FCC High Entropy Alloys Research Articles

The hot deformation behavior of a L12 precipitation strengthened face centered cubic high entropy alloy

In this work, the hot compressive deformation behavior of a homogenized Ni45Co15Cr15Fe15Al8Ti2 high entropy alloy was investigated at deformation temperatures of 900–1200°C and strain rates of 10−3-1 s−1. Initially, combined the true stress-strain curves and hyperbolic sine function model, the constitutive equation for the flow stress of the alloy during the hot deformation process was established. Additionally, the hot processing map of the alloy at a true strain of ε = 0.5 was plotted based on the DMM model. Coupled with microstructural observations, the optimal hot processing parameters for the alloy were determined to be T = 1070–1200 °C and ε̇= 0.001–0.05 s−1. Finally, the deformed microstructure of the alloy under different temperatures and strain rates was characterized using EBSD, and the dynamic recrystallization mechanisms was revealed. The flow stress decreases with the increasing deformation temperature and decreasing strain rate. As the deformation temperature increases from 900°C to 1200°C, the dynamic recrystallization mechanism transitions from continuous dynamic recrystallization (CDRX) to discontinuous dynamic recrystallization (DDRX). The findings of this study will provide valuable guidance for the development of hot forging parameters of the L12 strengthened FCC high-entropy alloys.

Influence of Annealing Treatment on the Microstructure and Mechanical Properties of Cold-Sprayed CoCrFeNiMn High Entropy Alloy

AbstractIn this study, we developed a ~ 2 mm thick deposit of CoCrFeNiMn high entropy alloy (HEA) from cold spray. After cold-spraying, annealing at 600, 800 and 1000 °C for 5 hrs was conducted to improve and consolidate the microstructure. The influence of the annealing treatment on the microstructure, hardness and tensile strength of the HEA deposit was studied. The results showed that annealing treatment increased the fraction of metallurgical bonded areas due to diffusion, which resulted in enhanced mechanical performances of the deposit. The examined fractured surfaces of the tensile test samples revealed that the annealing treatment changed the failure behavior of the as-sprayed deposit from mostly particle-particle interface failure to void coalescence (ductile failure). Interestingly, a distinct microstructure was observed for the deposited annealed at 600 °C; a partially recrystallized microstructure with a small volume fraction of Cr-rich phase formed along grain boundaries, whereas fully recrystallized microstructure at higher two temperatures. The strengthening effect of partial recrystallisation, with a small volume fraction of the Cr-rich phase led to a greater reduced modulus and tensile strength (~196.7 GPa and 51.7 MPa) of the deposit annealed at 600 °C when compared with that annealed at 800 °C (~182.5 GPa and 43.6 MPa). It is believed that the small volume fraction of the Cr-rich phase partly constrained the deformation of the surrounding FCC HEA matrix during mechanical loading, leading to better mechanical properties as compared to the deposit annealed at 800 °C.

Enhancing strength and ductility synergy through heterogeneous laminated structure design in high-entropy alloys

Heterogeneous laminated structure (HLS) design offers new opportunities to enhance the mechanical performance of high-entropy alloys (HEAs) through synergistic effects from heterogeneity. However, it remains challenging to introduce the HLS into HEAs via severe plastic deformation due to their strong work-hardening capacity. In this study, a specially designed multi-level HLS, characterized by alternatively stacked micro-grained soft CoCrFeNi layers and nanostructured ultra-hard Al0.3CoCrFeNi layers containing a three-phase microstructure (composed of nanograined face-centered cubic matrix, (Al, Ni)-rich B2 precipitates, and Cr-rich σ precipitates), is controllably introduced into FCC HEAs via a conventional thermo–mechanical processing involving hot-pressing, cold-rolling, and annealing. Meanwhile, thermo–mechanical processing induces Al element diffusion across the layer interface, resulting in the formation of an interfacial transition layer and the establishment of a strong interface bonding between the neighboring CoCrFeNi and Al0.3CoCrFeNi layers. As a result, the multi-level HLSed CoCrFeNi/Al0.3CoCrFeNi composite exhibits a yield strength as high as 1127±25.4 MPa while maintaining a large fracture elongation (up to (26.3±2.4)%). Such an excellent strength–ductility synergy surpasses that of most previously reported high-performance monolithic bulk CoCrFeNi and Al0.3CoCrFeNi HEAs prepared through careful chemical composition optimization and/or thermo–mechanical processing. Strong hetero-deformation induced strengthening benefited from the apparent microstructural/microhardness difference and the strong interface bonding between the neighbouring CoCrFeNi and Al0.3CoCrFeNi layers, together with simultaneous activation of multiple strain hardening mechanisms containing mechanical twinning, stacking faults and precipitation strengthening, is responsible for the excellent strength–ductility combination. This multi-level HLS and its fabrication strategy provide an enlightening way to develop strong and ductile HEAs and can also be applied to high-performance designs of other metallic materials.

Cryogenic deformation strengthening mechanisms in FeMnSiNiAl high-entropy alloys

The mechanical properties and deformation mechanisms of a newly developed Co-free FeMnSiNiAl high entropy alloy (HEA) at room and cryogenic temperatures were systematically investigated. The initial tensile deformation at room temperature was dominated by dislocation slipping, with modest strengthening from the Transformation-Induced Plasticity (TRIP) effect due to the deformation-induced FCC → HCP martensitic transformation. Subsequently, the TRIP effect was markedly enhanced during the middle and later stages of deformation, leading to an excellent combination of yield strength (σy, 315.1 MPa), ultimate tensile strength (σu, 773.4 MPa), and fracture elongations (εf, 78.3%). The strengthening by the TRIP effect was significantly enhanced at cryogenic temperatures as a result of enhanced FCC → HCP martensitic transformation. This resulted in a synergetic improvement in strength and ductility at 223 K, with σy of 363.6 MPa, σu of 832.1 MPa, and εf of 87.2%. The enhanced ductility at 223 K was linked to the FCC → HCP → BCC sequential martensitic transformation during the middle and later stages of deformation, which acted as an additional way to accommodate plastic strain and delay strain localization. However, the rapid FCC → HCP transformation at the early stage of deformation at 173 K and 77 K impeded the FCC → HCP → BCC sequential martensitic transformation during subsequent deformation stages, thus remarkably enhancing strength but reducing ductility. Our findings provide new insights into the design and development of TRIP-assisted single-phase FCC HEAs for cryogenic applications.

Elastic constants from charge density distribution in FCC high-entropy alloys using CNN and DFT

While high-entropy alloys (HEAs) present exponentially large compositional space for alloy design, they also create enormous computational challenges to trace the compositional space, especially for the inherently expensive density functional theory calculations (DFT). Recent works have integrated machine learning into DFT to overcome these challenges. However, often these models require an intensive search of appropriate physics-based descriptors. In this paper, we employ a 3D convolutional neural network over just one descriptor, i.e., the charge density derived from DFT, to simplify and bypass the hunt for the descriptors. We show that the elastic constants of face-centered cubic multi-elemental alloys in the Ni–Cu–Au–Pd–Pt system can be predicted from charge density. In addition, using our recent PREDICT approach, we show that the model can be trained only on the charge densities of simpler binary and ternary alloys to effectively predict elastic constants in complex multi-elemental alloys, thereby further enabling easier property-tracing in the large compositional space of HEAs.

Unusual deformation mechanisms evoked by hetero-zone interaction in a heterostructured FCC high-entropy alloy

Understanding the synergistic mechanical effects of heterostructured materials remains challenging due to the complexities in the underlying deformation mechanisms, which are usually diverse, activated at different length scales and possibly interacting. Here, we unravel a deformation fundamental for heterostructures: in addition to the direct contribution on strength, hetero-zone interaction and the development of long-range internal stress could assist in evoking extra plastic mechanisms that are difficult to activate in their homogeneous counterparts. Specifically, the deformation of a heterostructure in Al0.1CoCrFeNi alloy, featuring nanostructured hard lamellae embedded in fine-grained soft matrix, is taken as an example for study. Drawing from experimental insights, the long-range internal stress buildup at elastoplastic transition stage due to intense dislocation pile-ups against hetero-zone boundary, which increases yield strength significantly, is theoretically analyzed. At the plastic stage, the high internal stress helps to activate phase transformation in the fine-grained zones, and the inter-zone constraint leads to form dispersed stable strain bands in the nanostructured zones. These extra mechanisms, together with enhanced deformation twinning, facilitate work hardening and coordinate the strain partitioning between zones, imparting improved ductility at high flow stress. These findings indicate a principle for heterostructural design: introducing strong hetero-zone interaction to enhance internal stress and thereby invoke new deformation mechanisms.

Origin of Excellent Strength-Ductility Balance Unique to FCC High-Entropy Alloys: A Plaston-Based Mechanism Derived from Electronic Structure Calculations

Origin of Excellent Strength-Ductility Balance Unique to FCC High-Entropy Alloys: A Plaston-Based Mechanism Derived from Electronic Structure Calculations

Effect of crystallographic orientation on the deformation and mechanical behavior of CoCrFeNi in Berkovich nanoindentation

The hardness and plastic deformation behavior of single-phase concentrated solid solution alloy CoCrFeNi were studied for selected crystallographic orientations (⟨001⟩, ⟨011⟩ and ⟨111⟩) using Berkovich nanoindentation. The integration of transmission electron microscopy (TEM) and molecular dynamics (MD) simulations was carried out to elucidate comprehensively the distinction in the deformation mechanisms, the evolution of dislocation structures, and the pile-up of differently oriented CoCrFeNi alloys subjected to indentation by a Berkovich indenter. In MDs, a Berkovich indenter, meticulously crafted to replicate its real geometry, was effectively employed in investigating the nanoindentation. The ⟨001⟩-oriented CoCrFeNi exhibited the highest hardness and the lowest magnitude of pop-in events, followed by the ⟨011⟩ and then the ⟨111⟩ orientations. The pile-up behavior was strongly dependent on the crystallographic orientation and occurred by three sequential processes. The deformation twinning was activated only in the ⟨001⟩ orientation and inclined toward the center direction of the indent due to the force inclining toward the radial direction of the indent. The clear manifestation of the evolution of dislocations in the three orientations during indentation was elucidated. More SFTs or Stair-rods were observed in the ⟨001⟩ orientation, attributable to the intensified reaction of Shockley partials stemming from its higher density of Shockley partials, followed by the ⟨011⟩ and then the ⟨111⟩ orientations. The anisotropic nanoindentation-hardness was correlated with the microstructures. The highest hardness and lowest magnitude of pop-in events of the ⟨001⟩ orientation could be ascribed to its intense dislocation interactions, high-density SFTs and the formation of twins. In the ⟨111⟩ orientation, the three-fold symmetry emission of prismatic dislocation loops along the three slip directions not parallel to the surface leaded to a weak interaction between dislocations, and then resulted in the lowest hardness. This research may be helpful to enhance our comprehension of the intricate mechanical behaviors exhibited by different oriented FCC high entropy alloys under Berkovich indentation.

Tuning mechanical behavior of an FCC high entropy alloy: Insights from roll deformation and texture

Severe plastic deformation (SPD) is introduced as a significant approach in designing and tailoring the mechanical characteristics of high entropy alloys (HEAs). This research focuses on investigating the microstructure evolution, deformation mechanisms, and their influence on texture components and mechanical behavior in a single solid solution Ni1.5FeCrCu0.5 HEA subjected to cold rolling. For this purpose, microstructure evolution and texture expansion were studied in 25, 45, 65, and 85 % thickness reductions using electron backscatter diffraction (EBSD) as well as transmission electron microscopy (TEM). The finding illustrates that alongside dislocation density, deformation twins and shear bands increase with higher strain; in particular, nanotwins with a thickness of approximately 50 nm were observed in the 85 % cold rolled (85 % CR) alloy. Bulk texture analysis indicates that the presence of shear bands and twins leads to the Goss {110}<001> and Brass {110}<112> texture orientations in the cold-rolled samples, which is ascribed to the low SFE of the alloy. With increased strain, the alloy's hardness and yield strength increased from ∼150 Hv and ∼236 Mpa (As-cast sample) to ∼467 Hv and ∼1097 Mpa (85 % CR), respectively, while the elongation decreased by ∼66 %. By examining the alloy's strengthening mechanisms, it has been determined that increasing the dislocation density and the presence of twins are the two main strengthening mechanisms of the alloy.

Strengthening a non-equiatomic quaternary high-entropy alloy via concurrent gradient structures of nanotwin and dislocation cell

One main drawback of FCC high-entropy alloys (HEAs) is their insufficient room temperature yield strength. In this work, concurrent gradient structures of nanotwins and dislocation cells have been introduced into a non-equiatomic quaternary Fe40Ni20Co20Cr20 (at. %) high-entropy alloy by ultrasonic shot peening to meet this challenge. The gradient alloy exhibits a significantly enhanced yield strength, i.e., from the original 204 MPa to 559 MPa, combining with a well-retained ductility of 29 % and a good work hardening ability. Strengthening mechanisms at different depths (5, 30, 200, 400 μm) are also carefully discussed. The current work presents a strategy for hardening FCC HEAs significantly while maintaining the single solid solution microstructure.

Microstructural evolution in doped high entropy alloys NiCoFeCr-3X (X=Pd/Al/Cu) under irradiation

Commonly studied equatomic single-phase FCC high entropy alloys based on 3d transition metals like NiCoFeCr do not provide adequate strength and radiation resistance at high doses for nuclear structural applications. In the current study, the major alloying effects like lattice distortion, ordering and clustering tendencies were investigated by adding low concentration of Pd, Al, or Cu respectively to study the doping effects on the ion irradiation response of NiCoFeCr alloy. The alloys were irradiated with 3 MeV Ni2+ ions at 500 °C to a fluence of 1 × 1017/cm2 at a beam flux of approximately 2.8 × 1012 ions/cm2/s. The microstructural evolution upon irradiation i.e., formation of dislocation networks, radiation induced segregation and precipitation, and void formation were studied in detail. Post-irradiation characterization results showed that a Pd addition leads to a high void nucleation rate but controlled void growth, which may be attributed to increased lattice distortion. In Al added HEA, our microstructural analysis indicates that radiation induced ordered L12 precipitates do not affect void swelling significantly. Cu addition led to Cu precipitation that drastically suppressed dislocation density and void swelling of the alloy. Additionally, a model was developed to qualitatively describe the trend in void swelling of typical FCC alloys under ion irradiation. This model was able to qualitatively explain the suppression and reappearance of void swelling in ion irradiated alloys that generally occurs near the region with peak implanted ion concentration.

New insights on bonding mechanism of FCC and BCC high entropy alloy microparticles upon supersonic impact using micromechanical adhesion test

Coating buildup in cold spraying (CS) relies on particle bonding upon impact, thus making it a significant area of study. High entropy alloys (HEAs) are a class of materials with superior work-hardening ability and resistance to softening, making their particle impact and bonding particularly important to understand for their cold sprayability. In this study, the splat adhesion strength is measured with a scratch test for FCC CoCrFeMnNi and BCC AlCrFeMnNi HEA microparticles deposited on mirror-polished SS304 substrates. Nanoindentation tests were conducted to estimate the tensile and shear strengths of the HEA powders. The results showed that particle deformation and jetting were dominant in the FCC HEA/SS304 system, while substrate deformation was dominant in the BCC HEA/SS304 system. Focused ion beam cross sections of particles confirmed that the FCC HEA had good bonding and underwent significant deformation and grain refinement at the substrate/particle interface, while the BCC HEA had limited bonding only in the sheared zone of the particles.In spite of the very limited deformation of BCC splats, their average adhesion strength was measured to be 451 ± 141 MPa, which is significantly higher than that of FCC splats, with an average value of 308.7 ± 69.3 MPa. Fracture surface analysis revealed that FCC splats consistently exhibit ductile fracture, regardless of whether it occurs at the particle, substrate, or interface. In contrast, for BCC splats, fracture was ductile when occurring at the interface and substrate sides, while it was cleavage when occurring on the particle side. These findings suggest that the toughness and shear strength of the bonding developed at the interface were higher than those of the particle. It was found that the flattening ratio (FR) of the particles decreases for both FCC and BCC HEA splats with an increase in particle size, resulting in a decrease in adhesion strength. Metallurgical bonding has been identified as the primary factor contributing to the adhesion strength of both FCC and BCC HEA splats. Particle penetration, indicative of mechanical interlocking, appears to have little or no effect on adhesion strength. However, it does positively influence the increase in adhesion energy of the splats.

Role of multi-variant deformation-induced HCP plates in cryogenic mechanical properties of an FCC high entropy alloy

We investigated the temperature-dependent mechanical properties and deformation mechanisms of the Fe49.5Mn30Co10Cr10C0.2Ti0.1V0.1Mo0.1 alloy from room temperature to cryogenic temperatures. As the deformation temperature decreased, both the strength and strain-hardening rate increased, while ductility tended to decrease. The higher strength of the material deformed at 123 K, compared to that deformed at 223 K, can be attributed to the larger number of multivariant HCP plates and higher hetero deformation induced (HDI) strengthening in the former. Multivariant deformation-induced HCP plates were the primary deformation mechanisms at all deformation temperatures, and cryogenic deformation activated intersecting shear, defined as the primary HCP plate being sheared by the secondary HCP plate. This mechanism contributed to stress relaxation and enhanced deformation accommodation. The reduced ductility observed in the material deformed at 123 K may be attributed to coarse edge and grain boundary cracks, which were more pronounced when the TRIP effect was enhanced.

Cryo-pre-straining contributes to achieving high yield strength in high-entropy alloys

In this study, we propose an effective and feasible strategy to significantly improve the room-temperature yield strength of the Co35Cr20Fe15Mn15Ni15 HEA by 338%, from 281 MPa (as-recrystallized, before CR) to 1231 MPa (30% CR) through cryo-pre-strain-inducing defects such as dislocations, stacking-faults (SFs), nano-twins, Lomer-Cottrell (L-C) locks, and HCP martensite, while maintaining good ductility (16.4% elongation in 30% CR). Additionally, the contribution of these defects to the yield strength enhancement was quantified. This study offers a new perspective for the preparation of advanced FCC high-entropy alloys.

Cryogenic strengthening of Fe27Co24Ni23Cr26 high-entropy alloys via hierarchical nanotwin-driven mechanism

FCC-based high entropy alloys (HEAs) with exceptional cryogenic mechanical properties have the potential for use in strategic applications. In this regard, we have studied via transmission electron microscopy (TEM) the microstructural evolution of hierarchical nanotwins in tensile-strained Fe27Co24Ni23Cr26 HEA alloys as a function of true strain at cryogenic temperature and compared the behavior at room temperature. At cryogenic temperature, the alloy had a high strength of ∼1211 MPa and large elongation of 87.2% compared to the room temperature tensile strength of 746 MPa and elongation of ∼61%. At 25 °C, the deformation initiated with the entanglement of wavy dislocations, which transformed into planar configuration and dislocation walls, and lastly to deformation nanotwins. In striking contrast, at −150 °C, numerous nanotwin bundles were formed through the coalescence of deformation nanotwins, which divided the matrix into microscale closed blocks. At higher strain, dense deformation nanotwins refined annealing twins into nanoscale closed blocks. Subsequently, in the interior of annealing twins, deformation nanotwin variants interpenetrated, producing serrated/curved interfacial structures. These interfaces are envisaged to promote the formation of ultra-fine deformation nanotwins and closed blocks that facilitate accommodation of a higher degree of deformation strain at cryogenic temperatures. The study provides new insights and guidelines in the futuristic design of FCC high entropy alloys for use at cryogenic temperatures by embracing the hierarchical nanotwin-driven mechanism.

Physics-based model to predict yield strength of single-phase FCC high-entropy alloys over wide temperature range

A physics-based temperature-dependent yield strength model without fitting parameters was developed for single-phase FCC high-entropy alloys. The model considered the temperature dependence of lattice friction stress, solid solution strengthening, grain boundary strengthening, dislocation strengthening, and their evolution with temperature to the overall yield strength. The results show that a quantitative relationship between temperature, material parameters, and yield strength was successfully captured by the model. This model can predict the yield strength at different temperatures only by using the easily available material parameters at room temperature. The accuracy of model was well verified by 17 sets of available experimental data over a wide temperature range (4.2–1273 K). Moreover, the contribution of different strengthening mechanisms to the yield strength was quantitatively analyzed and discussed from 4.2 to 1273 K, and some suggestions for improving the temperature-dependent yield strength were put forward.

Superior strength-ductility balance of an extremely low stacking fault energy (SFE) high-entropy alloy (HEA) processed by novel hybrid-rolling

An extremely low SFE Co20Cr26Fe20Mn20Ni14 FCC HEA was processed by warm-rolling (WR) and a novel hybrid-rolling (HR) (cryo-rolling followed by warm-rolling). The HR material showed finer nanostructure and a larger fraction of Cr-rich precipitates due to the finer microstructure and greater driving force resulting from the prior cryo-rolling step. Annealing resulted in finer recrystallized FCC grain size (GS) and a significantly higher fraction of in-grain precipitate in the HR material. The finer GS and higher in-grain precipitate fraction in the annealed HR material resulted in remarkably high strength (∼1125 MPa) and elongation (∼20%). The results highlighted the significant potential of this novel processing route for tailoring microstructure and properties of HEAs.

Improving ductility by coherent nanoprecipitates in medium entropy alloy

Controlling precipitates in spatial density and size distribution is essential for tailoring the microstructure and mechanical properties through precipitation hardening. We herein obtained heterogeneous grain structures with coherent L12 nanoprecipitates in (CrCoNi)94Al4Ti2 medium entropy alloy (MEA) by annealing and aging. Additional pre-aging leads to a high spatial density and more random distribution nucleation sites of the coherent L12 nanoprecipitates. The pre-aging doubled the ductility without apparently sacrificing the strength. Transmission electron microscopy (TEM) revealed that, in pre-aged MEA, finely dispersed L12 nanoprecipitates with higher spatial density were sheared by dislocation, promoting planar slips, which favors geometrically necessary dislocations (GNDs) piling up to increased hetero-deformation-induced (HDI) stress and work-hardening. Stacking faults, Lomer-Cottrell locks, and 9R structures were formed in aged and pre-aged MEA after tensile deformation. The formation of these defects enormously enhanced strain hardening by blocking dislocation movements and accumulating dislocations. Moreover, a higher frequency of interactions between defects and coherent L12 nanoprecipitates can be observed in the pre-aged MEA due to the more randomly distributed L12 nanoprecipitates, substantially increasing ductility. This work demonstrates a new route to achieving a super strength-ductility combination of single-phase FCC high entropy alloys by nanoscale coherent precipitation strengthening.

Effect of stacking fault energy on the thickness and density of annealing twins in recrystallized FCC medium and high-entropy alloys

This work aims to predict the microstructure of recrystallized medium and high-entropy alloys (MEAs and HEAs) with a face-centered cubic structure, in particular the density of annealing twins and their thickness. Eight MEAs and five HEAs from the Cr-Mn-Fe-Co-Ni system are considered, which have been cast, homogenized, cold-worked and recrystallized to obtain different grain sizes. This work thus provides a database that could be used for data mining to take twin boundary engineering for alloy development to the next level. Since the stacking fault energy is known to strongly affect recrystallized microstructures, the latter was determined at 293 K using the weak beam dark-field technique and compared with ab initio simulations, which additionally allowed to calculate its temperature dependence. Finally, we show that all these data can be rationalized based on theories and empirical relationships that were proposed for pure metals and binary Cu-based alloys.

Achieving superior combined cryogenic strength and ductility in a high-entropy alloy via the synergy of low stacking fault energy and multiscale heterostructure

A Co30Cr20Fe18Mn18Ni11Si3 high-entropy alloy (HEA) with low stacking fault energy and high strengthen was designed based on the principle of heterostructure strengthening. The alloy was processed into a multiscale heterogeneous microstructure containing hierarchical twins. The alloy achieved an ultrahigh yield strength of 1500 MPa, ultimate tensile strength of 1750 MPa and a large ductility of 20 % at 77 K. These mechanical properties were superior to those of most FCC HEAs reported in the literature, breaking the strength-ductility trade-off of conventional metal alloys. Such extraordinary mechanical properties were attributed to a suitable strain hardening capability, stemming from the synergistic effect of hetero-deformation-induced hardening, twinning-induced plasticity, and deformation-induced phase transformation during tensile deformation.