Surface Segregation of Sulfur and Its Effect on Surface-Energy-Induced Selective Grain Growth in Magnetostrictive Fe-Ga-B Alloy

The surface-energy-induced selective grain growth with a specific plane can be governed in polycrystalline Fe-Ga-B alloys doped with sulfur. The segregated sulfur during texture annealing played an important role in controlling the surface energy to induce the selective growth of {100} or {110} grains, corresponding to maximum magnetostrictive performance, along <001> orientation with respect to rolling direction. The results show that sulfur diffuses (adsorbs) from bulk interior (sulfur atmosphere) then segregates on the surface. The amount of segregated sulfur increases with an increase of annealing time at the temperature of 1200°C. Auger electron spectroscopy (AES) and X-ray photoelectron spectroscopy (XPS) data on the surface as well as selective development of {100}<001> and {110}<001> preferred textures are presented in this work. The XPS fitted peaks of S 2p_3/2 at binding energy of 161.2 and 163.2 eV for annealed Fe-Ga-B doped with sulfur represent the presence of stoichiometric FeS and FeS_n (polysulfide), respectively. For all of the sulfur-free Fe-Ga-B sheets annealed in the ampoule with sulfur element, XPS indicated contributions centered at approximately 161.7 (S 2p) that has been assigned to iron sulfide as well. The presence of FeS was clearly confirmed by XRD patterns and XPS fitted peak positions at 161.5 eV (S 2p_3/2) and 710.2 eV (Fe 2p_3/2). The segregation of sulfur and boron during annealing were also confirmed by AES depth profile results, which exhibited peak concentrations of 10 at.%S and 20 at.%B at the surface, respectively. The peak magnetostriction of 201 ppm was obtained at annealed (Fe_81.3Ga_18.7)₉₉B₁ alloy with near {100}<001> orientation under sulfur atmosphere containing the amounts of 6.4 mg S. On the other hand, the texture of sulfur-free Fe-Ga-B alloy was close to {110}<001> after annealing at 1200°C for 6h under flowing argon, corresponding to the magnetostriction of 160 ppm.