Length-scale Dependent Dislocation Nucleation during Nanoindentation on Nanosized Gold
Islands
Alex Gonzalez
Department of Mechanical EngineeringUniversity of Arkansas
Fayetteville, AR 72701 USA
REU Advisor: Dr. Douglas Spearot
REU 10th Week Seminar – Monday July 20th, 2009
Multiscale Mechanics of Materials Laboratory, Department of Mechanical Engineering
Index
Objective: The objective of this project is to make indentations on nanosized islands of Au to generate dislocations and study the onset of plasticity through the four steps listed below…
Step 1: Make and set boundary conditions.
Step 2: Determine how to control thermo oscillations.
Step 3: Indent sample and plot force v. depth curves to study plasticity.
Step 4: Systematically indent closer to the free surface to study dislocation nucleation.
Multiscale Mechanics of Materials Laboratory, Department of Mechanical Engineering
Make and set boundary conditions
Simulation cell dimensions are N x 30 x N (N=30,40,50,60) lattice units.
Indenter size is 5 angstrom radius. Simulations
First: NVE – updates positions and velocities for groups of atoms. V is volume and E is energy.
– Run simulation for certain amount of time periods
Second: NVT - uses Nose/Hoover thermostat with Tdamp and drag values. V is volume and T is temperature.
– Run simulation for certain amount of time periods
Third: Energy Minimization – iteratively adjusting atom coordinates for lowest possible local potential energy
– Lower indenter, minimize, lower indenter, minimize
Plot force as a function of depth. Create free surface on YZ plane and indent closer
to the free surface at steps of 5 lattice units.
Multiscale Mechanics of Materials Laboratory, Department of Mechanical Engineering
Role of thermo oscillation
Multiscale Mechanics of Materials Laboratory, Department of Mechanical Engineering
Role of thermo oscillation
Multiscale Mechanics of Materials Laboratory, Department of Mechanical Engineering
Force v. Depth Curves and Plasticity
**Both cases have box dimensions of 40x30x40
Multiscale Mechanics of Materials Laboratory, Department of Mechanical Engineering
Atomistic displacement simulation
Before, at, and after the peak close ups of Energy Minimization of a 40x30x40 Simulation cell
Before Peak
At Peak
After Peak
Raise indenterIndentation on
surface
Multiscale Mechanics of Materials Laboratory, Department of Mechanical Engineering
Free surface dislocations
30 L.U.
25 L.U.
20 L.U.
15 L.U. 10 L.U. 5 L.U.
Multiscale Mechanics of Materials Laboratory, Department of Mechanical Engineering
Free surface analysis
No free surface interaction – Indenter 10 lattice units away from free surface
Dislocations forming from the free surface – Indenter 5 lattice units away from free surface.
Multiscale Mechanics of Materials Laboratory, Department of Mechanical Engineering
Conclusions
Objective steps: Step 1: Make and set boundary conditions.
Step 2: Determine how to control thermo oscillations.
Step 3: Indent gold surface and plot force versus depth to understand the onset of plasticity.
Step 4: Indent closer to the free surface to study dislocation nucleation in the presence of the free surface.
Outcome Better understanding of plastic behavior. Produced results similar to that of published papers for pure Au. Analyzed force v. depth curves and proved when plasticity occurs. Gave evidence of activity before “peak” in force depth curve. Showed evidence of dislocation nucleation from free surface.
Alex Gonzalez
REU 10th Week Seminar – Monday July 20th, 2009