Multi-principal Element Alloys Research Articles

Research progress and development tendency on storage mechanism of multi-principal element alloys for hydrogen/tritium storage

Research progress and development tendency on storage mechanism of multi-principal element alloys for hydrogen/tritium storage

Microstructural evolution in a precipitate-hardened (Fe0.3Ni0.3Mn0.3Cr0.1)94Ti2Al4 multi-principal element alloy during high-pressure torsion

Microstructural evolution in a precipitate-hardened (Fe0.3Ni0.3Mn0.3Cr0.1)94Ti2Al4 multi-principal element alloy during high-pressure torsion

Unraveling dislocation-based strengthening in refractory multi-principal element alloys

Refractory multi-principal element alloys (RMPEAs) draw great interest with their superior mechanical properties and extremely high melting points, yet the strengthening mechanism remains unclear. Here, we calculate the critical resolved shear stress (CRSS) for a single dislocation to move in RMPEAs consisting of 4 or 5 elements with or without short-range order (SRO) to represent strength by a machine learning-based interatomic potential. The increased CRSS is then attributed to high lattice distortion, elastic mismatch, and SRO strengthening, all of which originate from the solid solution strengthening theory. After detailed research of the CRSS across many RMPEAs systems with different composition ratios, we construct an XGBoost model to predict the CRSS from a few parameters and rank their importance. We find that lattice distortion strongly influences both dislocation types and reduces the screw-to-edge ratio in CRSS, while the elastic mismatch has a more significant impact on the screw dislocation than the edge one.

Formation of a protective oxides-nitrides compound layer with enhanced wear properties, corrosion resistance, and cytocompatibility on novel Ti-Zr-Nb-Ta-Mo multi-principal element alloy

The insufficient surface properties of multi-principal element alloys (MPEAs) hinder their potential biomedical applications. Here, a facile thermal oxidation plus nitridation treatment is conducted to fabricate a protective layer on a novel Ti-Zr-Nb-Ta-Mo MPEA. The total surface layer of NT5-ON consists of a top compound layer, including TiO2, TiN, TiON, ZrO2, and ZrN, and an underneath N-diffusion zone, leading to an exceptionally high wear performance. Also, electrochemical testing and in-vitro evaluations demonstrated enhanced corrosion resistance and cytocompatibility of NT5-ON. As a result, thermal oxidation plus nitridation treatment is an effective technique for designing high-performance hip implant materials.

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.

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.

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.

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 in fcc MPEAs.

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.

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.

Tensile Properties of a Non-Equiatomic Ni–Co–V Medium Entropy Alloy at Cryogenic Temperature

The development of strong and ductile alloys for application in cryogenic temperatures has long been sought after. In this work, we have developed a face-centered cubic Ni10Co56.5V33.5 multi-principal element alloy (MPEA) that exhibits a balanced combination of high strength and good ductility at 77 K, based on the considerations of large local lattice distortion (LLD) and low stacking fault energy. The small-grained Ni10Co56.5V33.5 MPEA exhibits a yield strength of 1400 MPa and an ultimate tensile strength of 1890 MPa, while preserving a good ductility of 23%. Moreover, precession electron diffraction and transmission electron microscopy revealed multiple deformation mechanisms, including wavy dislocations, atypically severely twisted dislocation bands, hierarchical stacking faults, and deformation twins, which are implicated in the alloy’s outstanding mechanical performance. These insights offer a strategic guide for the design of strong and ductile alloys, particularly for utilization in extreme environments.

Corrosion and Wear Resistance of HVOF-Sprayed Ni-Cr-Co Multi-principal Element Alloy Coating on Copper Plate

Corrosion and Wear Resistance of HVOF-Sprayed Ni-Cr-Co Multi-principal Element Alloy Coating on Copper Plate

Formation mechanism of AlNbTiVZr high entropy alloy/Ti multi-principal element alloy by laser direct energy deposition: Microstructure evaluation and mechanical properties

A novel refractory multi-principal element alloy with high strength and toughness matching characteristics was successfully prepared by mixing TA15 alloy with AlNbTiVZr high-entropy alloy. The effects of the composition ratio of the high-entropy alloy on the microstructure evolution and its influence on properties were investigated in this study. The tensile strength of the 70 wt % AlNbTiVZr HEA/Ti multi-principal element alloy is 1.18 times more than that of TA15 alloy, which has a value of 1068.0 ± 2.1 MPa. The increase in strength is mainly due to the production of equiaxed fine grains and the development of a robust and resilient body-centered cubic (BCC) phase in the alloy following the introduction of the high-entropy alloy. The attainment of the acceptable tensile strain is the result of the combined influence of the BCC matrix phase and the dispersed second-phase particles. The exceptional mechanical capabilities of the 70 wt % AlNbTiVZr/Ti multi-principal element alloy undeniably enhance its potential for use as a structural component in aerospace load-bearing constructions.

In situ neutron diffraction study and electron microscopy analysis of microstructure and texture evolution during annealing of rolled CoCrFeNi alloy doped with 1 at.%C

In situ neutron diffraction studies of changes in the dislocation density, volume fraction of carbides, and crystallographic texture have been performed during annealing of a cold-rolled multi-principal element CoCrFeNi alloy doped with 1 at.%C. In addition, microstructures of the as-rolled and several annealed samples of this alloy have been investigated using electron microscopy-based techniques. It is found that cold rolling to a thickness reduction of 74% results in a brass-type rolling texture with a spread of orientations between the Goss and brass components and with a pronounced 〈111〉//ND fiber. The deformed microstructure is characterized by a high density of dislocations and shows large variations in deformation structures in regions of different crystallographic orientations. In particular, bundles of deformation twins and high frequencies of shear bands are observed in regions having orientations along the 〈111〉//ND fiber. Annealing at 700 °C–1000 °C leads to recovery and recrystallization, during which shear bands and regions of mixed orientations act as preferential nucleation sites for recrystallized grains. Precipitation of M23C6 particles also takes place during annealing. The particles retard boundary migration, thus slowing down recrystallization and restricting grain growth. The average size of recrystallized grains does not exceed 5 μm even after annealing at 1000 °C for 60 min. While rolling textures are retained in some annealed samples, other samples demonstrate distinct recrystallization textures with new texture components. The results obtained in this work are compared with the literature data for several carbon-doped face-centered cubic multi-principal element alloys and for a carbon-free CoCrFeNi alloy.

Investigations on microstructure and properties of five TiZr-based refractory multi-principal element alloys for clinical metallic biomaterials

The development of new materials with high strength, ductility, low modulus, wear resistance, and good biocompatibility remains a requirement in the field of clinical biomaterials. Based on the concept of high-entropy alloys, five TiZr-based refractory multi-principal element alloys (RMPEAs), i.e. MoTiZr, NbTiZr, MoTiVZr, NbTiVZr, and NbMoTiZr were designed and their microstructure, mechanical properties, elastic properties, and biocompatibility were assessed in this study. Results indicate that NbTiZr and NbTiVZr RMPEAs are single-phase BCC alloys, while the other alloys exhibit multiphase structures. In terms of mechanical behaviors, NbMoTiZr RMPEA has the highest hardness (496±15 HV) among the five alloys and also combines a high yield strength and good ductility, with values of 1275 MPa and 21.66%, respectively. According to atomic calculations, the ideal shear strengths of the five alloys on the {110} plane are all lower than those on the {112} plane. Furthermore, NbMoTiZr RMPEA also exhibits a lower Young’s modulus than that of Ti6Al4V, which can reduce “stress shielding” effects. The in vitro biocompatibility evaluation revealed that NbMoTiZr RMPEA can support cell adhesion and proliferation of MC3T3-E1 cells and exhibits good biocompatibility comparable to Ti6Al4V. These results demonstrate the remarkable potential of NbMoTiZr RMPEA for orthopedic bioimplant applications.

An experimental estimation method of diffusion coefficients in ternary and multicomponent systems from a single diffusion profile

Until recently, it was textbook knowledge that the diffusion coefficients could not be estimated in a multi-component system following the widely practised diffusion couple method. The recently proposed constrained diffusion couple method largely solved the problems. The need to produce two intersecting diffusion paths in a multi-component space with very well-controlled compositions of the diffusion couple end members may be tricky in certain situations, depending on the complications of diffusion paths. In this study, we have shown the estimation of all types of diffusion coefficients from a single diffusion profile without neglecting Onsager's cross terms in concentrated ternary Ni-Co-Fe, Fe-rich quaternary Fe-Ni-Co-Cr and concentrated Ni-Co-Fe-Cr-Al quinary alloys. This is applicable in ternary and multicomponent component systems (n≥3) at a stable Kirkendall marker plane position. One can even estimate the impurity diffusion coefficients utilizing the composition profiles at the ends of the diffusion couples following the Hall method, which has been rarely practised in multicomponent systems until now. We have further demonstrated the importance of estimating the tracer and intrinsic diffusion coefficients in a concentrated or multi-principal element alloy in which interdiffusion coefficients can be vague and misleading for understanding the elements' diffusional interactions and relative mobilities. We have also shown the importance of considering the vacancy wind effect in concentrated alloys. The method proposed in this study will help to generate a mobility database with relative ease in various multicomponent systems important for various applications, which was considered impossible until recently.

High-Throughput Multi-Principal Element Alloy Exploration Using a Novel Composition Gradient Sintering Technique

This work demonstrates the capabilities and advantages of a novel sintering technique to fabricate bulk composition gradient materials. Pressure distribution calculations were used to compare several tooling geometries for use with current-activated, pressure-assisted densification or spark plasma sintering to densify a gradient along the long dimension of the specimen. The selected rectangular tooling design retains a low aspect ratio to ensure a uniform pressure distribution during consolidation by using a side loading configuration to form the gradient along the longest dimension. Composition gradients of NixCu1−x, MoxNb1−x, and MoNbTaWHfx (x from 0 to 1) were fabricated with the tooling. The microstructure, composition, and crystal structure were characterized along the gradient in the as-sintered condition and after annealing to partially homogenize the layers. The successful fabrication of a composition gradient in a difficult-to-process material like the refractory multi-principal element alloy system MoNbTaWHfx shows the utility of this approach for high-throughput screening of large material composition spaces.

Preferential composition during nucleation and growth in multi-principal element alloys

The crystallization of complex, concentrated alloys can result in atomic-level short-range order, composition gradients, and phase separation. These features govern the properties of the resulting alloy. While nucleation and growth in single-element metals are well understood, several open questions remain regarding the crystallization of multi-principal component alloys. We use molecular dynamics to model the crystallization of a five-element, equiatomic alloy modeled after CoCrCuFeNi upon cooling from the melt. Stochastic, homogeneous nucleation results in nuclei with a biased composition distribution, rich in Fe and Co. This deviation from the random sampling of the overall composition is driven by the internal energy and affects nuclei of a wide range of sizes, from tens of atoms all the way to super-critical sizes. This results in short-range order and compositional gradients at nanometer scales.

Short-range-order degree dominated physical and mechanical properties of refractory multi-principal element alloys: a first-principles study

The degree of short-range order (SRO) can influence the physical and mechanical properties of refractory multi-principal element alloys (RMPEAs). Here, the effect of SRO degree on the atomic configuration and properties of the equiatomic TiTaZr RMPEA is investigated using the first-principles calculations. Their key roles on the lattice parameters, binding energy, elastic properties, electronic structure, and stacking fault energy (SFE) are analyzed. The results show the degree of SRO has a significant effect on the physical and mechanical properties of TiTaZr. During the SRO degree increasing in TiTaZr lattice, the low SRO degree exacerbates the lattice distortion and the high SRO degree reduces the lattice distortion. The high degree of SRO improves the binding energy and elastic stiffness of the TiTaZr. By analyzing the change in charge density, this change is caused by the atomic bias generated during the formation of the SRO, which leading to a change in charge-density thereby affecting the metal bond polarity and inter-atomic forces. The high SRO degree also reduces SFE, which means the capability of plastic deformation of the TiTaZr is enhanced.