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

Abstract

• Crystallization kinetics of Zr 45.5 Cu 45.5 Al 9 bulk metallic glass was investigated. • Simultaneous DSC and SANS was applied to study in situ the crystallization process. • A three-stage structural evolution during crystallization was revealed. • Continuous chemical redistribution following structural ordering was observed. • A new feature, i.e., the transition region, was identified and discussed. 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 Zr 45.5 Cu 45.5 Al 9 BMG 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) 50 Zr 50 , served as an intermediate step facilitating the formation of a thermodynamically stable crystalline phase with a composition of (Cu,Al) 10 Zr 7 .