Thickness dependence of solid-state single crystal conversion in magnetostrictive Fe-Ga alloy from thin foil to thick sheet

Abstract

Abnormal grain growth (AGG) is a highly promising process for solid state conversion of polycrystals into single crystals in ceramic and in-metallic materials. In this paper we investigate AGG behaviors in NbC-added Fe-Ga rolled sheets with varying thicknesses for converting polycrystalline ingots of Fe-Ga alloy into single-crystal-like rolled sheet for application as the transduction components in low-cost magnetostrictive devices. Magnetostriction values of greater than 200 ppm resulted from AGG of a large single Goss grain, (011)[100], in 0.35-mm to 0.60-mm thick sheet samples annealed at 1200oC. Samples thinner than 0.35 mm or thicker than 0.60 mm developed (113)[uvw]-oriented grain growth or random orientations with low magnetostriction values. In order to understand how thickness influences grain growth behaviors, a total interface energy model has been developed that combines both grain boundary and surface energies. We investigated the hypothesis that surface energy differences between grains in conjunction with grain boundary energy act as the driving force underlying the ability to selectively develop AGG and even to promote single-crystal-like grain growth. Results obtained from modeling simulation demonstrate that the extent of the matrix consumed with AGG was determined by controlling surface energy which plays a major role in accelerating AGG beyond what is achieved with the effects of just grain boundary energy.