Short-range order patterns in Mg66Zn29Ca5 metallic glass: Phase transformations and crystallization kinetics

Abstract

Thanks to outstanding mechanical and chemical properties, metallic glasses have been recently alluring for biomedical applications. However, inadequate knowledge of atomic rearrangements as well as kinetics and mechanisms of crystallization prevents further microstructure improvement controlling the properties. In this work, we reveal that hierarchical transformations happen during the crystallization of Mg66Zn29Ca5 metallic glass ribbons which were investigated using differential scanning calorimetry (DSC), electron microscopy (SEM and TEM), and X-ray diffraction (XRD). The activation energies corresponding to the crystallization of the glassy ribbons were evaluated by different thermodynamic models, i.e., Kissinger, and Augis and Bennett analysis. Our findings prove the formation of short-range order patterns in Mg66Zn29Ca5 metallic glass with a unit size of ∼ 0.247 nm. Furthermore, the applied heating rate affects the glass transformation only slightly but accelerates the growth process quickly. The phase transformations occurring in annealed samples were studied by XRD, proving that crystallization starts with the formation of α-Mg + metastable Mg51Zn20. The metastable Mg51Zn20 gradually transfers to Mg7Zn3 with increasing temperature, and also Ca2Mg5Zn13 precipitates. Close to the end of crystallization, Ca2Mg6Zn3 precipitates by consuming Ca2Mg5Zn13. The Johnson–Mehl–Avrami–Kolmogorov (JMAK) model was employed to obtain the Avrami exponent of the crystallization reaction. The average slope of the Avrami plots is close to 2, indicating that crystallization progresses with diffusional growth and decreasing nucleation rate.