Effects of Au-addition on plastic deformation ability of Zr-Cu-Ni-Al bulk metallic glasses

Effects of Au-addition on plastic deformation ability of Zr-Cu-Ni-Al bulk metallic glasses (BMGs) during compression and tensile tests have been examined. Zr₆₅Cu₂₀Ni₅Al₁₀ BMG(Zr65) and Zr_60+xCu_22-xNi₅Al₁₀Au₃(x=0, 5, 6.5, 8, 9 at%) BMGs (Zr60Au3-Zr69Au3) have been prepared by a tilt casting method. By the Au-addition on the BMGs containing Zr-content of 65 at% and more, precipitation of icosahedral quasicrystalline phase (I-phase) in supercooled liquids was observed, resulting in large increase of viscosity in the supercooled liquids. Compression and tensile tests were carried out in these BMGs under various strain rates of 1∼5 × 10^-4/s at room temperature. In the compression test of the Zr65 BMG, plastic strain at fracture was about 10% and only several localized shear bands were observed. In the Zr65Au3 and Zr66.5Au3 BMGs, plastic strains at fracture were largely increased to 20% and above. In these cases, initial plastic strains of about 5% with large number of shear band were observed as macroscopically homogeneous deformation, and successive plastic strains along main shear band was observed. In the tensile test, large plastic strain at fracture of about 0.5% was also observed for the Zr66.5Au3 BMG. By transmission electron microscopic observations of the Au-addition BMGs, precipitation of nano-crystallites having about 10 nm in diameter was observed near the fractured surfaces of the compressive and tensile test specimens. Viscosity of supercooled liquids in shear bands was estimated by the stress-drop of serrations in the compressive stress-strain curves. It is considered that the large increase of the plastic strain is due to the drastic increase of the viscosity of the supercooled liquids formed inside shear bands during plastic deformation.