Non-planar grain boundary structures in fcc metals and their role in nano-scale deformation mechanisms

Abstract

This work presents the results of a comparative molecular dynamics study showing that relaxed random grain boundary structures can be significantly non-planar at the nano-scale in fcc metals characterized by low stacking fault values. We studied the relaxed structures of random [1 1 0] tilt boundaries in a polycrystal using interatomic potentials describing Cu and Pd. Grain boundaries presenting non-planar features were observed predominantly for the Cu potential but not for the Pd potential, and we relate these differences to the stacking fault values. We also show that these non-planar structures can have a strong influence on dislocation emission from the grain boundaries as well as on grain boundary strain accommodation processes, such as grain boundary sliding. We studied the loading response in polycrystals of 40 nm grain size to a level of 9% strain and found that the non-planar grain boundaries favour dislocation emission as a deformation mechanism and hinder grain boundary sliding. This has strong implications for the mechanical behaviour of nano-crystalline materials, which is determined by the competition between dislocation activity and grain boundary accommodation of the strain. Thus, the two interatomic potentials for Cu and Pd considered in this work resulted in the same overall stress–strain curve, but significantly different fractions of the strain accommodated by the intergranular versus intragranular deformation mechanisms. Strain localization patterns are also influenced by the non-planarity of the grain boundary structures.

Keywords:

• molecular dynamics simulations
• mechanical behaviour
• copper
• palladium
• heterogeneous strain
• LAMMPS
• grain boundary
• grain boundary structure
• stacking fault
• unstable stacking fault energy
• stacking fault energy
• molecular dynamics
• modelling

Acknowledgements

This work was supported by the Department of Energy, Office of Basic Energy Sciences, under grant DE-FG02-08ER46525 and the National Science Foundation, IRD program. The authors acknowledge Advanced Research Computing at Virginia Tech for providing computational resources and technical support that have contributed to the results reported within this paper. URL: http://www.arc.vt.edu