Phase separation in Zr56-xGdxCo28Al16 metallic glasses (0 ⩽ x ⩽ 20)

The influence of Gd addition on the microstructure of Zr56Co28Al16 metallic glasses was investigated for the exchange of Zr by up to 20 at.% Gd. Due to the large positive enthalpy of mixing between Zr and Gd, liquid–liquid phase separation occurs during rapid quenching of the melt. For a low concentration of Gd (x = 2 at.%), a homogeneous amorphous structure is obtained for the as-quenched state. Early stages of spinodal decomposition are observed in the as-quenched state of the glasses with x = 5 and 10 at.% Gd. Gd-enriched clusters 4–7 nm in size are formed, as shown by atom probe tomography (APT). Annealing below the crystallization temperature Tx leads to an increase in the amplitude of compositional fluctuations and the analysis of the spatial atomic distribution by APT provides direct evidence of the spinodal character of the decomposition by uphill diffusion of Gd into the clusters. For higher Gd content (x = 15 and 20 at.%), a coarsened microstructure of the phase-separated glass is obtained due to growth and coalescence while quenching the melt. The microstructure formation is essentially determined by the thermodynamic properties of the metastable undercooled liquid.