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

Suok-Min Na,Alison Flatau

doi:10.1063/1.4973286

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.

Highlights

INTRODUCTIONMagnetostrictive Fe-Ga (Galfenol) alloys have become the focus of extensive research due to their useful structural magnetostrictive properties with high magnetostriction of ∼400 ppm along a [100] orientation parallel to a magnetic easy axis, good magneto-mechanical coupling and sufficient ductility for transformation into thin sheets through thermo-mechanical deformation processing. Since highly textured Galfenol alloys have been developed with magnetostrictive performances of ∼250 ppm via directional solidification process, they have been of great interest in actuator, sensor and energy harvesting applications. Within the past decade, there have been several studies of Galfenol texture development along the [100] orientation and significant improvement in achieving abnormal grain growth (AGG) with a high yield of Goss, (011)[100], texture from polycrystalline ingot materials through rolling and annealing processes. The highly textured rolled sheets with a preferred [100] orientation in Galfenol provide a cost-effective alternative to magnetostrictive single crystal Galfenol
To advance our understanding of the conversion of single crystal from the polycrystalline rolled sheet, we investigate the hypothesis that surface energy differences between grains with different crystal orientations in conjunction with grain boundary energy by introducing a total interface energy model developed by combining both grain boundary and surface energies
Magnetostriction values of greater than 200 ppm resulted from abnormal grain growth (AGG) of a large single Goss grain, (011)[100], in 0.35-mm to 0.6-mm thick sheet samples annealed at 1200oC

Summary

INTRODUCTION

Magnetostrictive Fe-Ga (Galfenol) alloys have become the focus of extensive research due to their useful structural magnetostrictive properties with high magnetostriction of ∼400 ppm along a [100] orientation parallel to a magnetic easy axis, good magneto-mechanical coupling and sufficient ductility for transformation into thin sheets through thermo-mechanical deformation processing. Since highly textured Galfenol alloys have been developed with magnetostrictive performances of ∼250 ppm via directional solidification process, they have been of great interest in actuator, sensor and energy harvesting applications. Within the past decade, there have been several studies of Galfenol texture development along the [100] orientation and significant improvement in achieving abnormal grain growth (AGG) with a high yield of Goss, (011)[100], texture from polycrystalline ingot materials through rolling and annealing processes. The highly textured rolled sheets with a preferred [100] orientation in Galfenol provide a cost-effective alternative to magnetostrictive single crystal Galfenol. Since highly textured Galfenol alloys have been developed with magnetostrictive performances of ∼250 ppm via directional solidification process, they have been of great interest in actuator, sensor and energy harvesting applications.. There have been several studies of Galfenol texture development along the [100] orientation and significant improvement in achieving abnormal grain growth (AGG) with a high yield of Goss, (011)[100], texture from polycrystalline ingot materials through rolling and annealing processes.. The AGG phenomenon occurs for solid state conversion of polycrystals into single crystals in ceramic and in metallic materials when grains of a specific orientation grow at a rate that is faster than the growth rate of neighboring grains.. To advance our understanding of the conversion of single crystal from the polycrystalline rolled sheet, we investigate the hypothesis that surface energy differences between grains with different crystal orientations in conjunction with grain boundary energy by introducing a total interface energy model developed by combining both grain boundary and surface energies

EXPERIMENTAL PROCEDURES

RESULTS AND DISCUSSION

CONCLUSIONS