Maximum strength and dislocation patterning in multi-principal element alloys

Abstract

Multi-principal element alloys (MPEAs) containing three or more components in high concentrations render a tunable chemical short-range order (SRO). Leveraging large-scale atomistic simulations, we probe the limit of Hall-Petch strengthening and deformation mechanisms in a model CrCoNi alloy and unravel chemical ordering effects. The presence of SRO appreciably increases the maximum strength and lowers the propensity for faulting and structure transformation, accompanied by intensification of planar slip and strain localization. Deformation grains exhibit notably different microstructures and dislocation patterns that prominently depend on their crystallographic orientation and the number of active slip planes. Grain of single-planar slip attains the highest volume fraction of deformation-induced structure transformation, and grain with double-slip planes develops the densest dislocation network. These results advancing the fundamental understanding of deformation mechanisms and dislocation patterning in MPEAs suggest a mechanistic strategy for tuning mechanical behavior through simultaneously tailoring grain texture and local chemical order.

Fig. 1.. Microstructures and Hall-Petch strengthening in CrCoNi alloys.

(A to D) Snapshots of the polycrystalline microstructure and the corresponding mapping of constituent elements from MC-MD annealing simulations. The detailed results of SRO analysis and element distributions in grains and grain boundaries are summarized in figs. S1 and S2. (E) Grain size dependence of plastic flow strength for CrCoNi alloys with RSS and SRO. The two panels correspond to two independent polycrystalline structures consisting of 27 and 8 grains, respectively.

Fig. 2.. Deformation microstructure and local plastic strain.

(A and B) Atomic structures and local strain for RSS and SRO samples stretched to 20% strain, respectively. Atoms are color-coded by structural type, where the red color represents the hcp-coordinated atoms and blue indicates the fcc structure. (C) Volume fraction of hcp-coordinated atoms as a function of tensile strain. (D) Probability distributions of local strain in RSS and SRO. The SRO system exhibits long tail and extreme local strain after deformation.

Fig. 3.. Deformation characteristics and dislocation pattern in grain of a single active slip plane.

(A and B) Deformation microstructure and local strain map of grain deformed at 20% strain, respectively. Thompson tetrahedron is shown to illustrate active slip plane ABC and view direction CB. The hcp-coordinated atoms are colored in red. (C) Magnification of the deformation microstructure in the circled region in (A). The hcp-coordinated atoms are identified as SF, hcp phase, and TB. (D) Volume fraction of deformation structures as a function of strain for the grain with single planar slip. (E) Dislocation configurations in the deformed grain. Smoothly curved Shockley partial dislocations (green) lie in the parallel ABC planes, and stair-rod (purple) and Hirth partial (red) dislocations are scattered.

Fig. 4.. Deformation characteristics and dislocation pattern in grain of double active slip planes.

(A and B) Deformation microstructure and local strain map of grain at 20% strain, respectively. Thompson tetrahedron is shown to illustrate active slip planes ABC and BCD. (C) Magnification of deformation microstructure in the dashed circle region in (A). The hcp-coordinated atoms are characterized as SF, hcp phase, and TB. (D) Volume fraction of deformation structures as a function of strain for the grain with double-planar slip. (E) Dense dislocation network in the deformed grain. Highly curved Shockley partial dislocations (green) lie in two intersecting slip planes, and dense stair-rod (purple) and Hirth partial (red) dislocations appear in the grain interior.

Fig. 5.. Deformation characteristics and dislocation pattern in grain of multiple active slip planes.

(A and B) Deformation microstructure and local strain map of grain at 20% strain, respectively. (C) Magnification of deformation microstructure in the dashed circle region in (A). The hcp-coordinated atoms are mainly SFs. (D) Volume fraction of deformation structures as a function of strain for the grain with multiplanar slip. (E) Dislocation network in the deformed grain shows some local regions with concentrated dislocations. Shockley partial dislocations (green) lie in multiple slip planes, with variously arranged stair-rod (purple) and Hirth partial (red) dislocations.