Nanocrystallization of the amorphous Co-B-Si alloys formed by melt spinning and mechanical alloying

Abstract

X-ray diffraction, differential scanning calorimetry, and transmission electron microscopy were used to investigate the thermal stability and peculiarities of nanocrystallization upon heating of amorphous Co65.5S18B10 M 6.5 (M = Cr, Fe) alloys obtained by melt-spinning (MS) and mechanical alloying (MA) of mixtures of elemental components. It is shown that, irrespective of the method of manufacture, the amorphous iron-containing alloys, in general, have a lower thermal stability than the chromium-containg alloys. The thermal stability of MA alloys is lower than that of the MS alloys of the same chemical composition. The processes of nanocrystallization during heating of MS and MA amorphous alloys differ. The first crystallize, when heating, through the formation of intermediate phases, and the second, with the formation of stable crystalline phases directly from the amorphous matrix. After the crystallization, the alloys have a nanocrystalline structure with a crystallite size of about 20 nm and the same phase composition of crystallization products for the alloys of the same nominal composition obtained by MA and MS: ɛ-Co, Co2B, and β-Co2Si for Co65.5Si18B10Cr6.5 and α-Co, Co2B, β-Co2Si, and (Fe,Co)3Si for Co65.5Si18B10Fe6.5. The results are discussed in terms of the excess free volume of the amorphous phase and of the value of its specific surface area.