Comparison of the Strength of Binary Dislocation Junctions in fcc Crystals

Discrete dislocation dynamics were used to determine the relative strengths of binary dislocation junctions in fcc crystals. Equilibrium junctions of different types Lomer, glissile, coplanar, and collinear were formed by allowing parallel dislocations of unequal length to react. The strengths were determined from the computed minimum strain rate versus the applied shear stress plots. The collinear configuration was found to be the strongest and coplanar the weakest. It was seen that the glissile junction could exist as two variants depending on which parent slip system the shear stress is applied. One variant of the glissile junction was found to be as strong as the collinear configuration. 1. Introduction Under externally applied stress metals plastically deform and work-harden through the movement, interaction, and multiplication of dislocations. The interactions of dislocations in fcc metals on the slip systems result in formation of Lomer, Hirth, collinear, glissile, and coplanar junctions [1]. Lomer and Hirth junctions are sessile, the glissile and coplanar junctions are glissile, and collinear is an interaction between dislocations with parallel Burgers vectors leading to annihilation. The formation of these junctions, their subsequent interaction with dislocations and their destruction is known to control the work-hardening behaviour of materials [2]. The modelling of the mechanical behaviour of materials at different length scales requires an understanding of the movement and interactions of dislocations. In the last decade dislocations have been modelled as lines of singularity in an elastic continuum and their dynamics solved using a discrete dislocation dynamics (DDD) framework formulation [3–10] as individual dislocation interactions [11–14], as dislocation-forest interactions [2, 15, 16] and as dislocation-boundary interactions [17, 18]. Of the individual dislocation interactions the Lomer junction has been studied the most [11, 12, 15, 19, 20] as its formation and destruction is thought to be the primary cause for strain hardening in fcc metals. In most studies on fcc crystals the junction strength is correlated with the length of junctions, that is, a higher junction length signifies higher strength [1, 20, 21]. Analytical calculations made using line tension model [11] showed that the junction strength was the highest for Lomer, followed by glissile junction and the least for the Hirth junction; the collinear interaction and coplanar junction were not considered in that study. However, in later studies [1, 15] the collinear interaction