First Principles Molecular Dynamics Study Of Solidification And Nucleation Processes In Metallic Melts

Solidification, which is initiated by nucleation, is a very important first-order phase transition due to both fundamental and technological applications. In order for liquid to solidify, it is necessary to supercool the liquid to a certain temperature below the melting point. Detailed knowledge of the local atomic structural evolution during quenching plays a vital role in understanding of solidification process. Despite the importance, it is very challenging to experimentally study the microscopic structural evolution of liquid and supercooled (amorphous) state, since the current experimental techniques cannot clearly characterize the local structural arrangement of disordered systems in contrast to crystals. In addition, the short length and time scale associated with nucleation events makes understanding of nucleation pathways challenging. However, computer simulations using state of the art ab initio molecular dynamics method enable us to explore the structural features of novel amorphous materials (such as liquid metals and metallic glasses) in addition to the detailed the crystallization process.The presented thesis covers two important subjects of rapid quenching process. i) During supercooling of pure metallic melts and metallic alloys, the evolution of local short range order is explored using various structural analysis methods. ii) The nucleation pathways in pure metallic melts are revealed by isothermally annealing the supercooled liquid state.We have performed a series of ab intio molecular dynamics simulations of various pure body centered cubic (BCC), face centered cubic (FCC) and hexagonal close packed (HCP) metallic melts in order to investigate both local atomic structure evolution and nucleation pathways. In addition, the effect of alloying on phase selection during solidification is explored. Various structural analysis methods including the bond orientational order analysis, which is the most accurate method to detect the crystalline structures, are used to quantify the local atomic-structure evolutions. It is found that the fraction and types of short range order and their evolution upon supercooling depend on the system under consideration. Further investigations into the nucleation pathways, by performing time-temperature-transformation (TTT), reveal that extended structured regions of high bond orientational order are spontaneously formed in a supercooled state by thermal fluctuations and play a critical role during crystallization process. This order mainly has BCC-type local symmetry in the amorphous region with small portion of HCP/FCC-like local symmetries for all systems. For BCC systems, this preexisting order further selects BCC crystalline polymorph to be formed upon future crystallization. On the other hand, for BCC systems, the process of polymorph selection already happens in the supercooled liquid state. But in the closed-packed (FCC and HCP) systems it starts at the time of onset of crystallization. Moreover, the effect of alloying Mg with 10 at.% of Ca or Al on the structural (both topological and chemical ordering) evolution during solidification is investigated. Overall the presented study advances our understanding of crystallization in metallic melts for industrial applications, which is linked to formation of novel materials such as metallic glasses.