1. Length-scale Dependent Dislocation Nucleation during Nanoindentation on Nanosized Gold Islands Alex Gonzalez Department of Mechanical Engineering University of Arkansas Fayetteville, AR 72701 USA REU Advisor: Dr. Douglas Spearot REU 10 th Week Seminar – Monday July 20 th, 2009

2. 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.

3. 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.

4. Multiscale Mechanics of Materials Laboratory, Department of Mechanical Engineering Role of thermo oscillation

5. Multiscale Mechanics of Materials Laboratory, Department of Mechanical Engineering Role of thermo oscillation

6. Multiscale Mechanics of Materials Laboratory, Department of Mechanical Engineering Force v. Depth Curves and Plasticity **Both cases have box dimensions of 40x30x40

7. 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 indenter Indentation on surface

8. 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.

9. 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.

10. 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.

11. Alex Gonzalez REU 10 th Week Seminar – Monday July 20 th, 2009