Evolution of the microstructure and magnetic properties of a cobalt-silicon-based alloy in the early stages of mechanical milling

Abstract

The early stages in the mechanical alloying of amount fractions x = 40 % cobalt (Co) and x = 60 % silicon (Si) powders were investigated using X-ray diffractometry (XRD), scanning electron microscopy (SEM), differential thermal analysis (DTA) and vibrating-sample magnetometry (VSM). After 2-8 h of ball-milling, the characteristic XRD peaks of the face-centered-cubic (fcc) Si and hexagonal close-packed (hcp) Co phases remained sharp without a cobalt-silicide phase. As the milling progressed, the particle size observed by SEM tended to reduce, being accompanied by smoother edges and a narrow size distribution. On the DTA curves, between 200°C and 1200°C, exothermic peaks indicated a ferromagnetic-to-paramagnetic transition, whereas endothermic peaks corresponded to the lattice recovery, the transition from a hcp to a fcc Co and melting. The longest milling of up to 8 h significantly increased the magnetic squareness and the coercive field. Keywords: Co-Si alloys, ball milling, XRD, VSM, DTA Preiskovan je bil za~etni stadij mehanskega legiranja zlitine, sestavljene iz prahov z mno`inskim delom x = 40 % kobalta (Co) in x = 60 % silicija (Si). Uporabljena je bila rentgenska difraktometrija (XRD), vrsti~na elektronska mikroskopija (SEM), diferencialno termi~na analiza (DTA) in vibracijska magnetometrija vzorcev (VSM). Po mletju od 2 h do 8 h v krogli~nem mlinu so {e ostali ostri zna~ilni XRD-vrhovi faz fcc Si in hcp Co brez Co-silicidne faze. S podalj{evanjem~asa mletja se s SEM opazi zmanj{anje velikosti delcev prahov, zo`anje njihove velikostne porazdelitve in pove~anje zaobljenja robov delcev. Na DTA-krivuljah se je med 200°C in 1200°C pojavil eksotermni vrh, ki je posledica prehoda iz feromagnetne v paramagnetno fazo, endotermni vrh pa je posledica poprave re{etke zaradi prehoda iz hcp v fcc Co in nato taljenja. Najdalj{i~asi mletja (do 8 h) pomembno vplivajo na magnetno koercitivnost in pove~anje kvadrati~nosti histerezne zanke. Klju~ne besede: zlitina na osnovi Co-Si, krogli~no mletje, XRD, VSM, DTA