Crystals and nanocrystals in rapidly solidified Al-Sm alloys

The formation, size and morphology of crystals in Al100-xSmx (8<x<12) alloys either melt spun or crystallized from the glass was analyzed with SEM, AFM, TEM, DSC, XPS, SIMS. AFM and SEM show crytalline fractions even in some ribbons appearing amorphous at XRD. On the surface which was in contact with the wheel, colonies of crystals were found, apparently nucleated on heterogeneities. In Al92Sm8 they were identified as a-Al, inAl88Sm12 and Al90Sm10 as Al11Sm3. On the opposite surface ofAl92Sm8 ribbons, AFM reveals crytals of 100-200 nm, regularly distributed, nucleated in the undercooling regime. In glass samples annealed between 423 K and 444 K, TEM shows nanocrystals reaching a density of 1022 m-3 in less than a minute with maximum size of 20 nm. The isothermal transformed fraction indicates a two step process of crystallization. The findings are discussed considering nucleation in various temperature ranges.