Spherulitic crystallization behavior of a metallic glass at high heating rates

• Published in: Intermetallics 2011-10, Vol.19 (10), p.1538-1545
• Main Authors: Sun, Hongqing, Flores, Katharine M.
• Format: Article
• Language: English
• Subjects: A. Multiphase intermetallics; Amorphous materials; B. Metallic glasses; B. Phase transformation; C. Crystal growth; C. Laser processing; Crystallization; Heating rate; Metallic glasses; Nanocrystals; Nucleation; Phase separation; Spherulites
• ISSN: 0966-9795; 1879-0216
• DOI: 10.1016/j.intermet.2011.05.022

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 Zr58.5Cu15.6Ni12.8Al10.3Nb2.8 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 1014 m−3. ► Investigated glass crystallization over 4 orders of magnitude in heating rate. ► Low heating rates produce nanocrystals; high heating rates produce spherulites. ► Spherulites form at heating rates 1000x higher than the critical cooling rate. ► Spherulite formation activation energy is much lower than nanocrystallization. ► The number density of quenched-in nuclei during casting is ∼1014 m−3.