Tunable stacking fault energies by tailoring local chemical order in CrCoNi medium-entropy alloys

Abstract

High-entropy alloys (HEAs) are an intriguing new class of metallic materials due to their unique mechanical behavior. Achieving a detailed understanding of structure-property relationships in these materials has been challenged by the compositional disorder that underlies their unique mechanical behavior. Accordingly, in this work, we employ first-principles calculations to investigate the nature of local chemical order and establish its relationship to the intrinsic and extrinsic stacking fault energy (SFE) in CrCoNi medium-entropy solid-solution alloys, whose combination of strength, ductility, and toughness properties approaches the best on record. We find that the average intrinsic and extrinsic SFE are both highly tunable, with values ranging from -43 to 30 mJ⋅m^-2 and from -28 to 66 mJ⋅m^-2, respectively, as the degree of local chemical order increases. The state of local ordering also strongly correlates with the energy difference between the face-centered cubic (fcc) and hexagonal close-packed (hcp) phases, which affects the occurrence of transformation-induced plasticity. This theoretical study demonstrates that chemical short-range order is thermodynamically favored in HEAs and can be tuned to affect the mechanical behavior of these alloys. It thus addresses the pressing need to establish robust processing-structure-property relationships to guide the science-based design of new HEAs with targeted mechanical behavior.

Keywords: local chemical order; medium-entropy alloys; stacking fault energy; transformation-induced plasticity.

Fig. 1.

Evolvement of energy and local chemical ordering in CrCoNi alloys. The CrCoNi ternary alloys (18 independent samples) were studied by DFT-based MC simulations (Methods). (A) Potential energy changes with the steps of the MC simulation with only data for four representative samples, out of a total of 18, shown. Configurations at the specific energy states are denoted by four groups as CH_0, CH_1, CH_2, and CH_F. (B) Schematic illustration of the first three nearest-neighbor (NN) shells in an fcc crystal. (C) The nonproportional number of local Cr–Cr pairs, $\mathrm{\Delta }{\delta }_{Cr-Cr}^k$, in the first three NN shells are plotted versus the steps of MC simulation. (D) Averaged over 18 independent configurations at CH_F, $\mathrm{\Delta }{\delta }_{ij}^k$ for all pairs are plotted at the first, second, and third nearest-neighbor shells. The dashed lines represent the case of a random solid solution.

Fig. 2.

Calculated stacking fault energy of the random solid-solution CrCoNi alloys. (A–C) The side view images of atomic configurations in the original fcc structure, with ISF and ESF. The yellow shade indicates the stacking fault; the plane labels of ABC… represent the sequence of close-packed (111) planes. (D) Top view image showing the close-packed (111) plane. (E) The average SFE with the number of random solid-solution CrCoNi samples (containing 360 atoms each) for both as-assigned and replicated supercells (Methods). (Inset) The ${\gamma }_{isf}$ of as-assigned supercells of CrCoNi alloys.

Fig. 3.

Stacking fault energy correlates strongly with local chemical ordering. (A) Distribution of intrinsic stacking fault energy, ${\gamma }_{isf}$, for the CrCoNi alloys in four specific states, which span from random solid solution to the highest degree of chemical ordering extracted from the MC simulations. In total, 108 stacking faults were considered for analysis in each group. The average stacking fault energies, (B) ${\overline{\gamma }}_{isf}$ and (C) ${\overline{\gamma }}_{esf}$, among those four groups are plotted versus $\mathrm{\Delta }{\delta }_{sum}$ for the first, second, and third nearest-neighbor shells.

Fig. 4.

Energies of fcc and hcp structures in CrCoNi solid-solution alloys. The (A) side and (B) top view images of atomic configurations for the CrCoNi alloys in both fcc and hcp phases. (C) The relative energy difference (E − E_fcc,0) for both fcc and hcp phase at various degrees of local chemical ordering, where E_fcc,0 is the average potential energy of the fcc CrCoNi alloys with random configurational disorder. (Inset) The correlation between $\mathrm{\Delta }{\delta }_{Cr-Cr}^1$ and energy difference between the hcp and fcc phases, $\mathrm{\Delta }{E}^{fcc\to hcp}$. (D) The correlation between ${\overline{\gamma }}_{isf}$ and $\mathrm{\Delta }{E}^{fcc\to hcp}$. Dashed lines indicate where ${\overline{\gamma }}_{isf}=0$ or $\mathrm{\Delta }{E}^{fcc\to hcp}=0$.