Size-Dependent Viscosity in the Super-Cooled Liquid State of a Bulk Metallic Glass

We experimentally investigate the role of sample size on the viscosity of a bulk metallic glass by examining pressure driven flows in nm-scale confinement. A Pt57.5Cu14.7Ni5.3P22.5 metallic glass in the super-cooled liquid state is extruded into isolated cylindrical pores of varying nano-scale dimensions, down to 40 nm. The apparent viscosity of the liquid as a function of sample size is determined from the filling depth by appropriate corrections to the Hagen-Poiseuille equation. We observe a striking, sudden increase of the apparent viscosity for dimensions below approximately 100 nm. Results are discussed in the framework of confinement of collective shear events. The unusually high viscosity of bulk metallic glass (BMG) -forming liquids is generally believed to be the main origin for their high resistance against crystallization, hence their extraordinary glass forming ability. By comparison with pure metals, at the liquidus temperature the viscosity of BMG forming liquids (~ 1 Pa.s) is considerably higher, by about 2 orders of magnitude, and increases faster with decreasing temperature. Due to its crucial contribution to glass formation, tremendous efforts have been made to determine and understand the viscosity of BMG-forming liquids. Viscosity also plays a crucial role in thermoplastic forming (TPF) of BMGs. TPF is based on the rapid yet continuous softening of BMGs when heated above their glass transition temperature. For some BMGs, viscosities as low as 10 Pa.s can be accessed on experimentally practical time scales (seconds to minutes) before they crystallize. Access to such viscosities allows forming or molding of these BMGs using only moderate pressures, very similar to thermoplastic forming of engineering polymers. TPF-based processing has been demonstrated over a wide range of length scales, from Angstroms to meters. 11-16 Due to the ease and versatility of TPF based molding and the absence of size limitations imposed by the