Dislocation depinning from ordered nanophases in a model fcc crystal: From cutting mechanism to Orowan looping

Based on the embedded atom method we have studied dislocation bypassing of nanophases in a model for face-centered cubic (fcc) alloys. A system in which either a purely screw or a purely edge dislocation crosses Ni(3)Al nanophases with L1(2) order in a Ni single crystal is employed as an archetypal case for strengthened fcc alloys. For a radius up to 1.5 nm the dislocations cut the nanophase and the depinning stress is found to be proportional to the area of the nanophase. For larger radii, the dislocation circumvents the nanophase and leaves an Orowan loop around the inclusion with the depinning stress increasing as the logarithm of the inclusion radius, in agreement with predictions drawn from an analytical theory proposed by Bacon, Kocks and Scattergood (Phil Mag 1973; 28: 1241). The theory is extended to determine the logarithm pre-factor for the looping regime and the depinning stress needed to cut through the nanophase. The theoretical predictions are then compared to atomistic simulations. (C) 2010 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.