Single Face-centered Cubic Phase Research Articles

Design of novel high entropy alloys based on the end-of-life recycling rate and element lifetime for cryogenic applications

The equiatomic face-centred cubic (FCC) CoNiCrFeMn alloy, known as the Cantor alloy, is renowned for its high ductility under extreme conditions, such as cryogenic temperatures. Despite this, it suffers from low hardness and yield strength (YS) and includes elements with significant supply concerns. This study introduces novel non-equiatomic CoNiCrFeMn alloys, designed using machine learning (ML)-assisted-high-throughput atomistic simulations to enhance sustainability and mechanical properties such as hardness and YS while maintaining the alloy's single-phase FCC structure. We incorporated two critical sustainability indicators, end-of-life recycling rate (EOL−RR) and lifetime (τ), to guide the alloy design process. Mean-flow stress (MFS), measured from the yield point to 15% strain during tensile tests, was used to assess and predict the mechanical properties. Two alloys, with cobalt contents of 12.5% and 21.5%, were developed and analysed. The Co12.5 alloy showed better sustainability (44% improvement in τ with almost the same mechanical properties), while the Co21.5 alloy exhibited better mechanical properties (20% improvement in MFS with almost the same sustainability) as the equiatomic system. Their stable FCC microstructures were confirmed through CALPHAD modelling and X-ray diffraction (XRD) analysis of vacuum arc melted, as-cast samples. The results highlight the potential of integrating sustainability metrics into high-performance alloy design.

Microstructure and mechanical properties of laminated ferrous medium-entropy alloys fabricated by direct energy deposition and post-rolling annealing treatment

Laminated ferrous medium entropy alloy (Fe-MEAs), Fe75(CoCrNi)25/Fe60(CoCrNi)40, was successfully fabricated by direct energy deposition (DED). Due to the component mixing during DED process, a transition layer with face-centered cubic (FCC) and body-centered cubic (BCC) dual-phase was obtained in the Fe60(CoCrNi)40 layer after rolling and annealing. This dual-phase exhibits higher strength compared to a single FCC phase, thereby enhancing the overall strength of the material. During the deformation process of the laminated material, interface constraint will lead to high yield strength, resulting in performance superior to that predicted by the rule of mixtures. Moreover, the transition layer plays an important role in strain transferring and shows greater strain-hardening ability than the other two layers, enhancing the overall ductility of the laminated alloy.

Effect of Cu upon recrystallization and mechanical properties of TRIP-assisted high entropy alloy

The face-centered cubic (FCC) high-entropy alloys (HEAs) have been hindered in their engineering applications due to their low yield strength. Combining multiple strengthening mechanisms with phase transformation-induced plasticity (TRIP) may serve as an effective approach to overcome the strength-ductility trade-off. In this study, (Co3Cr1.6FeNi)100-xCux (x = 0,2) high entropy alloys (HEAs) were designed. A straightforward processing route, involving hot rolling followed by annealing, was utilized to produce single-phase FCC HEAs. The alloy doped with 2 at.% Cu achieved a perfect combination of strength and ductility. It was also found that the addition of Cu suppresses the recrystallization process during the hot rolling and annealing of the alloy. The increase in strength is attributed to the synergistic effects of various strengthening mechanisms dominated by dislocation strengthening and heterogeneous deformation induced strengthening, while the TRIP effect ensures continuous strain hardening capability of the alloy while maintaining high yield strength and ensuring good ductility. These results offer valuable insights into the design and development of high-performance HEAs with exceptional strength and ductility.

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.

Impact of copper and manganese on phase transformation, microstructural development and material strength in CoCrFeNi high-entropy alloys through mechanical alloying

CoCrFeNiX (X = Cu, Mn) equiatomic high-entropy alloys (HEAs) were prepared through mechanical alloying (MA) followed by sintering at moderate temperature. The prepared HEAs were subjected to X-ray diffraction and microscope techniques to evaluate their phase evolution, microstructure, and elemental composition. Alloyed CoCrFeNiCu powder consists of two face centered cubic (FCC) phases (FCC1 and FCC2), whereas, CoCrFeNiMn powder showed a major FCC phase with a minor body centered cubic (BCC) phase. After sintering, bulk CoCrFeNiMn alloy showed only a single solid solution FCC phase, and the bulk CoCrFeNiCu still contained dual FCC phases. The elemental composition analysis revealed that both HEAs are equiatomic in composition. The average microhardness of CoCrFeNiCu and CoCrFeNiMn HEAs was 85 ± 15 HV and 147 ± 20 HV, respectively. It was observed that the presence of Cu in CoCrFeNiCu reduces the hardness of the HEA due to segregation. This study demonstrates the influence of the fifth metal on the phase evolution, microstructure, and mechanical properties of the HEAs.

Hydrogen-Stabilized ScYNdGd Medium-Entropy Alloy for Hydrogen Storage.

The research on the functional properties of medium- and high-entropy alloys (MEAs and HEAs) has been in the spotlight recently. Many significant discoveries have been made lately in hydrogen-based economy-related research where these alloys may be utilized in all of its key sectors: water electrolysis, hydrogen storage, and fuel cell applications. Despite the rapid development of MEAs and HEAs with the ability to reversibly absorb hydrogen, the research is limited to transition-metal-based alloys that crystallize in body-centered cubic solid solution or Laves phase structures. To date, no study has been devoted to the hydrogenation of rare-earth-element (REE)-based MEAs or HEAs, as well as to the alloys crystallizing in face-centered-cubic (FCC) or hexagonal-close-packed structures. Here, we elucidate the formation and hydrogen storage properties of REE-based ScYNdGd MEA. More specifically, we present the astounding stabilization of the single-phase FCC structure induced by the hydrogen absorption process. Moreover, the measured unprecedented high storage capacity of 2.5 H/M has been observed after hydrogenation conducted under mild conditions that proceeded without any phase transformation in the material. The studied MEA can be facilely activated, even after a long passivation time. The results of complementary measurements showed that the hydrogen desorption process proceeds in two steps. In the first, hydrogen is released from octahedral interstitial sites at relatively low temperatures. In the second, high-temperature process, it is associated with the desorption of hydrogen atoms stored in tetrahedral sites. The presented results may impact future research of a novel group of REE-based MEAs and HEAs with adaptable hydrogen storage properties and a broad scope of possible applications.

Phase stability of a eutectic high entropy alloy under extremes of pressures and temperatures

Additively manufactured high-entropy alloys are of interest because of their unique combination of high yield strength and large ductility achieved with far-from-equilibrium crystalline phases and micro/nanostructure morphology. We report on the phase transformation and thermal equation of state of the eutectic high-entropy alloy (EHEA) Al18Co20Cr10Fe10Ni40W2, produced by laser powder-bed fusion (L-PBF). The EHEA was studied in a large-volume Paris–Edinburgh cell using energy-dispersive x-ray diffraction to a pressure of 5.5 GPa and a temperature of 1723 K. Static compression studies in diamond anvil cells using angle-dispersive x-ray diffraction extended the high-pressure structural data to 317 GPa at ambient temperature. The initial dual-phase nanolamellar face-centered cubic (FCC) and body-centered cubic (BCC) structure of Al18Co20Cr10Fe10Ni40W2 transforms into a single FCC phase under high pressure, with the BCC-to-FCC phase transformation completing at 9 ± 2 GPa. The FCC phase remained stable up to the highest pressure of 317 GPa. The measured thermal equation of state for the FCC phase of Al18Co20Cr10Fe10Ni40W2 is presented up to 5.5 GPa and 1473 K. We observed melting of the EHEA at 1698 ± 25 K at a pressure of 5.5 GPa, and the recrystallized sample shows an increased fraction of the CsCl-type (B2) phase at ambient conditions following release from the high-pressure high-temperature state. The BCC-to-FCC phase transition completion pressure is correlated with the nanolamellae thickness of the BCC layer in this diffusion-less transformation at ambient temperature.

Breaking the strength-ductility trade-off via heterogeneous structure in FeCoCrNiMo0.2 high-entropy alloy

The synergistic effect between heterogeneous structures has been proved to enhance the strength and plasticity of alloys, especially in the context of single-phase face-centered cubic (FCC) high-entropy alloys with low stacking fault energy. This study explores the synergistic effects of heterogeneous structures on the strength and plasticity of single-phase FCC FeCoCrNiMo0.2 high-entropy alloys. Through a combination of cryogenic rolling and annealing at 1000 °C for 0.5 h (RA-1000), the alloy demonstrates exceptional strength-ductility synergy and work-hardening ability. The heterogeneous structure comprises fine grains, nano-scale rich- (Cr, Mo) σ phase, and high-density annealing twins. The interaction of σ precipitation and FCC matrix induces heterogeneous deformation-induced strengthening (HDI), while annealing twins and stacking faults act as barriers to dislocation movement, enhancing strength and ductility through a dynamic Hall-Petch effect. Additionally, chemically ordered structures, ordered L12 phase, and type 63 topologically close-packed phases in RA-1000 alloys contribute to improved strain hardening via anti-phase boundaries. This work provides valuable insights for designing multi-scale heterogeneous structures to strengthen high-entropy alloys.

Synthesis of High-Entropy Alloys with a Tailored Composition and Phase Structure Using a Single Configurable Target.

Previously, refractory high-entropy alloys (HEAs) with high crystallinity were synthesized using a configurable target without heat treatment. This study builds upon prior investigations to develop nonrefractory elemental HEAs with low crystallinity using a novel target system. Different targets with various elemental compositions, i.e., Co20Cr20Ni20Mn20Mo20 (target 1), Co30Cr15Ni25Mn15Mo15 (target 2), and Co15Cr25Cu20Mn20Ni20 (target 3), are designed to modify the phase structure. The elemental composition is varied to ensure face-centered cubic (FCC) or body-centered cubic (BCC) phase stabilization. In target 1, the FCC and BCC phases coexist, whereas targets 2 and 3 are characterized by a single FCC phase. Thin films based on targets 1 and 2 exhibit crystalline phases followed by annealing, as indicated by X-ray diffraction (XRD) and transmission electron microscopy (TEM) analyses. In contrast, target 3 yields crystalline thin films without any heat treatment. The thin-film coatings are classified based on the atomic size difference (δ). The δ value for the target with the elemental composition CoCrMoMnNi is 9.7, i.e., ≥6.6, corresponding to an HEA with an amorphous phase. However, the annealed thin film is considered a multiprincipal elemental alloy. In contrast, δ for the CoCrCuMnNi HEA is 5, i.e., ≤6.6, upon the substitution of Mo with Cu, and a solid solution phase is formed without any heat treatment. Thus, the degree of crystallinity can be controlled through heat treatment and the manipulation of δ in the absence of heat treatment. The XRD results clarify the crystallinity and phase structure, indicating the presence of FCC or a combination of FCC and BCC phases. The outcomes are consistent with those obtained through the analysis of the valence electron concentration based on X-ray photoelectron spectroscopy. Furthermore, a selected area electron diffraction analysis confirms the presence of both amorphous and crystalline structures in the HEA thin films. Additionally, phase evolution and segregation are observed at 500 °C.

Grain boundary strengthening in nanocrystalline Mo0.2CoNi medium-entropy alloys produced via high-pressure torsion

In this study, we produced nanocrystalline (NC) MoCoNi-based medium-entropy alloys (MEAs) with a single face-centered cubic (FCC) phase, which exhibit exceptional hardness through combining compositional and microstructural features of alloys. The alloys were prepared via powder metallurgy and high-pressure torsion, and the processing condition was optimized to achieve full densification and prevent grain growth in order to maximize the effect of grain boundary strengthening. Consequently, NC Mo0.2CoNi MEAs revealed the refined microstructures with a grain size of approximately 50 nm and outstanding hardness of 857 HV (corresponding to approximately 2.86 GPa in yield strength). This remarkable property, even compared to the single FCC phase alloys with similar grain size, is attributed to the existence of alloying elemental atoms of Mo with large atomic radius, which leads to severe lattice distortion allows to improve friction stress as well as the Hall-Petch coefficient. This study aims to discuss the potential of developing hard metallic materials by using grain boundary strengthening from alloys with large atomic size difference.

Enhancing strength and ductility of CrCoNi medium-entropy alloy through the 9R phase and doping

The 9R phase model based on experimental orientation relationship are constructed, the mechanical moduli of the face-centered cubic (FCC) and 9R phases in CrCoNi medium-entropy alloy (MEA) are investigated by first-principles calculations with the meta-generalized gradient approximation (meta-GGA) functional. This functional accurately describes CrCoNi MEA, captures localized electronic states, and overcomes the challenge of DFT + U parameters in special quasi-random structure (SQS) based transition metal atom models. Results indicate that the 9R phase exhibits higher mechanical moduli than the FCC phase, with a 12% increase in bulk modulus and a 7% increase in Poisson's ratio. Consequently, the strength and toughness of CrCoNi MEA are significantly enhanced. Doping Mo, Ta, and W in the MEA structures improves strength by promoting the formation of the 9R phase compared to the doped single FCC phase. Calculations of the work of adhesion and fracture toughness for the 9R/FCC interface reveal improved interface properties with Mo and W doping. These enhancements are attributed to the hybridization of electronic orbitals in the doped CrCoNi MEA, resulting in changes in bond types between neighboring atoms and subsequent modifications of mechanical properties in the 9R phase. Comparative computational values strongly support an alloy design strategy involving Mo doping and the construction of the 9R phase to achieve superior mechanical properties in MEA.

Computational and experimental investigation of a new non equiatomic FCC single-phase Cr15Cu5Fe20Mn25Ni35 high-entropy alloy

The structural, mechanical, and thermal properties of a new non-equiatomic face-centered cubic (FCC) structured high entropy alloy (HEA) were examined computationally using density functional theory (DFT) calculations and validated experimentally. To identify potential single phase HEA candidates, the composition space of the CrCuFeMnNi alloy system, including 1451 alloys, was investigated using the CALculation of PHAse Diagrams (CALPHAD) method and Thermo-Calc software. The CALPHAD approach successfully predicted an FCC single-phase Cr15Cu5Fe20Mn25Ni35 alloy. Elastic constants and thermal properties of the Cr15Cu5Fe20Mn25Ni35 HEA were determined through DFT calculations using a 100-atom supercell. To validate the DFT calculations, HEA samples were prepared by the arc melting technique, and X-ray diffraction (XRD) analysis confirmed a single-phase FCC structure.The DFT calculated elastic constants values satisfied the Born stability conditions for stability of a cubic crystal structure. Pugh's theory indicated that this alloy exhibits ductility and metallic behavior due to its B/G ratio exceeding 1.75 and positive Cauchy pressure. Additionally, Poisson's ratio exceeded 0.26, further confirming its ductile nature. Experimental tests confirmed these findings by measuring bulk modulus, shear modulus, Young's modulus, and Poisson's ratio. The calculated CV values showed good agreement with experimental data at different temperatures. The linear coefficient of thermal expansion increased rapidly at lower temperatures before becoming linear. Both XRD as well as DFT calculations yielded similar lattice constant results for this HEA. These findings provide insights into various properties of the Cr15Cu5Fe20Mn25Ni35 HEA, including its structural stability, mechanical behavior and thermal characteristics.

Phase formation, microstructure evolution and strengthening mechanism of Au27.9Ag20.3Cu51.8 noble metal medium-entropy alloy during cold deformation and aging

Au27.9Ag20.3Cu51.8 noble metal medium-entropy alloy (MEA) is commercially applied in the electrical contact field due to the unique advantages, such as excellent chemical stability, outstanding electrical property and good processability. However, the low strength brings the poor deformation resistance and wear resistance in service. Herein, the cold deformation and aging treatment were applied to strengthen Au27.9Ag20.3Cu51.8 noble metal MEA, and the related evolutions of the microstructure, tensile property, and electrical resistivity were experimentally investigated. The phase formation and strengthening mechanisms were theoretically discussed. The results show that Au27.9Ag20.3Cu51.8 MEA is composed of the single-phase face-centered cubic (FCC) solid solution at the initial solution treatment of 750 °C. After the aging at 400 °C for 0.5 h, lamellar discontinuous precipitation (DP), consisting of Cu-rich and Ag-rich phases, forms at the grain boundaries (GBs), and L12-type chemical short-range order (CSRO) precipitates in both the matrix and the Cu-rich phase of DP. After 20% cold deformation, the L12-type CSRO similarly emerges in the matrix. After 20% cold deformation prior to aging at 400 °C for 0.5 h, DP at the GBs and L12-type CSRO in the matrix and the Cu-rich lamellae of DP are observed. The growth kinetics of DP and L12-type CSRO are both slightly promoted by prior cold deformation. Au27.9Ag20.3Cu51.8 MEA wire with single-phase FCC sold solution achieves the yield strength of ∼422 MPa. And, 20% cold deformation prior to aging at 400 °C for 0.5 h raises the yield strength of Au27.9Ag20.3Cu51.8 MEA wire to a peak value of ∼696 MPa. The solid solution strengthening mechanism invariably offers the largest strength contribution during the cold deformation and aging, the strain and precipitation strengthening mechanisms almost equivalently contribute to the strength improvement. The electrical resistivity maintains stable with a small variation during the cold deformation and aging. The findings in this study can provide insights for strengthening the noble metal HEAs or MEAs via traditional strengthening methods, and theoretically support for tailoring the microstructure and properties of the high-performance noble metal HEAs or MEAs.

The effect of stacking fault energy on the formation of nano twin/HCP during deformation in FCC concentrated solid solution (CSS) alloys

The exceptional mechanical properties of face-centered cubic (FCC) concentrated solid solution (CSS) alloys generally correlate various deformation mechanisms, such as deformation induced nano twins and/or hexagonal close packed (HCP) phases, which is normally determined by stacking fault energy (SFE). To systemically change the SFE but keep the influence of atomic size and modulus mismatches minimum, a series of NiCo-based alloys are designed by adjusting the Ni/Co ratio. All alloys are fabricated to be single phase FCC with grain size carefully controlled to be ∼60 μm. Deformation substructures after tensile tests over temperature ranging from 77 K to 1073 K are carried out to reveal the effect of SFE on mechanical properties and deformation mechanism. SFE has little effect on yield strength (YS) at all test temperature, but do affect the ultimate tensile strength (UTS) and elongation to fracture (EF) of the alloys. At temperature lower than 873 K, it seems that the lower the SFE, the higher the UTS. At 1073 K, there is no obvious trend for UTS and EF, indicating that SFE has little effect on deformation behavior of alloys at high temperature. Formation of deformation nano twin/HCP depends on not only the SFE of the alloys but also the temperature. As testing temperature and/or SFE of the alloys decreases, deformation induced nano twin/HCP are favorable and become dominant deformation modes for the FCC CSS alloys, resulting in a combination of high strength and good plasticity due to the great work hardening capacity.

Effect of silicon addition on the corrosion resistance of Al0.2CoCrFe1.5Ni high-entropy alloy in saline solution

In the paper, the effect of Si content on the microstructure evolution and corrosion properties of Al0.2CoCrFe1.5NiSix (x = 0, 0.1, 0.2, 0.3) high entropy alloys (HEAs) was systematically investigated. With the addition of Si element, the microstructure changed from single face-centered cubic (FCC) phase in the Al0.2CoCrFe1.5Ni and Al0.2CoCrFe1.5NiSi0.1 HEAs to a dual-phase structure composed of FCC and body-centered cubic (BCC) phases in the Al0.2CoCrFe1.5NiSi0.2 and Al0.2CoCrFe1.5NiSi0.3 HEAs. In addition, the corrosion mechanism has been analyzed by elemental analysis and corrosion morphology. The electrochemical measurement results indicated that the Al0.2CoCrFe1.5NiSi0.1 HEA possessed the best corrosion resistance (Ecorr = − 215 mVAg/AgCl, Icorr = 256 nA/cm2) because the small amount of Si element improves the corrosion resistance of the single-phase FCC HEA. When the Si content increased to 4.08 at% (x = 0.2) and 6.00 at% (x = 0.3), the galvanic corrosion was caused by BCC, resulting in the selective dissolution of the BCC phase and the decrease of corrosion resistance. This research results will help to understand the potential applications of doped trace Si element as a matrix element in single-phase FCC HEAs with good corrosion resistance.

Achieving High Strength and Good Ductility in a Nb-Containing CoCrNi-Based High-Entropy Alloy by Grain Boundary and Precipitates Strengthening

Face-centered cubic (FCC) high-entropy alloys (HEAs) have attracted considerable attention due to their excellent mechanical properties; however, an insufficient yield strength (YS) limits their widespread engineering applications. To improve the strength of FCC HEAs, the present work aims to develop fine-grained Nb-containing CoCrNi-based HEAs with precipitates. In the present work, a single-phase FCC CoCrNi1.5Nb0.2 HEA was processed by cold rolling followed by annealing at a higher temperature and aging at a lower temperature, yielding fine- and ultra-fine-grained FCC matrices and two types of precipitates: ultra-fine granular C15 Laves phase CoCrNb-based precipitates and ultra-fine lath-shaped D019-structured ε-Ni3Nb-based precipitates. The resultant alloy exhibits a combination of high strength (approximately 1409 MPa of yield strength) and good ductility (10.1% of uniform elongation). The contributions of grain boundary and precipitation strengthening to YS were analyzed and calculated. The mechanisms underlying good ductility were discussed.

Achieving superb strength in single-phase FCC alloys via maximizing volume misfit

Single-phase face-centered cubic (SP-FCC) alloys normally possess low strength. Conventionally strengthening strategies inevitably cause significant ductility sacrifice. Here, a single-phase Ni-based FCC alloy with a superb yield strength of ∼1.05GPa and a good ductility of 37% is designed through maximizing the volume misfits. The misfit of the purposely targeted Ni80Mo20 alloy is severer than all existing FCC alloys, bringing the alloy a highest-ever Hall-Petch coefficient (kHP = 1034 MPa·μm1/2) and a pronounced solid solution strengthening (Δσss = 224 MPa). Current work yields two surprising and novel findings for SP-FCC alloys. First, volume misfit is a good pertinent indicator of kHP. Second, the conventional impression about the sole contribution of edge dislocations to strengthening in SP-FCC alloys may no longer hold; instead, screw dislocations can also kick in once the nonsphericity of the solute-induced stress field reaches a critical value. Altogether, this work paves a new avenue of pursuing ultimate strengthening without significant ductility sacrifice for SP-FCC alloys relying on the volume-misfit-maximization strategy.

Nanolamellar phase transition in an additively manufactured eutectic high-entropy alloy under high pressures

Much is unknown about how phase transitions link to micro-/nano-structures in high-entropy systems, especially under extreme pressure and temperature conditions. This work studies the evolution of dual-phase nanolamellar eutectic high-entropy alloy phases of AlCoCrFeNi2.1 generated by laser powder-bed fusion (L-PBF) for pressures up to 42 GPa. We compare quasi-hydrostatic high pressure synchrotron x-ray diffraction studies on L-PBF printed cylindrical samples up to 5.5 GPa (large-volume Paris–Edinburgh cell) to those carried out on an L-PBF printed foil in a diamond anvil cell where the pressure reached 42 GPa. Our results show that the initially alternating face-centered cubic (FCC) and body-centered cubic (BCC) nanolamellar structure of AlCoCrFeNi2.1 transformed into single-phase FCC nanolamellae under high pressure with BCC–FCC phase transformation completion at 21 ± 3 GPa. Our results indicate a diffusionless BCC–FCC transformation in this additively manufactured far-from-equilibrium microstructure and demonstrate that the FCC phase is stable up to very high pressures. The measured equation of state for the FCC phase of AlCoCrFeNi2.1 is presented up to 42 GPa and shows excellent agreement between the data obtained in large-volume press and diamond anvil cell experiments.

Investigation on microstructure, superior tensile property and its mechanism in Al0.3CoCrFeNi high-entropy alloy via thermo-mechanical processing

In this study, the fully annealed Al0.3CoCrFeNi high entropy alloy (HEA) with single-phase face-centered cubic (FCC) was cryo-rolled and isochronal annealed at different temperatures from 1123 K to 1473 K for 1 h. The effects of annealing temperature on microstructure, grain boundary evolution, and mechanical properties, especially the deformation mechanism of cryo-rolled Al0.3CoCrFeNi alloys were systematically investigated using XRD, EBSD, TEM, microhardness, and tensile tests. A partial recrystallization was achieved after annealing in the 1123 K and 1223 K temperature ranges, which was mainly attributed to that the precipitation of B2 phase can effectively improve the nucleation rate of phase boundary and play a barrier role in growth of grain. Additionally, The remarkable combination of strength (yield strength of ∼911 MPa and ultimate tensile strength of ∼1146 MPa) and ductility (fracture elongation of ∼23%) achieved in cryo-rolled Al0.3CoCrFeNi alloy annealed at 1123 K with heterogeneous microstructure was mainly due to the contributions of back stress and grain size. It was noticeable that the FCC → 9R phase → twin transformation was found in cryo-rolled Al0.3CoCrFeNi HEA annealed at 1473 K during the tensile deformation, which indicated that twin-induced plasticity and phase transformation induced plasticity were reason for the outstanding ductility and strain hardening ability of this sample. This research are important not only for understanding the strengthening mechanisms of HEAs with various microstructure in general, but also for guiding thermo-mechanical processing of single-phase FCC HEAs.

Enhanced strengthening effect via nano-twinning in cryo-rolled FeCoCrNiMo0.2 high-entropy alloys

High-entropy alloys (HEAs) with face-centered cubic (FCC) structure generally exhibit extraordinary ductility but relatively low strength, which limits its widespread applications as engineering structural materials. Enhancing the strength of FCC HEAs is crucial to broaden its application. In present work, by performing rolling at cryogenic temperature (77 K), we obtained a significantly enhanced ultimate strength as high as 1557.7 MPa in a typical single-phase FCC FeCoCrNiMo0.2 HEA. A high intensity of Brass texture and Goss texture are formed in the cryo-rolled specimens. With the increasing of cryo-rolling reduction, twins are generated inside grains with the Copper texture, forming the Copper-twin texture. Due to the low stacking fault energy of FeCoCrNiMo0.2 HEA, abundant nanoscale deformation twins and stacking faults are generated inside the cryo-rolled specimens. Besides, a high density of dislocation networks and Lomer-Cottrell dislocation locks are also observed. Hence, the interactions among the dislocations, nano-twins, stacking faults, and Lomer-Cottrell dislocation locks are speculated to effectively contribute to the enhanced ultimate strength and work hardening. Our results are forward to improve the strength of single-phase FCC HEAs as structural materials.