Build Nanocrystalline Materials and MD Simulations

Most metallic materials are polycrytalline, rather than a perfect single crystal. The properties of these materials have dependence on the size of the grains (polycrystals), such as yield strength. Conventionally, the Hall-Petch effect captures the relation between yield strength and grain sizes. However, at the nanoscale, materials behave differently compared to at scales larger than millimeters, showing inverse Hall-Petch effect in many nanocrystalline materials. In this tutorial, the generation of nanocrytalline Aluminum using Atomsk and its MD simulations using LAMMPS is demonstrated.

Initial Nanocrystalline Al System Buildup

Atomsk was used to build the system. The system (a cubic box) size is 5 nm wide. The number of grains is 4, 10, 30, and 100, giving rise to the average grain size: 2 nm, 1.5 nm, 1.0 nm, and 0.667nm, respectively. Here are the initial systems.

Here is a sample code to generate these systems:

  
#!/bin/bash

# generate the unit cell
atomsk --create fcc 4.046 Al aluminium.xsf

for i in 4 10 30 100
do
	input=seed$i.txt
	
	# generation
	atomsk --polycrystal aluminium.xsf $input final$i.cfg

	# convert to lammps data file
	atomsk final$i.cfg lammps

done
  

where seed$i.txt is as follows

  
box 50 50 50
random i // i from 4/10/30/100 
  

MD Simulation in Lammps

Lammps (version Aug-7-2019) was used as the MD simulation engine for the uniaxial tensile tests of the nanocrystalline Al systems and a single crystalline for comparison. The simulation protocol follows the classical tutorial given here. The tension test was carried out at 300 K and 1 bar using the EAM potential. Here is the result of the stress-strain curves for single crystalline Al and nanocrystalline Al (inset for zoom in of the nanocrystalline systems for comparison of size effect).

References

  1. Uniaxial tensile test of Aluminum using LAMMPS