The foundation of materials science is the structure-property relationship of materials. The structure of materials can be described at various length scales like macroscopic, microscopic, atomic and subatomic structure. Since the properties of materials are ultimately founded in the underlying atomic structure, the design of novel materials with optimized properties requires an increasing focus on the atomic lengthscale. There, computational methods need to be developed that can quickly and efficiently screen the nearly infinite number of possibilities of mixing different elements into alloys for optimized compositions.
In this study we have used atomistic modeling to examine the structure and properties of complex materials with the goal of identifying structures and properties of materials for finding improved and optimized processes and compositions. With the availability of parameters to study almost all the systems, density functional theory (DFT) is probably the most widely used theory to model the atomic and electronic structure of materials. While DFT is very accurate, it is computationally very expensive which limits the system size to ~1000 atoms. Molecular dynamics can be used with empirical potentials to study larger-scale materials which have a less periodic arrangement of atoms such as glasses, cracks and fracture in metals, polymers, proteins etc. In this work we have used both of these atomistic techniques to understand materials such as metallic glasses and processes involved in corrosion.
In the first study, we have examined the stability and transformation of metavanadate and decavanadate which is important to understand their role as corrosion inhibitors for aluminum alloys. We have used DFT and ab-initio molecular dynamics to understand their solvation behavior in water. The water cell size effect on the solvation behavior of metavanadate was investigated as well. In this thesis, we have also investigated if the effect of changing the number of electrons can be correlated to the effect of changing the pH value of the solution on the vanadates.
In the second study, we have developed an empirical embedded atom method (EAM) potential (which takes into account the effect of electron clouds in metals) of beryllium, which among others is an important element in common bulk metallic glass alloys. This task was necessary since all EAM and modified EAM potentials (which takes the angular nature of the bonding into account in addition to the electron cloud effect) of beryllium available in the literature failed to predict the properties of beryllium to a sufficient degree of accuracy. The developed potential predicts the relative stabilities of various lattices of beryllium correctly when compared to values calculated using DFT. It also predicts the elastic moduli of beryllium with good accuracy and without negative moduli, which was a major drawback of the previously reported potentials. Additionally, it predicts the vacancy and vacancy cluster formation energy and the interstitial formation energy with good accuracy.
The third study in this thesis is concentrated on the study of metallic glasses using molecular dynamics with embedded atom method potentials. We have addressed the issue of predicting the glass forming ability in ternary systems without using any experimental input, solely based on a computational study which was still missing in the literature. Our results suggest that the modeled fragility is sufficient to predict the glass forming ability in ternary systems, which we validate by calculating the fragility as a function of composition in the ternary Cu-Zr-Ti system. We also show that the icosahedral fraction of the nearest-neighbor shells, which has been frequently suggested to provide information on glass forming ability, does not correlate well with it while its differential with respect to the composition may correlate well with the glass forming ability. However, due to the limited number of test cases examined in the present study, this needs to be further probed.
The deformation behavior of metallic glasses is very different from that of their crystalline counterparts owing to the absence of long-range order. We have also explored the deformation behavior of metallic glasses using molecular dynamics for two glass compositions, Cu65Zr35Ti5 and Cu45Zr45Al10. The effect of system size, annealing temperature, testing temperature and strain rate on the deformation behavior of the glasses was investigated. We found that annealing temperature and strain rate do not affect the elastic modulus to a great extent, while the modulus increases with decreasing the testing temperature from 300 K to 50 K. The compressive and tensile yield strength were also investigated. The results suggest that the yield strength is higher for compressive load. The deformation of glass is discussed in terms of change in icosahedron fraction and von Mises shear strain.
Finally, the deformation behavior of metal-metallic glass composites was explored for the cases where the crystal is the stronger and weaker phase, respectively. For this we have studied Cu-(Cu65Zr35Ti5) nano-laminates with copper as the stronger phase and Al-(Cu45Zr45Al10) nano-laminates where the glass is the stronger phase. The effect of orientation of the crystal relative to the interface, the loading direction, the annealing temperature and the crystal volume percent was also investigated.