Multi-principal Element Research Articles

Strengthening CrFeNi multi-principal element alloy through grain refinement, introducing partially recrystallized structure, and constructing fine heterostructures

The excellent corrosion resistance exhibited by CrFeNi multi-principal element alloys makes them highly desirable for the development of novel stainless alloys, but they exhibit a low strength. This work explores a simple processing route to enhance the strength by cold-rolling and annealing a CrFeNi alloy to introduce a partially recrystallized structure, refine its grain, and construct fine heterostructures. These changes increased the yield strength from 402 MPa to 1025 MPa while maintaining acceptable ductility. Dislocation strengthening, grain boundary strengthening, and hetero-deformation-induced strengthening were responsible for the strength enhancement. A smaller soft phase in the heterostructure enhanced the effectiveness of hetero-deformation-induced strengthening. The findings presented in this article offer additional strengthening methods for CrFeNi multi-principal element alloys.

Unveiling the deformation micro-mechanism for mechanical anisotropy of a CoCrFeNi medium entropy alloy

The equiatomic Cr-Co-Fe-Ni medium-entropy alloy has the face-centered cubic structure. Single crystals of this alloy were tested by in-situ micropillar compression along different loading axes under scanning electron microscope. The transmission electron microscopy characterization and molecular dynamics simulation were incorporated for quantitative analysis of the effects of different crystal orientations on the deformation mechanisms. The <001>-oriented pillar not only exhibited extensive deformation-induced nano twinning, but also has been identified for the first time to undergo the FCCHCP phase transformation at room temperature. The strain localization tendency of <011>-oriented samples was confirmed through uniaxial tests to interpret the significant serration on stress-strain curves. The prominent strain hardening of <111>-oriented pillars was attributed to intense intersection between slip planes as evidenced by the extra density of Lomer-Cottrell locks. Such a high hardening rate has caused subsequent kinking of pillars. Functional division of different regions of kink band was conducted based on Orowan model. In principle, multi-principal element alloys can theoretically be designed and developed to combine a variety of excellent properties, which is an important class of candidate structural materials for advanced engineering systems. These findings provide promising guidance for understanding the mechanical anisotropy and application of these alloys.

Serrated flow stress in the CoCrFeNi multi-principal element alloys with different grain sizes and added Re

Multi-principal element alloys have attracted extensive attention as a new alloy design concept. In particular, the CoCrFeNi alloys with low stacking fault energy have excellent tensile plasticity and fracture toughness at both low and room temperature, and good phase stability at high temperatures, considered a new type of structural material with great potential. Multi-principal element alloys exhibit plastic instability termed serrated flow, at certain strain rates and temperature ranges. The serrated flow stress is usually closely related to dynamic strain aging, which reflects dislocation and barrier processes and impacts the mechanical properties of alloys. In the present study, CoCrFeNi alloys were selected as the research object, and the impact of grain size and the addition of Re elements on the serrated flow stress behavior was investigated. The results showed that in the tensile tests at 600 °C, the serrated flow stress behavior occurred in the fine-grain specimen only at a lower strain rate. The tensile curves of the coarse-grain specimen at different strain rates all show serration. Overall, the serrated type changes with the decreasing grain size, the increasing strain rate, and the addition of Re element, all of which result in a decrease in the magnitude and number of serrations and a weakening of the serrated behavior.

Enhanced irradiation tolerance of the body-centered cubic structured FeCrW medium-entropy alloy as revealed from primary damage process

Multi-principal element alloys have attracted much attention in the development of nuclear reactor materials due to their potentially excellent irradiation resistance. Here, we employed Molecular Dynamics (MD) simulations to investigate displacement cascades resulting from Primary Knock-on Atoms (PKA) energies ranging from 10 to 50 keV in body-centered cubic structured FeCrW, FeCr and α-Fe. The analysis encompasses irradiation-induced defect generation and evolution, aiming to elucidate mechanism of irradiation tolerance. The simulations results indicate that FeCrW medium-entropy alloys generate more Frenkel defects during the thermal spike stage compared to α-Fe, yet fewer residual defects are observed in FeCrW at the end of the cascade. The suppression of the increase in defects number in FeCrW compared to α-Fe is significantly enhanced with the increasing PKA energies. It is foreseeable that the number of residual defects in FeCrW will be much smaller than that of α-Fe with a dramatic increase of PKA energy, which will be attributed to the improved defect self-self-healing ability in FeCrW. Moreover, the mechanism of enhanced defect self-self-healing is attributed to the size of the defect cluster, the formation and distribution of defects, the enhanced thermal spike, and the chemical disorder caused by the increase in composition leading to low thermal conductivity. The spectral energy density (SED) curves spikes of α-Fe, FeCr and FeCrW reveal an important role in the enhanced phonon scattering in improving defect self-repairing ability due to chemical disorder. Our research provides valuable insights for guiding the development of advanced multi-principal element alloy structural materials in nuclear industry, particularly for the fourth-generation fast reactor.

Optimizing high-entropy alloys using deep neural networks

Exceptional mechanical properties of multi-principal element alloys have been typically achieved through manual trial-and-error approaches. The advances in artificial neural networks (ANN) have recently allowed for the development of predictive alloy design ANN models that take into account a large variety of prior experimental findings. However, when mechanical performance is considered, prior processing protocols (e.g. annealing, cold working) are almost impossible to track across different research groups and material classes. In this work, we propose to reverse the use of the yield stress point, setting it as an explicit input (instead of predictive output) parameter that is then used for the quantification of the material state, towards predicting material properties, such as the Ultimate Tensile Strength (UTS). This novel approach specifically addresses the challenge of predicting and improving ductility, a common issue in existing methods focused on yield stress, hardness, and phases. By combining available, validated databases of eleven elements and 734 total materials, we develop an ANN that receives as input the compositions out of 11 possible elements and the "observed" yield stress. Then, we utilize this model to perform predictions for 5-element concentrated alloys with exceptional UTS. We show that this ANN modeling strategy leads to exceptional confidence scores, and then, we use this ANN model for the design of HEAs, which we further discuss.

Vanadium boosted high-entropy amorphous FeCoNiMoV oxide for ampere-level seawater oxidation

A series of multi-principal elements composites (FeCoNiMoVOx-n) were synthesized on Ni foam (FeCoNiMoVOx-n/NF) to investigate the role vanadium (V) plays in improvingtheir electrocatalytic activity for oxygen evolution reaction (OER). Surprisingly, it was discovered that the addition of V not only changed the morphology of the catalysts from microsphere to three dimensional (3D) microflower shape, but also optimized the proportion of high-valent metal species that enhance the formation of active metal oxy(hydroxide) intermediates. The as-synthesized high-entropy amorphous FeCoNiMoVOx-1.5 coated Ni foam electrode (FeCoNiMoVOx-1.5/NF) with a 3D microflower structure exhibited higher OER activity than its counterparts. It exhibited extremely low over-potentials of 216 and 236 mV which are required at 10 mA cm−2in alkaline water and natural seawater, respectively. Particularly, this high-entropy amorphous catalyst showed long-time stability for a continuous 100-h high-performance OER at an ampere-level current density of 1 A cm−2in alkaline natural seawater. This shows that it can beapplied for large-scale seawater electrolysis.

Enhanced strength-ductility synergy in a Mo and W co-doping NiCoCr multi-principal element alloy at ambient and cryogenic temperatures

A novel designed Ni48Co33Cr9Mo4W6 multi-principal element alloy (MPEA) with single face-centered cubic (FCC) solid-solution phase was fabricated by casting, homogenization, cold rolling, and various post deformation annealing (PDA) heat treatments. The single-phase FCC structured samples with homogeneous grains exhibited average grain sizes ranging from ∼1.4 to ∼47.8 μm. In addition, a single-phase FCC structured heterogeneous sample consisting of partially un-recrystallized region (∼47.1 vol%) and fully recrystallized region (∼52.9 vol%) was also achieved. Among all the samples with homogeneous grains, the sample processed using PDA treatment at 900 °C for 3 min (named as PDA900-3min) showed an average grain size of ∼1.4 μm, displaying the best mechanical properties. In detail, it exhibited a yield strength of ∼811 MPa and a total elongation of ∼40.2 % at ambient temperature (298 K), and these values were increased to ∼1085 MPa and ∼42.7 % at LN2 (77 K) temperature. Due to heterogeneous deformation induced (HDI) strengthening, the hetero-structured sample (named as PDA900-90s) present markedly higher strength and slightly lower ductility in comparison with the PDA900-3min sample at 298 K and 77 K. These two samples demonstrated synergetic improvement of strength and ductility through Mo/W co-doping and PDA heat treatments both at ambient and LN2 temperatures, attributing to solid solution strengthening and grain boundary strengthening, as well as heterogeneous deformation induced (HDI) strengthening. Additionally, Mo/W co-doping, reduces the stacking fault energy (SFE) of the alloy, facilitates the generation of stacking faults (SFs) and twins, which is prone to maintaining the higher work hardening rate. Therefore, our work suggests that Mo and W co-doping is an effective strategy for achieving enhanced strength-ductility synergy in FCC structured MPEAs.

Role of Electronegativity on the Elemental Diversity in High-Entropy Alloys.

High-entropy alloys (HEAs) or multi-principal element alloys (MPEAs) have found extensive applications in high-precision devices. While the increased configurational entropy for HEAs favors more elemental diversity, it also increases the possibility of phase separation into multiple heterogeneous systems. This article reports that these two mutually competing effects are balanced for 3- and 4-component alloys. Analysis of all of the n-component ABCD···-type (∼5 × 105) available compounds in the materials' database shows that more than 70% are either 3- or 4-component ones. Their high propensity is explained on the basis of their optimal average difference of electronegativity (EN) ∼0.5-1.0 and the average sum of electronegativity (EN) ∼5.0-6.5 between the constituent atoms in the Oganov scale. Effectively, these 3- and 4-component alloys lie in the intermediate (centroid) region of the van Arkel-Ketelaar triangle, indicating their metalloid nature.

A simple model for short-range ordering kinetics in multi-principal element alloys

Short-range ordering (SRO) in multi-principal element alloys influences material properties such as strength and corrosion. While some degree of SRO is expected at equilibrium, predicting the kinetics of its formation is challenging. We present a simplified isothermal concentration-wave (CW) model to estimate an effective relaxation time of SRO formation. Estimates from the CW model agree to within a factor of five with relaxation times obtained from kinetic Monte Carlo (kMC) simulations when above the highest ordering instability temperature. The advantage of the CW model is that it only requires mobility and thermodynamic parameters, which are readily obtained from alloy mobility databases and Metropolis Monte Carlo simulations, respectively. The simple parameterization of the CW model and its analytical nature makes it an attractive tool for the design of processing conditions to promote or suppress SRO in multicomponent alloys.

Mechanisms of phase decomposition in a non-equimolar CrFeMnNi alloy during thermal aging

While high entropy alloys and multi principal element alloys have been largely studied and developed as single solid solutions, fewer investigations have focused on phase transformations in these multicomponent alloys. This study reports on the aging behavior at 500 °C of a non-equimolar CrFeMnNi alloy. The results unveil a complex evolution, leading to complete decomposition into three main phases, namely an L10 NiMn phase, an FCC Fe-rich phase, and a BCC Cr-rich phase. The intricate microstructure is achieved in stages through continuous precipitation of L10 Ni-Mn nanoscale precipitates, which are progressively overtaken by a sweeping discontinuous front involving the co-precipitation of the L10 NiMn and BCC Cr-rich phases in addition to the parent FCC Fe-rich phase. Furthermore, the Cr-rich phase initially forms with metastable compositions leading to further precipitation of Fe-rich precipitates within the Cr-rich region behind the transformation front. Understanding these transformation pathways through metastable states emphasizes the potential for expanding the range of achievable properties through deliberate microstructure tailoring of multi principal element alloys.

Electrochemical behavior and passivation film characterization of TiZrHfNb multi-principal element alloys in NaCl-containing solution

In this work, the electrochemical behavior and passivation film characteristics of a TiZrHfNb multi-principal element alloy (MPEA) with an equal atomic ratio in Cl− solution were studied. The effect of constituent metals on the overall corrosion resistance of alloys focused on different aspects. Ti could reduce the point defect density of the passivation film, while Nb kept the passivation film stable at a high potential, Hf and Zr provided high resistance and charge transfer resistance to the passivation film, and hydroxides enhanced the repair ability of the passivation film. There was also a synergistic effect between the alloying elements.

Understanding the effect of refractory metal chemistry on the stacking fault energy and mechanical property of Cantor-based multi-principal element alloys

Multi-principal-element alloys (MPEAs) based on 3d-transition metals show remarkable mechanical properties. The stacking fault energy (SFE) in face-centered cubic (fcc) alloys is a critical property that controls underlying deformation mechanisms and mechanical response. Here, we present an exhaustive density-functional theory study on refractory- and copper-reinforced Cantor-based systems to ascertain the effects of refractory metal chemistry on SFE. We find that even a small percent change in refractory metal composition significantly changes SFEs, which correlates favorably with features like electronegativity variance, size effect, and heat of fusion. For fcc MPEAs, we also detail the changes in mechanical properties, such as bulk, Young's, and shear moduli, as well as yield strength. A Labusch-type solute-solution-strengthening model was used to evaluate the temperature-dependent yield strength, which, combined with SFE, provides a design guide for high-performance alloys. We also analyzed the electronic structures of two down-selected alloys to reveal the underlying origin of optimal SFE and strength range in refractory-reinforced fcc MPEAs. These new insights on tuning SFEs and modifying composition-structure-property correlation in refractory- and copper-reinforced MPEAs by chemical disorder, provide a chemical route to tune twinning- and transformation-induced plasticity behavior.

A deformation twin mediated sliding-opening zig-zag fracture mechanism in multi-principal element alloys

Multi-principal element alloys (MPEA) have emerged as a class of high-performance materials. Of particular interest is the excellent fracture toughness in face-centered cubic MPEAs with low stacking fault energy, where abundant deformation twins are commonly observed. To understand the fracture mechanism in such MPEAs, here we perform in situ nanomechanical testing inside transmission electron microscope, atomistic simulation, and theoretical analysis to investigate the mechanism of interaction between a propagating crack and a coherent twin boundary (TB) in CoCrFeNi MPEA. We unveil a hitherto unknown microscopic mechanism of crack-TB interaction in MPEAs, where sliding along secondary nanotwins (NTs) nucleated from the primary TB followed by sub-crack nucleation some distance ahead of the main crack constitute a consecutive sliding-opening crack propagation mechanism, leading to zig-zag fracture around the primary TB. The origin of this mechanism lies in the frequent nucleation of NTs from steps on the primary TB, which can effectively blunt and deflect a propagating crack. Quantitative analysis indicates nearly doubled fracture resistance associated with this unique mechanism of crack-TB interaction in MPEAs. These findings bring about a missing aspect in TB-modulated fracture in MPEAs and enrich our understanding of the enhanced damage tolerance of alloys with low stacking fault energy.

Lead-bismuth eutectic corrosion behavior of NbMoVCr refractory multi-principal element alloys coating synthesized by magnetron sputtering

Nanocrystalline NbMoVCr refractory multi-principal element alloys coatings are fabricated by magnetron sputtering technology to improve lead-bismuth eutectic (LBE) corrosion resistance of structural materials, and their corrosion behavior in oxygen-saturated liquid LBE at 600 and 650 °C has been investigated. The coatings exhibit good phase structural stability even at 650 °C exposure. Moreover, the coatings show promising LBE corrosion resistance with parabolic oxidation rate constant 3–5 orders of magnitude lower than other candidate steels (such as 316 L, HT9, T91, SIMP, CLAM, etc.) which is due to the formation of protective (CrV)2O3 solid solution layer. The oxidation process of the coating is mainly dominated by kinetic factors, including diffusion driving force (related to LBE solubility) and diffusion energy barrier (related to atomic radius). By the way, at the grain boundaries with much lower oxygen potential, thermodynamic factors dominate the oxidation process, and V with the highest oxygen activity shows evidence of selective oxidation.

Effects of Zr and Hf on superelasticity, shape memory effect and microstructure of β-type (Ti–Zr–Hf)–Nb–Sn multi-principal element alloys with low magnetic susceptibility

(Ti–Zr–Hf)(97−x)–xNb–3Sn (Ti:(Zr + Hf) = 1:1) alloys were designed for biomedical superelastic alloys with magnetic resonance imaging (MRI) compatibility. The effects of Zr and Hf on constituent phases, superelasticity, shape memory effect and magnetic susceptibility were clarified. The specific Nb content for superelasticity and shape memory effect was also explored. It was found that the (Ti–Zr)–6.5Nb–3Sn alloy exhibited a superelastic recovery strain of 5.2 % with lower magnetic susceptibility compared to conventional Ti–Ni and Ti–Nb alloys. The magnetic susceptibility decreased with increasing Hf content. The (Ti–Hf)–9Nb–3Sn alloy showed a superelastic recovery strain of 0.9 % and very low magnetic susceptibility less than half of that of the conventional alloys. Based on the lattice parameters of these alloys, it was found that these alloys exhibited larger transformation lattice strain between a β phase and an α” phase than those of the Ti–Nb alloys, suggesting high potential to exhibit a large superelastic recovery strain. According to orientation analyses of constituting grains, the larger superelastic recovery strain in the (Ti–Zr)–6.5Nb–3Sn alloy than that of the (Ti–Hf)–9Nb–3Sn alloy is considered to be caused by a strong recrystallization texture of {001}β <110>β. By optimization of heat-treatment temperature, superelastic recovery strain of 3.6 % was achieved in the (Ti–0.5Zr–0.5Hf)–7.5Nb–3Sn alloy.

Achieving ultrafine grain strengthening Cf/SiC-GH3536 joints brazed with Cu-Ti-WC composite interlayer via cold spray additive manufacturing

Cold spray additive manufacturing (CSAM), as a solid-phase additive manufacturing technology, exhibits enormous potential in the fabrication of metal matrix composite. The conventional preparation methods of composite filler often have some problems, such as agglomeration of reinforcements and poor fluidity, which have limited the further application of composite filler. In this study, we proposed a novel method to prepare the Cu-Ti-WC composite interlayer for brazing Cf/SiC and GH3536 using CSAM. The synergistic deposition mechanism of matrix particles and reinforcing particles in the composite interlayer was revealed. The deformation and recrystallisation of Cu and Ti particles and the crushing and tamping of WC particles achieved the deposition of the cold sprayed Cu-Ti-WC interlayer. The CSAM achieved a tight bonding of the composite filler, generating the initial TiC before reaching the brazing temperature. And the Fe-W-Ni-Cr-Cu multi-principal element alloys (MPEA) generated from the reaction between WC and Cr(s,s), and formed ultrafine grain regions (∼200 nm) with [2–1 −1 0]HCP-MPEA // [0 0 1]TiC and [-1 1 1]FCC-MPEA // [1 1 3]TiC orientation due to the hindering effect of the increased TiC. The shear strength of Cf/SiC-GH3536 joint brazed with cold sprayed Cu-Ti-WC interlayer was improved to 112.0 MPa, which is 1.5 times higher than that of Cu-Ti-WC powder filler. This study shows the great potential of CSAM in fabricating metal-based composite interlayers for brazing and aids in composite-metal joining for higher performance.

Shearing brittle intermetallics enhances cryogenic strength and ductility of steels.

Precipitates are crucial for crafting mechanically strong metallic materials. In this work, we report the dislocation cutting of B2 (ordered body-centered cubic) nanoprecipitates, typically considered nonshearable intermetallics, in a lightweight compositionally complex steel during cryogenic tensile loading. Shearing is enabled by the high strength level for dislocation glide within the austenitic matrix, attributed to the substantial strengthening from subnanoscale local chemical ordering zones and the pronounced solid solution strengthening from the multiprincipal elements in the matrix. This mechanism not only harnesses the intense strengthening and strain hardening provided by otherwise impenetrable brittle nanoprecipitates but also introduces ductility through their sequential shearing with ongoing deformation. Our steel thus showcases ultrahigh cryogenic tensile strength up to 2 gigapascal at a remarkable tensile elongation of 34%. This study reveals a new strategy for designing high-performance structural materials.

On the Phase Configurational Entropy of Dual-Phase γ/γ′ Multi-principal Element Alloys: A Case Study on the Al–Cu–Fe–Ni–Ti System

On the Phase Configurational Entropy of Dual-Phase γ/γ′ Multi-principal Element Alloys: A Case Study on the Al–Cu–Fe–Ni–Ti System

Nanostructuring of an additively manufactured CoCrFeNi multi-principal element alloy using severe plastic deformation: Comparison of two materials processed by different laser scan speeds

Experiments were conducted to reveal the refinement of the microstructure and the evolution of the hardness of an additively manufactured (AM) CoCrFeNi multi-principal element alloy (MPEA) processed by severe plastic deformation (SPD) using high pressure torsion (HPT) technique. AM was carried out by laser powder bed fusion (L-PBF) technique at two different laser scan speeds. The as-built alloys for both laser scan speeds have a single-phase face-centered cubic (fcc) structure with <110> fiber texture parallel to the building direction. X-ray line profile analysis (XLPA) revealed that the dislocation density was considerably high even in the AM-processed state before HPT (3 × 1014 m−2) which increased by two orders of magnitude during HPT. The saturation of the lattice defects (dislocation density and twin fault probability) as well as the crystallite size occurred at a shear strain of about 10 during HPT. In both AM-processed alloys, <111> fiber texture developed parallel to the normal of the HPT-processed disks. For both laser scan speeds, the initial grain size in the AM-processed samples was refined from 70 to 90 μm to the nanocrystalline regime after 10 turns of HPT. Additionally, nanotwins formed with a probability of about 3 %. The initial hardness of the AM-processed MPEA samples for both laser scan speeds was 2700–2800 MPa, which is superior to that of CoCrFeNi produced by casting (about 1380 MPa). This can be explained by the high dislocation density in the AM-processed specimens. The formation of nanostructure with high lattice defect density during HPT resulted in a very high hardness value of about 5500 MPa in the AM-processed CoCrFeNi MPEA samples for both laser scan speeds.

High-temperature tensile properties and deformation behavior of a multi-phase FeNiCrAlTi alloy

Single-phase multi-principal elements alloys (MPEAs) within the elevated temperature experience particularly severe strength loss, and even a distinct ductile-to-brittle transition. Herein, we report on a non-equiatomic, multi-phase FeNiCrAlTi MPEA which possesses remarkable combinations of mechanical properties across a wide range of temperatures from 400 °C to 800 °C, especially its specific yield strength possesses nearly 100 MPa/g∙cm−3 at 600 °C. The excellent mechanical behavior benefits from the synergistic effect of the special phase structure and precipitation strengthening by the ordered Ni (Al, Ti) B2 nanoparticles. The transformation of deformation mode at 600 °C should be considered the dominant of the resistance to intermediate or high-temperature embrittlement. This work provides a new methodology for regulating mechanical properties of superior alloys that involve high-temperature conditions.