Zr-Co(Cu)-Al bulk metallic glasses with optimal glass-forming ability and their compressive properties

The formation of bulk metallic glasses (BMGs) in the ternary Zr₅₆Co₂₈Al₁₆ and quaternary Zr₅₆Co_28–xCu_xAl₁₆ (x=2, 4, 5, 6, 7, mole fraction, %) glassy alloys was investigated via the copper mold suction casting method. The main purpose of this work was to locate the optimal BMG-forming composition for the quaternary ZrCo(Cu)Al alloys and to improve the plasticity of the parent alloy. The X-ray diffractometry (XRD), transmission electron microscopy (TEM) and differential scanning calorimetry (DSC) were used to investigate the glassy alloys structure and their glass forming ability (GFA). In addition, the compression test, microhardness, nano-indentation and scanning electron microscopy (SEM) were utilized to discuss the possible mechanisms involved in the enhanced plasticity achievement. The highest GFA among Cu-containing alloys was found for the Zr₅₆Co₂₂Cu₆Al₁₆ alloy, which was similar to that of the base alloy. Furthermore, the plasticity of the base alloy increased significantly from 3.3% to 6% for the Zr₅₆Co₂₂Cu₆Al₁₆ BMG. The variations in the plasticity and GFA of the alloys were discussed by considering the positive heat of mixing within Cu and Co elements.