Cross-slip of long dislocations in FCC solid solutions

Abstract

Cross-slip of screw dislocations is a dislocation process involved in dislocation structuring, work hardening, and fatigue. Cross-slip nucleation in FCC solid solution alloys has recently been shown to be strongly influenced by local fluctuations in spatial arrangement of solutes, leading to a statistical distribution of cross-slip nucleation barriers. For cross-slip to be effective macroscopically, however, small cross-slip nuclei (~40b) must expand across the entire length of typical dislocation segments (10 2-10 3 b). Here, a model is developed to compute the relevant activation energy distribution for cross-slip in a random FCC alloy over arbitrary lengths and under non-zero Escaig and Schmid stresses. The model considers cross-slip as a random walk of successive flips of adjacent1b segments, with each flip having an energy consisting of a deterministic contribution due to constriction formation and stress effects, plus a stochastic contribution. The corresponding distribution is computed analytically from solute-dislocation and solute-solute binding energies. At zero stress, the probability of high activation energies increases with dislocation length. However, at stresses of just a few MPa, these barriers are eliminated and lower barriers are dominant. For increasing segment length, the effective energy barrier decreases according to a weak-link scaling relationship and good analytic predictions can be made using only known material properties. Overall, these results show that the effective cross-slip barrier in a random alloy is significantly lower than estimates based on average elastic and stacking fault properties of the alloy.

Origin of high strength in the CoCrFeNiPd high-entropy alloy

“…[13,14] on CoCrFeNiPd and CoCrNi reported significant cross-slip observed in TEM in both alloys but only CoCrFeNiPd has nonrandom concentration fluctuations. Recent theoretical work [34] suggests that the barriers for cross-slip can be much lower in HEAs as compared to elemental metals with the same average stacking fault energy due to the energy fluctuations associated with random concentration fluctuations. Thus, enhanced cross-slip does not appear to be significantly altered by concentration fluctuations.…”

Design using randomness: a new dimension for metallurgy

“…They observed an essentially-spontaneous cross-slip event at 300 K [9] even though the A-atom stacking fault energy is very low (γ sf = 14.8 mJ m −2 ) and the cross-slip barrier very high 4.6-4.9 eV. We have computed the distribution of cross-slip energy barriers in this same random alloy using atomistic minimum energy path calculations (the modified String method; see details in [7,8]). Briefly, we consider a straight dislocation in a tubular domain of length 2ζ = 130b and radius 15 √ 3a that is sufficient to capture the transition state and the overall energetics with good accuracy, see Fig.…”

“…A dislocation encountering these random low-barrier environments will have a very high cross-slip rate -almost spontaneous -in spite of the fact that the barriers elsewhere are much higher. Once cross-slip is initiated, it can spread along the entire length, particularly if assisted by a (small) Schmidt stress on the cross-slip plane [8]. These findings are consistent with the observation of spontaneously cross-slip configurations in the simulations of Rao et al In spite of the immense and essentially insurmountable average cross-slip barrier in this alloy, cross-slip can occur with nearly zero energy barrier due to favorable local fluctuations of the solutes on the cross-slip plane relative to the initial glide plane.…”

Influence of the stress state on the cross-slip free energy barrier in Al: An atomistic investigation

“…These atomistic strategies have been used to determine the influence of different chemical species, such as hydrogen [23] or solid solution Al atoms in Ni [24], on the energy barrier for cross-slip. More recently, an atomistically based model has been proposed to compute the energy barrier for cross-slip as a function of the type and volume fraction of solute atoms [25,26]. Additionally, Chen et al [27] studied the effect of vacancy clusters on the cross-slip activation energy in pure Ni, employing the free-end nudged elastic band methodology.…”