Reasons for the superior mechanical properties of medium-entropy CrCoNi compared to high-entropy CrMnFeCoNi

Laplanche, Guillaume; Kostka, Aleksander; Reinhart, C.; Hunfeld, Julian; Eggeler, Gunther; George, E.P.

doi:10.1016/j.actamat.2017.02.036

Cited by 987 publications

(305 citation statements)

References 53 publications

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“…Because of the favorable composition, the SFE in these HEAs is low (29,30,32), thereby allowing a transition from deformation by full dislocations to partials with SFs and twinning (28,33). As the neutron diffraction data demonstrated, the critical strain for SF decreases nearly linearly with temperature.…”

Section: Discussionmentioning

confidence: 99%

Cooperative deformation in high-entropy alloys at ultralow temperatures

et al. 2020

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High-entropy alloys exhibit exceptional mechanical properties at cryogenic temperatures, due to the activation of twinning in addition to dislocation slip. The coexistence of multiple deformation pathways raises an important question regarding how individual deformation mechanisms compete or synergize during plastic deformation. Using in situ neutron diffraction, we demonstrate the interaction of a rich variety of deformation mechanisms in high-entropy alloys at 15 K, which began with dislocation slip, followed by stacking faults and twinning, before transitioning to inhomogeneous deformation by serrations. Quantitative analysis showed that the cooperation of these different deformation mechanisms led to extreme work hardening. The low stacking fault energy plus the stable face-centered cubic structure at ultralow temperatures, enabled by the high-entropy alloying, played a pivotal role bridging dislocation slip and serration. Insights from the in situ experiments point to the role of entropy in the design of structural materials with superior properties.

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Section: Discussionmentioning

confidence: 99%

Cooperative deformation in high-entropy alloys at ultralow temperatures

et al. 2020

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“…Additionally, the equiatomic, three-element (medium-entropy) CrCoNi alloy has been observed to display even better properties; at 77 K, this alloy displays tensile strengths of 1.4 GPa, tensile ductilities of ~90%, and fracture toughness values of 275 MPa√m, approaching the best damage-tolerance on record (7). A synergy of deformation mechanisms appears to be the basis of the exceptional mechanical behavior of these alloys (8)(9)(10)(11)(12)(13). For instance, in the stronger medium-entropy CrCoNi alloy, nanotwinning occurs at room temperature and leads to the formation of a hierarchical twin network, where the twin boundaries act as barriers to dislocation motion, providing for strength, whereas both full and partial dislocations can move rapidly along the boundaries themselves for ductility (11).…”

mentioning

confidence: 99%

“…Such unique mechanical behavior for the CrCoNi and CrMnFeCoNi alloys appears to be associated with their low stacking fault energies (SFE), as the SFE is one of the most significant parameters influencing plastic deformation, dislocation mobility, deformation twinning as well as occurrence of phase transformations in conventional crystalline alloys (14)(15)(16)) (see the SI Text for further discussions). For the CrCoNi alloy, experimental measurements of the separation of partial dislocations suggested values of the SFE of 22±4 mJ.m -2 (9); conversely, first-principles densityfunctional-theory (DFT) calculations have yielded negative values of the SFE for this alloy, e.g., -24 mJ.m -2 (11), -62 mJ.m -2 (17) or -40 mJ.m -2 (18), which are apparently at odds with the measured finite dislocation dissociation widths. Given the importance of the SFE in governing the nanoscale mechanisms active during plastic deformation, this discrepancy between measurement and computation clearly warrants further investigation.…”

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confidence: 99%

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

Ding

Asta

et al. 2018

Proc. Natl. Acad. Sci. U.S.A.

629

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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 and from -28 to 66 mJ⋅m, 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 () and hexagonal close-packed () 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.

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“…Here [24], -24 mJ/m 2 [27] and -43 mJ/m 2 [18]. We mention here that one should not directly compare the theoretical SFE with the experimental values, e.g., 18±4 mJ/m 2 [62] and 22±4 mJ/m 2 [63]. The latter ones are usually calculated based on the measured partial dislocation separations ( ) using the transmission electron microscopic (TEM) method according…”

Section: Stacking Fault Energymentioning

confidence: 99%

Designing Transformation-Induced Plasticity and Twinning-Induced Plasticity Cr-Co-Ni Medium Entropy Alloys: Theory and Experiment

Yang

Tian

et al. 2020

SSRN Journal

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In order to efficiently explore the nearly infinite composition space in multicomponent solid solution alloys, it is important to establish predictive design strategies and use computation-aided methods. In the present work, we demonstrated the density functional theory calculations informed design routes for realizing transformation-induced plasticity (TRIP) and twinning-induced plasticity (TWIP) in Cr-Co-Ni medium entropy alloys (MEAs). We systematically studied the effects of magnetism and chemical composition on the generalized stacking fault energy surface (-surface) and showed that both chemistry and the coupled magnetic state strongly affect the -surface, consequently, the primary deformation modes. Based on the calculated effective energy barriers for the competing deformation modes, we constructed composition and magnetism dependent deformation maps at both room and cryogenic temperatures. Accordingly, we proposed various design routes for achieving desired primary deformation modes in the ternary Cr-Co-Ni alloys. The deformation mechanisms predicted by our theoretical models are in nice agreement with available experimental observations in literature. Furthermore, we fabricated two non-equiatomic Cr-Co-Ni MEAs possessing the designed TWIP and TRIP effects, showing excellent combinations of tensile strength and ductility. Corresponding authors: a Song Lu, songlu@kth.se b Yanzhong Tian,

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Reasons for the superior mechanical properties of medium-entropy CrCoNi compared to high-entropy CrMnFeCoNi

Cited by 987 publications

References 53 publications

Cooperative deformation in high-entropy alloys at ultralow temperatures

Cooperative deformation in high-entropy alloys at ultralow temperatures

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

Designing Transformation-Induced Plasticity and Twinning-Induced Plasticity Cr-Co-Ni Medium Entropy Alloys: Theory and Experiment

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