Interface Friction Behavior Of A Zr-Based Metallic Glass During Thermoplastic Forming In The Supercooled Liquid Region

In this dissertation, the interfacial friction behaviors during micro-extrusion atdifferent strain rates and temperatures in the supercooled liquid region (SCLR), as well asthe effect of interfacial friction on the thermoplastic-formability of a Zr-based metallicglass have been systematically investigated through X-ray diffraction (XRD), differentialscanning calorimetry (DSC), scanning electron microscopy (SEM) and mechanical testingsystem.Zr35Ti30Be26.75Cu8.25metallic glass was fabricated by arc-melting by combiningCu-mould casting. Double-cup mold, Extrusion mold and Si master mold were also beendesigned and fabricated. The constitutive equations of the Zr-based metallic glass in thesupercooled liquid region was been fitted based on the data obtained byuniaxial-compression test. The results of the double-cup experiment showed that theinterfacial friction coefficient can be improved by temperature increase or strain ratedecrease, which provides a theoretical basis for the definition of material properties andthe choice of the coefficient of friction during the finite element simulation process.Correlation between flow characteristics and interfacial friction behavior of theZr-based metallic glass was investigated through a series of micro-extrusion experimentsat a wide range of strain rates and temperatures in SCLR. The results showed that at highttemperature (such as at390℃) or low strain rate (such as at1×10-3s-1), a closed interfaceemerges between the steel mould and the metallic glass can be observed, belongs to aadhesion regime; while at low temperature (such as at330℃) or hight strain rate (such asat1×10-1s-1), the material completely can not fill the pits and exhibited a mechanicalengagement regime; Between these two conditions, the metallic glass sweeps over theprotrusion and leaves a big cavity, shows a furrow regime. Compared the tribologicalmechanism map with the typical deformation map, the results revealed that the Newtonianflow favors the adhesion mechanism, while the non-Newtonian flow fascinates the furrowregime, which was well analyzed by combining the viscosity theory with the finiteelement simulations. Thermoplastic-formability of a Zr-based metallic glass in the supercooled liquidregion was studied by hot-embossed experiment on silicon circular grooves with differentwall thickness. It was found that filling length decreased and top curvature increased withthe reduction of groove thickness, exhibiting a size-dependent thermoplastic-formability.This phenomenon can be well demonstrated in terms of thermoplastic forming kinetics,atomics-scale shear transformation zone theory and finite element simulation, respectively.