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