Multi-principal Element Research Articles

Advanced Polycrystalline γ′-Strengthened CoNiCr-Based Superalloys

Novel compositionally complex CoNiCr-based superalloys with excellent mechanical properties have been developed, which combine the multiprincipal element nature of high-entropy alloys with the precipitation strengthening in superalloys. A series of advanced polycrystalline γ′-strengthened CoNiCr-based superalloys, called CoWAlloys, with varying contents of Al, W, Ti, Ta, Mo, and Nb are investigated in terms of microstructure, thermophysical properties, yield, and creep strength. The microstructure of all CoWAlloys consists of an fcc solid solution matrix phase (approximate γ composition in at. pct: 50Co–20Ni–20Cr–10X (X = other alloying elements)), which is strengthened by a multicomponent γ′ (Ni,Co)3(Al,Ti,Ta,W,Nb)-based precipitate phase with a very high-volume fraction of around 60 vol pct (approximate γ′ composition in at. pct: 45Ni–30Co–25X). These alloys have high solidus temperatures above 1300 °C and moderate γ′ solvus temperature between 985 °C and 1080 °C leading to a large processing window. The increasing content of γ′-forming elements Ti, Ta, W, and Nb decreases this window, but increases the γ/γ′ lattice misfit and the anti-phase boundary energy, which contribute to a significantly higher yield and creep strength. Their properties are discussed in comparison with conventional polycrystalline Ni-base superalloys and so-called L12-strengthened high-entropy alloys, revealing that the creep strengths of the CoWAlloys are significantly higher. This is due to the reduced strain rate sensitivity of the CoWAlloys due to different underlying deformation mechanisms: By increasing the anti-phase boundary energy, a transition to stacking fault shearing and microtwinning occurs, which leads to the enhanced creep strength. Based on these results, guidelines and strategies for the design of next-generation advanced high-temperature polycrystalline superalloys are proposed.Graphical

Multi-objective optimization and machine learning assisted design and synthesis of magnesium based novel non-equiatomic medium entropy alloy

Non equiatomic compositionally complex alloys have gained significant attention due to their unique combinations of properties including high strength, good ductility, excellent thermal stability and corrosion resistance. The non equiatomic compositions provide flexibility in tailoring the mechanical, thermal, magnetic, biomedical and other functional properties to meet the specific application requirements. In this study, a novel design approach was proposed to identify quaternary non equiatomic medium entropy alloy for biomedical application. The design methodology employs multi-objective optimization of empirical phase formation parameters and ML algorithm to design solid solution containing non equiatomic medium entropy alloy. The suggested approach scans over a large composition space to find possible non-equiatomic compositionally complex alloy compositions (CCA) with desired phases. The proposed methodology is an alternative to trial-and-error alloy fabrication and CCA design approach reported in the literature. This step significantly reduced the time and resources utilized in CCA design by decreasing the number of experimental runs. Based on the design strategy, Mg60Ti24.24Zn9.66Nb6.1 alloy was designed and selected for fabrication through powder metallurgy route by employing microwave sintering. The Mg rich alloy exhibited the presence of two solid solution phases viz HCP and BCC in both prealloyed powder and final sintered specimen. The developed alloy showed a high microhardness value of 153.9 ± 0.85HV with a measured density of 2.82 ± 0.03 g/cm3. The mechanical properties such as yield strength, ultimate compressive strength, elastic modulus and fracture strain of the developed alloy were found to be superior to that of pure Mg and AZ31 fabricated through the same manufacturing route. Mechanical properties of local phases present within the alloy were assessed through nanoindentation technique. Elastic modulus (E) and hardness (H) values for the Ti rich BCC phase were 118.9±7.2 GPa and 4.65±1.1 GPa, respectively, whereas the Mg rich HCP phase showed values of 65.2±7 GPa and 1.6±0.43 GPa, respectively. Furthermore, with minor changes to the constraint equations and correct ML dataset selection, the suggested design technique can be applied to any multi-principal element alloy system to design non-equiatomic compositions.

Insights into a novel refractory multi principal element alloy (Mo98W2)85Nb10(TiHf)5 and advancements in oxidation resistance upto 1300 °C through silicide coatings

In the present work, a novel refractory multi-principal element alloy (Mo98W2)85Nb10(TiHf)5 was developed through comprehensive material testing, including high-temperature exposure and oxidation resistance assessments. It was found that after pack siliconizing, the samples could withstand high temperatures up to 1300 °C with a marginal weight change of 0.255 mg/cm2 during four hours holding time. The overall activation energy for the coating process was found to be 123 kJ/mol, which is a consequence of the decreased atomic mobility of silicon and other atoms to overcome sluggish diffusion phenomena. Minor addition of hafnium created a peg effect in the outer silica scale, which improved its oxidation resistance covering coating cracks. The findings of this research provide valuable insights into the potential applications of such alloys with silicide coatings in high-temperature environments, such as aerospace, energy, and high-temperature manufacturing processes.

Inhibited Surface Diffusion in Nanoporous Multi-Principal Element Alloy Thin Films Prepared by Vacuum Thermal Dealloying

Nanoporous structures with 3D interconnected networks are traditionally made by dealloying a binary precursor. Certain approaches for fabricating these materials have been applied to refractory multi-principal element alloys (RMPEAs), which can be suitable candidates for high-temperature applications. In this study, nanoporous refractory multi-principal element alloys (np-RMPEAs) were fabricated from magnesium-based thin films (VMoNbTaMg) that had been prepared by magnetron sputtering. Vacuum thermal dealloying (VTD), which involves sublimation of a higher vapor pressure element, is a novel technique for synthesizing nanoporous refractory elements that are prone to oxidation. When VMoNbTaMg was heated under vacuum, a nanoporous structure was created by the sublimation of the highest vapor pressure element (Mg). X-ray photoelectron spectroscopy depth profiling indicated significantly less ligament oxidation during VTD as compared to traditional dealloying methods. Furthermore, np-RMPEAs exhibited outstanding stability against coarsening, retaining smaller ligaments (~25 nm) at elevated temperature (700 °C) for a prolonged period (48 h).

Effects of Chemical Short-Range Order and Temperature on Basic Structure Parameters and Stacking Fault Energies in Multi-Principal Element Alloys

In the realm of advanced material science, multi-principal element alloys (MPEAs) have emerged as a focal point due to their exceptional mechanical properties and adaptability for high-performance applications. This study embarks on an extensive investigation of four MPEAs—CoCrNi, MoNbTa, HfNbTaTiZr, and HfMoNbTaTi—alongside key pure metals (Mo, Nb, Ta, Ni) to unveil their structural and mechanical characteristics. Utilizing a blend of molecular statics and hybrid molecular dynamics/Monte Carlo simulations, the research delves into the impact of chemical short-range order (CSRO) and thermal effects on the fundamental structural parameters and stacking fault energies in these alloys. The study systematically analyzes quantities such as lattice parameters, elastic constants (C11, C12, and C44), and generalized stacking fault energies (GSFEs) across two distinct structures: random and CSRO. These properties are then evaluated at diverse temperatures (0, 300, 600, 900, 1200 K), offering a comprehensive understanding of temperature’s influence on material behavior. For CSRO, CoCrNi was annealed at 350 K and MoNbTa at 300 K, while both HfMoNbTaTi and HfNbTaTiZr were annealed at 300 K, 600 K, and 900 K, respectively. The results indicate that the lattice parameter increases with temperature, reflecting typical thermal expansion behavior. In contrast, both elastic constants and GSFE decrease with rising temperature, suggesting a reduction in resistance to stability and dislocation motion as thermal agitation intensifies. Notably, MPEAs with CSRO structures exhibit higher stiffness and GSFEs compared to their randomly structured counterparts, demonstrating the significant role of atomic ordering in enhancing material strength.

Uncovering origin of grain boundary resistance to irradiation damage in NiCoCr multi-principal element alloys

Multi-principal element alloys (MPEAs) demonstrate significant promise as structural materials for nuclear energy equipment owing to their exceptional mechanical properties and radiation-resistant performances. In these alloys, the grain boundary (GB) serves as a crucial microstructure that typically mitigates irradiation damage by absorbing the irradiation-induced defect. However, the micromechanisms governing the anti-irradiation performance of GBs in MPEAs remain unclear. In this study, we investigate the irradiation defect production during collision cascade in the model NiCoCr bicrystal system through atomic simulations, aiming to unveil the atomic-scale origin of GB to resist irradiation damage in MPEAs. The results reveal that GBs effectively serve as sinks for irradiation defects in NiCoCr. The sink efficiency depends on the GB energetic state, including GB excess energy and defect segregation energy, as well as the energetic difference between interstitial and vacancy segregation. Statistical analysis identifies a universally exponent function between the defect absorption rate at GB and GB energetic state. In NiCoCr, the GB-disorder-induced-entropy increase leads to a biased reduction in interstitial segregation energy, narrowing the gap between interstitial and vacancy segregation energies by approximately 11 % compared to Ni. This improvement enhances the overall resistance of GBs to irradiation damage. Additionally, preferential segregation of Ni interstitial atoms is notably enhanced in NiCoCr, contributing to a high defect absorption rate at GBs. This study provides new insights into the resistance of GBs to irradiation defects in MPEAs and suggests GB engineering as an effective strategy for developing advanced alloys with enhanced radiation tolerance.

Development of FeCoMnAl multi-principal element alloys and soft magnetic powder cores with excellent magnetic properties and high temperature magnetic stability

In order to develop a multi-principal element alloy with outstanding soft magnetic properties and high thermal stability, a Fe35Co35Mn30−xAlx (x = 9, 12, 15, 18, 21, 24) alloy system was designed. The findings demonstrated that the optimal Al/Mn content ratio contributes to the formation of a BCC and B2 dual-phase structure, which leads to a significant increase in the saturation magnetic flux density (Bs) and a considerable decrease in the coercivity of the alloy. The as-cast Fe35Co35Mn15Al15 alloy with a co-lattice structure exhibits exceptional soft magnetic properties, with a resistivity of 324 μΩ·cm, a Bs of 1.46 T and a Hc of 167.2 A/m. Corresponding soft magnetic powder cores (SMPCs) were fabricated and the impact of heat treatment temperature on the properties of the SMPCs was examined. The SMPCs after annealing at 660 °C exhibits high stable permeability (μe) of 48 up to 10 MHz, a low core loss (Pcv) of 1208 mW/cm3 (at 1 MHz, 20 mT), and excellent direct current bias performance. In addition, these SMPCs possess excellent high temperature magnetic stability, and the variation of both μe and Pcv at a high frequency (1 MHz) is less than 2 % over a wide temperature range of 25–150 °C.

Chemical composition based machine learning model to predict defect formation in additive manufacturing

With a goal of exploiting additive manufacturing to improve the manufacturing of existing reactor materials, we developed a chemical composition-based machine learning model to predict the printability of any given alloy in laser powder bed fusion (L-PBF) using experimental data from peer-reviewed literature. We defined printability as the ability to avoid defects like cracking, balling, porosity, and lack of fusion, that are caused by thermal stresses (during solidification or liquation), molten pool disintegration into disconnected small beads or lack of heat input respectively. Our models predict the tendency of balling defect formation and porosity percentage for a given composition, under a given set of processing conditions. To predict the likelihood of balling defect, three models: a random forest classifier, a gradient boost regressor and a neural network were trained on a dataset containing both single and multi principal element alloys. The neural network model showed the highest accuracy of 92.3 % in predicting the balling defect formation. A random forest regressor, gradient boost regressor and neural network were trained and tested on a dataset of various alloys to predict porosity. The random forest regressor showed the best predictions with an R2 score of 0.97. The models also revealed the relative importance of the input descriptors on defect-formation tendency. Of particular significance was the identification of carbon as an important element in determining the occurrence of balling and percent porosity in alloys like steel, as well as being moderately important to the percentage porosity in other alloys as well as steel. Manganese was also identified as a key descriptor for the percentage of porosity in steel and other alloys. Manganese's low thermal conductivity and consistent presence in the dataset is the likely cause for its contribution. Carbon's role is attributable to its relatively high specific heat and high melting temperature. Our model serves as a swift, chemistry-based tool to design experiments and find modified compositions better suited for additive manufacturing.

Strength-ductility synergy and superior strain-hardening ability of Ni38Co25Fe13Cr10Al7Ti7 multi principal element alloy through heterogeneous L12 structure modulation

In this paper, heterogeneous L12 particles were successfully introduced into Ni38Co25Fe13Cr10Al7Ti7 MPEA by two-stage aging treatment. Compared with the homogeneous particles, the heterogeneous L12 particles strengthened alloy exhibits enhanced strength-plasticity synergy. The tensile elongation is 23.2%, which is higher than all samples with the homogeneous particles, and the yield strength remains at 942 MPa, which is equivalent to the micro particles (972 MPa) with the highest yield strength in the homogeneous particles. Further analysis of the strengthening and deformation mechanisms revealed that precipitation hardening, and heterogeneous deformation-induced hardening were the main factors for the enhanced strength. The deformation-induced layered fracture networks, high-density Lomo-Cottrell locks, and heterogeneous L12 particles microstructure characteristics significantly improve the strain hardening ability; GNDs accumulate at the heterogeneous interface to alleviate the stress concentration at the GBs and improve the tensile plasticity of the MPEA with heterogeneous L12 particle structure. The heterogeneous L12 particle structure obtained through two-stage aging treatment without severe mechanical deformation has attractive engineering application potential. These properties of Ni38Co25Fe13Cr10Al7Ti7 MPEA with heterogeneous L12 particle structure have important enlightening and practical value.

Rejuvenation as the origin of planar defects in the CrCoNi medium entropy alloy

High or medium- entropy alloys (HEAs/MEAs) are multi-principal element alloys with equal atomic elemental composition, some of which have shown record-breaking mechanical performance. However, the link between short-range order (SRO) and the exceptional mechanical properties of these alloys has remained elusive. The local destruction of SRO by dislocation glide has been predicted to lead to a rejuvenated state with increased entropy and free energy, creating softer zones within the matrix and planar fault boundaries that enhance the ductility, but this has not been verified. Here, we integrate in situ nanomechanical testing with energy-filtered four-dimensional scanning transmission electron microscopy (4D-STEM) and directly observe the rejuvenation during cyclic mechanical loading in single crystal CrCoNi at room temperature. Surprisingly, stacking faults (SFs) and twin boundaries (TBs) are reversible in initial cycles but become irreversible after a thousand cycles, indicating SF energy reduction and rejuvenation. Molecular dynamics (MD) simulation further reveals that the local breakdown of SRO in the MEA triggers these SF reversibility changes. As a result, the deformation features in HEAs/MEAs remain planar and highly localized to the rejuvenated planes, leading to the superior damage tolerance characteristic in this class of alloys.

Effect of Al related chemical short-range order on the irradiation resistance of Al0.5CoCrFeNi multi-principal element alloy: A molecular dynamics simulation

Multi-principal element alloys (MPEAs) have received much attention for future nuclear materials due to their exceptional radiation resistance. This work investigated the effect of Al related chemical short-range ordered (CSRO) which caused by its large atomic size in face-centered cubic (FCC) Al0.5CoCrFeNi alloy on its irradiation damage behavior through mixed molecular dynamics and Monte Carlo simulations. The calculation results indicate that the CSRO structure effectively suppresses the generation of point defects during irradiation in Al0.5CoCrFeNi alloy, and this inhibitory effect increases with increasing irradiation energy. The enhanced radiation resistance of CSRO alloy is attributed to the high vacancy formation energy, interstitial atom formation energy, and vacancy migration energy barrier of Al. Moreover, the Al atoms in CSRO alloy can also act as defect barriers, preventing the migration of vacancy defects; The formation energy of interstitial atoms in the CSRO alloy exhibits a strong element dependence, and there is a significant difference in the formation energy of interstitial atoms between different points, which strengthens the hysteresis diffusion effect of MPEAs and further enhances the radiation resistance of MPEAs.

Novel Al-Sc-Ti-Zr multi-principal element alloys with outstanding mechanical properties

Non-equiatomic multi-principal element alloys, namely Al5Sc25Ti35Zr35, Al10Sc20Ti35Zr35 and Al15Sc15Ti35Zr35, with a density of approximately 4.95 × 103 kg/m3, were prepared by casting and subsequently treated with dual-annealing. The microstructure and compressive mechanical properties of both the as-cast and annealed samples were investigated at room temperature, while the mechanical properties of those with good performance were also studied at 873 K. In the as-cast Al5Sc25Ti35Zr35 alloy, a single hcp phase was formed, while dual hcp phases were observed in the annealed state. With an increase in Al content, the amount of the bcc phase increased. The compressive strength initially decreased and then increased for the as-cast samples, whereas it consistently decreased for the annealed ones. Among all the tested samples, the Al15Sc15Ti35Zr35 alloy demonstrated the most outstanding mechanical properties, achieving a high compressive strength of 2584 MPa at a strain of 31.9% at room temperature, and a strength of 804 MPa with a strain exceeding 50% at 873 K. Regarding the other hcp-containing alloys, the Al5Sc25Ti35Zr35 alloy with a dual-hcp structure exhibited a high strength of 2040 MPa at a strain of 14.9% and a strength of 692 MPa with a strain of 45% at 873 K. The underlying reasons for the differences in mechanical properties were thoroughly discussed. This research provides valuable insights into the development of the lightweight multi-principal element alloys containing hcp or bcc phase, which exhibit outstanding properties for future applications as structural materials.

Impacts of local chemical ordering on the primary radiation damage in Cr–Fe–Ni multi-principal element alloys

Face-centered cubic (FCC) single-phase multi-principal element alloys (MPEAs) have demonstrated promising irradiation resistance. The generation and evolution of irradiation defects are integral aspects to comprehend, as they offer valuable insights into the radiation resistance of materials. In this work, we use molecular dynamics simulations to investigate the primary radiation damage of CrFeNi FCC MPEAs, while focusing on the impact of local chemical ordering (LCO) on the generation and evolution of defects. Our simulation results reveal that the LCO reduces the probability of defect production, consequently mitigating the maximum number of defects. Furthermore, the LCO increases defect migration barriers, thereby impeding the migration of interstitial atoms and leading to an increased number of residual defects subsequent to primary damage. However, the dispersed distribution of defects throughout space facilitates the uniform nucleation of dislocations in subsequent irradiation cascades. Additionally, the increased migration energy acts as a barrier to the movement of defect clusters, impeding their growth and thereby enhancing the radiation resistance of MPEAs. Our findings underscore the potential for enhancing the radiation resistance of MPEAs through the deliberate tailoring of LCOs.

Hydroxyl transport mechanisms upon passivation of Cr-Fe-Co-Ni-Mo multi-principal element alloy surfaces investigated by isotopic labelling

Time-of-flight secondary ion mass spectrometry (ToF-SIMS) combined with deuterium labelling (D2O), and X-ray photoelectron spectroscopy (XPS) were applied to investigate the interfacial transport mechanisms of hydroxyls between aqueous electrolyte and outer part of the protective oxide film during passivation of Cr15Fe10Co5Ni60Mo10 multi-principal element alloy surfaces. A ToF-SIMS signal treatment methodology was developed to retrieve the in-depth distribution of deuterated hydroxides in the nanometer-thick surface oxide film. After immersion at free potential or anodic passivation in acidified D2O, the initial bilayer structure is retained. The use of heavy water electrolyte has no effect on the composition alterations induced by passivation. Deuterated species were detected mostly in the outer layer of the surface oxide film. Pure inward hydroxyl diffusion from electrolyte/oxide to oxide/metal interface is excluded from being primarily responsible for oxide film growth induced by anodic passivation. The hydroxyls from the dissolving hydroxide species are retained in the oxide film and participate in forming Cr hydroxide, thus reducing the hydroxyl surface uptake from the electrolyte. This work provides deeper insight into the mechanisms of oxide growth and corrosion protection induced by anodic passivation.

Phase formation criteria for structurally ordered high-entropy intermetallics

Structurally ordered high-entropy intermetallics (SOHEI), as the latest paradigm of high-entropy materials, exhibit a variety of excellent and attractive mechanical and functional properties, which are mainly derived from the formation of multi-principal element intermetallics. Nevertheless, the phase formation criteria for SOHEI have not been systematically investigated so far. In this work, a series of parameter formulas related to the mixing entropy (ΔSmix*), mixing enthalpy (ΔHmix*, ΔHmixA−B), atomic size difference (δr*, δrA−B), electronegativity difference (Δχ*, ΔχA−B), standard deviation of valence electron concentration (VEC) (σVEC*, σVECA−B), and total VEC (VECtot) based on the sublattice model were established. The universal criteria for phase formation of SOHEI were proposed, which states that various atoms occupying the common and different sublattice need to satisfy: − 8 kJ∙mol−1 ≤ ΔHmix*≤ 4 kJ∙mol−1, δr* ≤ 6.5%, Δχ*≤ 8%, σVEC*≤ 1.6, − 60 kJ∙mol−1 ≤ ΔHmixA−B ≤ 0 kJ∙mol−1, 2.5% ≤ δrA−B ≤ 16.5%, ΔχA−B ≥ 2.5% and 0.5 ≤ σVECA−B ≤ 3.5. Importantly, the VECtot of alloy system was identified as the crucial factor for determining the phase types of SOHEI, which can be utilized to design and regulate the desired type of intermetallic phase. The correctness and effectiveness of the proposed criteria based on sublattice model were well verified by designed experiments. The quantitative insights of phase formation criteria are greatly useful for the preliminary screening alloy components, and this contribution can provide meaningful guidance for the future development of SOHEI.

Minimizing the diffusivity difference between vacancies and interstitials in multi-principal element alloys

Interstitial atoms usually diffuse much faster than vacancies, which is often the root cause for the ineffective recombination of point defects in metals under irradiation. Here, via ab initio modeling of single-defect diffusion behavior in the equiatomic NiCoCrFe(Pd) alloy, we demonstrate an alloy design strategy that can reduce the diffusivity difference between the two types of point defects. The two diffusivities become almost equal after substituting the NiCoCrFe base alloy with Pd. The underlying mechanism is that Pd, with a much larger atomic size (hence larger compressibility) than the rest of the constituents, not only heightens the activation energy barrier (Ea) for interstitial motion by narrowing the diffusion channels but simultaneously also reduces Ea for vacancies due to less energy penalty required for bond length change between the initial and the saddle states. Our findings have a broad implication that the dynamics of point defects can be manipulated by taking advantage of the atomic size disparity, to facilitate point-defect annihilation that suppresses void formation and swelling, thereby improving radiation tolerance.

Ultrahigh Degree of Cationic Disorder, Configurational Entropy in New Type of High-Entropy Pseudobrookite Phase.

High-entropy ceramics exhibit various excellent properties owing to their high configurational entropy, which is caused by multi-principal elements sharing one lattice site. The configurational entropy will further increase significantly if multi-principal elements randomly share two different lattice sites. For this purpose, pseudobrookite phase containing two cationic lattice sites (A and B sites) is selected, and corresponding high-entropy pseudobrookite (M2+ 0.4M3+ 1.2)Ti1.4O5 is synthesized. Herein, the distribution of the 2-valent and 3-valent cations in the A and B sites are analysed in depth. The distance between the A and B sites in the crystal structure models which are constructed by the Rietveld analysis is calculated and defined as distance d. Meanwhile, the atomic column positions in the STEM images are quantified by a model-based mathematical algorithm, and the corresponding distance d are calculated. By comparing the distance d, it is determine that the 2-valent and 3-valent cations are jointly and disorderly distributed in the A and B sites in high-entropy (M2+ 0.4M3+ 1.2)Ti1.4O5. The density functional theory (DFT) simulations also demonstrate that this type of crystal structure is more thermodynamically stable. The higher degree of cationic disorder leads to a higher configurational entropy in high-entropy (M2+ 0.4M3+ 1.2)Ti1.4O5, and endows high-entropy (M2+ 0.4M3+ 1.2)Ti1.4O5 with very low thermal conductivity (1.187-1.249Wm-1K-1).

The shear softening and dislocation glide competition due to the shear-induced short-range order degeneration in CoCrNi medium-entropy alloy

The short-range order (SRO) in multi-principal element alloys (MPEAs) is an intriguing topic in advanced alloy research. The crucial question related to this topic lies in the effect of the local chemical fluctuations on the deformation behavior of MPEAs. In this study, the large-scale molecular dynamics (MD) simulation is used to investigate the dislocation glide behavior and mechanical performance of CoCrNi medium-entropy alloy (MEA) with respect to the SRO and lattice distortion (LD) effects. The slip plane softening and dislocation glide competition are found in the models with SRO. The change of energy barrier caused by SRO degeneration is the dominant reason for the slip plane softening, while the combination of dislocation pinning and slip plane softening leads to the dislocation glide competition, which is the primary mechanism for the shear localization in the CoCrNi MEA with SRO. Moreover, the dislocation glide competition compensates for the strength loss induced by slip plane softening. The results provide a new proposition for the conflicting simulation and experimental results on the topic of the SRO effect in MPEAs.

Microstructure and mechanical properties of multi-principal component CoCrNiMo medium entropy alloys

In traditional high entropy alloy (HEA) systems, achieving a synergistic enhancement of alloy strength and ductility remains a challenge. However, dual-phase high entropy alloys, with their simultaneous high strength and high ductility, have shown potential for industrial applications. In this study, three multi-principal element CoCrNiMo alloys were synthesized by adding varying amounts of Mo to the base medium entropy Co45Cr27Ni28 alloy, which possesses a single face-centered cubic (FCC) phase. The impact of variations in Mo content on the alloy's microstructure and mechanical properties was investigated. Experimental data revealed that all three alloys formed dual-phase (FCC + μ) precipitation-strengthened medium entropy alloys (MEAs), where the μ phase is enriched with Mo, and its quantity increases with the increase in Mo content. With the rise in Mo content, the alloy's hardness increased from 240 ± 20 HV to 668 ± 35 HV, and tensile strength also increased, albeit at the cost of reduced plasticity. The single-phase medium entropy Co45Cr27Ni28 alloy exhibited a tensile strength and elongation rate of 591 ± 15 MPa and 44 ± 3%, respectively. The tensile strength of the dual-phase CoCrNiMo HEAs could be elevated to 857 ± 16 MPa and 1045 ± 25 MPa, while maintaining considerable elongation rates of 16 ± 2% and 5 ± 1%, respectively. This work provides a pathway for strengthening CoCrNi alloys and contributes positively to the development of high-performance HEAs.

The influences of vanadium addition and temperature on the microstructure, and mechanical properties of Al0.1CoCrFeNiVx multi-principal element alloys

The study aimed to examine the crystal structure, microstructure, microhardness, and tensile properties of Al0.1CoCrFeNiVx (x = 0.0, 0.2, 0.4, 0.6, 0.8, 1.0) multi-principal element alloys. Both Al0.1CoCrFeNi and Al0.1CoCrFeNiV0.2 alloys were single FCC phase solid solutions. σ-phase precipitated in Al0.1CoCrFeNiV0.4 alloy. The volume fraction of σ-phase increased as V content increased from x = 0.4 to 1.0. The volume fraction of σ-phase in Al0.1CoCrFeNiV alloy was found to be 48.4 vol%. It was shown that to predict the phase composition of multi-principal element alloys containing σ-phase employing solid solution formation model, coupling it with criteria of σ-phase formation was necessary. Moreover, when the V content was x ≤ 0.4, the alloys showed good toughness and ductility (> 49%). However, a continuous increase in V content led to a higher volume fraction of σ-phase, which enhanced the strengthening/hardening effect until reaching the embrittlement threshold. At x = 0.4, the alloy exhibited a better balance between strength and plasticity. Uniaxial tensile tests were performed on the annealed Al0.1CoCrFeNiV0.4 alloy at different temperatures. Results suggested that the alloy retains excellent plasticity (> 45%) at 500 ℃. Nevertheless, it exhibited a decrease in strength and plasticity with elevated temperatures. The addition of V caused a significant increase in ultimate strength and elongation at mid-temperatures (500–700 ℃) compared to Al0.1CoCrFeNi alloy. The engineering stress-strain curves of the Al0.1CoCrFeNiV0.4 alloy displayed serrated flow at temperatures from 300 ℃ to 700 ℃. The serration type transitioned from A-type to C-type as the temperature rose.