Crystallization kinetics of ultrafine Co74.4B25.6 amorphous powder prepared by chemical reduction

The ultrafine Co74.4B25.6 amorphous powder was successfully prepared by chemical reduction, whose amorphous structure was confirmed by X-ray diffraction (XRD) and selective area electronic diffraction (SAED), meanwhile the surface morphology and the particles size were observed by both of the transmission and the scanning electron microscope. The crystallization kinetics of this alloy was investigated by non-isothermal differential scanning calorimetry (DSC). The as-prepared sample has a net-like morphology, and the net is formed by many interconnected fine particles with sizes <100 nm. It is found that the crystallization of the amorphous Co74.4B25.6 powder occurred through a single exothermic peak, and it was confirmed by X-ray diffraction that the structure after crystallizing consisted of Co, Co2B and Co3B phases. The activation energy E was determined as 427.76 ± 4.89 kJ/mol by the means of Kissinger analysis. When using the Popescu's procedure, it is found that with the crystallized volume fraction α increasing from 0.1 to 0.35, the value of E decreased dramatically from 640.8 to 423.3 kJ/mol, then it maintained almost identical for the range 0.35 < α < 0.65, and during which the average value was 422.5 kJ/mol with an error of 2.45 kJ/mol. The value of E gradually decreased again with α exceeding 0.65 and remained 355.9 kJ/mol at α = 0.955. The Popescu's curve fitting procedure reveals that the crystallization of the Co74.4B25.6 amorphous powder follows the JMA-like kinetic model with the Avrami exponent varying from 0.5 to 0.985 under different temperatures, which indicates diffusion-controlled growth with an appreciable initial volume. The Kissinger analysis also got the conclusion of n ≈ 1, which was independent of the heating rate and implied the same transformation mechanism as well.