Continuous chemical redistribution following amorphous-to-crystalline structural ordering in a Zr-Cu-Al bulk metallic glass

Bulk metallic glasses(BMGs) are thermodynamically metastable. As such, crystallization occurs when a BMG is thermally annealed at a temperature above the glass transition temperature. While extensive studies have been performed on the crystallization kinetics of BMGs, most of them have focused on the amorphous-to-crystalline structural ordering, and little attention has been paid to chemical distribution and its relationship with the structural ordering during the crystallization process. In this paper, a new approach, with simultaneous differential scanning calorimetry(DSC) and small angle neutron scattering(SANS) measurements, was applied to study in situ the crystallization of a ZrCuAlBMG upon isothermal annealing at a temperature in the supercooled liquid region. Quantitative analysis of the DSC and SANS data showed that the structural evolution during isothermal annealing could be classified into three stages:(Ⅰ) incubation;(Ⅱ) amorphous-to-crystalline structural ordering;(Ⅲ) continuous chemical redistribution. This finding was validated by composition analysis with atom probe tomography(APT),which further identified a transition region formed by expelling Al into the matrix. The transition region, with a composition of(Cu,Al)Zr, served as an intermediate step facilitating the formation of a thermodynamically stable crystalline phase with a composition of(Cu,Al)Zr.