Solid Solution Phase Research Articles

Analysis of phase stability and chemical segregation in the Mo-V alloys using a generalized embedded atom method potential

A new interatomic potential for the Mo-V system is introduced to facilitate the study of phase stability and mechanical properties at lower temperatures. This potential is based on a generalization of the embedded atom method and includes contributions from embedding energy, explicit two- and three-body interactions and nonlocal many-body interaction terms. The parameters of the potential are optimized by using data from ab initio density functional theory (DFT) calculations. The potential is rigorously validated across a range of physical properties, such as elastic constants, equation of states, phonon dispersion curves, point defect properties and melting temperatures for different compositions. Even though our potential is trained on a small dataset, its accuracy is comparable to available machine learning potentials for Mo and V. Our results show that an ordered B2 phase is stable at low temperatures in alloys containing 50% V, but the solid solution phase is stable above 800 K. However, such long-range ordering is not observed in V-rich or Mo-rich alloys. In addition, our results show that V segregates to dislocation cores and grain boundaries.

The influence of SRB on corrosion behavior of Cu-based medium-entropy alloy coating sprayed by HVOF

In this study, a novel Cu-based (Cu55Al20Ni12Ti8Si5, at.%) medium-entropy alloy (MEA) coating was prepared by high-velocity oxygen-fuel (HVOF) spraying technology. Thermo-Calc was employed to simulate the phase diagram of the alloy system. Phase composition and microstructure of the as-sprayed coating were characterized by means of XRD, FESEM, TEM and STEM/EDX. The effect of sulfate-reducing bacteria (SRB) on the corrosion behavior of the coating and the as-cast Ni-Al bronze (NAB) was investigated using electrochemical measurements and surface characterization. The Thermo-Cala simulation results showed that the alloy system presented a single BCC solid solution phase, while the detailed characterization of microstructure indicated that a few NiTi-rich B2-ordered precipitates could be also found in the as-sprayed coating other than the Cu-rich BCC matrix. Electrochemical studies illustrated that the coating exhibited superior corrosion resistance than the NAB in SRB medium, the corrosion acceleration efficiency induced by SRB of the NAB (95.3 %) was more severe than that of the coating (63.8 %). Surface analysis results demonstrated the occurrence of pitting corrosion and the formation of Cu2S on the coating surface after corroded in SRB medium. Corrosive metabolite HS- induced microbiologically influenced corrosion was considered as the main corrosion acceleration mechanism caused by SRB.

Mechanical behavior of high entropy ceramic (TiZrHfVNb)C₅ under extreme conditions: A first-principles density functional theory study

Multi-component high-entropy carbides (HECs) have garnered extensive attention owing to their excellent serviceability in harsh environments characterized by ultra-high temperatures and high pressures. However, the microstructure, electronic structure, and chemical bonding of materials are significantly influenced by high pressure. This study investigates the effects of high-pressure conditions on the stability and properties of (TiZrHfVNb)C₅ phases using the first-principles calculation. According to the judgment analysis in thermodynamics, mechanics, and kinetics, HECs can form a single solid-solution phase which is stable in 0–100 GPa. When pressure is applied, HECs undergo more significant property changes. With increasing pressure, there are some increases in lattice distortion, elastic modulus, mechanical anisotropy, sound velocity, and Debye temperature of (TiZrHfVNb)C₅ ceramic. It is worth noting that hardness has a perverse behavior at high pressures, and the hardness decreases with increasing pressure. At 40–50 GPa, HECs also experience a brittle–ductile transition. A shift in HECs' electronic structure under pressure is what essentially causes the changes in brittle–ductile transition and hardness. This work provides instructive insights for predicting and designing high-performance high-entropy carbide ceramics tailored for extreme environmental conditions.

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

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

Improving the mechanical properties and oxidation resistance of Nb-16Si-20Ti alloys with B4C addition

Nb-Si-based alloys, as one of the candidate materials which may replace nickel-based alloys for the next generation aero-engines, its low fracture toughness (KQ) and poor oxidation resistance severely limited their application and development. Research has shown that Nb-Si-based alloys with a refined microstructure usually have good fracture toughness, whereas B and C elements are good microstructure refinement elements. So, the effects of B4C on the microstructure evolution and properties of Nb-16Si-20Ti alloys were studied. The results showed that the microstructure of Nb-16Si-20Ti was composed of Nb-based solid solution (Nbss) and Nb3Si phases. With the addition of B4C, the coarse Nb3Si decomposed and transformed into (α, γ)-Nb5Si3 + Nbss eutectic structure, refining the microstructure of the alloys. The room temperature fracture toughness and compressive strength of Nb-16Si-20Ti alloy were improved by adding B4C, among which the fracture toughness of 0.50B4C alloy was up to 11.5 MPa·m1/2, which increased by 98.3% compared with that of the 0B4C alloy. The improvement of the fracture toughness was mainly due to the increment of the ductile Nbss content in the alloy and the refinement of the microstructure. In addition, the addition of an appropriate amount of B4C (≤0.5 at.%) also improved the oxidation resistance of Nb-Si-based alloys.

Glass-Forming Ability and Magnetic Properties of Al82Fe16Ce2 and Al82Fe14Mn2Ce2 Alloys Prepared by Mechanical Alloying

Al82Fe16Ce2 and Al82Fe14Mn2Ce2 amorphous alloys were successfully synthesized by the mechanical alloying technique. The microstructural evolution of the milled powders was thoroughly investigated employing X-ray diffraction (XRD) and scanning electron microscopy (SEM). Additionally, their magnetic properties were quantitatively evaluated by a vibrating sample magnetometer (VSM). A full amorphous structure was obtained for both alloys after milling for 40 h. During the initial milling stage, extending from 5 to 20 h, an fcc solid solution phase was formed, coexisting with the residual Al phase. The partial substitution of 2 atomic percent (at.%) Mn for Fe in Al82Fe16Ce2 did not affect the alloy’s glass-forming ability. The amorphous Al82Fe16Ce2 and Al82Fe14Mn2Ce2 powders exhibited a nearly spherical shape, with diameters ranging from 1 to 3 µm and to 10 µm, respectively. Additionally, both the Al82Fe16Ce2 and Al82Fe14Mn2Ce2 alloys demonstrated characteristics of hard magnetism.

A Novel Preparation Method of (Ti,Zr,Nb,Mo,W)B2-SiC Composite Ceramic Based on Reactive Sintering of Pre-Alloyed Metals

High-entropy diboride-based (MeB2-based) ceramics are promising high-temperature structural materials because of their excellent mechanical properties, high-temperature stability, and oxidation resistance. In order to achieve low-temperature sintering of the high-entropy ceramics, a novel preparation method of high-entropy (Ti,Zr,Nb,Mo,W)B2-SiC ceramics based on reactive sintering of pre-alloyed solid-solution metals and nonmetals of Si, C, B4C was conducted in the current work. Mechanical alloying behavior of the mixed metal powders, as well as the phase composition, microstructure, mechanical properties, and oxidation behavior of the as-sintered MeB2-SiC ceramic were investigated. The XRD, SEM, and EPMA results indicated that the primary MeB2 solid-solution and SiC phases could be successfully formed during reactive sintering at a relatively low temperature of 1650 °C. The as-sintered MeB2-SiC ceramics had a high relative density of 97.8% and high mechanical properties (hardness of 19.74 ± 0.8 GPa, flexure strength of 533 ± 38 MPa, and fracture toughness of 6.01 ± 0.77 MPa·m1/2). Combining the oxidation behavior and microstructure evolution of the oxidation layer, a continuous and relatively dense MeOx-SiO2 oxidation layer was gradually formed and covered on the external surface, leading to decelerating oxidation behavior after an oxidation exposure time of 10 min.

Modeling of Crystal Structure in Li-Rich Layered Oxides for Rechargeable Lithium Cells

Lithium-Ion Batteries (LIBs) require further development in terms of safety and performance to establish also in the automotive market and new materials at both electrode sides as well as at the electrolyte side are necessary to overcome the state of-the-art and the commercial benchmarks. [1] The positive electrode materials constitute the bottleneck for present LIBs. They are based on layered structures (LiCoO2, NMC, NMA), spinel (LiMn2O4) and olivine LiFePO4, with rather moderate specific capacities (120–180 mAhg−1). [2] Therefore, the resulting specific energies are insufficient to meet market demand. Over-stoichiometric Li-rich layered oxides (LRLOs) are a family of positive electrode materials with promising theoretical electrochemical performance. [3] Indeed, they have received widespread attention due to their extremely high specific capacity (>250 mAhg−1) and low cost. Their extraordinary capacity has been confirmed to originate not only from the redox activities of transition metals, but also from the redox activities of oxygen anions. The crystal structure of LRLOs is an open playground of debate. There are two main hypotheses: a) a two phase nano-domain and b) a single phase solid solution. In this framework, the modeling of the crystal structure is quite challenging. Usually, the description of the structure of LRLOs can be done with rhombohedral or monoclinic unit cell. Nevertheless, the use of convention unit cells leads to a bad representation of the crystal structure because of the missing of structure features in the superstructure region (20<θ<30 in typical diffraction pattern with Cu Kα). In this communication, we demonstrate that extensive defectivities play a key role in the shape and intensity of diffractograms of LRLOs. In particular, we show how the use of supercells or defects (antisite defects and stacking faults) can help in the description of crystal structure of this class of materials.[1] Zubi, G.; Dufo-López, R.; Carvalho, M.; Pasaoglu, G. The Lithium-Ion Battery: State of the Art and Future Perspectives. Renewable and Sustainable Energy Reviews 2018, 89, 292–308.[2] Casas-Cabanas, M.; Ponrouch, A.; Palacín, M.R. Blended Positive Electrodes for Li-Ion Batteries: From Empiricism to Rational Design. Isr J Chem 2021, 61, 26–37.[3] Nie, L.; Chen, S.; Liu, W. Challenges and Strategies of Lithium-Rich Layered Oxides for Li-Ion Batteries. Nano Res 2022.

First principles-based design of lightweight high entropy alloys

Recently, the design of lightweight high entropy alloys (HEAs) with a mass density lower than 5 g/cm3 has attracted much research interest in structural materials. We applied a first principles-based high-throughput method to design lightweight HEAs in single solid-solution phase. Three lightweight quinary HEA families were studied: AlBeMgTiLi, AlBeMgTiSi and AlBeMgTiCu. By comprehensively exploring their entire compositional spaces, we identified the most promising compositions according to the following design criteria: the highest stability, lowest mass density, largest elastic modulus and specific stiffness, along with highest Pugh’s ratio. We found that HEAs with the topmost compositions exhibit a negative formation energy, a low density and high specific Young’s modulus, but a low Pugh’s ratio. Importantly, we show that the most stable composition, Al0.31Be0.15Mg0.14Ti0.05Si0.35 is energetically more stable than its metallic compounds and it significantly outperforms the current lightweight engineering alloys such as the 7075 Al alloy. These results suggest that the designed lightweight HEAs can be energetically more stable, lighter, and stiffer but slightly less ductile compared to existing Al alloys. Similar conclusions can be also drawn for the AlBeMgTiLi and AlBeMgTiCu. Our design methodology and findings serve as a valuable tool and guidance for the experimental development of lightweight HEAs.

Effects of milling time and sintering temperature on the mechanical properties of 8 wt% WC/AlCoCrFeNiTi0.5 high entropy alloy matrix composite

The present study investigates the effect of milling time, and sintering temperature on the microstructure and mechanical properties of the spark plasma sintered 8 wt% WC/AlCoCrFeNiTi0.5 high entropy alloys matrix composite (HEAC). The results show that the grain size is decreased first and then increased with an increase of milling time. The refined grain along with the homogeneous distribution of reinforcements is observed when milling for 15 h. It also shows both of relative density and compression strength of compacts are increased first and then decreased with an increase of sintering temperature. A highest relative density of 99.89 % and a fracture strength of 2683 MPa are obtained at 1150 °C. Another, sintering leads to crystal structure transition from BCC to FCC for the solid solution phase. The formation of (Fe, Co)6W6C complex carbide as a main strengthening phase is conducive to the enhancement of hardness. The strengthening mechanism is mainly attributed to dislocation strengthening and Orowan mechanism.

Preparation and lithium-storage performance of (Ti1−x,Vx)3C2 MXene solid solutions

Two-dimensional Ti3C2 MXene shows excellent lithium-storage performance as an anode of lithium-ion batteries. The replacement of Ti with V can enhance the lithium-storage performance, but decrease the stability of Ti3C2 MXene. To understand the impact of V replacement, in this study, a series of (Ti1−x,Vx)3C2 (x = 0, 0.1, 0.2…0.6) MXene solid solutions were prepared. The stability and lithium storage performance of the MXene solutions were investigated. The precursors to make (Ti1−x,Vx)3C2 MXene were MAX phase solid solutions of (Ti1−x,Vx)3AlC2, which were made from TiH2, V, Al and C powders at 1400 °C for 2 h under Ar atmosphere. Because of the replacement of > 60 at% Ti by V destroys the stability of the (Ti1−x,Vx)3AlC2, the MAX solid solutions with x ≤ 0.6 were synthesized in this study. (Ti1−x,Vx)3C2 MXene was made from the corresponding MAX phases by the etching in mixed solutions of NaF + HCl. (Ti1−x,Vx)3C2 MXene (x = 0–0.4) were synthesized at 60 °C for 2 days, and the MXene (x = 0.5–0.6) were synthesized at 70 °C for 6 days. All (Ti1−x,Vx)3C2 solid solutions had better electrochemical performance than pure Ti3C2 MXene. Especially, (Ti0.7,V0.3)3C2 exhibited the best performance. The capacity of (Ti0.7,V0.3)3C2 was 183.5 mAh/g after 100 cycles at a current density of 500 mA/g, which was ∼4.3 times of the capacity of the pure Ti3C2 (42.2 mAh/g at 500 mA/g). This is the first report to clarify the effect of V replacement on the stability and lithium performance on (Ti1−x,Vx)3C2 MXene solid solution. This work is critical for the synthesis of novel MXenes and MAX phases. It provides a way to make a MXene that is very close to V3C2 MXene, which is theoretically predicted but cannot be made. This work is vital for the enhancement of MXenes’ electrochemical performance.

Enhanced wear resistance of AlCoCrFeMo high entropy coatings (HECs) through various thermal spray techniques

The remarkable mechanical and wear properties of high entropy alloys (HEAs) have opened up exciting possibilities for their use in aerospace applications. This study explores the potential of AlCoCrFeMo high entropy coatings (HECs) fabricated using three thermal spray techniques, namely low-pressure cold spray (LPCS), flame spray (FS), and high-velocity oxy fuel spray (HVOF) as wear resistant surfaces. The coatings were investigated in terms of their microstructures, phases, mechanical properties, and sliding wear characteristics from room temperature up to 350 °C. Ex-situ characterization was performed using XRD for phase analysis and SEM-EDS for cross-sectional microscopy and phase compositions of the coatings. All three coatings exhibited a typical lamellar structure with the formation of BCC solid solution phase and variations in porosity and oxide content. The HVOF sprayed coatings showed higher hardness values compared to the FS and LPCS coatings, which can be attributed to their fine splats microstructure, lower porosity, and oxide inclusions compared to the other two coatings. In terms of tribological performance, the LPCS coatings exhibited overall lower frictional coefficient than the FS and HVOF coatings at both tested temperatures. However, the HVOF coatings exhibited relatively lower wear rates, correlating well with observations from ex-situ analysis, highlighting lower material transfer and decreased formation of debris particles compared to LPCS and FS coatings. These findings suggest that the HEA materials have great potential as next-generation tribological interfaces under high-temperature wear conditions, emphasizing the importance of designing materials with improved microstructural features.

Synthesis and characterization of a new (Ti1-yCuy)2(Al1-xCux)C MAX phase solid solution

New [Ti(1-y)Cuy]2[Al(1-x)Cux]C solid solutions have been synthesized by solid-state reaction performed at 760 and 800 °C on compacted Ti2AlC-40 vol % Cu composite particles produced by mechanical milling. Using XRD and EDXS, it is demonstrated that Cu atoms can enter the crystallographic structure of the Ti2AlC MAX phase, whereas a Cu(Al) solid solution is formed during thermal treatment. A selective chemical etching of the Cu(Al) matrix is performed to determine the composition of the MAX phase solid solution by analyzing the filtrate and the solid phase using ICP-OES end EDXS methods respectively. (Ti0.85Cu0.15)2(Al0.75Cu0.25)C and (Ti0.85Cu0.15)2(Al0.58Cu0.42)C solid solutions are formed after thermal treatment at 760 and 800 °C, respectively. The substitution rate on the A site of the MAX phase thus increases with temperature.

Zn Redistribution and Volatility in ZnZrOx Catalysts for CO2 Hydrogenation.

ZnO-ZrO2 mixed oxide (ZnZrOx) catalysts are widely studied as selective catalysts for CO2 hydrogenation into methanol at high-temperature conditions (300-350 °C) that are preferred for the subsequent in situ zeolite-catalyzed conversion of methanol into hydrocarbons in a tandem process. Zn, a key ingredient of these mixed oxide catalysts, is known to volatilize from ZnO under high-temperature conditions, but little is known about Zn mobility and volatility in mixed oxides. Here, an array of ex situ and in situ characterization techniques (scanning electron microscopy/energy dispersive X-ray spectroscopy (SEM/EDX), transmission electron microscopy (TEM), powder X-ray diffraction (PXRD), X-ray absorption spectroscopy (XAS), X-ray photoelectron spectroscopy (XPS), Infrared (IR)) was used to reveal that Zn2+ species are mobile between the solid solution phase with ZrO2 and segregated and/or embedded ZnO clusters. Upon reductive heat treatments, partially reversible ZnO cluster growth was observed above 250 °C and eventual Zn evaporation above 550 °C. Extensive Zn evaporation leads to catalyst deactivation and methanol selectivity decline in CO2 hydrogenation. These findings extend the fundamental knowledge of Zn-containing mixed oxide catalysts and are highly relevant for the CO2-to-hydrocarbon process optimization.

The Effects of the Al and Zr Contents on the Microstructure Evolution of Light-Weight AlxNbTiVZry High Entropy Alloy.

To investigate the comprehensive effects of the Al and Zr element contents on the microstructure evolution of the AlNbTiVZr series light-weight refractory high entropy alloys (HEAs), five samples were studied. Samples with different compositions were designated Al1.5NbTiVZr, Al1.5NbTiVZr0.5, AlNbTiVZr, AlNbTiVZr0.5, and Al0.5NbTiVZr0.5. The results demonstrated that the actual density of the studied HEA samples ranged from 5.291 to 5.826 g·cm-3. The microstructure of these HEAs contains a solid solution phase with a BCC structure and a Laves phase. The Laves phase was further identified as the ZrAlV intermetallic compound by TEM observations. The microstructure of the AlNbTiVZr series HEAs was affected by both the Al and Zr element contents, whereas the Zr element showed a more dominant effect due to Zr atoms occupying the core position of the ZrAlV Laves phase (C14 structure). Therefore, the as-cast Al0.5NbTiVZr0.5 sample exhibits the best room temperature compression property with a compression strength (σp) of 1783 MPa and an engineering strain of 28.8% due to having the lowest ZrAlV intermetallic compound area fraction (0.7%), as characterized by the EBSD technique.

Effect of La2O3–CeO2 codoping on the microstructure and mechanical properties of tailing glass–ceramics

The effect of the La2O3–CeO2 content on the microstructure and mechanical properties of CaO–MgO–Al2O3–SiO2 tailing glass–ceramics codoped with La2O3 and CeO2 was investigated to gain insight into the properties of glass–ceramics produced from solid wastes. The results showed that augite was the dominant crystalline phase in all of the samples. Augite formed a solid solution phase containing La and Ce as the La2O3–CeO2 content increased, and the shape of the augite phases changed from dendritic to island-shaped because La oxyapatite precipitated at the augite boundary. Among the glass–ceramics, the highest percentage of Si–O tetrahedrons with two nonbridging oxygen atoms (Q2) was observed for that containing 1 wt% La2O3–CeO2. In addition, this particular composition exhibited an optimal microstructure owing to its high crystallinity and interlocking dendrites. Consequently, this microstructure displayed superior mechanical properties, such as Vickers hardness, bending strength, and fracture toughness of 7.95 GPa, 250.13 MPa, and 1.97 MPa/m1/2, respectively.

Novel TaC0.8 incorporated WC composites with enhanced mechanical properties by spark plasma sintering

The WC-based composites incorporated with different concentrations of TaC0.8 were solidified by using ball milling and spark plasma sintering (SPS). The impact of TaC0.8 content on the densification, phase composition, microstructure and mechanical properties of WC-TaC0.8 composites was evaluated. The major constitutions of WC-TaC0.8 composites were WC matrix, (W1−x, Tax)Cn solid solution and W2C phase. The adequate amounts of TaC0.8 additive had promoter function on the generation of W2C and the densification of WC-based composites through carbon vacancies. The (W1−x, Tax)Cn and W2C phases were uniformly dispersed in the WC matrix, effectively bringing a refinement of WC grains. There were the coherent interface of WC (01̅10)/W2C (101̅0) and semi-coherent interface of (W1−x, Tax)Cn (200)/WC (101̅1) in the WC-7.5 wt%TaC0.8 (WTa7.5) composite. The nearly-full densified WTa7.5 composite (99.3%) rendered the best overall mechanical properties, featuring high hardness of 24.9 GPa and high fracture toughness of 8.3 MPa·m1/2. The high fracture toughness was justified by several factors including highly dense and fine microstructure, coherent and semi-coherent interfaces, and a combination of transgranular and intergranular fracture, as well as crack deflection and crack bridging.

Phase relationships at 1673 K and thermodynamic evaluation of ZrO2–SrO–CaO system

Twelve samples of the ZrO2–SrO–CaO system were prepared, quenched from 1673 K to room temperature, and characterized by XRD and SEM/EDS methods to investigate the phase relations of the ZrO2–SrO–CaO system at 1673 K. The results showed that four solid solution phases exist: (Sr, Ca)ZrO3, (Sr, Ca)2ZrO4, (Sr, Ca)3Zr2O7 and (Sr, Ca)4Zr3O10. Based on the experiments as well as the literature data, the ZrO2–SrO–CaO system was evaluated by the CALPHAD (CALculation of PHAse Diagram) method, and the isothermal sections at 1673 K and 2073 K and a liquidus projection were calculated. The calculation results were in good agreement with most of experimental results, which proves that the database is self-consistent and reliable, and this work can be applied to the design of novel refractories.

Elevated temperature tribological performance of non-equiatomic CoCrNiTiWx high entropy alloy coatings developed by mechanical alloying and high-velocity oxy-fuel spray

High entropy alloys (HEA) have applications in multiple fields owing to their exceptional mechanical and physical properties. In the current study, mechanical alloyed CoCrNiTiWx (x; a molar fraction, x = 0.5 and 1.5) HEA feedstock powders were deposited on maraging steel substrate using high-velocity oxy-fuel spray (HVOF). The phase evolution and the microstructure of the milled powders and as-sprayed coatings were analysed by X-ray diffraction (XRD) and scanning electron microscope (SEM). The tribological behaviour of CoCrNiTiW0.5 and CoCrNiTiW1.5 HEA coatings at elevated temperatures was studied extensively using a Pin-on-Disc tribometer. The CoCrNiTiW0.5 and CoCrNiTiW1.5 HEA coatings retained the BCC solid solution phases formed during the milling stage. However, additional oxide and intermetallic phases were formed owing to the in-flight oxidation and high temperatures experienced during the HVOF deposition. The deposited coatings exhibited a lamellar structure and good mechanical bonding with the substrate. The porosities of CoCrNiTiW0.5 and CoCrNiTiW1.5 HEA coatings were found to be 1.69 ± 0.32 % and 1.51 ± 0.37 % respectively.Consequently, the CoCrNiTiW0.5 and CoCrNiTiW1.5 HEA coatings displayed average microhardness values of 863 ± 52 HV0.3 and 1025 ± 39 HV0.3, respectively. Further, the wear rates of coatings exhibited a significant reduction at elevated temperatures, owing to the formation of TiO2, NiCr2O4 oxide tribofilms for CoCrNiTiW0.5, and CoCr2O4, NiWO4, WO3 oxides for CoCrNiTiW1.5. The specific wear rate of CoCrNiTiW0.5 HEA coating dropped by 73.6 % from 22.7 ± 2.6 × 10−6 mm3/N-m to 5.99 ± 1.9 × 10−6 mm3/N-m, while CoCrNiTiW1.5 dropped by 78.8 % from 11.86 ± 3.5 × 10−6 mm3/N-m to 2.51 ± 1.5 × 10−6 mm3/N-m, with a rise in the temperature from RT to 600 °C. Likewise, The frictional coefficients of CoCrNiTiW0.5 HEA dropped from 0.504 ± 0.015 to 0.397 ± 0.005, while CoCrNiTiW1.5 HEA dropped from 0.578 ± 0.025 to 0.471 ± 0.004, with a rise in temperature from RT to 600 °C. At room temperature, the wear mechanisms of the as-sprayed CoCrNiTiWx coatings were dominated by adhesive wear. However, at elevated temperatures, a shift towards oxidative wear was observed.

Microstructure and properties of the TiAl/GH3030 dissimilar joints vacuum-brazed with a Ti-based amorphous filler metal

TiAl intermetallic was vacuum brazed to GH3030 alloy with a Ti-based amorphous filler metal (Ti33.3Zr16.7Cu39Ni11, at.%). The effect of the brazing temperature on the microstructural evolution and mechanical properties of the brazed joints was investigated. Ti3Al and Al(Cu, Ni)2Ti phases composed the diffusion zone I at the TiAl side resulting from the diffusion of Cu and Ni into TiAl intermetallic, and the diffusion zone III at GH3030 side consisted of α-Ni and α-Cr solid-solution phases and a TiNi3 intermetallic phase, which rooted in the diffusion of Ti into GH3030 alloy. Increasing the brazing temperature accelerated the diffusion and migration of the constituents, and further thickened all reaction zones. The growth rate of zones I and III was wzoneI = 0.0789 μm/oC and wzoneIII = 0.0833 μm/oC, respectively. The formation of zone III was more difficult than that of zone I with the growth activation energy (Q) of QzoneI = 101.96 kJ·mol−1 and QzoneIII = 165.58 kJ mol−1. The maximum shear strength was 260 MPa obtained at 960 °C/10 min. All brazed joints fractured at the interface of Al3(Cu, Ni)Ti2/Al(Cu, Ni)2Ti, because of the mismatch of their lattice structure and hardness, and all failed joints demonstrated zonal and brittle features.