Abstract
In contrast to pure metals and most non-glass forming alloys, metallic glass-formers are moderately strong liquids in terms of fragility. The notion of fragility of an undercooling liquid reflects the sensitivity of the viscosity of the liquid to temperature changes and describes the degree of departure of the liquid kinetics from the Arrhenius equation. In general, the fragility of metallic glass-formers increases with the complexity of the alloy with differences between the alloy families, e.g., Pd-based alloys being more fragile than Zr-based alloys, which are more fragile than Mg-based alloys. Here, experimental data are assessed for 15 bulk metallic glasses-formers including the novel and technologically important systems based on Ni-Cr-Nb-P-B, Fe-Mo-Ni-Cr-P-C-B, and Au-Ag-Pd-Cu-Si. The data for the equilibrium viscosity are analyzed using the Vogel–Fulcher–Tammann (VFT) equation, the Mauro–Yue–Ellison–Gupta–Allan (MYEGA) equation, and the Adam–Gibbs approach based on specific heat capacity data. An overall larger trend of the excess specific heat for the more fragile supercooled liquids is experimentally observed than for the stronger liquids. Moreover, the stronger the glass, the higher the free enthalpy barrier to cooperative rearrangements is, suggesting the same microscopic origin and rigorously connecting the kinetic and thermodynamic aspects of fragility.
Highlights
Starting from the early 1990s, several bulk metallic glass (BMG) compositions with critical casting thickness greater than 1 mm and robust supercooled liquid against crystallization were discovered [1,2,3,4,5,6,7].These BMG-formers are multicomponent alloys based on transition metals like Zr, Cu, Fe, Ni, Mg, Pd, Pt, Au, and La and with a large size mismatch between the constituents
Many BMG compositions can be cast with a thickness up to 30 mm by conventional casting due to their excellent glass forming ability (GFA)
A common characteristic of high-GFA of BMGs is the location at time greater than 100 s of the nose of the nucleation curve in the time temperature transformation (TTT) diagram [13]
Summary
Starting from the early 1990s, several bulk metallic glass (BMG) compositions with critical casting thickness greater than 1 mm and robust supercooled liquid against crystallization were discovered [1,2,3,4,5,6,7].These BMG-formers are multicomponent alloys based on transition metals like Zr, Cu, Fe, Ni, Mg, Pd, Pt, Au, and La and with a large size mismatch between the constituents. Starting from the early 1990s, several bulk metallic glass (BMG) compositions with critical casting thickness greater than 1 mm and robust supercooled liquid against crystallization were discovered [1,2,3,4,5,6,7]. A common characteristic of high-GFA of BMGs is the location at time greater than 100 s of the nose of the nucleation curve in the time temperature transformation (TTT) diagram [13]. This ability results from the interplay between the thermodynamic and the kinetics properties of the undercooled liquid. The shape of the nucleation curve in the TTT diagram for BMGs is connected, on one side, to their low driving force for crystallization, represented by the Gibbs
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