Linear complexions are defect states that have been recently discovered along dislocations in body centered cubic Fe-based alloys. In this work, we use atomistic simulations to extend this concept and explore segregation-driven structural transitions at dislocations in face centered cubic alloys. We identify a variety of stable, nanoscale-size structural and chemical states, which are confined near dislocations and can be classified as linear complexions. Depending on the alloy system and thermodynamic conditions, such new states can preserve, partially modify, or completely replace the original defects they were born at. By considering different temperatures and compositions, we construct linear complexion diagrams that are similar to bulk phase diagrams, defining the important conditions for complexion formation while also specifying an expected complexion size and type. Several notable new complexion types were predicted here: (1) nanoparticle arrays comprised of L12 phases in Ni-Fe, Ni-Al, and Al-Zr, (2) replacement of stacking faults with layered complexions comprised of (111) planes from the Cu5Zr intermetallic phase in Cu-Zr, (3) platelet arrays comprised of two-dimensional Guinier-Preston zones in Al-Cu, and finally (4) coexistence of multiple linear complexions containing both Guinier-Preston zones and L12 phases in ternary Al-Cu-Zr. All of these new complexion states are expected to alter material properties and affect the stability of the dislocations themselves, offering a unique opportunity for future materials design.
Amorphous complexions in nanocrystalline metals have the potential to improve mechanical properties and radiation tolerance, as well as resistance to grain growth. In this study, the stability of amorphous complexions in binary and ternary Cu-based alloys is investigated by observing the effect of cooling rate from high temperature on the occurrence of amorphous-to-ordered complexion transitions. Bulk Cu-Zr and Cu-Zr-Hf alloy samples were annealed to induce boundary premelting and then quenched through a procedure that induces a gradient of local cooling rate through the sample height. Amorpho…
Linear complexions are defect phases that form in the presence of dislocations and thus are promising for the direct control of plasticity. In this study, atomistic simulations are used to model the effect of linear complexions on dislocation-based mechanisms for plasticity, demonstrating unique behaviors that differ from classical dislocation glide mechanisms. Linear complexions impart higher resistance to the initiation and continuation of dislocation motion when compared to solid solution strengthening in all of the face-centered cubic alloys investigated here, with the exact strengthening …
Building on the recent discovery of tough nanocrystalline Cu-Zr alloys with amorphous intergranular films, this paper investigates ternary nanocrystalline Cu-Zr-Hf alloys with a focus on understanding how alloy composition affects the formation of disordered complexions. Binary Cu-Zr and Cu-Hf alloys with similar initial grain sizes were also fabricated for comparison. The thermal stability of the nanocrystalline alloys was evaluated by annealing at 950 °C (>95% of the solidus temperatures), followed by detailed characterization of the grain boundary structure. All of the ternary alloys exhibi…
Amorphous complexions have recently been demonstrated to simultaneously enhance the ductility and stability of certain nanocrystalline alloys. In this study, three quinary alloys (Cu-Zr-Hf-Mo-Nb, Cu-Zr-Hf-Nb-Ti, and Cu-Zr-Hf-Mo-W) are studied to test the hypothesis that increasing the chemical complexity of the grain boundaries will result in thicker amorphous complexions and further stabilize a nanocrystalline microstructure. Significant boundary segregation of Zr, Nb, and Ti is observed in the Cu-Zr-Hf-Nb-Ti alloy, which creates a quaternary interfacial composition that limits average grain …
Grain boundary (GB) enthalpies in nanocrystalline (NC) $\mathrm {Pd_{90}Au_{10}}$ are studied after preparation, thermal relaxation and plastic deformation. By comparing results from atomistic computer simulations and calorimetry, we show that increasing plastic deformation of equilibrated NC $\mathrm {Pd_{90}Au_{10}}$ specimen causes an increase of the stored GB enthalpy $Δγ$. We interpret this change of $Δγ$ as stress-induced complexion transition from a low-energy to a high-energy GB-core state. In fact, GBs behave not only as mere sinks and sources of zero- and one-dimensional defects or a…
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…
Grain boundary-based mechanisms are known to control the plastic deformation and failure of nanocrystalline metals, with manipulation of the boundary structure a promising path for tuning this response. In this study, the role of interfacial structural disorder on plasticity and failure of nanocrystalline Cu-Zr alloys is investigated with in situ scanning electron microscopy tensile deformation experiments. Two model materials are created, one with only the typical ordered grain boundaries and another with amorphous intergranular films interspersed into the boundary network, while the microstr…
Using large-scale atomistic simulations, dislocation mechanics in the presence of linear complexions are investigated in an Fe-Ni alloy, where the complexions appear as nanoparticle arrays along edge dislocation lines. When mechanical shear stress is applied to drive dislocation motion, a strong pinning effect is observed where the defects are restricted by their own linear complexion structures. This pinning effect becomes weaker after the first dislocation break-away event, leading to a stress-strain curve with a profound initial yield point, similar to the static strain ageing behavior obse…
A computational alloy design framework for the promotion of amorphous grain boundary complexions
Hopkins Extreme Materials Institute, Johns Hopkins University（约翰斯·霍普金斯大学极端材料研究所）；Department of Materials Science and Engineering, Johns Hopkins University（约翰斯·霍普金斯大学材料科学与工程系）；Department of Materials Science and Engineering, University of California Los Angeles（加州大学洛杉矶分校材料科学与工程系）；Department of Mechanical and Aerospace Engineering, University of California Los Angeles（加州大学洛杉矶分校机械与航空航天工程系）；Department of Materials Science and Chemical Engineering, Stony Brook University（石溪大学材料科学与化学工程系）；Institute for Advanced Computational Science, Stony Brook University（石溪大学高级计算科学研究所）；Materials Science and Technology Division, Oak Ridge National Laboratory（橡树岭国家实验室材料科学与技术部）
Complexions are phase-like interfacial features that can influence a wide variety of properties, but the ability to predict which material systems can sustain these features remains limited. Amorphous complexions are of particular interest due to their ability to enhance diffusion and damage tolerance mechanisms, as a result of the excess free volume present in these structures. In this paper, we propose a set of materials selection rules aimed at predicting the formation of amorphous complexions, with an emphasis on (1) encouraging the segregation of dopants to the interfaces and (2) lowering…
Nanocrystalline Cu-3 at.% Zr powders with ~20 nm average grain size were created with mechanical alloying and their thermal stability was studied from 550-950 °C. Annealing drove Zr segregation to the grain boundaries, which led to the formation of amorphous intergranular complexions at higher temperatures. Grain growth was retarded significantly, with 1 week of annealing at 950 °C, or 98% of the solidus temperature, only leading to coarsening of the average grain size to 54 nm. The enhanced thermal stability can be connected to both a reduction in grain boundary energy with doping as well as …
Stress field modification near linear complexions increases the effective obstacle size and strengthening effect
Hopkins Extreme Materials Institute, Johns Hopkins University（霍普金斯极端材料研究所，约翰斯·霍普金斯大学）；Materials Department, University of California, Santa Barbara（加州大学圣塔芭芭拉分校材料系）；Department of Materials Science and Engineering, Johns Hopkins University（约翰斯·霍普金斯大学材料科学与工程系）
The unique three-phase coexistence of metastable B2-FeNi with stable L10-FeNi and L12-FeNi3 is discovered near edge dislocations in body-centered cubic Fe-Ni alloys using atomistic simulations. Stable nanoscale precipitate arrays, formed along the compression side of dislocation lines and defined as linear complexions, were observed for a wide range of compositions and temperatures. By analyzing the thermodynamics associated with these phase transitions, we are able to explain the metastable phase formation and coexistence, in the process defining new research avenues for theoretical and exper…
Atomistic simulations are used to study linear complexion formation at dislocations in a body-centered cubic Fe-Ni alloy. Driven by Ni segregation, precipitation of the metastable B2-FeNi and stable L10-FeNi phases occurs along the compression side of edge dislocations. If the Ni segregation is not intense enough to ensure precipitate growth and coalescence along the dislocation lines, linear complexions in the form of stable nanoscale precipitate arrays are observed. Critical conditions such as global composition and temperature are defined for both linear complexion formation and dislocation…
Hybrid Monte Carlo (MC)/molecular dynamics (MD) simulations are conducted to study the microstructures of nanocrystalline (nc) Cu/Ag alloys with various Ag concentrations. When the Ag concentration is below 50 Ag atoms/nm!, an increase in Ag concentration leads to a gradual growth of monolayer grain boundary (GB) complexions into nanolayer complexions. Above the concentration of 50 Ag atoms/nm!, wetting layers with a bulk crystalline phase are observed. The effects of Ag on mechanical properties and deformation mechanisms of nc Cu/Ag alloys are investigated in MD simulations of uniaxial tensio…
Grain boundary complexions have been observed to affect the mechanical behavior of nanocrystalline metals, improving both strength and ductility. While an explanation for the improved ductility exists, the observed effect on strength remains unexplained. In this work, we use atomistic simulations to explore the influence of ordered and disordered complexions on two deformation mechanisms which are essential for nanocrystalline plasticity, namely dislocation emission and propagation. Both ordered and disordered grain boundary complexions in Cu-Zr are characterized by excess free volume and prom…
Magnetic and transport properties of near-stoichiometric metastable FexMnyGaz alloys (46<x<52, 17<y25, 26<z<30) with face-centered cubic (FCC), body-centered cubic (BCC), and two-phase (FCC+BCC) structures are investigated. The experimental results are analyzed in terms of first-principles calculations of stoichiometric Fe2MnGa alloy with the L21, L12, and the tetragonally distorted L21 structural orderings. It is shown that the pure BCC and FCC phases have distinct magnetic and transport properties. Two-phase Fe2MnGa alloys have magnetic and transport properties typical of the mixed BCC and F…
A model lattice ab initio parameterised Hamiltonian spanning a broad range of alloy compositions and a large variety of chemical and magnetic configurations has been developed for face-centered cubic Fe-Ni alloys. Thermodynamic and magnetic properties of the alloys are explored using configuration and magnetic Monte Carlo simulations in a temperature range extending well over 1000 K.
A Magnetic Cluster Expansion (MCE) model for ternary face-centered cubic Fe-Ni-Cr alloys has been developed using DFT data spanning binary and ternary alloy configurations. Using this MCE model Hamiltonian, we perform Monte Carlo simulations and explore magnetic structures of alloys over the entire range of alloy compositions, considering both random and ordered alloy structures. In random alloys, the removal of magnetic collinearity constraint reduces the total magnetic moment but does not affect the predicted range of compositions where the alloys adopt low temperature ferromagnetic configur…
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…
Energy efficiency is motivating the search for new high-temperature metals. Some new body-centered-cubic random multicomponent "high entropy alloys (HEAs)" based on refractory elements (Cr-Mo-Nb-Ta-V-W-Hf-Ti-Zr) possess exceptional strengths at high temperatures but the physical origins of this outstanding behavior are not known. Here we show, using integrated neutron-diffraction (ND), high-resolution transmission electron microscopy (HRTEM), and theory, that the high strength and strength retention of a NbTaVTi alloy and a new high-strength/low-density CrMoNbV alloy are attributable to edge d…
In traditional body-centered cubic (bcc) metals, the core properties of screw dislocations play a critical role in plastic deformation at low temperatures. Recently, much attention has been focused on refractory high-entropy alloys (RHEAs), which also possess bcc crystal structures. However, unlike face-centered cubic high-entropy alloys (HEAs), there have been far fewer investigations on bcc HEAs, specifically on the possible effects of chemical short-range order (SRO) in these multiple principal element alloys on dislocation mobility. Here, using density functional theory, we investigate the…
A simple fitting approach is used for modeling the compressive yield strength of body centered cubic (bcc) solid solution high entropy alloys in Al-Hf-Nb-Mo-Ta-Ti-V-Zr system. It is proposed that the yield strength could be modeled by a polynomial where the experimental data can be used for finding the polynomial coefficients. The results show that the proposed polynomial could model the yield strength of solid solution alloys relatively well. The developed polynomial is used for predicting the strength of RHEAs in Hf-Mo-Nb-Ta-Ti-V-Zr system. It is observed that the yield strength of alloys wi…
With the aim of the discovery of face-centered-cubic (fcc) high-entropy alloy (HEA) superconductor, we have carried out materials research on Nb or Pb-containing multi-component alloys. Although the X-ray diffraction (XRD) patterns of some Nb-containing samples exhibited the dominant fcc phases, no superconducting signals were observed down to 3 K. Examination with an energy dispersive X-ray spectrometer revealed that all samples were multi-phase, but the existence of several new Nb-containing HEA phases was found in them. It was confirmed that the synthesis of Pb-containing an HEA or quaterna…
Edge Dislocation Mediated Anomalous Charge Transfer in Face Centered Cubic High Entropy Alloys
School of Materials Science and Technology, IIT-BHU（印度理工联合学院瓦拉纳西分校材料科学与技术学院）；National Centre for Computational Sciences, Oak Ridge National Laboratory（橡树岭国家实验室国家计算科学中心）；Department of Metallurgy and Materials Engineering, IIEST Shibpur（IIEST Shibpur冶金与材料工程系）
The yield strengths of High Entropy Alloys have recently been correlated with measured picometer-scale atomic distortions. Here, the root mean square microdistortion in a multicomponent alloy is shown to be nearly proportional to the misfit-volume parameter that enters into a predictive model of solute strengthening. Analysis of two model ternary alloy families, face-centered cubic Cr-Fe-Ni and body-centered cubic Nb-Mo-V, demonstrates the correlation over a wide composition space. The reported correlation of yield strength with microdistortion is thus a consequence of the correlation between …
We here show by first principles theory that it is possible to achieve a structural and magnetic phase transition in common steel alloys like Fe$_{85}$Cr$_{15}$, by alloying with Ni or Mn. The predicted phase transition is from the ferromagnetic body centered cubic (bcc) phase to the paramagnetic face centered cubic (fcc) phase. The relatively high average magnetic moment of $\sim1.4μ_{B}$/atom predicted at the transition suggests that stainless steel potentially can present a magnetocaloric effect strong enough to make these alloys good candidates for refrigeration applications operating at a…
The influence of the physical mechanisms activated during deformation and annealing on the microstructure and texture evolution as well as on the mechanical properties in the equiatomic CoCrFeMnNi high-entropy alloy (HEA) were investigated. A combination of cold rolling and annealing was used to investigate the HEA in the deformed, recovered, partially recrystallized, and fully recrystallized states. Detailed microstructure and texture analysis was performed by electron backscatter diffraction, transmission electron microscopy, and X-ray diffraction. The mechanical properties were evaluated us…
In this study, we report on face-centered cubic structured CoCrFeNi high-entropy alloy thin films with finely dispersed nano-oxide particles which are formed by internal oxidation. Analytical scanning transmission electron microscopy imaging found that the particles are Cr2O3. The oxide particles contribute to the hardening of the film increasing its hardness by 14% compared to that of the film without precipitates, through the Orowan-type strengthening mechanism. Our novel approach paves the way to design medium- and high-entropy alloys with high strength by making use of oxide phases.
We have grown a single crystal of the equiatomic FeCoCrMnAl high-entropy alloy. The crystal was grown by means of the Bridgman technique and is about 1 cm in diameter and 6.6 cm in length. X-ray Laue images taken at various positions on the surface are sharp and mutually consistent, reflecting its single-crystal nature. We thus report on the first successful growth of a single crystal of a high-entropy alloy with a volume of the order of cubic centimeters. The material has a microstructure consisting of B2 inclusions in a body-centered cubic matrix on the 10 nm length scale.
Phase diagrams supported by density functional theory methods can be crucial for designing high-entropy alloys that are subset of multi-principal$-$element alloys. We present phase and property analysis of quinary (MoW)$_{x}$Zr$_{y}$(TaTi)$_{1-x-y}$ refractory high-entropy alloys from combined Calculation of Phase Diagram (CALPHAD) and density-functional theory results, supplemented by molecular dynamics simulations. Both CALPHAD and density-functional theory analysis of phase stability indicates a Mo-W-rich region of this quinary has a stable single-phase body-centered-cubic structure. We rep…
The formation energy of the interface between face-centered cubic (fcc) and hexagonal close packed (hcp) structures is a key parameter in determining the stacking fault energy (SFE) of fcc metals and alloys using thermodynamic calculations. Often the contribution of the planar fault energy to the SFE has the same order of magnitude as the bulk part, and thus the lack of a precise information about it can become the limiting factor in thermodynamic predictions. Here, we differentiate between the actual interfacial energy for the coherent fcc(111)/hcp(0001) interface and the "pseudo-interfacial …
We investigate the formation of stress hotspots in polycrystalline materials under uniaxial tensile deformation by integrating full field crystal plasticity based deformation models and machine learning techniques to gain data driven insights about microstructural properties. Synthetic 3D microstructures are created representing single phase equiaxed microstructures for generic copper alloys. Uniaxial tensile deformation is simulated using a 3-D full-field, image-based Fast Fourier Transform (FFT) technique with rate-sensitive crystal plasticity, to get local micro- mechanical fields (stress a…
New approaches for the design of alloy systems with multiprincipal elements is recently researched in refractory materials field. However, most research aimed at arc melting process with weakness of coarsening of grains and inhomogeneous microstructure of segregation of elements during the cooling. This study aims to design and fabricate high-entropy alloy with powder metallurgy. In this study, a refractory high entropy alloys with composition near AlMo0.5NbTa0.5TiZr were produced by powder metallurgy. The alloy consists of two body-centered cubic (BCC) phases. One phase was disordered BCC enr…
Dimension reduction procedure is the recipe to represent defects in two dimensional dislocation dynamics according to the changes in the geometrical properties of the defects triggered by different conditions such as radiation, high temperature, or pressure. In the present study, this procedure is extended to incorporate further features related to the presence of defects with a special focus on face-centered cubic/body-centered cubic alloys used for diverse engineering purposes. In order to reflect the microstructural state of the alloy on the computational cell of two dimensional dislocation…
In a narrow composition range centered at Zn74.5Au10.5Yb15.0, Tsai-type icosahedral quasicrystal is formed in alloys quenched from 880 C. This quasicrystal belongs to the primitive type with the 6-dimensional lattice parameter a6D=7.378 A. The quasicrystal was not formed in the slowly cooled specimen, and was considered a metastable phase. The stable phase is a 2/1 approximant of the lattice parameter a2/1=23.271 A. This approximant forms exclusively in Zn76.0Au9.0Yb15.0 alloy annealed at 530 C. In addition, Zn70.5Au15.5Tb14.0 alloy annealed at 505 C forms a Tsai-type 1/1 approximant (a1/1=14.…
High-entropy alloys, near-equiatomic solid solutions of five or more elements, represent a new strategy for the design of materials with properties superior to those of conventional alloys. However, their phase space remains constrained, with transition metal high-entropy alloys exhibiting only face- or body-centered cubic structures. Here, we report the high-pressure synthesis of a hexagonal close-packed phase of the prototypical high-entropy alloy CrMnFeCoNi. This martensitic transformation begins at 14 GPa and is attributed to suppression of the local magnetic moments, destabilizing the ini…
High-entropy alloys are an intriguing new class of metallic materials that derive their properties from being multi-element systems that can crystallize as a single phase, despite containing high concentrations of five or more elements with different crystal structures. Here we examine an equiatomic medium-entropy alloy containing only three elements, CrCoNi, as a single-phase face-centered cubic (fcc) solid solution, which displays strength-toughness properties that exceed those of all high-entropy alloys and most multi-phase alloys. At room temperature the alloy shows tensile strengths of al…
High entropy alloys (HEAs) are multicomponent compounds whose high configurational entropy allows them to solidify into a single phase, with a simple crystal lattice structure. Some HEA's exhibit desirable properties, such as high specific strength, ductility, and corrosion resistance, while challenging the scientist to make confident predictions in the face of multiple competing phases. We demonstrate phase stability in the multicomponent alloy system of Cr-Mo-Nb-V, for which some of its binary subsystems are subject to phase separation and complex intermetallic-phase formation. Our first-pri…
Binary alloy phases of the noble metals with main group elements are analyzed in relation to body centered cubic structure with orthorhombic and hexagonal distortions. Stability of these distorted phases is considered on the base of the Fermi sphere - Brillouin zone interaction to understand the important part of the band structure energy contribution to the overall crystal energy. Examination of Brillouin zone - Fermi sphere configurations for several representative phases has shown significance of the electron energy contribution in the formation of distorted structures with superlattices an…
The structure and properties of the ferromagnet Tb_{1-x}Dy_xFe_2 (Terfenol-D) are explored through the morphotropic phase boundary (MPB) separating ferroic phases of differing symmetry. Our synchrotron data support a first order structural transition, with a broadening MPB width at higher temperatures. The optimal point for magnetomechanical applications is not centered on the MPB but lies on the rhombohedral side, where the high striction of the rhombohedral majority phase combines with the softened anisotropy of the MPB. We compare our findings with single ion crystal field theory and with f…
Plastic deformation mechanisms during nanoindentation of W, Mo, V body-centered cubic single crystals and their corresponding W-Mo, W-V equiatomic random solid solutions
Developing a single-phase and nanograined refractory high-entropy alloy ZrHfNbTaW with ultrahigh hardness by phase transformation via high-pressure torsion
Phase selection in Ti-Zr-Hf-Al high-entropy alloys was investigated by in-situ high-energy X$-$ray diffraction, single-crystal X$-$ray diffraction, and density-functional theory based electronic-structure methods that address disorder and vacancies, predicting formation enthalpy and chemical short-range order (SRO). Samples with varying Al content were synthesized, characterized, and computationally assessed to ascertain the composition-dependent phase selection, as increased Al content often acts as a stabilizer of a body-centered-cubic structure. Equiatomic TiZrHfAl was especially interestin…
