites as Ni and Fe–Co spinel, whose magnetic easy axes are 〈100〉 axes rather than 〈111〉 axes.

Effect of porosity on the effective magnetostriction of polycrystals

The effective magnetostriction of a two-phase fibrous composite and a two-dimensional polycrystal assembled from cylindrical magnetostrictive cubic crystals is studied. For the considered systems, we show that there exist exact microstructure-independent relations between the effective magnetostriction and the effective elastic moduli. The fibers could be aligned identically or randomly oriented in the transverse plane. There is no restriction on the cross-sectional shapes of the fibers, nor on the arrangement of transverse geometry of the composite aggregate. These connections imply that knowledge of the effective elastic moduli will readily provide the effective magnetostriction of the composite medium.

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Exact connections between effective magnetostriction and effective elastic moduli of fibrous composites and polycrystals

The universally used approximation for the magnetostriction constant of a polycrystalline material is based on the assumption of uniform stress through the aggregate In this article we calculate the alternative approximation of uniform strain The calculation is performed in terms of the grains orientation distribution (texture) function, and single crystal constants describing the elastic and magnetostrictive anisotropy These bounds represent the optimum solution of the incompletely specified problem of crystallites shape distribution in a polycrystalline aggregate The available experimental data are compared with the two approximations

M. Birsan

Papers

The bounds of saturation magnetostriction in polycrystalline materials