Date of Award

Fall 11-17-2018

Document Type

Thesis

Degree Name

Master of Science (MS)

Department

Molecular Science and Nanotechnology

First Advisor

Gergana Nestorova

Abstract

Titanium-Aluminum alloys are one of the widely used alloys in multiple engineering applications. They are highly preferred in Selective Laser Melting (SLM) processes due to their low density, high melting temperature, and good strength. Segregation occurs during the solidification of most alloys and produces a non-uniform distribution of atoms. In SLM, segregation may depict the type of adhesion between the two deposited interfacial layers and the strength between the interphase between an already solidified layer and a new one, and overall, the quality of the printed part. In order to avoid segregation, the understanding of the segregation behavior at atomistic level is important. The main goal of this work is to understand the metal segregation in titanium-aluminum alloys for 3D printing applications using molecular simulations. For this the solidification of metal alloys is computationally simulated and the atomic ordering and structural transformations of the systems are studied. The thermal stability of different compositions of titanium-aluminum metal alloys and the size effects of the system on atomic ordering, structural transformation, and thermal stabilization of the alloys using Molecular Dynamics (MD) simulations is done. MD simulations are widely used in the studies of atomic transformations and structural evolutions of molecular systems and require the use of forcefields to describe the forces between atoms. In addition to the main goal, in this the performance of the Zope-Mishin Embedded Atom Method (EAM) and the Sun-Ramachandranan-Wick Modified Embedded Atom Method iv (MEAM) potential developed for the titanium-aluminum systems is evaluated. This work is divided into two parts. In the first part, the simulations are carried out in nine different compositions of 3.4 nm titanium-aluminum spherical nanoclusters with the timestep of 2 fs, the equilibration time of 50 ps, and the total simulation time of 2 ns and in the second part the simulations are carried out in two titanium-rich clusters of the same composition but different sizes are done at the timestep of 1 fs, the equilibration time of 100 ps, and the total simulation time of 1 ns. Each study is done twice using two different forcefields each time. The clusters are melted at high temperature and solidified at room temperature (300K) using simulated annealing. The analysis is done using the radial density distribution of atoms, the structural evolutions from the trajectories, and the melting temperature calculations using the combination of caloric and heat capacity curves using both forcefields. From the EAM potential, the solidified titanium-rich clusters show an inclination to icosahedral geometry whereas the MEAM potential shows the solidified aluminum-rich clusters to have an inclination to truncated octahedral geometry. Moreover, MEAM calculated the melting temperature of pure aluminum nanocluster at 711.76K and pure titanium nanocluster at 1185.36K. In the clusters except for the ones with high titanium concentration, aluminum migrates to the surface upon solidification. Due to the presence of titanium in the grain boundary making the adhesion between two SLM interfacial layers stronger. These high titanium clusters are found to have low segregation and suggested in SLM printing applications. The size effect study done shows no significant change in the properties of the structures or the melting temperatures.

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