Kinetic Characterization of Nanocrystal Formation in Metallic Glasses

Abstract

Fe-Mo-Cu-B, FINEMET-type and Al-based ribbons were investigated by differential scanning calorimetry (DSC) in both scanning and isothermal regimes. The devitrification of rapidly quenched ribbons is a multistage process. Our studies have established that the kinetics of the nanocrystal formation stage (being the primary crystallization, R1, or the main transformation stage) is characterized by four principal peculiarities: (i) The scanning exothermal DSC peak has the large high temperature part. (ii) Any pre-annealing at temperatures below the R1 peak shifts the peak to higher temperatures, decreasing its enthalpy. (iii) The isothermal transformation exotherm does not shape any peak. (iv) The thermograms taken at all heating rates and at each annealing temperature follow the unique Surifiach plot master curve. This curve is a straight line indicating the normal-grain-growth exponent < 2. The nanocrystal formation in metallic glasses does not follow the conventional Johnson-Mehl-Avrami kinetics. The normal-grain-growth kinetic law, proposed for coarsening of extremely fine crystalline grains in heterogeneous thin films, could also in the case of R1 nanocrystalline formation stage in metallic ribbons rationalize all peculiar results without any correction.