In-situ scattering and calorimetric studies of crystallization pathway and kinetics in a Cu-Zr-Al bulk metallic glass

The crystallization route and kinetics of a Cu-Zr-Al bulk metallic glass (BMG), based on Cu50Zr50, were investigated using in-situ synchrotron small- and wide-angle X-ray scattering (SAXS/WAXS), in-situ high-energy X-ray diffraction (HEXRD), and differential scanning calorimetry (DSC). The acquisition of kinetic diagrams of SAXS/ WAXS enables the direct construction of the overall phase transformation route from room temperature to the onset temperature of solid state phase transformation upon heating. It is revealed that the studied alloy undergoes multiple steps (at least five steps) before reaching the high-temperature thermodynamic equilibrium. This is distinct from the commonly believed simple path of glass -> Cu10Zr7 + CuZr2 -> B2-CuZr predicted by the Cu-Zr equilibrium phase diagram. In addition to thermodynamics, we emphasize that kinetic factors play a prominent role in the phase transformation process of Cu-Zr-Al BMGs. Furthermore, isothermal crystallization kinetics analyses conducted by in-situ HEXRD and DSC indicate that the formation of the initial crystalline phase, specifically the Cu10Zr7, follows an interface-controlled quasi-polymorphic process with an increasing nucleation rate. The findings of this study may provide novel insights into the crystallization mechanism of glass-forming supercooled liquids from a thermophysical perspective.