Mg grain boundary (GB) segregation and GB diffusion can impact the processing and properties of Al-Mg alloys. Yet, Mg GB diffusion in Al has not been measured experimentally or predicted by simulations. We apply atomistic computer simulations to predict the amount and the free energy of Mg GB segregation, and the impact of segregation on GB diffusion of both alloy components. At low temperatures, Mg atoms segregated to a tilt GB form clusters with highly anisotropic shapes. Mg diffuses in Al GBs slower than Al itself, and both components diffuse slowly in comparison with Al GB self-diffusion. Thus, Mg segregation significantly reduces the rate of mass transport along GBs in Al-Mg alloys. The reduced atomic mobility can be responsible for the improved stability of the microstructure at elevated temperatures.
Solute segregation plays an important role in formation of long-period stacking ordered (LPSO) structure in Mg-M-RE (M: Zn, Ni etc., RE: Y, Gd, etc.) alloy systems. In this work, the planar segregation in Mg-Al-Gd alloy is characterized by high angle annular dark field (HAADF) scanning transmission electron microscopy (STEM) and three-dimensional atom probe (3DAP). It is found there is no planar fault accompanying the segregation, and the spatial distribution of segregation may resemble the periodicity of LPSO structure. The segregation is further quantified by 3DAP, and it mainly enriches wit…
Bimetallic alloys have drawn extensive attentions in materials science due to their widespread applications in electronics, engineering and catalysis. A very fundamental question of alloy is its surface segregation phenomenon. Many recent observations have shown that reactive gases or supports may have strong effects on alloy segregation. However, segregation in water, the most common solvent and environment, has not received enough attention. In this paper we give the quantitative descriptions on the surface segregation energies of 23 transition-metal impurities in Cu hosts under the conditio…
We present a first-principles atomistic thermodynamics framework to describe the structure, composition and segregation profile of an alloy surface in contact with a (reactive) environment. The method is illustrated with the application to a Ag3Pd(111) surface in an oxygen atmosphere, and we analyze trends in segregation, adsorption and surface free energies. We observe a wide range of oxygen adsorption energies on the various alloy surface configurations, including binding that is stronger than on a Pd(111) surface and weaker than that on a Ag(111) surface. This and the consideration of even …
In magnesium alloys with multiple substitutional elements, solute segregation at grain boundaries (GBs) has a strong impact on many important material characteristics, such as GB energy and mobility, and therefore, texture. Although it is well established that GB segregation is inhomogeneous, the variation of GB solute composition for random boundaries is still not understood. In the current study, atomic-scale experimental and simulation techniques were used to investigate the compositional inhomogeneity of six different GBs. Three-dimensional atom probe tomography results revealed that GB so…
Microalloying elements tend to segregate to the matrix-precipitate phase boundaries to reduce the interfacial energy. The segregation mechanism is emerging as a novel design strategy for developing precipitation-hardened alloys with significantly improved coarsening resistance for high temperature applications. In this paper, we report a nanoscopic diffuse-interface thermodynamic model that describes multicomponent segregation behavior in two-phase substitutional alloys. Following classical approaches for grain boundaries, we employ the regular solution thermodynamics to establish segregation …
This paper studies how solute segregation and its relationship to grain boundary energy in binary alloys is captured in the phase field crystal (PFC) formalism, a continuum method that incorporates atomic scale elasto-plastic effects on diffusional time scales. Grain boundaries are simulated using two binary alloy PFC models --- the original binary model by Elder et al (2007) and the XPFC model by Greenwood et al (2011). In both cases, grain boundary energy versus misorientation data is shown to be well described by Read-Shockley theory. The Gibbs Adsorption Theorem is then used to derive a se…
Grain boundary segregations were investigated by Atom Probe Tomography in an Al-Mg alloy, a carbon steel and Armco\trademark Fe processed by severe plastic deformation (SPD). In the non-deformed state, the GBs of the aluminium alloy are Mg depleted, but after SPD some local enrichment up to 20 at.% was detected. In the Fe-based alloys, large carbon concentrations were also exhibited along GBs after SPD. These experimental observations are attributed to the specific structure of GBs often described as "non-equilibrum" in ultra fine grained materials processed by SPD. The grain boundary segregat…
Atomistic simulations were carried out to analyze the interaction between $< a>$ basal dislocations and precipitates in Mg-Al alloys and the associated strengthening mechanisms.
Grain boundary engineering via dopant segregation can dramatically change the properties of a material. For metallic systems, most current studies concerning interfacial segregation and subsequent transitions of grain boundary structure are limited to binary alloys, yet many important alloy systems contain more than one type of dopant. In this work, hybrid Monte Carlo/molecular dynamics simulations are performed to investigate the behavior of dopants at interfaces in two model ternary alloy systems: Cu-Zr-Ag and Al-Zr-Cu. Trends in boundary segregation are studied, as well as the propensity fo…
While it is known that alloy components can segregate to grain boundaries (GBs), and that the atomic mobility in GBs greatly exceeds the atomic mobility in the lattice, little is known about the effect of GB segregation on GB diffusion. Atomistic computer simulations offer a means of gaining insights into the segregation-diffusion relationship by computing the GB diffusion coefficients of the alloy components as a function of their segregated amounts. In such simulations, thermodynamically equilibrium GB segregation is prepared by a semi-grand canonical Monte Carlo method, followed by calculat…
The segregation behavior of Cr in dilute Fe-Cr alloys is known to be anomalous since the main barrier for surface segregation of Cr in these alloys arises not from the topmost surface layer but from the subsurface layer where the solution energy of Cr is much more endothermic as compared to the topmost surface layer. The Fe-Cr alloys are candidate structural materials for the new generation of nuclear reactors. The surfaces of these alloys will be exposed to hydrogen or its isotopes in these reactors, and although hydrogen is soluble neither in Fe nor in Fe-Cr alloys, it is known that the adso…
X-ray reflectivity studies reveal atomic-level surface-segregation at the free surface of the eutectic Au71Sn29 liquid alloy. The surface-segregation extends up to three layers, in which the top layer is almost a pure monolayer of Sn, the second layer is almost a pure monolayer of Au and the third layer appears to be slightly enhanced in Au. Although the surface-segregation concentration profiles can be qualitatively accounted for by the theories of Defay-Prigogine and Strohl-King, they cannot satisfactorily account for the measured surface tension. PACS numbers: 61.20.-p, 61.10.-i, 68.03.-g
The dynamic behaviour of thermally driven segregation of Cr to the surface of WCrY smart alloy is studied with low energy ion scattering (LEIS). Sputtering the WCrY sample with 500 eV D$_2^+$ ions at room temperature results in preferential removal of the lighter alloy constituents and causes an almost pure W surface layer. At elevated temperatures above 700 K the segregation of Cr atoms towards the surface sets in and prevents the formation of a pure W layer. The simultaneous heating and sputtering of the sample leads to a surface state which reflects the balance between sputter removal and s…
An age-hardenable Al-Cu-Mg alloy (A2024) was processed by high-pressure torsion (HPT) for producing an ultrafine-grained structure. The alloy was further aged for extra strengthening. The tensile strength then reached a value as high as ~1 GPa. The microstructures were analyzed by transmission electron microscopy and atom probe tomography. The mechanism for the high strength was clarified in terms of solid-solution hardening, cluster hardening, work hardening, dispersion hardening and grain boundary hardening. It is shown that the segregation of solute atoms at grain boundaries including subgr…
Based on ab-initio simulations, we report on the nature of principally different mechanisms for interaction of Mg and Zn atoms with grain boundaries in Al alloys leading to different morphology of segregation. The Mg atoms segregate in relatively wide GB region with heterogeneous agglomerations due to the deformation mechanism of solute-GB interaction. In contrast, in the case of Zn atoms an electronic mechanism associated with the formation of directional bonding is dominating in the solute-GB interaction. As a result, for Zn atoms it is energetically beneficial to occupy interstitial positio…
The segregation behavior of the bimetallic alloys PtPd and CoCr in the case of bare surfaces and in the presence of an oxygen ad-layer has been studied by means of first-principles modeling based on density-functional theory (DFT). For both systems, change of the d-band filling due to charge transfer between the alloy components, resulting in a shift of the d-band center of surface atoms compared to the pure components, drives the surface segregation and governs the chemical reactivity of the bimetals. In contrast to previous findings but consistent with analogous PtNi alloy systems, enrichmen…
Bimetallic catalysts can undergo segregation or redistribution of the metals driven by oxidizing and reducing environments. Bragg coherent diffraction imaging (BCDI) was used to relate displacement fields to compositional distributions in crystalline Pt-Rh alloy nanoparticles. 3D images of internal composition showed that the radial distribution of compositions reverses partially between the surface shell and the core when gas flow changes between O2 and H2. Our observation suggests that the elemental segregation of nanoparticle catalysts should be highly active during heterogeneous catalysis …
Segregation of impurities to grain boundaries plays an important role in both the stability and macroscopic behavior of polycrystalline materials. The research objective in this work is to better characterize the energetics and length scales involved with the process of solute and impurity segregation to grain boundaries. Molecular dynamics simulations are used to calculate the segregation energies for carbon within multiple grain boundary sites over a database of 125 symmetric tilt grain boundaries in Fe. The simulation results show that the majority of atomic sites near the grain boundary ha…
Refractory multi-principal element alloys exhibiting promising mechanical properties such as excellent strength retention at elevated temperatures have been attracting increasing attention. Although their inherent chemical complexity is considered a defining feature, a challenge arises in predicting local chemical ordering, particularly in grain boundary regions with enhanced structural disorder. In this study, we use atomistic simulations of a large group of bicrystal models to sample a wide variety of interfacial sites (grain boundary) in NbMoTaW and explore emergent trends in interfacial se…
The occurrence of segregation in highly dilute alloys under irradiation is an unusual phenomenon that has so far eluded theoretical explanation. Using ab initio calculations, we are able to explain the origin of radiation-induced rhenium segregation in dilute tungsten-rhenium alloys.
The energy of segregation of Cr from the bulk to the topmost surface layer in dilute Fe-Cr alloys is endothermic but small. On the other hand, the segregation energy from the bulk to the subsurface layer is not only endothermic but relatively large. Thus, the subsurface layer presents a huge barrier, not only for the segregation of Cr from the bulk to the surface, but also for its diffusion from the surface into the bulk. This means that the topmost layer alone cannot be used to determine the barrier for Cr segregation (the segregation energy), which is determined in these alloys by the subsur…
We study order transitions and defect formation in a model high-entropy alloy (CuNiCoFe) under ion irradiation by means of molecular dynamics simulations. Using a hybrid Monte-Carlo/molecular dynamics scheme a model alloy is generated which is thermodynamically stabilized by configurational entropy at elevated temperatures, but partly decomposes at lower temperatures by copper precipation. Both the high-entropy and the multiphase sample are then subjected to simulated particle irradiation. The damage accumulation is analyzed and compared to an elemental Ni reference system. The results reveal …
Grain boundaries (GBs) can critically influence the microstructural evolution and various materials properties. However, a fundamental understanding of GBs in high-entropy alloys (HEAs) is lacking because of the complex couplings of the segregations of multiple elements and interfacial disordering, which can generate new phenomena and challenge the classical theories. Here, by combining large-scale atomistic simulations and machine learning, we demonstrate the feasibility of predicting the GB properties as functions of four independent compositional degrees of freedoms and temperature in a 5D …
The control of solute segregation at grain boundaries (GBs) is essential in engineering alloy properties, however the structure-activity relationship of the key parameter-the segregation energies-still remains elusive. Here we propose the electronic and geometric descriptors of GB segregation based on the valence, electronegativity and size of solutes and the non-local coordination number of free surfaces, with which we build a predictive framework to determine the segregation energies across different solutes, matrices, GB structures and segregation sites. This framework uncovers not only the…
Solute clusters affect the physical properties of alloys. Knowledge of the atomic structure of solute clusters is a prerequisite for material optimisation. In this study, solute clusters in a rapid-hardening Al-Cu-Mg alloy were characterised by a combination of atom probe tomography and density functional theory, making use of a hybrid data type that combines lattice rectification and data completion to directly input experimental data into atomistic simulations. The clusters input to the atomistic simulations are thus observed experimentally, reducing the number of possible configurations. Ou…
Grain size engineering, particularly reducing grain size into the nanocrystalline regime, offers a promising pathway to further improve the strength-to-weight ratio of Al alloys. Unfortunately, the fabrication of nanocrystalline metals often requires non-equilibrium processing routes, which typically limit the specimen size and require large energy budgets. In this study, multiple dopant atoms in ternary Al alloys are deliberately selected to enable segregation to the grain boundary region and promote the formation of amorphous complexions. Three different fully dense bulk nanocrystalline Al a…
Compositionally complex alloys or concentrated solid solutions are the latest frontier in catalyst design, but mixing different elements in one catalyst may result in surface segregation. Atomistic simulations can predict segregation patterns, but standard approaches based on mean-field models, cluster expansion, or classical interatomic potentials are often limited for the description of multicomponent alloys. We present machine learning potentials that can describe surface segregation with near DFT accuracy. The method is used to study a complex Co-Cu-Fe-Mo-Ni quinary alloy. For this alloy, …
In this work, a multiscale approach based on phase field was developed to simulate the microstructure's evolution under irradiation in binary systems, from atomic to microstructural scale. For that purpose, an efficient numerical scheme was developed. In the case of AgCu alloy under Krypton ions irradiation, phenomenological parameters were computed using atomistic methods, as a function of the temperature and the irradiation flux. As a result, we predicted the influence of the irradiation flux and the temperature on the formation of patterned microstructures. In the case of AgCu, our model wa…
In this paper we report phonon dynamics in chemically synthesized Zn1- xMgxO (0\leqx\leq0.07) and Zn1-yCdyO (0\leqy\leq0.03) alloy nanostructures of sizes ~10 nm using non-resonant Raman and Fourier Transformed Infrared Spectroscopy (FTIR). Substitution by Mg makes the unit cell compact while Cd substitution leads to unit cell expansion. On alloying, both A1(LO) and E1(LO) mode of wurtzite ZnO show blue shift for Zn1-xMgxO and red shift for Zn1-yCdyO alloy nanostructures due to mass defect and volume change induced by the impurity atoms. Significant shift has been observed in E1(LO) mode for Z…
Grain boundaries have been shown to dramatically influence the behavior of relatively simple materials such as monatomic metals and binary alloys. The increased chemical complexity associated with multi-principal element alloys is hypothesized to lead to new grain boundary phenomena. To explore the relationship between grain boundary structure and chemistry in these materials, hybrid molecular dynamics/Monte Carlo simulations of a faceted Σ11 <110> tilt boundary, chosen to sample both high- and low-energy boundary configurations, are performed in face-centered cubic CrFeCoNiCu and CrFeCoNi equ…
A scale bridging strategy based in molecular statics and molecular dynamics simulations in combination with transition state theory has been developed to determine the flow stress of Al-Cu alloy containing Guinier-Preston zones. The athermal contribution to the flow stress was determined from the Taylor model, while the thermal contribution was obtained from the obstacle strength and the free energy barrier. These two magnitudes were obtained by means of molecular statics and molecular dynamics simulations of the interaction of edge dislocations with Guinier-Preston zones in two different orie…
We have applied extended X-ray absorption fine structure (EXAFS) analyses and ab-initio molecular dynamics (AIMD) simulations to study the atomic structure of Ag-doped (up to 42%) Ge1Sb2Te4 alloys. Analysis of the models that are consistent with the EXAFS experiment reveals that the Ge environment is significantly modified by Ag doping whereas those of Sb and Te are barely affected (except for high Ag concentrations), and suggests that Ag prefers bonding with Te to Ge or Sb. Doping with Ag promotes the conversion of tetrahedral Ge sites to octahedral Ge sites and enhances the speed of crystall…
This paper compares two approaches for investigating the near-surface composition profile that results from surface segregation in the so-called Cantor alloy, an equi-molar alloy of CoCrFeMnNi. One approach consists of atomistic computer simulations by a combination of Monte Carlo, molecular dynamics and molecular statics techniques, and the other is a nearest neighbor analytical calculation performed in the regular solution approximation with a multilayer model, developed here for the first time for a N-component system and tested for the 5-component Cantor alloy. This type of comparison is u…
θ' (Al2Cu) precipitates in Al-Cu-Ag alloys were examined using high angle annular dark field scanning transmission electron microscopy (HAADF-STEM). The precipitates nucleated on dislocation loops on which assemblies of γ' (AlAg2) precipitates were present. These dislocation loops were enriched in silver prior to θ' precipitation. Coherent, planar interfaces between the aluminium matrix and θ' precipitates were decorated by a layer of silver of two atomic layers in thickness. It is proposed that this layer lowers the chemical component of the Al-θ' interfacial energy. The lateral growth of the…
Due to their high strength and advantageous high-temperature properties, tungsten-based alloys are being considered as plasma-facing candidate materials in fusion devices. Under neutron irradiation, rhenium, which is produced by nuclear transmutation, has been found to precipitate in elongated precipitates forming thermodynamic intermetallic phases at concentrations well below the solubility limit. Recent measurements have shown that Re precipitation can lead to substantial hardening, which may have a detrimental effect on the fracture toughness of W alloys. This puzzle of sub-solubility preci…
We predict general trends for surface segregation in a binary metal cluster based on the difference between the atomic properties of the constituent elements. Considering the attractive and repulsive contributions of the cohesive energy of an atom in a cluster, energetically most favorable sites for a guest atom on a pure metal cluster is determined. It is predicted that for adjacent elements in a row of the periodic table, the bimetallic system would be more stable if the component with smallest valence electron density is placed on the surface. On the contrary, for well separated components …
