Source: Dissertation Abstracts International, Volume: 79-05(E), Section: B.

Adviser: Corey S. O'Hern.

This thesis presents three computational studies of Bulk Metallic Glasses (BMGs) about asymmetric crystallization, crystallization kinetics in isothermal simulations, and the correlation of material's ductility and its potential energy landscape in quasi-static uniaxial deformations.

In the first project, we perform molecular dynamics (MD) simulations of binary Lennard-Jones mixture to investigate atomic-scale crystallization kinetics in glass-forming materials. The systems are quenched or heated linearly to measure the critical cooling and heating rate R* c and R*h, below which the systems begin to form a substantial fraction of crystalline clusters. Glasses can be relaxed into different states depending on the thermal history which is characterized by the glass preparation rate Rp. We find that R*h > R c and that the asymmetry ratio R* h / R*c includes an intrinsic contribution that increases with the glass-forming ability (GFA) of the system and a preparation-rate dependent contribution that increases strongly as Rp → R* c from above. We also shown that the predictions from classical nucleation theory (CNT) can qualitatively describe the dependence of the asymmetry ratio on the GFA and preparation rate Rp from the MD simulations and results for the asymmetry ratio measured in Zr- and Au-based BMGs.

In the second project, we investigate the effect of pre-critical nuclei on the nano-scale crystallization kinetics. The systems are prepared by quenching rapidly from equilibrium liquid into glassy state with embedded crystal inside the material that serves as seed for crystallization. We measure the crystallization rate during isothermal simulation by accurately determining the growing crystal boundary at a given time. We find that the quenched-in pre-critical nuclei enhance the crystallization rate, and the fluctuation of the crystallization rate is larger when more pre-critical nuclei are present. This result is consistent with experiment, and supports the argument that the crystal growth in nano-scale is stochastic in nature and depends on the thermal history and diameter of the metallic glass nano-rods.

In the third project, we perform quasi-static simulation of binary mixtures with ranged potentials and asymmetric bumpy potential to determine the ductility of glasses prepared at cooling rates spanning over three orders of magnitudes. The ductility is measured in quasi-static uniaxial deformation with periodic boundary condition applied in direction perpendicular to the strains. We aim to correlate ductility with the characteristics in the potential energy landscape. We find that the average energy drop size in elastic regime is positively correlated with its ductility, and this correlation seems to be universal for glasses with different potentials and quenched at different cooling rate. It provides the physical understanding of ductility from energy landscape perspective that ductile material is able to release accumulated internal energy before it causes a catastrophic failure.