Complex Concentrated Alloys Research Articles

Cold Spray Coatings of Complex Concentrated Alloys: Critical Assessment of Milestones, Challenges, and Opportunities

Complex concentrated alloys (CCAs) are structural and functional materials of the future with excellent mechanical, physical, and chemical properties. Due to the equiatomic compositions of these alloys, cost can hinder scalability. Thus, the development of CCA-based coatings is critical for low-cost applications. The application of cold spray technology to CCAs is in its infancy with emphasis on transition elements of the periodic table. Current CCA-based cold spray coating systems showed better adhesion, cohesion, and mechanical properties than conventional one-principal element-based alloys. Comprehensive mechanical behavior, microstructural evolution, deformation, and cracking of cold spray CC-based coatings on the same and different substrates are reviewed. Techniques such as analytical models, finite element analysis, and molecular dynamic simulations are reviewed. The implications of the core effects (high configurational entropy and enthalpy of mixing, sluggish diffusion, severe lattice distortion, and cocktail behavior) and interfacial nanoscale oxides on the structural integrity of cold spray CCA-based coatings are discussed. The mechanisms of adiabatic heating, jetting, and mechanical interlocking, characteristics of cold spray, and areas for future research are highlighted.

Helium diffusion and bubble evolution in single-phase tungsten-based W-Ta-Cr-V complex concentrated alloy

Tungsten-based complex concentrated alloy (CCA) W-Ta-Cr-V films with bcc single-phase have been investigated for its exceptional radiation tolerance. This work aims to investigate the formation of helium (He) bubble and He retention behavior in the CCA film under 100 eV He plasma at a fluence of 2.02 × 1025 He/m2. The results show that the size of He bubbles in the CCA film is smaller than that in the pure W film, and the depth distribution is shallower, which indicates that the evolution of He bubbles is inhibited. As for the underlying mechanism of resistance to the formation of He bubbles in CCAs, it is believed that the characteristic of low He mobility reduces the diffusion depth of He, and the sluggish diffusion of He inhibits the aggregation of He atoms or He clusters to a certain extent. In other words, the unique energy and microstructure characteristics also inhibit the diffusion of He in the CCA film, which affects the nucleation and growth of He bubbles. Furthermore, the release of He is mainly from He adsorption of the near surface, and the desorption peaks shift to lower temperatures in the CCA film within the desorption temperature of 1273 K.

Development of a high strength, low density and corrosion resistant novel FeCrMoNb1.5Ti0.5 complex concentrated alloy

The present work deals with the microstructure, mechanical properties and corrosion behavior of a FeCrMoNb1.5Ti0.5 complex concentrated alloy (CCA). The aforementioned alloy was fabricated by vacuum arc melting technique. The observed density and lattice parameters of the as-cast alloy are 7.821 g/cm3 and 3.179 ± 0.003 Å, respectively, which follow the rule of mixture. The phase evolution, chemical composition, and microstructure in the as-cast condition, as well as after annealing treatment at 800°C, 1000°C, and 1200°C, were investigated through XRD, TEM, and SEM-EDS. The XRD and TEM analyses confirmed the presence of BCC structure with minor amount of TiCr2 type and NbCr2 type C14 Laves phases. Instrumented micro-indentation and corrosion testing were performed in the as-cast and annealed specimens. The microhardness of the as-cast alloy was 825 HV and it increased to 1102 HV after annealing at 1200ºC for 20 h. This peak hardness was achieved after annealing at 1200ºC due to the presence of a large volume fraction of Laves phase. Potentiodynamic polarization tests of the as-cast and as-annealed specimens revealed outstanding corrosion resistance of the CCA compared to stainless steel. This is attributed to better microstructural homogeneity and reduced elemental segregation after annealing.

Multistage strain-hardening behavior of ultrastrong and ductile lightweight refractory complex-concentrated alloys

Lightweight high-entropy alloys or complex-concentrated alloys have demonstrated great potential for engineering applications due to their high strength and lightweight. However, a weak strain-hardening ability and a limited tensile ductility remain their major hindrance. Here, a multistage strain-hardening effect is developed to ensure a high strength and still a sufficient ductility in a rolled and annealed (Ti44V28Zr14Nb14)98.5Mo1.5 (at.%) lightweight refractory complex-concentrated alloy (M1.5A-LRCCA). This multistage strain-hardening behavior is related to the microstructure and the corresponding initial average dislocation density and distribution by comparison with rolled and annealed Ti44V28Zr14Nb14 (M0-LRCCA) and as-cast (Ti44V28Zr14Nb14)98.5Mo1.5 (M1.5C-LRCCA). The microstructure, with homogeneously distributed submicron precipitations, a moderate initial average dislocation density, and uniform dislocation distribution (e.g., M1.5A-LRCCA), is susceptible to producing various deformation substructures, such as dislocation substructures (slip bands, Taylor lattices, microbands, DDWs), shear bands, and deformation twins, which results in the multistage strain-hardening behavior. This method of achieving multistage strain hardening behavior through a microstructure modulation is significant for engineering applications of lightweight high-entropy alloys or complex-concentrated alloys, and it might be extended to other lightweight and high-strength alloys.

PySSpredict: A python-based solid-solution strength prediction toolkit for complex concentrated alloys

The emergence of solid solution high entropy alloys (HEAs) and complex concentrated alloys (CCAs) offers opportunities to design novel alloys with tailored strength and ductility. The growing community of integrated-computational materials engineering (ICME) can benefit from implementing state-of-the-art solid-solution strengthening models to alloy design practices. This paper introduces pySSpredict, an open-source python-based toolkit that automates high-throughput calculations of solid-solution strengths of CCAs and thermodynamic properties. We present the functions of the pySSpredict code: (1) automating high-throughput calculations of strength for CCAs, (2) managing the data of thermodynamic calculations from databases or other software, and (3) visualizing and filtering the data to identify candidate alloys. The toolkit implements the latest theoretical edge dislocation model for face-centered cubic (FCC), and edge/screw dislocation models for body-centered cubic (BCC) alloys. The pySSpredict code is hosted on GitHub and deployed on nanoHUB for demonstrations.

Role of heterophase interfaces on local coercivity mechanisms in the magnetic Al0.3CoFeNi complex concentrated alloy

Microstructural features across different length scales have a profound influence on the coercivity of magnetic alloys. Whereas the role of homophase boundaries on the pinning of magnetic domain walls is well established, the influence of heterophase interfaces on domain wall motion is complex and poorly understood. Here, we use state-of-the-art electron microscopy techniques to show that the magnetization reversal process in an Al0.3CoFeNi magnetic complex concentrated alloy (CCA), which is responsible for its coercivity, changes dramatically from a nucleation-type mechanism in the FCC+L12 state of the CCA, with a domain wall width of 171 nm, to a pinning type mechanism in the microstructure with colonies of FCC/L12 nanorods embedded in a BCC/B2 matrix, with a domain wall width of 35 nm. Our work reveals that heterophase FCC/BCC interfaces have a much stronger effect on coercivity than isostructural chemically ordered/disordered interfaces and provides a powerful guide to the rational design of microstructure to tune magnetic properties in both complex concentrated alloys and conventional magnetic alloys.

Remarkable Wear Resistance in a Complex Concentrated Alloy with Nanohierarchical Architecture and Composition Undulation.

Sustained wear damages on the sliding surfaces of alloys are generally the culprit responsible for the failure of various mechanical systems. Inspired by high-entropy effects, here we deliberately deploy nanohierarchical architecture with composition undulation in a Ni50(AlNbTiV)50 complex concentrated alloy, which yields ultralow wear rate within the order of 10-7 to 10-6 mm3/Nm between room temperature and 800 °C. Such remarkable wear resistance heretofore represents one of the highest wear resistance reported for the bulk alloys or composites, and originates from the multi-type adaptive friction interface protection governed by intrinsically nano-coupled grains and nanoprecipitates. This cooperative heterostructure releases gradient frictional stress in stages upon wear at room temperature through the coexistence of multiple deformation pathways while activating a dense nanocrystalline glaze layer upon wear at 800 °C to minimize adhesive and oxidative wear. Our work uncovers a practical avenue for tailoring wear properties with multicomponent heterostructures over a wide temperature range.

Chemical order transitions within extended interfacial segregation zones in NbMoTaW

Interfacial segregation and chemical short-range ordering influence the behavior of grain boundaries in complex concentrated alloys. In this study, we use atomistic modeling of a NbMoTaW refractory complex concentrated alloy to provide insight into the interplay between these two phenomena. Hybrid Monte Carlo and molecular dynamics simulations are performed on columnar grain models to identify equilibrium grain boundary structures. Our results reveal extended near-boundary segregation zones that are much larger than traditional segregation regions, which also exhibit chemical patterning that bridges the interfacial and grain interior regions. Furthermore, structural transitions pertaining to an A2-to-B2 transformation are observed within these extended segregation zones. Both grain size and temperature are found to significantly alter the widths of these regions. An analysis of chemical short-range order indicates that not all pairwise elemental interactions are affected by the presence of a grain boundary equally, as only a subset of elemental clustering types are more likely to reside near certain boundaries. The results emphasize the increased chemical complexity that is associated with near-boundary segregation zones and demonstrate the unique nature of interfacial segregation in complex concentrated alloys.

Antibacterial properties of Cu containing complex concentrated alloys

The present work demonstrates the antibacterial properties of CoCrCuNi and CoCrCuFeNi complex concentrated alloys (CCAs) against the Escherichia coli bacteria. A non-Cu-containing CoCrFeNi alloy has also been investigated to gain a comprehensive understanding of the effect of Cu addition on the antibacterial properties. All the alloys have been studied with reference to 316 L stainless steel and pure Cu. The alloys' phase structure, wettability, bacterial adhesion studies, and apatite forming ability are analyzed. Antibacterial efficiency of CoCrCuNi and CoCrFeNiCu are 99.99% and 98.11%, respectively which are superior to CoCrFeNi (40%) and SS316L. The Cu ion release mechanism facilitated the improved antibacterial efficiency in the Cu-containing CCAs. The CCAs in the study showed apatite formation on the surface of the coupons. However, the Cu-containing alloys showed high surface degradation with selective oxidation and loss of the copper-rich phase in CoCrCuFeNi and CoCrCuNi, respectively, which can be attributed to the phase separation in these CCAs. The Vickers microhardness of the alloys is in the range of 130–145 VHN. The results of this work pave the way for further development of these alloys for several antibacterial applications.

Design of metastable complex-concentrated alloys through composition tailoring

To develop alloys with high strength and reasonable ductility, CoCrFeNi-based metastable complex-concentrated alloys were designed using composition–property contour maps. The map was constructed by exploring the phase stability and mechanical behaviors of a series of CoCrFeNi alloy thin films synthesized via solid-state alloying of multilayer thin films. The concentrations of Co and Ni were key to activate metastable deformation behaviors by reducing the stacking fault energy of alloys and improve solid-solution strengthening, as expected from the atomic-level complexity related to the electronegativity difference. By optimizing the Co and Ni concentrations based on the composition-phase/mechanical property contour maps, we activated the combined deformation behavior of mechanical twinning and phase transformation. This resulted in a Co33Cr25Fe25Ni17 metastable complex-concentrated alloy with excellent tensile properties—yield strength of 234 MPa, ultimate tensile strength of 720 MPa, and elongation to failure of 80%. The proposed approach provides a useful guideline for the design of complex-concentrated alloys with customized properties through property predictive control.

Machine learning prediction of the mechanical properties of refractory multicomponent alloys based on a dataset of phase and first principles simulation

In this work, a dataset including structural and mechanical properties of refractory multicomponent alloys was developed by fusing computations of phase diagram (CALPHAD) and density functional theory (DFT). The refractory multicomponent alloys, also named refractory complex concentrated alloys (CCAs) which contain 2–5 types of refractory elements were constructed based on Special Quasi-random Structure (SQS). The phase of alloys was predicted using CALPHAD and the mechanical property of alloys with stable and single body-centered cubic (BCC) at high temperature (over 1,500°C) was investigated using DFT-based simulation. As a result, a dataset with 393 refractory alloys and 12 features, including volume, melting temperature, density, energy, elastic constants, mechanical moduli, and hardness, were produced. To test the capability of the dataset on supporting machine learning (ML) study to investigate the property of CCAs, CALPHAD, and DFT calculations were compared with principal components analysis (PCA) technique and rule of mixture (ROM), respectively. It is demonstrated that the CALPHAD and DFT results are more in line with experimental observations for the alloy phase, structural and mechanical properties. Furthermore, the data were utilized to train a verity of ML models to predict the performance of certain CCAs with advanced mechanical properties, highlighting the usefulness of the dataset for ML technique on CCA property prediction.

Synergistic effect of Ti, B, Si, and C on microstructure and mechanical properties of as-cast Al0.4Co0.9Cr1.2Fe0.9Ni1.2(Si, Ti, C, B)0.375 complex concentrated alloy

The synergistic effect of Ti, B, Si, and C on microstructure and mechanical properties of as-cast Al0.4Co0.9Cr1.2Fe0.9Ni1.2(Si, Ti, C, B)0.375 complex concentrated alloy (CCA) was studied. The as-cast alloy was prepared by vacuum induction melting in a ceramic crucible followed by a tilt casting to a graphite mould. The effect of alloying elements on the primary solidification phase, solidification path, and phase transformation temperatures is evaluated. The solidification of the alloy starts with the growth of columnar FCC(A1) (face-centred cubic crystal structure A1) dendrites and is finalised by the formation of lamellar FCC(A1) + (Ni,Co,Fe)16(Ti,Cr)6(Si,Al)7 eutectic in the interdendritic region. During solidification, Al, Ti, B, Si, and C segregate preferentially to liquid, which leads to the formation of BCC(B2) (ordered body-centred cubic crystal structure B2), BCC(A2) (body-centred cubic crystal structure A2), FCC(A1), (Ni,Co,Fe)16(Ti,Cr)6(Si,Al)7, Cr2B, and TiC phases in the interdendritic region. The highly anisotropic microstructure of tensile specimens prepared from the as-cast alloy consists of columnar FCC(A1) dendrites (89 vol%) oriented at an angle ranging from 75 to 90 degrees to the longitudinal axis of specimens and multiphase interdendritic region (11 vol%). The tensile deformation curves show only the strain hardening stage at test temperatures up to 700 °C. The strain hardening coefficient decreases from 0.20 to 0.17 by increasing the test temperature from 20° to 700°C. At temperatures ranging from 725° to 900°C, the tensile curves show the strain hardening stage, which is followed by the strain-softening stage up to a fracture. The strain softening stage results from dynamic recrystallisation of FCC(A1) dendrites and fracture of brittle high elastic modulus phases in the interdendritic region.

Sub-micron grains and excellent mechanical properties of low-density Ti40Nb30Mo30-C refractory complex concentrated alloy reinforced with in-situ (Ti, Nb)C

A novel Ti40Nb30Mo30–C refractory complex concentrated alloy (RCCA) reinforced with in-situ (Ti, Nb)C carbide was fabricated via mechanical alloying (MA) and hot press sintering (HPS). The results showed that the alloy consisted of the BCC crystal-structured solid solution phase (TNM phase) and FCC crystal-structured carbide (Ti, Nb)C with submicron grain sizes of 0.79 μm and 0.62 μm, respectively. The ultra-fine microstructure benefitted from the inhabitation of in-situ (Ti, Nb)C on grain growth. The ab-initio calculations were conducted to study the formation process of in-situ composite carbide (Ti, Nb)C. Here, the lightweight alloy with a density of 7.04 g/cm3 exhibited promising compressive mechanical properties, such as yield strength of 2550 MPa at room temperature (RT), 1508 MPa at 800 °C, and 485 MPa at 1000 °C. According to the deformation microstructures at RT and 1000 °C, the dislocation-dominated deformation mechanism in TNM phase was invariable at different temperatures. A brittle-to-ductile translation of (Ti, Nb)C was found to happen at around 1000 °C supported by the stress-strain curve and TEM observation. The excellent mechanical properties of the alloy were attributed to the grain boundary strengthening and the strengthening effect of (Ti, Nb)C that hindered the movement of dislocations. The strategy that incorporating in-situ carbides into light RCCA system could broaden and inspire the design of lightweight and high-strength materials.

3D printable low density B2+BCC refractory element based complex concentrated alloy with high compressive strength and plasticity

While complex concentrated alloys (CCAs), involving large fraction of refractory elements, are promising candidates for structural applications, their relatively high densities, processing challenges, and low plasticity, has retarded their development. Here, we report a 3D printable low-density precipitation strengthened CCA, Al10Nb15Ta5Ti30Zr40. The chemical homogeneity in the alloy deposited using laser bed powder fusion process is substantially better as compared to as-cast ingot. This homogeneity is attributed to the enhanced solubility at the high temperatures experienced during laser melting, small melt pool dimensions, and the high cooling rates. The as-built CCA exhibited an excellent balance of room temperature mechanical properties, a compressive yield stress ∼1400 MPa, peak stress ∼1700 MPa, and plastic strain exceeding 45%. These properties can be attributed to its unique as-processed microstructure consisting of refined grains incorporating a high density of sub-grain boundaries, containing a nanoscale two-phase mixture of an ordered B2 and disordered BCC solid solution phases.

Magnetic and mechanical properties of additively manufactured Alx(CoFeNi) complex concentrated alloys

Varying the Al content, strongly influences the microstructure, magnetic and microhardness of additively manufactured Alx(CoFeNi) (x = 0, 10, 30) complex concentrated alloys (CCA). Compared to the single FCC phase of CoFeNi, the hierarchical FCC/L12+BCC/B2 heterostructure of heat treated Al10(CoFeNi) CCA displayed substantially improved saturation magnetization, Curie temperature and microhardness. However, there was no significant change in the properties of heat treated CoFeNi and Al30(CoFeNi) CCA. These findings can be rationalized via thermodynamic modelling of the phase stability. We have demonstrated the feasibility of exploiting additive manufacturing for rapidly screening and developing novel high-performance alloys for next generation rotating electrical machines.

Hierarchical machine learning based structure–property correlations for as–cast complex concentrated alloys

Efficient design of complex concentrated alloys (CCAs) requires knowledge of constituent elements and their fundamental properties, phase selection, phase-fractions and microstructure and their correlations with properties. We present a novel framework incorporating a feature selection scheme to extricate the factors deciding phase formation and mechanical properties — both compressive and tensile for as-cast compositions. These features have been harnessed using classifier and regressor models to predict the phases and properties of compositions for which they are as yet undocumented. The stacking fault energies and density regressor models augment the above feature set. The phases are predicted using a novel multi-output phase classifier model that could predict up to 4 phases simultaneously with accuracy in the range of (95–99)%. The hardness, yield strength, peak strength and fracture strain regressor models also perform well with testing metrics in the range (0.91–0.96)R2. The regressors are subsequently operated upon an artificially generated composition space of 5-element CCAs comprising a select few elements, for which the properties in the as-cast condition are predicted.

Microstructure, dislocation density and microhardness of 1 %C-doped CoCrFeNi complex concentrated alloys during isochronal annealing

In this study, a microstructure of moderately deformed 1 %C-doped CoCrFeNi complex concentrated alloys (CCA) was investigated in a correlation with microstructure evolution at different annealing temperature for 1 h. After 60 % thickness reduction of cold rolling, the dislocation density accumulated as large as 19 × 1015 m−2. The interstitial carbon atom was a key factor inducing large dislocation storage and caused large stored strain energy of 368 J/mol. The recrystallization was completed at 1173 K for 1 h and the average recrystallized grain size was as fine as 3.7 µm. During the annealing between 973 and 1173 K, the carbides were increasingly precipitated with an increase in annealing temperature. The carbon addition delayed the recrystallization process due to solute drag effect and carbide precipitations. The conventional fabrication method produced the fine recrystallized microstructure attributed to the large dislocation density of the deformed microstructure and carbide precipitation.

Creating heterogeneous lamella structure in a multiphase Al-Cr-Fe-Co-Ni complex concentrated alloy

The present work reports strength-ductility synergy achieved in an Al–Cr–Fe–Co–Ni complex concentrated alloy (CCA) through utilizing the heterogeneous lamella (HL) structure produced by a simple two-step thermo-mechanical processing route (cold rolling + annealing). The as-cast microstructure consists of dendritic FCC phase and inter-dendritic BCC phase having coherent precipitation of B2 particles. Compared to the as-cast specimen, nearly all the processed materials in this study showed significant simultaneous enhancement in both strength and ductility, which has not been achieved previously on CCAs with this type of microstructure. In addition, their mechanical properties can be tuned by changing the annealing time and resulted in superior properties compared to other CCAs of similar phase constitution. Moreover, the multiphase HL structure also exhibited a stronger Bauschinger effect compared to other HL-structured materials with heterogeneity among the lamellae generated solely by grain size difference. The strategy demonstrated in this work provides a cost-efficient and less composition-dependent direction for fabricating high-performance structural materials.

Microstructure and Mechanical Properties of Ni-Based Complex Concentrated Alloys under Radiation Environment

The rapid development of fusion-reactor technology calls for excellent anti-irradiation materials. Complex concentrated alloy (CCA) is a newly proposed alloy concept which is a promising candidate of nuclear fusion materials by virtue of its great phase stability under irradiation. This article summarizes anti-radiation mechanism and the microstructure evolution in HEAs. The effective factors on irradiation behavior of HEAs, including entropy, sample size and temperature, are discussed. Finally, the article introduces the potential ways to solve the economic and environmental problems which the HEAs faced for their applications in the future. In summary, the HEAs usually show better irradiation resistance than traditional alloys, such as less swelling, smaller size of defects, and more stable mechanical properties. One possible reason for the irradiation resistance of HEA is the self-healing effect induced by the high-entropy and atomic-level stress among the metal atoms. The activation of the principal element should be considered when selecting components of HEA, and the high throughput technique is a potential way to reduce the design and fabrication cost of HEAs. It is reasonable to expect that coming years will see the application of novel HEAs in fusion reactors.

Synthesis and Characterization of High‐Entropy CrMoNbTaVW Thin Films Using High‐Throughput Methods

High‐entropy alloys (HEAs) or complex concentrated alloys (CCAs) offer a huge research area for new material compositions and potential applications. Since the combination of several elements sometimes leads to unexpected and unpredictable material properties. In addition to the element combinations, the optimization of the element proportions in CCAs and HEAs is also a decisive factor in tailoring desired material properties. However, it is almost impossible to achieve the composition and characterization of CCAs and HEAs with a sufficient number of compositions by conventional experiments. Therefore, an optimized high‐throughput magnetron sputtering synthesis to fabricate a new HEA gradient layer of six elements is presented. With this approach, the compositional space of the HEA system CrMoNbTaVW can be studied in different subsections to determine the influence of the individual elements and their combinations on the structure, morphology, and physical properties (hardness and resistivity). It is found that the Cr‐, Ta‐, and W‐rich phases, which have a grain size of 10–11 nm, exhibit the hardest mechanical properties, whereas V‐, Ta‐, and Cr‐rich compounds exhibit the highest electrical resistivity. The combination of high‐throughput synthesis, automated analysis tools, and automated data interpretation enables rapid and time‐efficient characterization of the novel CrMoNbTaVW gradient film.