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Understanding the thermal stability of metallic glasses is critical to determining their safe temperatures of service. In this paper, the crystallization mechanism in spark plasma sintered Fe₄₈Cr₁₅Mo₁₄Y₂C₁₅B₆metallic glass is established by analyzing the crystal size distribution using x-ray diffraction, transmission electron microscopy andin-situsmall angle neutron scattering. Isothermal annealing at 700 °C and 725 °C for 100 min resulted in the formation of (Fe,Cr)₂₃C₆crystals, measured from transmission electron micrographs, to be from 10 to 30 nm. The small angle neutron scattering intensity measuredin-situ, over a Q-range of 0.02 to 0.3 Å⁻¹, during isothermal annealing of the sintered samples, confirmed the presence of (Fe,Cr)₂₃C₆crystals. The measured scattering intensity, fitted by the maximum entropy model, over the Q-range of 0.02 to 0.06 Å⁻¹, revealed that the crystals had radii ranging from 3 to 18 nm. The total volume fraction of crystals were estimated to be 0.13 and 0.22 upon isothermal annealing at 700 °C and 725 °C for 100 min respectively. The mechanism of crystallization in this spark plasma sintered iron based metallic glass was established to be from pre-existing nuclei as confirmed by Avrami exponents of 0.25 ± 0.01 and 0.39 ± 0.01 at the aforesaid temperatures.

In this paper, the composition, structure, morphology and kinetics of evolution during isothermal oxidation of Fe₄₈Cr₁₅Mo₁₄Y₂C₁₅B₆metallic glass powder in the supercooled region are investigated by an integratedex-situandin-situcharacterization and modelling approach. Raman and X-ray diffraction spectra established that oxidation yielded a hierarchical structure across decreasing length scales. At larger scale, Fe₂O₃grows as a uniform shell over the powder core. This shell, at smaller scale, consists of multiple grains. Ultra-small angle X-ray scattering intensity acquired during isothermal oxidation of the powder over a wide Q-range delineated direct quantification of oxidation behavior. The hierarchical structure was employed to construct a scattering model that was fitted to the measured intensity distributions to estimate the thickness of the oxide shell. The relative gain in mass during oxidation, computed theoretically from this model, relatively underestimated that measured in practice by a thermogravimetric analyzer due to the distribution in sizes of the particles. Overall, this paper presents the first direct quantification of oxidation in metallic glass powder by ultra-small angle X-ray scattering. It establishes novel experimental environments that can potentially unfold new paradigms of research into a wide spectrum of interfacial reactions in powder materials at elevated temperatures.