Carbon transformations in rapidly solidified nickel–carbon ribbon

Abstract

We report the structural transformations of carbon via the melt spinning and subsequent annealing of nickel-carbon alloys. We attained metastable solid solubility of carbon in nickel ribbon by achieving a rapid solidification rate of up to 1.6 × 106 K/s. Excess carbon atoms were found to be dissolved in the nickel lattice causing up to 1.4% strain for an alloy spun at 70 m/s tangential wheel speeds. High temperature heat treatments led to precipitation of carbon from the nickel lattice on the ribbon free surfaces but also led to growth of spherical precipitates within the nickel matrix, an effect consistent with bulk diffusion-driven Ostwald ripening. Carbon was excavated from the ribbons via chemical dissolution of the metal and characterized by electron microscopy and Raman spectroscopy. We found that the microstructure of carbon precipitated from the rapidly quenched ribbon could be tuned by varying the carbon content from 4 to 12 at. % in the precursor and annealing the ribbon at temperatures that ranged from 400 to 1200 °C. Via the step-wise variation of these two parameters, we sequentially transformed amorphous carbon nanospheres with a high BET surface area of 203 m2/g into thick, highly crystalline flakes of graphite that conformed to the shape of the as-spun ribbon.