The effect of Ag addition on the non-isothermal crystallization kinetics and fragility of Zr56Co28Al16 bulk metallic glass

The non-isothermal crystallization kinetics and fragility of Zr56Co28Al16 and Zr56Co24Ag4Al16bulk metallic glasses (BMGs) were studied by differential scanning calorimetry (DSC). The X-ray diffraction (XRD), transmission electron microscopy (TEM) and microhardness test were used to investigate the glassy alloys structure. DSC traces exhibited two crystallization processes for both alloys. The activation energies of characteristic temperatures were obtained by Kissinger and Ozawa methods. Also, the heating rate sensitivity of characteristic temperatures was determined. The Ag-bearing Zr-based BMG presented higher activation energies and lower heating rate sensitivity in regard to characteristic temperatures, indicating a higher stabilization of the supercooled liquid, which can be correlated with the existence of strong icosahedral short range order (ISRO) clusters in the structure. In addition, the local Avrami exponents at various heating rates were calculated by the Johnson-Mehl-Avrami equation. The n value variations for each crystallization process pursued almost similar trends for both alloys with n > 2.5, revealing that the transformation kinetics is dominated by a diffusion-controlled three-dimensional growth, as well as an increase in the nucleation rate. Moreover, both BMGs were classified into “strong glasses”, depending on the fragility index calculated values.