High or medium entropy alloys (HEAs/MEAs) are multiple principal element alloys with the same atomic element composition, some of which have shown record-breaking mechanical properties. However, the link between short-term order (SRO) and the special mechanical properties of these alloys remains elusive. It has been predicted that the local failure of SRO caused by dislocation slip will lead to a recovery state with increased entropy and free energy, creating softer regions within the boundaries of matrix and plane faults, thereby enhancing ductility, but this has not been verified.
在此，来自美国加州伯克利劳伦斯伯克利国家实验室&宾夕法尼亚州立大学的Yang Yang和加州大学伯克利分校的Andrew M. Minor等研究者，将原位纳米力学测试与能量过滤四维扫描透射电子显微镜(4D-STEM)相结合，直接观察室温下单晶CrCoNi在循环力学加载过程中的再生。 相关论文以题为“Rejuvenation as the origin of planar defects in the CrCoNi medium entropy alloy”发表在Nature Communications上。
Paper Links:
https://www.nature.com/articles/s41467-024-45696-z
In crystalline materials, plastic deformation occurs through structural changes called defects, such as dislocations, SFs (slip zones), and TBs (twin boundaries). The reversibility of these defects usually depends on the defect formation energy. When the defect formation energy is high and positive, the system tends to remove the defect after the external driving force is released, thus minimizing its total free energy. When the defect formation energy is relatively small with a positive or even negative value, the defect tends to retain its original shape after the release of the external driving force, resulting in irreversible changes in the material.
Defect formation can have an important impact on the deformation mechanism and mechanical properties of materials. With conventional alloys, where there are only one or two major elements and a few trace elements, defect formation energy is usually fixed when the temperature is fixed. Recent studies on high-entropy alloys (HEAs)/medium-entropy alloys (MEAs) have broadened this view, with theoretical studies showing that short-range ordered (SRO) can modulate SF energy in the same material to vary over a wide range from negative to positive, stabilizing nanoscale heterostructures through the coexistence of multiple dominant deformation mechanisms dependent on local SF energy. The presence of SRO in HEAs/MEAs reduces the entropy and free energy of the system, and the increase in SF energy is confirmed by experimental observations. However, how this interaction of SROs with crystal defects in HEAs/MEAs leads to their superior mechanical properties remains a fundamental question.
Paradoxically, as the SRO decreases, the system will reach a higher energy state. This state has a lower SF energy and is more malleable in nature. Therefore, the destruction of SROs in HEAs/MEAs can be seen as a form of renewal, which has been widely discussed as a beneficial structural transition in disordered materials such as bulk metallic glasses (BMGs). When the BMG is renewed, the structure exhibits a lower order and a higher energy state than its relaxed state, resulting in enhanced ductility and toughness. In the CrCoNi MEA system, Li et al. predicted that using molecular dynamics (MD) simulations, only three Burgers vector dislocation slips would be required to completely disrupt SRO. Therefore, it is expected that the collective movement of dislocations can randomize the atomic arrangement and update the system, increasing the tendency of HEAs/MEAs to have long and stable SFs due to the low SF energy. Although a great deal of effort has been made to understand SRO in HEAs/MEAs through advanced characterization techniques, there is still a lack of experimental evidence to show how updates in HEAs/MEAs are linked to a series of deformation mechanisms that produce long and thin planar defects characteristic of HEAs/MEAs.
To address this critical knowledge gap, there is an urgent need to probe the evolution of SFs and TBs in HEAs/MEAs in situ under controlled strain conditions, combined with high spatial resolution to resolve individual defects and a wide field of view for accurate statistical analysis. However, previous techniques such as high-resolution transmission electron microscopy (TEM), TEM darkfield imaging, or electron channel contrast imaging (ECCI) have encountered various challenges, such as limited field of view, high radiation damage rates, low spatial resolution, or inability to map strain distributions or distinguish between different types of planar defects.
Here, the researchers combined in-situ nanomechanical testing with energy-filtering four-dimensional scanning transmission electron microscopy (4D-STEM) to directly observe the regeneration of single crystal CrCoNi during cyclic mechanical loading at room temperature. Surprisingly, stacking faults (SFs) and twin boundaries (TBs) were reversible in the initial cycle, but became irreversible after 1000 cycles, indicating that stacking faults are reduced and regenerated in energy. Molecular dynamics (MD) simulations further revealed that the local collapse of SRO in MEA triggered these SF reversibility changes. As a result, the deformation characteristics in HEAs/MEAs are still planar and highly confined to the regenerated surface, and these alloys have superior damage tolerance characteristics.
Fig.1 The evolution of lamination faults (SFs) and twin grain boundaries (TBs) during mechanical deformation was directly observed by combining in-situ energy-filtered four-dimensional scanning transmission electron microscopy (4D-STEM) with nanomechanical testing.
Fig.2 Evolution of SFs and TBs in CrCoNi MEA and pure Ni over 1000 cyclic loadings. Time (t) is defined in (q).
Fig.3 Quantitative analysis of CrCoNi MEA rebound during cyclic mechanical deformation.
Fig. 4 MD simulations reveal the evolution of SRO and SFs in isoatomic CrCoNi MEA during cyclic loading.
Figure 5 Schematic diagram showing the source of nanostructures with superior damage tolerances for MEAs and HEAs during deformation.
In summary, by integrating in-situ energy filtration 4D-STEM, nanomechanical testing, and MD modeling, our study reveals and elucidates the reversible to irreversible transformation of SF dynamics in CrCoNi MEA during cyclic deformation. This phenomenon is mainly due to the failure of SRO along the slip surface, which leads to the formation of local anti-aging and composite systems. The system includes the "soft" zone where the SRO is reduced and the "hard" zone where the SRO is maintained. Interestingly, it is SRO that harmonizes the sequence of deformation mechanisms and promotes the synergy of multiple mechanisms, thereby improving the mechanical properties of CrCoNi MEA. Identifying strategies to optimize the extent and distribution of SROs in MEAs/HEAs may be key to coordinating the local regeneration process and thereby improving their mechanical properties. (Text: Aquatic)
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