Multi-principal Element Research Articles

Microstructure, mechanical behavior, and cavitation erosion-corrosion resistance of the BCC/B2 strengthened Fe44.2Ni27Cr23Mo1.8Al4 multi-principal element alloys

Microstructure, mechanical behavior, and cavitation erosion-corrosion resistance of the BCC/B2 strengthened Fe44.2Ni27Cr23Mo1.8Al4 multi-principal element alloys

Improving strength and plasticity via pre-assembled dislocation networks in additively manufactured refractory high entropy alloy

Refractory high entropy alloys (RHEAs), as a novel class of multi-principal element alloys, have attracted significant attention owing to their excellent properties. However, their low plasticity limits their potential applications, while the high melting points of the alloying elements face challenges to additive manufacturing (AM). Herein, RHEA, with extensively distributed cellular structure within their grains, was successfully fabricated using AM. Furthermore, we proposed a simple strategy to form a complete dislocation network within the cellular structure region in advance through cyclic deformation processing in the elastic stage (microplastic deformation). Dislocation networks are entangled with other dislocations, creating numerous pinned points adjacent cell walls, which impede dislocation motion. As a result, the cyclic deformation processing of RHEA achieves a yield strength of 1136 MPa while maintaining 50% deformation strain without fracturing. The cyclic deformation processing method provides a route to strengthen additively manufactured alloys, offering a solution to overcome the trade-off between strength and plasticity.

Influence of grain size on strain-induced phase transformation in a CrCoNi multi-principal element alloy

Influence of grain size on strain-induced phase transformation in a CrCoNi multi-principal element alloy

Correlations between local chemical fluctuations and grain boundary strength in Ti-Zr-Nb-Ta-Mo refractory multi-principal element alloys

Ti-Zr-Nb-Ta-Mo refractory multi-principal element alloys typically exhibit high yield strength while tensile ductility tends to be poor. In this study, we found that even after solid solution treatment of Ti-Zr-Nb-Ta-Mo alloys, significant change in ductility occur due to the local chemical fluctuations. Ta's dramatic chemical fluctuations cause variations in atomic-scale strain fields, leading to reduced grain boundary strength and brittle fracture. First-principles calculations show that (Ti, Zr)-rich at the grain boundaries increases the delocalization of valence electrons, leading to longer bond lengths and reduced crystal orbital bond index values, thereby causing grain boundary embrittlement. Our findings explore the root causes of brittleness in Ti-Zr-Nb-Ta-Mo alloys at the atomic and electronic scales, providing not only a method to analyze grain boundary embrittlement using bond length, electronic localization function and crystal orbital bond index, but also a theoretical guidance for improving the mechanical properties via grain boundary structure optimization.

Influence of Al and Ti Alloying and Annealing on the Microstructure and Compressive Properties of Cr-Fe-Ni Multi-Principal Element Alloy

This study meticulously examines the influence of aluminum (Al) and titanium (Ti) on the genesis of self-generated ordered phases in high-entropy alloys (HEAs), a class of materials that has garnered considerable attention due to their exceptional multifunctionality and versatile compositional palette. By meticulously tuning the concentrations of Al and Ti, this research delves into the modulation of the in situ self-generated ordered phases’ quantity and distribution within the alloy matrix. The annealing heat treatment outcomes revealed that the strategic incorporation of Al and Ti elements facilitates a phase transformation in the Cr-Fe-Ni medium-entropy alloy, transitioning from a BCC (body-centered cubic) phase to a BCC + FCC (face-centered cubic) phase. Concurrently, this manipulation precipitates the emergence of novel phases, including B2, L21, and σ. This orchestrated phase evolution enacts a synergistic enhancement in mechanical properties through second-phase strengthening and solid solution strengthening, culminating in a marked improvement in the compressive properties of the HEA.

Novel as-cast HfNbTaTiAl refractory multi-principal element alloys with superior strength-ductility combination at room temperature

Refractory multi-principal element alloys (RMPEAs) have attracted much attention due to their excellent high temperature mechanical properties. However, extremely low room temperature tensile ductility hinders further engineering applications. In this work, we designed novel HfNbTaTiAl RMPEAs and studied the structure-properties relation and deformation mechanism at room temperature. Results show that the yield strength of the as-cast alloy is up to 1029 MPa and the tensile ductility at room temperature is between 17.8 % and 22.8 %. The high yield strength is mainly attributed to the solid solution strengthening effect. Cross-slip provides ultimate tensile strength while maintaining the tensile ductility. Kink bands have been observed in Al9.0Hf30.3Nb15.0Ta15.3Ti30.3 alloy, which is the main reason for its excellent tensile ductility at room temperature. Overall, this study elucidated the deformation mechanism of Al9.0Hf30.3NbxTa30.3-xTi30.3 (x = 0, 10, 15) as-cast RMPEAs at room temperature, and provides meaningful insights for developing RMPEAs with excellent strength-ductility combinations.

Microstructures and properties of a novel cemented carbide prepared using a Co–Ni–Cu multiprincipal element alloy as the binder

A Co–Ni–Cu multiprincipal element alloy (MPEA) powder was successfully synthesized through coprecipitation. Results of energy dispersive spectroscopy (EDS) and X-ray fluorescence spectroscopy (XRF) revealed nearly equal atomic ratios for Co, Ni, and Cu in the Co–Ni–Cu MPEA powder. Moreover, X-ray diffraction (XRD) confirmed a single-phase face-centered cubic (fcc) structure in the Co–Ni–Cu MPEA powder. To explore the differences in microstructure and mechanical properties, we compared a novel WC–2Co–2Ni–2Cu cemented carbide, utilizing the Co–Ni–Cu MPEA as a binder, with conventional WC–6Co and WC–3Co–3Ni cemented carbides prepared via liquid phase sintering. This comparison was conducted using scanning electron microscopy, XRD, and mechanical properties testing. Observations revealed that the surfaces of WC grains in WC–2Co–2Ni–2Cu and WC–6Co appeared smooth and flat, while those in WC–3Co–3Ni exhibited roughness with numerous terraces. In addition, the average grain size was 0.96, 1.14, and 1.22 μm in WC–2Co–2Ni–2Cu, WC–6Co, and WC–3Co–3Ni, respectively. Notably, the Co–Ni–Cu MPEA inhibited WC grain growth. WC–2Co–2Ni–2Cu (1388 HV30) exhibited hardness similar to that of WC–6Co (1406 HV30) but the highest fracture toughness (9.04 MPa∙m1/2), while WC–3Co–3Ni (1332 HV30) presented the lowest hardness. Bending strength values were recorded at 2406 MPa for WC–2Co–2Ni–2Cu, 2816 MPa for WC–6Co, and 2645 MPa for WC–3Co–3Ni, with WC–6Co exhibiting the highest bending strength. Considering their microstructures, physical properties, and mechanical properties comprehensively, the novel cemented carbides exhibited only slightly reduced bending strength compared to conventional counterparts. Thus, the Co–Ni–Cu MPEA binder demonstrates promising potential as a replacement for the conventional Co metal binder.

Role of grain refinement and phase metastability on deformation behavior of additively manufactured FeMnCoCr multiprincipal element alloys

This study investigates the deformation mechanisms of three additively manufactured FeMnCoCr multiprincipal element alloys (MPEAs), each with distinct grain sizes achieved through metastability engineering and unique deformation mechanisms. The Fe35Mn45Co10Cr10 composition serves as a baseline for comparison, exhibiting intermediate mechanical properties and deformation primarily through slip. In contrast, the Fe40Mn40Co10Cr10 composition relies on twinning mechanisms to accommodate strain and enhance work hardening. However, the Fe30Mn50Co10Cr10 composition demonstrates that grain refinement, facilitated by a body-centered cubic to face-centered cubic transformation during solidification, is more effective in improving the yield strength of the material. This comparative analysis underscores the effectiveness of grain refinement and twinning in optimizing the mechanical properties of additively manufactured MPEAs.

Enhancing cryogenic mechanical properties of a cost-effective FeCrNi dual-phase multi-principal element alloy by fully constrained heterostructure and deformation twinning

This paper reports a cost-effective Fe40Cr40Ni20 (at%) dual-phase multi-principal element alloy with excellent cryogenic mechanical properties. The high strength primarily originated from an ideal fully constrained heterostructure, which enhanced back stress strengthening and dislocation strengthening. Twinning enhanced the deformability and the strength of the alloy simultaneously.

Characteristic deformation microstructure evolution and deformation mechanisms in face-centered cubic high/medium entropy alloys

Face-centered cubic (FCC) high/medium entropy alloys (HEAs/MEAs), novel multi-principal element alloys, are known to exhibit exceptional mechanical properties at room temperature; however, the origin is still elusive. Here, we report the deformation microstructure evolutions in a tensile-deformed Co20Cr40Ni40 representative MEA and Co60Ni40 alloy, a conventional binary alloy for comparison. These FCC alloys have high/low friction stresses, fundamental resistance to dislocation glide in solid solutions, respectively, and share similar other material properties, including stacking fault energy. The Co20Cr40Ni40 MEA exhibited higher yield strength and work-hardening ability than in the Co60Ni40 alloy. Deformation microstructures in the Co60Ni40 alloy were marked by the presence of coarse dislocation cells (DCs) regardless of grain orientation and a few deformation twins (DTs) in grains with the tensile axis (TA) near <1 1 1>. In contrast, the MEA developed three distinct deformation microstructures depending on grain orientations: fine DCs in grains with the TA near <1 0 0>, planar dislocation structure (PDS) in grains with other orientations, and a high density of DTs along with PDS in grains oriented <1 1 1>. Three-dimensional electron tomography revealed that PDS in the MEA confined dislocations within specific {1 1 1} planes, indicating suppression of cross-slip of screw dislocations and dynamic recovery. In-situ X-ray diffraction during tensile deformation showed a higher dislocation density in the MEA than in the Co60Ni40 alloy. These findings demonstrate that FCC HEAs/MEAs with high friction stresses naturally develop unique deformation microstructures which is beneficial for realizing superior mechanical properties compared to conventional materials.

Enhancing mechanical and anti-corrosion properties in a L12 nanoprecipitates reinforced Fe35.4Ni24Cr21Co9Mo2Cu1.6Al3Ti4 multi-principal element alloy via tailoring grain size

A Fe35.4Ni24Cr21Co9Mo2Cu1.6Al3Ti4 multi-principal element alloy (MPEA) reinforced by L12 nanoprecipitates was fabricated with four different grain sizes (~19, ~50, ~127 and ~361 μm) to optimize mechanical and corrosion properties. All samples exhibit a similar microstructure, showing spherical L12 nanoprecipitates embedded in the FCC matrix at grain interiors, accompanied by a small fraction of plate-like L12 and B2-ordered phases at grain boundaries. As grain size decreases, both yield strength and ultimate tensile strength progressively increase, while ductility decreases. The sample with a grain size of ~50 μm demonstrates the most excellent combination of strength and ductility, displaying a yield strength of ~700MPa, an ultimate tensile strength of ~1130MPa, and a total elongation of ~29.4%. Concurrently, corrosion resistance in 3.5wt.% NaCl improves as grain size decreases from ~361 μm to ~50 μm, but notably deteriorates at ~19 μm. These results suggest that the L12 nanoprecipitates reinforced Fe35.4Ni24Cr21Co9Mo2Cu1.6Al3Ti4 MPEA with grain size of ~50 μm has achieved outstanding combination of mechanical and anti-corrosion properties. Precipitation strengthening and grain boundary strengthening are responsible for the enhanced mechanical properties in the studied samples. Electrochemical impedance spectroscopy (EIS) and X-ray photoelectron spectroscopy (XPS) revealed that the passive film on the ~50 μm grain size sample is the thickest (3.09nm), composed primarily of Cr2O3 and Fe2O3, which acts as an effective barrier against Cl- penetration. In a word, tailoring grain size proves to be an effective means to achieve enhanced mechanical and anti-corrosion properties.

Exploring the effect of Cr and Mn on the intrinsic strength of the tensile properties of FeCoNi, FeCoNiMn, FeCoNiCr, and FeCoNiCrMn multi-principal element alloys using in-situ EBSD

As the composition elements in multi-principal element alloys increase, it can bring excellent mechanical properties. However, the strengthening mechanism of the additional element is still unclear. In this work, we establish a method based on the in-situ EBSD technology to explore the possible effect of additional elements on the intrinsic strength of tensile properties. We prepared four different multi-principal element alloys, including FeCoNi, FeCoNiMn, FeCoNiCr, and FeCoNiCrMn with similar initial status. We systematically investigated the evolution of the microstructure, dislocation density, twin boundary, grain size, and element distribution during the tensile process by in-situ EBSD and EDS. By carefully analyzing the results of four different multi-principal element alloys, the strength effects of the solid-solution hardening, grain-boundary hardening, twin boundary hardening, precipitate hardening, and dislocation hardening were peeled. The effect of the Cr and Mn element addition on the intrinsic strength can be explored. It is found that the element addition indeed increases the intrinsic strength from quaternary to quinary but not very clear from ternary to quaternary no matter Cr or Mn, which indicated that the intrinsic strength was more related to the number of elements in the alloy than to which element was present. This can be explained using the mixing entropy theory, which states that the intrinsic strength is enhanced when the mixing entropy is over a threshold between the MEA and HEA. This paper presents a method to study the individual factors affecting the tensile properties, which can help other researchers to better investigate HEA.

Recrystallization kinetics and mechanical properties of cold-rolled and annealed CoCrFeNi multi-principal element alloy

The microstructure and mechanical properties have been investigated in a CoCrFeNi alloy cold-rolled to 80 % thickness reduction and subsequently annealed at 600 °C. It is observed that the as-rolled microstructure comprises extended regions of different dominant crystallographic orientations along with layers of mixed orientations. Shear bands are also present in this microstructure, with the susceptibility to shear banding varying significantly from region to region. Shear bands are most pronounced in extended regions containing narrow deformation twins, and are a crucial source of recrystallization nuclei. Analysis of the recrystallization kinetics indicates that the Avrami exponent is 1.6 for the first 30 min at 600 °C and that it decreases during further annealing. Tensile test data provide evidence that the sample annealed for 8 min, with a recrystallized fraction (fRX) of 13 % and an average recrystallized grain size of 0.8 μm, does not show any significant improvement in ductility compared to that in the as-rolled condition. However, the ductility is considerably improved in the sample annealed for 15 min, where fRX is 43 % and the average recrystallized grain size is 1.1 μm. This sample demonstrates a yield strength of 850 MPa and a total elongation to failure of 25 %. The data obtained in this work and in previous publications on partially recrystallized CoCrFeNi indicate that for samples annealed after 80–85 % deformation optimized combinations of strength and ductility are obtained when the recrystallized fraction is in the range 30 % < fRX ≤ 50 %.

He irradiation resistance performance in CrNiCo, CrFeNiCo, and CrFeMnNiCo multi-principal element alloys

In this study, the He irradiation resistance of CrFeMnNiCo high-entropy alloy (HEA) and CrNiCo and CrFeNiCo medium-entropy alloys (MEAs), which have better mechanical properties than HEAs, was investigated. Thin-film samples of CrFeMnNiCo, CrNiCo, and CrFeNiCo were irradiated with up to 2 × 1020 He/m2 of 5 keV ions at 673, 773, and 873 K, respectively. In all samples, He bubble formation was observed when irradiated at 1–3 × 1019 He/m2, which depended on the irradiation temperature. At low temperatures of 673 and 773 K, even when as the irradiation dose increased to 2 × 1020 He/m2, the differences in cavity swelling due to He bubble formation among the three alloys were not large. The relative resistance (degree) to cavity swelling for the three investigated alloys was as follows: CrNiCo MEA, CrFeMnNiCo HEA, and CrFeNiCo MEA. First-principles calculation results revealed that the formation of He-di-vacancy clusters was not possible in the CrFeNiCo MEA. This is believed to be the cause of the cavity swelling decrease in the CrFeNiCo MEA. In contrast, after 2 × 1020 He/m2 at 873 K, the cavity swelling of the CrNiCo and CrFeNiCo MEAs, especially CrNiCo, was approximately four times higher than that of the CrFeMnNiCo HEA. It is believed that the He irradiation resistance of the MEAs deteriorated because of element segregation owing to high–temperature irradiation.

Plastic deformations in NiCoFe medium-entropy alloy investigated using nanoindentation simulations

Medium-entropy alloys (MEA) have potential load-bearing applications as high-performance structural materials. In this work, the plastic deformations in NiCoFe MEAs were investigated using nanoindentation atomistic simulations. The nanoindentation responses of three typical orientations of NiCoFe single-crystals were investigated, i.e., [001], [011] and [111]. The results show that, during nanoindentation, Shockley partials on (111) slip planes remarkably affect dislocation-related activities including nucleation, gliding, and interactions. The form of atomic pile-ups is highly non-uniform and strongly asymmetrical due to the presence of multi-principal elements. The dislocation nucleation mechanism of the alloys during nanoindentation is proposed in detail. We find evidence that a hexagonal close-packed phase is formed from the face-centered cubic structure during nanoindentation.

Revealing the oxidation behavior of AlCrxFeNi lightweight multi-principal element alloys via experimental and first-principles calculations

Developing alloys capable of withstanding high temperatures is crucial for modern engineering applications. Introducing multi-component alloys (MPEAs) has provided new strategies for designing high-temperature alloys. We have successfully prepared a series of AlCrxFeNi lightweight MPEAs in this study. The oxidation behavior of AlCxFeNi lightweight at 800 °C was investigated systematically through a combination of experimental methods and first-principles calculations. The microstructure characterization before oxidation reveals that the lightweight AlCrxFeNi MPEAs consist of BCC and B2 phases. In contrast, the AlCrFeNi alloy exhibited a microstructure characterized by typical eutectic features. Oxidation tests indicate that all three alloys exhibited oxidation kinetics following a parabolic law, with oxidation rates decreasing as the Cr content increased. Post-oxidation microstructural characterization revealed an oxide layer composed primarily of Al2O3 and Cr2O3 on the alloy surfaces. First-principles calculations provided insights into the microscopic mechanisms underlying the oxidation process. The results demonstrate that the adsorption energy of the O atom at the top position of the Cr atom is significantly lower than that of the Al atom at the top position. Electronic structure analysis shows that the Cr-O covalent bonding is stronger than the Al-O covalent bonding, which accounts for the oxidation resistance improvement as the Cr element content increases. This study advances MPEAs for high-temperature environments by elucidating these microscopic mechanisms.

Exceptional hardness in multiprincipal element alloys via hierarchical oxygen heterogeneities

Refractory multiprincipal element alloys (RMPEAs) are potential successors to incumbent high-temperature structural alloys, although efforts to improve oxidation resistance with large additions of passivating elements have led to embrittlement. RMPEAs containing group IV and V elements have a balance of properties including moderate ductility, low density, and the necessary formability. We find that oxidation of group IV-V RMPEAs induces hierarchical heterogeneities, ranging from nanoscale interstitial complexes to tertiary phases. This microstructural hierarchy considerably enhances hardness without indentation cracking, with values ranging between 12.1 and 22.6 GPa from the oxide-adjacent metal to the surface oxides, a 3.7 to 6.8× increase over the interstitial-free alloy. Our fundamental understanding of the oxygen influence on phase formation informs future alloy design to enhance oxidation resistance and obtain exceptional hardness while preserving plasticity.

Towards quantifying (meta-)stability of multi-principal element alloys: From configurational entropy to characteristic temperatures

Understanding and hence predicting the stability and metastability of multi-principal-element alloys (MPEAs) is crucial for their design and applications, but it remains a complex and time-consuming task. Descriptors based on the configurational entropy alone are often insufficient in determining the relative stability of MPEA solid solutions, since they predict, against experimental evidence, that alloys containing a large number of elements will be eventually stable. Here we introduce two characteristic temperatures, derived from the temperature dependence of the configurational entropy, which effectively act as (meta-)stability indicators. These can be further combined in a dimensionless quantity, Td0, which enables us to rank alloys according to their compositional and structural (meta-)stability across a broad composition range. In particular, we are able to map equiatomic and non-equiatomic alloys, and even regions covered by conventional alloys. Our proposed descriptors are validated against a large body of experimental results and compared to other (meta-)stability descriptors. Furthermore, they allow us to revise the alloys classification scheme into high-entropy, medium-entropy and low-entropy. Our work sheds new light into the thermodynamic origin of alloying metastability, it provides a potential tool to correlate metastability thermodynamics and kinetics, and ultimately may help in alloys design and discovery.

Composition driven machine learning for unearthing high-strength lightweight multi-principal element alloys

High-strength, lightweight alloys are highly desired in the aerospace, automotive and marine industries. The optimization of such alloys is a multifaceted process, characterized by the consideration of diverse objectives and constraints. Various optimization methodologies exist, spanning from intricate first-principle approaches to the application of sophisticated machine learning algorithms. These algorithms might incorporate input features encompassing elemental composition, microstructural attributes, and thermodynamic properties to enhance prediction accuracies. In this work, we aim to streamline this complexity by employing solely the alloy's elemental composition as the input feature for the machine learning algorithm, improving the hardness while reducing the density of the alloy. We have curated a comprehensive database comprising 544 multi-principal element alloys and developed a robust surrogate model based on these compositions. This composition-driven model is subsequently coupled with principal component analysis (PCA) to facilitate the selection process. Remarkably, through a mere three iterations involving 14 samples, we successfully identified an alloy with an effective specific hardness surpassing the training database maximum by 8.6 %. The proposed composition-driven machine learning delineates a simplified approach for conducting optimization across multiple target material properties.

A review on recent progress of refractory high entropy alloys: From fundamental research to engineering applications

Refractory high-entropy alloys (RHEAs) have emerged as frontier materials for high-temperature structural applications due to their exceptional mechanical properties and thermal stability. This review aims to provide a comprehensive and in-depth summary of the latest research progress in the field of RHEAs. By systematically analyzing 253 publications since 2022, this review presents a panoramic overview of the current research status in the field of RHEAs, covering aspects from alloy design, microstructure, processing techniques, mechanical behavior, to physicochemical properties. Key strengthening and toughening mechanisms, such as solid solution strengthening, precipitation strengthening, and grain boundary strengthening, are extensively analyzed. The high-temperature mechanical performance, oxidation resistance, and adaptability of RHEAs in extreme environments including corrosion and irradiation, are critically reviewed, and the potential application value of these research findings in aerospace, nuclear energy, biomedical, and other fields is discussed. Simultaneously, the multidisciplinary characteristics of RHEAs research has revealed the trend of the field evolving from fundamental research to practical applications. Furthermore, with the aid of advanced characterization techniques and computational methods, the review elucidates the controlling effects of chemical short-range order, defect structures, and other factors on the performance evolution of RHEAs, highlighting the importance of multi-principal element synergistic effects. Based on summarizing the key scientific issues and technological bottlenecks faced by RHEAs, the article provides a forward-looking perspective on future research directions, emphasizing development strategies that integrate computation and experiments, and accelerate the transformation of fundamental research into engineering applications, thus providing insights and guidance for developing a new generation of high-performance RHEAs.