Understanding the composition evolution of grain boundaries and grain boundary precipitation at near-atomic scale in aluminum alloys is crucial to tailor mechanical properties and to increase ...resistance to corrosion and stress corrosion cracking. Here, we elucidate the sequence of precipitation on grain boundaries in comparison to the bulk in a model Al-Zn-Mg-Cu alloy. We investigate the material from the solution heat treated state (475 °C), through the very early stages of aging to the peak aged state at 120 °C and further into the overaged regime at 180 °C. The process starts with solute enrichment on grain boundaries due to equilibrium segregation accompanied by solute depletion in their vicinity, the formation of Guinier–Preston (GP) zones in the solute-enriched grain boundary regions, and GP zones growth and transformation. The equilibrium segregation of solutes to grain boundaries during aging accelerates this sequence compared to the bulk. Analysis of the ∼10 nm wide precipitate-free zones (PFZs) adjacent to the solute-enriched grain boundaries shows that the depletion zones are determined by (i) interface equilibrium segregation; (ii) formation and coarsening of the grain boundary precipitates and (iii) the diffusion range of solutes in the matrix. In addition, we quantify the difference in kinetics between grain boundary and bulk precipitation. The precipitation kinetics, as observed in terms of volume fraction, average radius, and number density, is almost identical next to the depletion zone in the bulk and far inside the bulk grain remote from any grain boundary influence. This observation shows that the region influenced by the grain boundaries does not extend beyond the PFZs.
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Severe lattice distortion is a core effect in the design of multiprincipal element alloys with the aim to enhance yield strength, a key indicator in structural engineering. Yet, the yield strength ...values of medium‐ and high‐entropy alloys investigated so far do not substantially exceed those of conventional alloys owing to the insufficient utilization of lattice distortion. Here it is shown that a simple VCoNi equiatomic medium‐entropy alloy exhibits a near 1 GPa yield strength and good ductility, outperforming conventional solid‐solution alloys. It is demonstrated that a wide fluctuation of the atomic bond distances in such alloys, i.e., severe lattice distortion, improves both yield stress and its sensitivity to grain size. In addition, the dislocation‐mediated plasticity effectively enhances the strength–ductility relationship by generating nanosized dislocation substructures due to massive pinning. The results demonstrate that severe lattice distortion is a key property for identifying extra‐strong materials for structural engineering applications.
Ultrastrong VCoNi equiatomic medium‐entropy alloys are developed via severe lattice distortion. The distortion enhances the lattice friction stress and also increases the sensitivity of the yield stress on grain size changes. These design ideas lead to a new alloy with near 1 GPa yield strength despite its precipitation‐free and fully recrystallized fcc structure.
Machine learning–enabled high-entropy alloy discovery Rao, Ziyuan; Tung, Po-Yen; Xie, Ruiwen ...
Science (American Association for the Advancement of Science),
10/2022, Letnik:
378, Številka:
6615
Journal Article
Recenzirano
Odprti dostop
High-entropy alloys are solid solutions of multiple principal elements that are capable of reaching composition and property regimes inaccessible for dilute materials. Discovering those with valuable ...properties, however, too often relies on serendipity, because thermodynamic alloy design rules alone often fail in high-dimensional composition spaces. We propose an active learning strategy to accelerate the design of high-entropy Invar alloys in a practically infinite compositional space based on very sparse data. Our approach works as a closed-loop, integrating machine learning with density-functional theory, thermodynamic calculations, and experiments. After processing and characterizing 17 new alloys out of millions of possible compositions, we identified two high-entropy Invar alloys with extremely low thermal expansion coefficients around 2 × 10
−6
per degree kelvin at 300 kelvin. We believe this to be a suitable pathway for the fast and automated discovery of high-entropy alloys with optimal thermal, magnetic, and electrical properties.
A little expansive
Invar alloys have extremely low thermal expansion, making them attractive for several types of applications. Finding these types of alloys in a complex compositional space, however, is challenging. Rao
et al
. used an iterative scheme that combines machine learning, density functional theory, experiments, and thermodynamic calculation to find two new invar alloys out of millions of candidates (see the Perspective by Hu and Yang). The alloys are both compositionally complex, high entropy materials, thus demonstrating the power of this approach for materials discovery. —BG
Two high-entropy alloys with extremely low thermal expansion were found with the help of machine learning.
Elemental segregation to grain boundaries (GBs) can induce structural and chemical transitions at GBs along with significant changes in material properties. The presence of multiple principal ...elements interacting in high-entropy alloys (HEAs) makes the GB segregation and interfacial phase transformation a rather challenging subject to investigate. Here, we explored the temporal evolution of the chemistry for general high-angle GBs in a typical equiatomic FeMnNiCoCr HEA during aging heat treatment through detailed atom probe tomography (APT) analysis. We found that the five principal elements segregate heterogeneously at the GBs. More specifically, Ni and Mn co-segregate to some regions of the GBs along with the depletion of Fe, Co and Cr, while Cr is enriched in other regions of the GBs where Ni and Mn are depleted. The redistribution of these elements on the GBs follow a periodic characteristic, spinodal-like compositional modulation. The accumulation of elements at the GBs can create local compositions by shifting their state from a solid solution (like in the adjacent bulk region) into a spinodal regime to promote interfacial phase-like transitions as segregation proceeds. These results not only shed light on phase precursor states and the associated nucleation mechanism at GBs in alloy systems with multiple principal elements but also help to guide the microstructure design of advanced HEAs in which formation of embrittling phases at interfaces must be avoided.
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There are still debates regarding the mechanisms that lead to hot cracking in parts build by additive manufacturing (AM) of non-weldable nickel-based superalloys. This lack of in-depth understanding ...of the root causes of hot cracking is an impediment to designing engineering parts for safety-critical applications. Here, we deploy a near-atomic-scale approach to investigate the details of the compositional decoration of grain boundaries in the coarse-grained, columnar microstructure in parts built from a non-weldable nickel-based superalloy by selective electron-beam melting. The progressive enrichment in Cr, Mo and B at grain boundaries over the course of the AM-typical successive solidification and remelting events, accompanied by solid-state diffusion, causes grain boundary segregation induced liquation. This observation is consistent with thermodynamic calculations. We demonstrate that by adjusting build parameters to obtain a fine-grained equiaxed or a columnar microstructure with grain width smaller than 100 μm enables to avoid cracking, despite strong grain boundary segregation. We find that the spread of critical solutes to a higher total interfacial area, combined with lower thermal stresses, helps to suppress interfacial liquation.
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Since its first emergence in 2004, the high‐entropy alloy (HEA) concept has aimed at stabilizing single‐ or dual‐phase multi‐element solid solutions through high mixing entropy. Here, this strategy ...is changed and renders such massive solid solutions metastable, to trigger spinodal decomposition for improving the alloys’ magnetic properties. The motivation for starting from a HEA for this approach is to provide the chemical degrees of freedom required to tailor spinodal behavior using multiple components. The key idea is to form Fe‐Co enriched regions which have an expanded volume (relative to unconstrained Fe‐Co), due to coherency constraints imposed by the surrounding HEA matrix. As demonstrated by theory and experiments, this leads to improved magnetic properties of the decomposed alloy relative to the original solid solution matrix. In a prototype magnetic FeCoNiMnCu HEA, it is shown that the modulated structures, achieved by spinodal decomposition, lead to an increase of the Curie temperature by 48% and a simultaneous increase of magnetization by 70% at ambient temperature as compared to the homogenized single‐phase reference alloy. The findings thus open a pathway for the development of advanced functional HEAs.
A novel strategy for improving magnetic properties of high‐entropy alloys (HEAs) is demonstrated in a prototype magnetic FeCoNiMnCu HEA by triggering bulk spinodal decomposition based on both theory and experiments. The key idea is to form Fe‐Co enriched regions which have an expanded volume (relative to unconstrained Fe‐Co), due to coherency constraints imposed by the surrounding HEA matrix.
In the current work, Mn enrichment at dislocations in Fe–Mn alloys due to segregation and spinodal decomposition along the dislocation line is studied via modeling and experimental characterization. ...To model these phenomena, both finite-deformation microscopic phase-field chemomechanics (MPFCM) and Monte Carlo molecular dynamics (MCMD) are employed. MPFCM calibration is carried out with the same Fe–Mn MEAM-based potential used in MCMD, as well as CALPHAD data. Simulation results for Mn segregation to, and spinodal decomposition along, straight screw and edge dislocations as well as dislocation loops, are compared with characterization results from atom probe tomography (APT) for two Fe–Mn alloy compositions. In contrast to classical Volterra (linear elastic) dislocation theory, both MPFCM and MCMD predict a non-zero hydrostatic stress field in screw cores. Being of much smaller magnitude than the hydrostatic stress in straight edge cores, much less solute segregates to screw than to edge cores. In addition, the segregated amount in screw cores is below the critical concentration of 0.157 for the onset of spinodal decomposition along the line. On the other hand, results from MPFCM-based modeling imply that the concentration dependence of the solute misfit distortion and resulting dependence of the elastic energy density on concentration have the strongest effect. The maximum amount of Mn segregating to straight edge dislocations predicted by MPFCM agrees well with APT results. On the other hand, the current MPFCM model for Fe–Mn predicts little or no variation in Mn concentration along a straight dislocation line, in contrast to the APT results. As shown by the example of a dislocation loop in the current work, a change in the hydrostatic stress along the line due to changing character of dislocation does lead to a corresponding variation in Mn concentration. Such a variation in Mn concentration can also then be expected along a dislocation line with kinks or jogs.
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Abstract
Segregation to grain boundaries affects their cohesion, corrosion, and embrittlement and plays a critical role in heterogeneous nucleation. In order to quantitatively study segregation and ...low-dimensional phase separation at grain boundaries, here, we apply a density-based phase-field model. The current model describes the grain-boundary thermodynamic properties based on available bulk thermodynamic data, while the grain-boundary-density profile is obtained using atomistic simulations. To benchmark the performance of the model, Mn grain-boundary segregation in the Fe–Mn system is studied. 3D simulation results are compared against atom probe tomography measurements conducted for three alloy compositions. We show that a continuous increase in the alloy composition results in a discontinuous jump in the segregation isotherm. The jump corresponds to a spinodal phase separation at grain boundary. For alloy compositions above the jump, we reveal an interfacial transient spinodal phase separation. The transient spinodal phenomenon opens opportunities for knowledge-based microstructure design through the chemical manipulation of grain boundaries. The proposed density-based model provides a powerful tool to study thermodynamics and kinetics of segregation and phase changes at grain boundaries.
We introduce an efficient, automated computational approach for analyzing interfaces within atom probe tomography datasets, enabling quantitative mapping of their thickness, composition, as well as ...the Gibbsian interfacial excess of each solute. Detailed evaluation of an experimental dataset indicates that compared with the composition map, the interfacial excess map is more robust and exhibits a relatively higher resolution to reveal compositional variations. By field evaporation simulations with a predefined emitter mimicking the experimental dataset, the impact of trajectory aberrations on the measurement of the thickness, composition, and interfacial excess of the decorated interface are systematically analyzed and discussed.
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