Spherulitic crystallization behavior of a metallic glass at high heating rates

Understanding crystallization in metallic glasses is an important consideration in the development of processing and manufacturing techniques which will utilize these unique alloys to full advantage. In the present work, the crystalline morphology and crystallization mechanisms of a Zr _58.5Cu_15.6Ni_12.8Al_10.3Nb _2.8 metallic glass are examined at heating rates spanning 4 orders of magnitude. In contrast to the multi-phase nanocrystalline structure formed at low heating rates, high heating rates (>2.5 K/s) result in the formation of a non-equilibrium crystalline phase with spherulitic microstructure and uniform composition. Heating rates as fast as 10³ K/s are found to be insufficient to avoid the formation of spherulites. The microstructure suggests that rapid heating suppresses phase separation and nucleation processes at the initial stage of crystallization, resulting in a growth-dominated crystallization behavior. The activation energy for spherulite formation is found to be less than that for nanocrystallization, which is attributed to the lack of phase separation and nucleation steps. Calculation of the Avrami exponent also indicates that spherulites form via the growth of quenched-in nuclei. The number density of quenched-in nuclei is estimated to be 10 ¹⁴ m^-3.