Effect of Boron Addition on Magnetostrictive and Mechanical Properties in Rolled Polycrystalline Fe-18.7%Ga Alloy
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0888-V06-07.1
Effect of Boron Addition on Magnetostrictive and Mechanical Properties in Rolled Polycrystalline Fe-18.7%Ga Alloy Suok-Min Na and Alison B. Flatau Dept. of Aerospace Engineering, University of Maryland, College Park, MD 20742, U.S.A. ABSTRACT During attempts to roll polycrystalline Fe-18.7%Ga alloy (atomic%), the alloy of interest because single crystal samples exhibit magnetostriction of ~400 ppm, cracks formed along grain boundaries and the samples subsequently fractured during hot rolling such that the alloy was too brittle to make thin sheets. While variations to rolling schedules (temperatures, times, roll diameters, etc.) may improve rollability, in this paper we investigate the effect of boron addition on Fe-18.7%Ga alloy magnetostrictive and mechanical properties. Thin sheets of Fe-18.7%Ga alloy plus 0.5 and 1.0 at.%B were successfully fabricated to thickness of 0.35 mm using rolling processes. It was observed that the fracture surface of the B-free alloy clearly appeared as an intergranular fracture mode and that of the B-added alloys was changed to a transgranular fracture mode with grain refinement. The annealed sheets with boron content of 0.5 and 1.0 at.% exhibit the maximum magnetostriction values of 103 and 184 ppm, respectively. In the case of Fe-18.7%Ga plus 1.0 %B, Fe2B phase presents throughout the α-iron matrix in small amount.
INTRODUCTION Fe-Ga solid solutions with 15~20 %Ga may be useful in magnetostrictive actuators and sensors due to their large saturation magnetostriction potential, high mechanical strength, and good ductility [1]. Properly textured polycrystalline Fe-Ga alloys obtained through rolling and texture annealing processes may provide even more desirable magnetic and mechanical properties and at a lower cost than single crystal. It was recently reported that textured polycrystalline Fe-15%Ga with 1 mol% NbC and Fe-17%Ga alloys was changed from a strong as-rolled {hkl} to preferred orientations along the rolling direction, but the values of magnetostriction are not sufficient [2,3]. In order to achieve maximum strain in polycrystalline form, we initially tried rolling Fe-18.7%Ga alloy, targeting a stoichiometry of peak magnetostriction in single crystal Fe-Ga alloys. However, the alloy cracked and fractured along the grain boundaries during hot rolling and was too brittle to make thin sheets. In order to improve the ductility, boron was introduced to Fe-18.7%Ga alloy, where boron is known to suppress grain boundary fracture due to its segregation to grain boundaries in metal alloys such as Fe-Al, FeCo-2V, Ni-Al, Ti-6Al(-4V), Cr-Mo steel, etc. [4-8]. The non-equilibrium segregation of boron in ultra low carbon bainitic (ULCB) steels influences recrystallization and precipitation when quenching from 1200oC [9]. This work motivated our investigation of the effect of B addition on magnetostrictive and mechanical properties for Fe-18.7%Ga alloy.
EXPERIMENTAL DETAILS
0888-V06-07.2
The Fe-18.7%Ga alloy buttons plus 0.5 and 1.0 at.%B were prepared by arc-melting system
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