Revealing the crystallization kinetics and phase transitions in Mg65Zn30Ca5 metallic glass by nanocalorimetry

As a potential candidate for biomedical applications, Mg65Zn30Ca5 (at%) metallic glass has attracted considerable attention. To guide the development of MgZnCa alloys with improved mechanical and biomedical properties, it is necessary to understand the phase transitions of this metallic glass. In this study, controlling phase transitions in Mg65Zn30Ca5 metallic glass is realized by nanocalorimetry. The underlying thermodynamic and kinetic features at heating rates ranging from 102 to 105 K s-1 are therefore revealed. Upon rapid heating, an intermediate amorphous phase is detected. This metastable phase facilitates the formation of the Mg7Zn3 crystalline phase with a remarkably reduced reaction temperature and activation energy. With the combination of nanocalorimetry and structural characterization, the continuous heating transformation (CHT) diagram for Mg65Zn30Ca5 metallic glass is plotted, and crystallization trajectories under various heating conditions are distinguished. Meanwhile, the crystallization kinetics covering seven orders of magnitude is estimated, giving the growth rate of the MgZn crystalline phase in the undercooled liquid with temperatures ranging from Tg to Tm. Accordingly, the maximum crystal growth rate and corresponding temperature are estimated at umax= 0.49 m s-1 and Tu,max= 563 K, respectively. With the decrease in undercooling, the transition from kinetics-controlled crystallization to thermodynamics-controlled crystallization is illustrated, causing divergence of the estimated growth rate from the experimental results. (c) 2021 Elsevier B.V. All rights reserved.