Effect of time on the isothermal viscosity of metallic glass supercooled liquids

Abstract

The viscosity of metallic glasses is considered to remain constant during thermoplastic forming operations. Here, we quantify the time dependent change in isothermal viscosity of Pt57.5Cu14.7Ni5.3P22.5 and Zr35Ti30Cu8.25Be26.75 glass formers in the supercooled liquid state. The samples were isothermally held prior to thermoplastic molding to probe the viscosity at different times. The viscosity values were extracted from the filling length of mold cavities. The viscosity increased with annealing time and the rate of increase was found to be largely independent of temperature and type of glass former. However, the viscosity increase was higher in the outer parts of samples which experienced higher shear rate during molding. Crystal nucleation and growth contributions are considered to explain the observed changes in effective viscosity of remaining amorphous phase. Our results suggest that thermoplastic forming is an effective tool in analyzing subtle changes in structure and viscosity of metallic glass supercooled liquids.