Designing metallic glasses : Alloying, properties, and degrees of freedom

Sammanfattning: The present design rules for metallic glasses with respect to glass formation, thermal stability (resistance to crystallisation), and material properties can still be improved. A main design rule is to use thermodynamic calculations to determine the composition of eutectic points in the phase space. The work in this thesis shows that thermodynamic calculations can be utilised to a larger extent than calculating liquidus temperatures. To gain additional insight about metallic glass forming alloys, they are herein classified into systems with a small and large negative enthalpy of mixing of the liquid. The mixing enthalpy influences the number of compound phases (intermetallic or ceramic) forming at equilibrium. A small negative enthalpy of mixing results in few compound phases at equilibrium, whereby the formation range of metallic glasses is dominated by the atomic size mismatch, for instance in ZrNbCrMo and VZr. A low thermal stability is observed for glasses with a chemical composition close to the amorphous-to-crystalline boundary if the alloys have a small negative enthalpy of mixing. Moreover, thermodynamic calculations are herein used to relate the crystallisation temperature to the thermodynamic driving force for crystallisation. A lower thermodynamic driving force in VZrH metallic glasses is related to a higher crystallisation temperature, where the configurational entropy is found to have a stabilising effect. The thermodynamic phase equilibrium is used to identify where the constituent atoms separate or mix in an alloy, which influences glass formation, thermal stability, and material properties. However, a negative enthalpy of mixing of the liquid can be misleading since the chemical interactions between the constituents in the solid state can nonetheless be repulsive. This is seen in the VZr and TaNiSiC metallic glasses investigated herein. There the enthalpy in the liquid and solid have opposite signs, whereby separation tendencies are observed in the solid amorphous state. The separation tendencies can result in variations in the local chemical composition on the nanometer scale and possibly larger length scales. The variations are found to be the origin of reduced thermal stability and corrosion resistance in TaNiC glasses. Furthermore, the occurrence of the variations may also lead to a larger extent of hydrogen embrittlement in metallic glasses.