Unveiling the mechanisms of motion of synchro-Shockley dislocations in Laves phases

Abstract

In Laves phases, synchroshear is the dominant basal slip mechanism. It is accomplished by the glide of synchro-Shockley dislocations. However, the atomic-scale mechanisms of motion of such zonal dislocations are still not well understood. In this work, using atomistic simulations, two 30°synchro-Shockley dislocations with different Burgers vectors and core structures and energies are identified. We demonstrate that nucleation and propagation of kink pairs is the energetically favorable mechanism for the motion of the synchro-Shockley dislocation (partial I). Vacancy hopping and interstitial shuffling are identified as two key mechanisms related to kink propagation and we investigated how vacancies and antisite defects assist kink nucleation and propagation, which is crucial for kink mobility. Additionally, we identified a mechanism of non-sequential atomic shuffling for the motion of the synchro-Shockley dislocation (partial II). These findings provide insights into the dependency on temperature and chemical composition of plastic deformation induced by zonal dislocations in Laves phases and the many related topologically close-packed phases.

Thermally activated nature of synchro-Shockley dislocations in Laves phases

“…In Laves phases, two synchro-Shockley partial dislocations (partial I and II) exist on the basal (or {111}) triple-layer with different core structures and energies, as well as distinct mechanisms of motion and corresponding activation energies. Our previous NEB calculations suggest that the motion of 30°synchro-Shockley dislocations is a multi-step reaction that resolves into multiple transition and intermediate states [13]. For partial I dislocation, we identified kink-pair nucleation and following kink propagation by vacancy-hopping and interstitial shuffling as the mechanisms of motion.…”

“…This triple-kagomé slip was demonstrated as a competing mechanism to synchro-shear slip in the Laves crystal building block of µ phases in experiments and ab-initio calculations [8,26]. The triple-kagomé full slip dissociates into two mirror symmetric partial slip events in CaMg 2 , and a stacking fault state with a high fault energy 409 mJ/m 2 compared to the low fault energy of the synchro-shear induced stacking fault (14 mJ/m 2 [13]) was obtained, see Figure S5(a-b). In addition, the energy barrier of the triple and kagomé slip vanishes when the applied shear stress σ xz reaches 5.39 GPa (Figure S5(cd)).…”

“…The motion of the synchro-Shockley dislocation requires the cooperative motion of these two coupled Shockley partial dislocations. Recently, the authors reported the mechanisms of motion of synchro-Shockley dislocations in C14 CaMg 2 and C15 CaAl 2 using atomistic simulations [13]. Two 30°synchro-Shockley dislocations, partial I and II, with different Burgers vectors, core structures, and energies were identified.…”

“…The interatomic interaction was modeled by the modified embedded atom method potential by Kim et al [20] for Mg-Ca. The synchro-Shockley partial dislocations in C14 CaMg 2 were constructed following the method detailed in [13,21,22], and then relaxed using the conjugate gradient with box relaxation and the FIRE [23,24] algorithm. OVITO [25] was used to visualize and analyze the atomistic simulation results.…”

“…The shear stress value was derived from the applied shear strain and shear modulus of the potential (25.6 GPa). For more details of the simulation setup and methods, see Figure S1 in the Supplementary Material and [9,13].…”