Nanomechanics Simulation Tool- Dislocations Make or Break Materials Michael Sakano, Mitchell Wood, David Johnson, Alejandro Strachan Department of Biomedical Engineering, University of North Carolina at Chapel Hill School of Materials Engineering, Purdue University

Failure in Materials 2 2 Why do we have materials engineers? We study materials because they are not ideal and eventually fail. It is our goal to understand how and why failure occurs. Liberty hull crack Failure does not occur the same way in all materials Significant deformationMinimal deformation Environment concernsChoice of material

Failure example Dislocations Bulk metal has a yield stress approximately one order of magnitude less than the nanowire MPa (10 6 Pa) 8 GPa (10 9 Pa) What causes there to be a difference between the two?

What are Dislocations? 4 Dislocations are atomic linear defects in the crystal structure. Either strengthen or weaken material structure behavior (mechanical and/or electrical) Transistor dislocation defect 4 ml/ Semiconductor Transistor /21/1/008/fulltext/

Dislocations in Crystals 5 Edge Dislocation Screw Dislocation 5

Atomistic Movement 6 Dislocation type depends on relative orientation of: Burgers vector (direction of atom shift) dislocation line slip plane (closed packed) 6 atoms Edge Dislocation Stress

Molecular Dynamics 7 7

Motivation 8 Our objective is to create a user friendly interface to enable wide spread use of simulation tools to predict the behavior of defects in metals Saves time and money compared to experimental work Click here for results Click here for creating personalize d run 8

Output- Animation 9 9 strain Stress (GPa) Copper Edge Iron Edge Gliding dislocations Stress (GPa) strain Nucleating dislocations (Creating/Forming) Bulk Atoms Dislocation Cores Dislocation Cores Bulk Atoms Not Shown Bulk Atoms Not Shown Applied Stress

Output – Dislocation Motion 10 Stress (GPa) strain Stress (GPa) strain Edge Dislocation FCC Copper - Oscillation around 0 GPa conveys gliding motion Edge Dislocation BCC Iron – Increasing stress implies single dislocation takes more stress to move than two partials for an FCC crystal 10 Face Centered Cubic (FCC) Body Centered Cubic (BCC)

Output – Dislocation Nucleation 11 strain Stress (GPa) Copper Edge Iron Edge Fixed Strain Rate and Temperature – Different magnitudes because dislocations in copper are partials compared to iron. Reduced Temperature – Yielding occurs at much later strain, BCC material acts more brittle Iron Edge Copper Edge strain Stress (GPa) 11

Conclusion 12 Dislocation behavior dependent on crystal and orientation Shear stress causes dislocations to glide or nucleate Replicates the behavior of bulk material by modeling at the nanoscale GUI presents the topic in a user-friendly manner Advanced users can freely create and define their own simulation parameters Questions? 12