Book Description

Metallic glasses, a class of metal alloys which lack a periodic crystal structure, exhibit exceptional property combinations not accessible by other classes of materials. In spite of promise for widespread application, metallic glasses are difficult to synthesize and understanding of their structure and behavior is limited compared to crystalline alloys. There is no predictive criterion for determining if a particular alloy is capable of forming glass. Numerous glass-forming alloys have been reported, spanning a wide range of possible properties largely through trial and error. Engineering of these materials is difficult, as the connection between atomic structure and macroscopic behavior is not sufficiently developed to exploit particular behaviors in any intentional capacity. Using Molecular Dynamics (MD) simulations, three metallic glass-forming systems, Al-La, Cu-Zr and Cu-Ti-Zr were investigated and compared with the intention of connecting structure to properties and illuminating differences in glass-forming behavior in different alloys. From these simulations a specific mechanism occurring in the liquid, the changing of nearest neighbor environments, was identified and correlated to liquid viscosity. The change in viscosity with temperature, called fragility, was connected to this atomic-scale behavior allowing glass formers and non-glass formers in the Al-La alloys system to be separated from each other. The structure of each glass is readily available from these simulations, and the changes to neighbor environments in Al-La and Cu-Zr alloys, were found to be very similar when comparing the smaller atom type (Al, Cu). Differences in system-wide behavior for Al-La and Cu-Zr can be described based upon the behavior of the larger atom type (La, Zr), where Zr causes a major change in behavior as the majority component not exhibited by even very La-rich alloys. This dissimilarity between La and Zr provides a plausible explanation for Cu-Zr’s superior glass-forming ability compared to Al-La. Experimental data indicated that Cu-Ti-Zr achieve maximum glass-forming ability near Cu51.7Zr36.7Ti11.6. The addition of Ti to the Cu-Zr binary system causes a decrease in nearest-neighbor-switching events and stabilizes structures formed in the liquid, rather than destroying them. Cu51.7Zr36.7Ti11.6 also divides two compositional regions of hardness dependence: above 37% Zr the hardness scales with the concentration of Cu, while below 37% Zr the hardness scales with the concentration of Ti. Based on concepts developed for Al-La and Cu-Zr it was revealed that removing Cu drastically reduced the number of efficiently-packed Cu-centered structures. Below 37% Zr this effect is compensated by an increase in other dense structures but above 37% the effect is both more potent and uncompensated. The loss of these structures is responsible for the changes in yield behavior, and has an effect on the GFA. Finally, extension of these simulations to additional systems requires new multi-component EAM potentials, an essential input for MD simulations. The Rapid Alloy Method for the Production of Accurate General Empirical Potentials (RAMPAGE) was developed to create new multi-component potentials from elemental potentials available in the literature. Using RAMPAGE, the characteristics identified in glass-forming systems can be investigated in other metallic systems.