Critical serrated flow features during nanoindentation in La-based bulk metallic glasses

La_57.6Al_17.5(Cu,Ni)_24.9 and La₆₄Al₁₄(Cu,Ni)₂₂ bulk metallic glasses (BMGs) were prepared by copper-mould casting method. Plastic deformation behavior of the two BMGs at various loading rates was studied by nanoindentation. The results showed that the La_57.6Al_17.5(Cu,Ni)_24.9 BMG with a glass transition temperature of 423 K exhibited prominent serrated flow at low loading rates, whereas less pronounced serrated flow at high rates during nanoindentation. In contrast, the La₆₄Al₁₄(Cu,Ni)₂₂ BMG with a glass transition temperature of 401 K exhibited prominent serrated flow at high loading rates. The different rate dependency of serrated flow in the two La-based BMGs is related to the different glass transition temperature, and consequently the degree of viscous flow during indentation at room temperature. A smoother flow occurs in the alloy with relatively lower glass transition temperature, due to the relaxation of stress concentration.