Energetics Calculation of {10-12} Twin Nucleation and Growth in Magnesium and Alloys using Density Functional Theory and Atomistic Simulation
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Authors
Wang, Fangxi
Issue Date
2019
Type
Dissertation
Language
Keywords
Atomistic simulation , Density Functional Theory , Mg alloys , Twinning
Alternative Title
Abstract
Deformation twinning strongly affects the mechanical properties of magnesium (Mg) and its alloys, as well as other alloys with hexagonal close-packed (HCP) structures. The influence of solute atoms such as aluminum (Al), zinc (Zn), and yttrium (Y) on {"10" "1" ̅"2" } twin nucleation and growth in magnesium alloys remains unclear. In this work, we perform energetics calculation for {"10" "1" ̅"2" } twin nucleation and growth using molecular dynamics (MD) simulations and Density Functional Theory (DFT) calculation for Mg and its alloys. For MD simulations, the embedded atom method (EAM) potential and modified embedded atom method (MEAM) potentials are used. All simulations are conducted with perfect single crystals with no defect in the initial systems. The result shows that the energy barrier for {"10" "1" ̅"2" } twin nucleation is approximately 33.5 meV/atom from the MEAM potential for pure Mg, which is close to 34.5 meV/atom from DFT calculation with 16 Mg atoms. The energy barrier for twin growth is approximately 12.2 meV/atom (EAM) and 18.8 meV/atom (MEAM) for pure Mg. The addition of solute atoms (Al, Zn, and Y) will decrease the energy barrier for twin nucleation at low concentration. In the Mg-12at%Al system, deformation twinning is dramatically suppressed in the MD simulation. The energy barrier for twin nucleation increases from 33.5 meV/atom (pure Mg) to 44.1 meV/atom (Mg-12at%Al), and the twin volume fraction decreases from 72.6% (pure Mg) to 1.1% (Mg-12at%Al) at 10% tensile strain applied along the [0001] direction of the systems. It appears that {"10" "1" ̅"2" } deformation twinning is hindered by the addition of Al, Zn, and Y solute atoms in Mg alloys with high concentrations. Al and Y atoms have a significant effect on suppressing twin nucleation. However, the effect of Zn contributes more to suppressing the twin growth because the energy barrier for twin growth increases from 12.2 meV/atom (pure Mg) to 28.9 meV/atom (Mg-3at%Zn). The stress-strain behavior obtained from MD simulations indicates that the addition of solute atoms will decrease the stress overshoot. The stress overshoot is much higher than the yield stress obtained from deforming the Mg single crystal in experiments. There are two main reasons for the above difference: 1) The initial structure in MD simulation is a perfect single crystal without any line defects and grain boundaries. 2) The strain rate in the MD simulation is 108/s, which is many orders of magnitude faster than that in the lab experiments. In MD simulation, the stress first increases with the strain increasing. The stress drops dramatically when dislocation or twin nucleates at the free surface. The nucleation of dislocation or twin causes the relaxation of the system and the stress drops. After the stress drop, a higher stress is required for twin growth in the system with a high content of alloying atoms. This result generally agrees with the experimental observation. The suppression of twinning behavior is due to the change in atomic bonding between Mg and solute atoms in terms of electron distribution. The charge density distribution results indicate that the addition of alloying atoms can change the charge density surrounding the alloying atoms. Electrons are more localized around Al and Y atoms. However, they are more delocalized around Zn atoms in Mg-Zn systems from the electron localization function (ELF) calculation. The crystal orbital Hamilton population (COHP) analysis indicates that the addition of Al and Y atoms will increase the average bond strength of Mg-Mg bonds in the alloy systems. The average Mg-Mg bond strength increases with increasing solute concentration. The weak Zn-Zn bonding results in a lower energy barrier for twin nucleation in Mg-Zn systems compared to that of pure Mg. The strong Al-Al bonding, Mg-Y bonding, and Y-Y bonding results in the higher energy barriers for twin nucleation. The bonding analysis reveals that solute atoms significantly influence the energetics of {"10" "1" ̅"2" } deformation twinning in Mg and Mg alloys. These results suggest that increasing the energy barrier for twinning with proper alloying may be an effective approach to designing and processing high strength commercial Mg alloys.