Bcc Solid Solution Research Articles

Effect of Vanadium Addition on the Wear Resistance of Al0.7CoCrFeNi High-Entropy Alloy.

High-entropy alloys are of interest to many researchers due to the possibility of shaping their functional properties by, among other things, the use of alloying additives. One approach to improving the wear resistance of the AlCoCrFeNi alloy is modification through the addition of titanium. However, in this study, an alternative solution was explored by adding vanadium, which has a completely different effect on the material's structure compared to titanium. The effect of vanadium additives on changes in the microstructure, hardness, and wear resistance of the Al0.7CoCrFeNi alloy. The base alloys Al0.7CoCrFeNi and Al0.7CoCrFeNiV0.5 were obtained by induction melting. The results showed that the presence of vanadium changes the microstructure of the material. In the case of the base alloy, the structure is biphasic with a visible segregation of alloying elements between phases. In contrast, the Al0.7CoCrFeNiV0.5 alloy has a homogeneous solid solution bcc structure. The presence of vanadium increased hardness by 33%, while it significantly reduced friction wear by 73%. Microscopic observations of friction marks indicate differences in the wear mechanisms of the two materials.

Comprehensive property combination for biomedical application achieved in a Ti35Zr35Nb15Mo5Fe5Cr5 complex concentrated alloy

A novel Ti35Zr35Nb15Mo5Fe5Cr5 complex concentrated alloy (CCA) with potential for biomedical application was developed by vacuum arc melting and its microstructural evolution, mechanical properties, corrosion and wear behavior, and cytocompatibility were systematically evaluated. The alloy has an experimentally measured density of 6.37 g/cm³, suitable to fulfill the requirement of light weight. XRD analysis revealed that the as-cast and annealed specimens contain two BCC solid solution phases and a TiCr2 type Laves phase. The average microhardness (H) and elastic modulus (E) of the as-cast CCA are 618.39 ± 9.26 HV and 97.32 ± 3.58 GPa, respectively. Moreover, the yield strength (YS) of the as-cast alloy, estimated from elastoplastic analysis of the microindentation data, is 1203.94 ± 30.28 MPa. However, the CCA annealed at 1100˚C exhibits a microhardness of 833.08 ± 7.58 HV and a YS of 1669.65 ± 24.79 MPa, with the elastoplastic stress-strain response revealing no significant loss of plasticity due to the increased hard Laves phase. Corrosion and wear tests conducted in simulated body fluid confirmed the alloy's excellent corrosion and wear resistance. Cell culture experiments with MG-63 and HEK-293 cells demonstrated superior cell viability and proliferation compared to CP-Ti and 316 L SS. Moreover, confocal fluorescence images of MG-63 cells stained with AO/EtBr, Rh-123, and DCFH-DA revealed that the present CCA exhibits good biocompatibility. Thus, a comprehensive combination of low density, compatible elastic modulus, good strength with adequate plasticity, and sufficient corrosion resistance and biocompatibility were successfully achieved, unraveling significant potential of the alloy for biomedical applications.

Properties of Al0.5CoCrFeNi2Ti High-Entropy Alloy System: From Gas-Atomized Powders to Atmospheric Plasma-Sprayed Coatings

The performance of the Al0.5CoCrFeNi2Ti HEA atmospheric plasma-sprayed coating was extended from characterizing the properties of its powder prepared via the gas atomization method. It was observed that the gas-atomized HEA powders possessed a solid solution BCC phase, while a major phase transformation to a FCC-L21 intermetallic phase occurred during the annealing process. The formation of the intermetallic phase resulted in an increase in average hardness from 6.28 to 7.64 GPa after annealing at 900 °C for 1 h. Afterward, HEA powders were applied in the atmospheric plasma spray technology. The phase constitution of Al0.5CoCrFeNi2Ti HEA coatings was investigated by varying powder size and applied current. It was observed that the smaller powder sizes prone to oxidation, whereas higher applied current facilitated the phase transformation from BCC to FCC phase. The nanoindentation test indicated distinct average microhardness values for the interlamellar oxide region, BCC unmelted particle and FCC phase lamellar region, which was measured at 12.35, 8.68 and 5.97 GPa, respectively. As a result, the adjustability of coating hardness was achieved by manipulating the relative phase ratio.

First Principles Calculation of the Influence of Alloying on the Phase Stability, Elasticity, and Thermodynamic Properties of the MoNbTiVX (X = Al/Cr) Refractory High-Entropy Alloy

In response to the poor wear resistance and high-temperature oxidation resistance of titanium alloys during service, a series of lightweight refractory high-entropy alloys (RHEAs) can be designed for the laser cladding coating of titanium alloy surfaces, with due consideration of the compositional and structural characteristics of titanium alloys. Firstly, the structural stability, mechanical and thermal properties of four lightweight RHEAs (MoNbTiV, AlMoNbTiW, CrMoNbTiV, and AlCrMoNbTiV) with equal atomic ratios were designed and calculated using first principles combined with quasi-harmonic approximation (QHA). The results indicate that all four RHEAs are stable BCC, exhibiting elastic anisotropy and ductility. The lightest density is 6.409 g/cm3. Adding Al/Cr can cause structural distortion and affect its mechanical properties. Their Young’s moduli are in the following order: AlCrMoNbTiV > MoNbTiV > CrMoNbTiV > AlMoNbTiV. The thermal expansion coefficients of the four RHEAs and titanium alloys are very close, with a difference in linear expansion coefficient of less than 1.16 × 10−5/K. Meanwhile, the metallurgical bonding of four types of RHEA coatings was successfully achieved on a Ti-6Al-4V(TC4) substrate through laser cladding technology, and all coatings exhibited a unique BCC solid solution phase.

Effects of laser energy density on the resistance to wear and cavitation erosion of FeCrNiMnAl high entropy alloy coatings by laser cladding

FeCrNiMnAl high entropy alloy (HEA) coatings are prepared on the surface of 304 stainless steel (304 SS) by laser cladding. The effects of laser energy density on the residual stress, microstructure, nanoindentation behavior, the resistance to wear, corrosion and cavitation erosion (CE) of FeCrNiMnAl HEA coatings are studied. Experimental results shows that when the laser energy density decreases from 40 J/mm2 to 24 J/mm2, the phase composition of the FeCrNiMnAl HEA coatings remains unchanged with a single BCC solid solution and the elements are uniformly distributed without obvious segregation. The average grain size of the HEA coatings is refined from 73.5 to 41.7 μm. When the laser energy density is 28 J/mm2 for S3 sample, the coating displays good forming quality and excellent comprehensive performance. The specific wear rate is only 8.2 % that of the 304 SS substrate. In addition, S3 exhibits the highest corrosion resistance as indicated by the highest corrosion potential (Ecorr) and the lowest corrosion current density (Icorr) in 3.5 wt% NaCl solution. After 10-h CE, the mean depth erosion rate (MDER) of S3 is the lowest (1.69 ± 0.03 μm/h), which is much lower than that of 304 SS (4.96 ± 0.13 μm/h), and the pure mechanical damage plays a dominant role in CE, followed by the synergistic damage effect. The excellent CE resistance of S3 is attributed to its excellent combination of corrosion resistance, mechanical properties and the self-recovery ability of passivation film.

Effect of Ti/Ta ratio on the microstructure and mechanical properties of Hf20Mo15Nb25Ta20-xTi20+x refractory high-entropy alloys

The limited deformation capacity at room temperature remains a significant challenge in refractory high-entropy alloys (RHEAs). In this study, the fine-grained Hf20Mo15Nb25Ta20-xTi20+x (x = 0, 5, 10, 15) alloys were prepared by mechanical alloying and subsequent spark plasma sintering (SPS), based on precise component regulation. The impact of altering the Ti/Ta ratio (R, R = (20+x)/(20-x)) on the pre-alloyed powders and the as-sintered alloys were investigated. An increase in R-value led to larger powder particle sizes and lower powder yields, which was attributed to the improvement in the plasticity of the pre-alloyed powders. After SPS sintering, the as-sintered alloys were comprised of two BCC solid solution matrices and nanoscale precipitated phases. As the R-value increases, the stability of the microstructure decreases slightly, while the yield strength shows a slight improvement, and the plastic strain experiences a relatively significant enhancement. The as-sintered Hf20Mo15Nb25Ta20-xTi20+x alloys all exhibited an excellent balance of strength and plasticity. Theoretical calculations revealed that the yield strength of the alloys was the result of the combined effect of several strengthening mechanisms, predominantly substitutional solid solution strengthening. Furthermore, a systematic discussion was conducted on the reasons for the improvement in the plasticity of the as-sintered alloys. This study presents an effective approach to enhancing the room temperature deformation capacity of RHEAs, thereby facilitating their broader application.

Effect of Nb on hydrogen storage properties of Ti–V–Cr-based alloys

This research investigates the impact of incorporating Nb into Ti–V–Cr-based alloys, with compositions ranging from Ti30-xV35Cr35Nbx (x = 0, 5, 10, 15, and 20 at. %), with the objective of enhancing hydrogen storage properties. Employing a comprehensive approach encompassing design, synthesis, and characterization, the study aimed to develop novel hydrogen storage alloys. Utilizing the CALPHAD method, the alloys were designed to anticipate the formation of multicomponent single-phase structures. Structural and microstructural analyses of the synthesized alloys were conducted using XRD, SEM, and EDS techniques. The hydrogen storage capabilities were evaluated utilizing a Sieverts’ type apparatus. The investigated alloys displayed a singular-phase BCC solid solution, characterized by dendritic microstructures. Notably, the Nb-free alloy, Ti30V35Cr35, demonstrated the highest hydrogen storage capacity, reaching 3.62 wt% (∼1.9 H/M) under 20 bar of H2 at room temperature. Among the Nb-containing alloys, Ti25V35Cr35Nb5 exhibited a capacity of 2.91 wt% (∼1.6 H/M) under identical conditions. Furthermore, the incorporation of Nb up to 10 at. % effectively bolstered cycle durability. This study underscores the presence of an optimal Nb content crucial for achieving the highest hydrogen capacity and cycle durability in these alloys.

Effect of annealing temperature on the microstructure and mechanical properties of Al0.2CrNbTiV lightweight refractory high-entropy alloy

The DSC analysis and heat treatment of the newly proposed Al0.2CrNbTiV lightweight refractory high-entropy alloy prepared by vacuum arc melting was investigated, and the evolutions of the microstructure and mechanical properties of the alloy after homogenization annealing at 650 °C, 850 °C and 1050 °C for 12 h were analyzed. The results show that the Al0.2CrNbTiV high-entropy alloy could maintain stable BCC solid solution structure from room temperature to 800 °C. The alloy annealed at 650 °C exhibited simple BCC structure with coarse equiaxed grains; after annealing at 850 °C, fine acicular and irregular block-like C14 Laves phases were uniformly precipitated in the grain and grain boundaries, meanwhile, the C14 Laves phase get coarser with the annealing temperature increased to 1050 °C. With the increase of annealing temperature, the microhardness of the Al0.2CrNbTiV alloy increased first and then decreased, reaching the maximum value of 692 HV after annealing at 850 °C. Due to the high dislocation density and the formation of kink bands, the alloy annealed at 650 °C showed a good combination of plastic and strength, with the work hardening ability strengthened simultaneously, the compressive yield strength could be up to 1454 MPa, with strain >50 %. Due to the precipitation of the hard and brittle C14 Laves phase, the load-bearing capacity of the alloy was reduced after annealing at 850 °C and 1050 °C. However, the wear resistance of the alloy also improved with the presence of the hard phase. The friction coefficient of Al0.2CrNbTiV alloy annealed at 650 °C is 0.67, with the abrasive wear acting as the main wear mechanism, and the alloy after annealing at 850 °C shew the best wear resistance, with the friction coefficient of 0.63, and delamination wear mechanism.

Microstructure evolution and catalytic activity of AlCo1−xFeNiTiMox high entropy alloys fabricated by powder metallurgy route

This investigation presents the synthesis of equiatomic and non-equiatomic AlCo1−xFeNiTiMox (x = 0, 0.1, 0.25 and 1.0) high entropy alloys fabricated by mechanical alloying. Mo partially replaced Co. Classic thermodynamic calculations, such as mixing enthalpy (ΔHmix), configurational entropy (ΔSmix), the atomic size difference (δ), entropy to enthalpy ratio (Ω), electronegativity difference (△χ), and valence electron concentration (VEC) were used. Considering δ, Ω and VEC parameters, a BCC solid solution and an intermetallic phase can be predicted due to the partial replacement of Co by Mo. X-ray and electron diffraction of equiatomic HEA without Mo content revealed that after 35 h of milling, a Fe-type BCC lattice phase was formed in the alloy and two L21 phases, in addition to a minimal amount of FCC phase. As the Mo content increased, the Fe-type BCC phase was steadily replaced by the Mo-type BCC phase and the Fe-type FCC phase, and two L21 phases were also developed. When the 5 at% Mo-containing (x = 0.25) alloy was further milled for 80 h, the amount of phases remained almost the same; only the grain size was strongly reduced. The influence of the Mo addition on the properties of studied alloys was also confirmed in the decolourisation of Rhodamine B using a modified photo-Fenton process. The decolourisation efficiency within 20 min was 72% for AlCoFeNiTi and 87% for AlCo0.75FeNiTiMo0.25 using UV light with 365 nm wavelength.

Microstructure and wear property of (TiN + NbC) double ceramic phase-reinforced in FeCrNiCoAl high-entropy alloy coating fabricated by laser cladding

This study delves into the influence of TiN and NbC ceramic particles on the phase structure, grain organization, microhardness, and wear resistance of FeCoNiCrAl High-Entropy Alloy (HEA) composite coatings produced through laser cladding. The integration of ceramic particles induced a dual BCC solid-solution phase structure (B2+BCC), with the formation of a TiNb phase upon the melting and interaction of TiN and NbC in the melt pool. The ceramic particles significantly modified the grain structure of the HEA coatings, disrupting the Columnar-to-Equiaxed Transition (CET) and favoring the emergence of equiaxed grains. The TiN particles induced a substantial refinement of grain size, albeit unevenly, while NbC had a milder effect. The combined presence of TiN and NbC particles resulted in a more uniform grain refinement, enhancing the mechanical properties of the coatings. Notably, the (TiN + NbC)/HEAs composite coating demonstrated superior mechanical performance under the synergistic effect of both ceramic particles. The average microhardness value increased by 55.80 % compared to 17-4Ph stainless steel, and the wear rate was reduced by 88.38 %, with the wear mechanism primarily involving abrasive and oxidative wear.

Microstructure evolution and magnetic characteristics of a novel high entropy alloy produced by mechanical alloying and spark plasma sintering

In this research, CoMoMnNiV high entropy alloys were successfully prepared by mechanical alloying (MA) and spark plasma sintering (SPS). The microstructure, phases and magnetic properties of the as-milled powders and bulk sample were examined, employing X-ray diffraction, differential scanning calorimeter (DSC), scanning electron microscopy, and Vibrating Sample Magnetometer (VSM) techniques. The resultant phase following MA exhibits a dual-phase microstructure comprising BCC and FCC solid solutions, with individual crystal dimensions below 18 nm. Thermal stability assessment via DSC reveals the alloy robustness up to 1216 °C. The SPS was performed on the MA samples at 980 °C and 50 MPa pressure, and their density was determined to be 89.64 %. The sample subjected to a milling duration of 40 h demonstrated a saturation magnetization of 25.977 electromagnetic units per gram (emu/g) and a coercivity of 371.5 Oersteds (Oe).

First-principles calculations to investigate phase stability, elastic and thermodynamic properties of TiMoNbX (X=Cr, Ta, Cr and Ta) refractory high entropy alloys

This work uses first-principles calculations to investigate the phase stability, thermophysical and mechanical properties of refractory high entropy alloys (RHEAs) at finite temperatures. On the basis of plane wave quasi-potential and density functional theory, construct the structure model of a solid solution. The TiMoNbX (X = Cr, Ta, Cr and Ta) RHEAs have been determined to preserve a single body-centered cubic solid solution structure by calculations and the equilibrium lattice parameters and elastic modulus are consistent with experimental data obtained by laser cladding, which is combined with TC4 (Ti-6Al-4V) substrate. Using the quasi-harmonic Debye-Grüneisen model, the thermophysical characteristics of three RHEAs are investigated. The Voigt-Reuss-Hill scheme is used for calculating the Young's modulus (E), bulk modulus (B), shear modulus (G), and Poisson's ratio (ν), which indicates that all three RHEAs are ductile materials. Additionally, the modulus and hardness of materials decrease as temperature rises, whereas the properties of TiMoNbX RHEAs are predicted, as the nanoindentation hardness values at room temperature are comparable to, and slightly higher than the calculated values.

Powder metallurgical processing of novel Al0.5CoCrFeNiNb0.5-Si0.1 high entropy alloys: Phase evolution and nanomechanical properties

In this research, the high entropy alloy (HEA) of Al0.5CoCrFeNiNb0.5-Si0.1 was successfully synthesized through mechanical alloying utilizing a high-energy planetary ball mill. The synthesized HEA powder was subsequently pressed and sintered at 1400 °C with varying holding times. The phase evolution was investigated using X-Ray Diffraction (XRD) technique. Furthermore, the microstructural, morphological, and compositional characteristics of the powders were examined using Transmission Electron Microscopy (TEM) with Selected Area Electron Diffraction (SAED), as well as Scanning Electron Microscopy (SEM) in conjunction with Energy Dispersive Spectroscopy (EDS). The nano-mechanical properties of the HEA compact were evaluated through nanoindentation to determine nano-hardness and elastic modulus.The mechanical alloying process was conducted for a duration of 30 h, resulting in the formation of a single-phase BCC solid solution, as confirmed by XRD and SAED analyses. The study investigated the criteria for phase stability based on the minimum Gibbs free energy and Hume-Rothery rules, which were consistent with the observed microstructural characteristics. Furthermore, the sintering process resulted in porosity levels of 6.01 % and 4.71 %, with corresponding densities of 6.96 g/cm³ and 7.12 g/cm³ for holding times of 3 and 4 h, respectively. It was noted that extended holding times improved the mechanical properties, with the alloy achieving a maximum hardness of 546 HV, nano-hardness of 5.57 GPa, elastic modulus of 265.47 GPa, and yield stress of 1.89 GPa after a holding time of 4 h.

Tribological behavior of a Ti42Nb42Mo6Fe5Cr5 complex concentrated alloy and prediction through response surface methodology based mathematical modeling

In the present work, a novel Ti42Nb42Mo6Fe5Cr5 complex concentrated alloy (CCA) was developed using the vacuum arc melting technique. The as-cast CCA underwent a two-stage heat treatment process. In the first stage of heat treatment (HT 1), the alloy was heated to 900˚C. Subsequently, in the second stage of heat treatment (HT 2), the HT 1 samples were annealed at various temperatures, including 700˚C, 900˚C, and 1100˚C, for 20 h. The microstructure and mechanical response of the as-cast, HT 1 and HT 2 (annealed) samples were thoroughly investigated. The phase evolution, microstructure, and chemical composition were analyzed using XRD and SEM-EDS. The XRD analysis revealed major solid solution BCC phases (BCC 1 and BCC 2) with a small amount of Laves phase; however, the amount of Laves phase increased with increase in annealing temperature. The microhardness and elastic modulus of the CCA specimens were evaluated using instrumented micro-indentation. The CCA annealed at 1100˚C exhibits the highest microhardness and elastic modulus, with values of 5.94 ± 0.38 GPa and 124.40 ± 7.17 GPa, respectively. Response surface methodology (RSM) was used to develop a mathematical model aimed at predicting tribological characteristics, specifically the specific wear rate (SWR) and coefficient of friction (COF). RSM proposes a quadratic model to represent the mathematical relationship between input parameters for evaluating SWR and COF. The desirability function approach is employed to optimize input parameters to minimize both SWR and COF. The optimized values of SWR and COF are 6.87 × 10−4 mm3/N.m and 0.30 under a 27.52 N load, 2.86 Hz oscillation frequency, and 1100˚C annealing temperature. Surface topography analysis of the worn surface was evaluated using SEM and a profilometer to understand the wear mechanism and surface characteristics.

Microstructure and wear properties of carbide reinforced high-entropy alloy coatings on EA4T steel via vibration assisted TIG-cladding using WC core spiral AlCuNiCrTi-CWW

Aiming at the difficulty preparing high-entropy alloy (HEA) coating by adding carbide powder using TIG-cladding, the WC core spiral CWW prefabricated vibration assisted TIG-cladding method is proposed in this work. The HEA coating (S1-coating) was prepared on the EA4T steel using AlCuNiCrTi cable-type welding wire (CWW) via TIG-cladding, then the WC reinforced HEA coating (S2-coating) was produced using spiral AlCuNiCrTi-CWW with WC powder core by TIG-cladding. Finally, another WC reinforced HEA coating (S3-coating) via vibration assistance TIG-cladding using the same material as S2-coating. The S1-coating is composed of BCC and FCC solid solutions. Except for the same phases as S1-coating, TiC appeared in the S2-coating and the S3-coating. Due to the grain refinement effect caused by vibration, the S3-coating has the maximum microhardness (725HV), lowest friction coefficient (0.42) and the smallest volume wear rate (0.1397 × 10−4 mm3/N·m). The addition of vibration assistance effectively improves the microhardness and wear resistance of the coating without changing CWW composition.

Development and Mechanical Characterization of a CoCr-Based Multiple-Principal-Element Alloy

The development and mechanical characterization of a CoCr-based multiple-principal-element alloy are presented and discussed. In this work, ab initio synthesis and mechanical characterization of the (CoCr)100-x(TiNbZr)x (x = 0, 48 60, 78 and 100 % at) alloy family is reported; these include the calculation of thermodynamic parameters such as mixing entropy (ΔSmix), mixing enthalpy (ΔHmix), valence electron concentration (VEC), Ω and δ factors. The alloys were melted in a vacuum arc furnace; rod-shaped ingots were produced by suction casting. Phase characterization was carried out using optical microscopy, scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), and X-ray diffraction. Mechanical characterization was done via compressive and hardness tests. Calculation of phase diagrams was performed using Thermo-Calc © software. Yang’s model for phase prediction predicted a BCC solid solution. Multicomponent simulations predicted a more complex structure, with Laves (C14 and C15), Theta (C16), BCC 1, 2 and 3, Mu, CoTi2, CoZr3 and TiZr2. Contrary to Yang’s model for phase prediction, the experimentally obtained phases agreed reasonability well with those obtained by the Thermo-Calc simulation. The suction cast process cooling rate suppressed the nucleation and growth of some equilibrium phases, i.e., Laves (C14) or BCC 3 for the (CoCr)100-x(TiNbZr)x (x = 0, 48 60, 78 and 100 % at) alloys. The hardness test results were strongly related to the intermetallic phase formation, showing an increase of 331% with the x = 78 alloy. The BCC 1 phase played an important role in the yield strength behavior, as the (CoCr)22(TiNbZr)78 alloy, with a considerable amount of this phase, showed the highest yield strength value.

Microstructures and Corrosion Behaviors of Non-Equiatomic Al0.32CrFeTi0.73(Ni1.50−xMox)(x = 0, 0.23) High-Entropy Alloy Coatings Prepared by the High-Velocity Oxygen Fuel Method

The non-equiatomic Al0.32CrFeTi0.73(Ni1.50−xMox) (x = 0, 0.23) high-entropy alloy (HEA) coatings were prepared by the high-velocity oxygen fuel (HVOF) method. The microstructures and corrosion behaviors of the HVOF-prepared coatings were investigated. The corrosion behaviors were characterized by polarization, EIS and Mott-Schottky tests under a 3.5 wt.% sodium chloride aqueous solution open to air at room temperature. The Al0.32CrFeTi0.73Ni1.50 coating is a simple BCC single-phase solid solution structure compared with the corresponding poly-phase composite bulk. The structure of the Al0.32CrFeTi0.73Ni1.27Mo0.23 coating, combined with the introduction of the Mo element, means that the (Cr,Mo)-rich sigma phase precipitates out of the BCC solid solution matrix phase, thus forming Cr-depleted regions around the sigma phases. The solid solution of large atomic-size Mo element causes the lattice expansion of the BCC solid solution matrix phase. Micro-hole and micro-crack defects are formed on the surface of both coatings. The growth of both coatings’ passivation films is spontaneous. Both passivation films are stable and Cr2O3-rich, P-type, single-layer structures. The Al0.32CrFeTi0.73Ni1.50 coating has better corrosion resistance and much less pitting susceptibility than the corresponding bulk. The corrosion type of the Mo-free coating is mainly pitting, occurring in the coating’s surface defects. The Al0.32CrFeTi0.73Ni1.27Mo0.23 coating with the introduction of Mo element increases pitting susceptibility and deteriorates corrosion resistance compared with the Mo-free Al0.32CrFeTi0.73Ni1.50 coating. The corrosion type of the Mo-bearing coating is mainly pitting, occurring in the coating’s surface defects and Cr-depleted regions.

Effect of Heat Treatment on Mechanical Properties and Corrosion Response of HVOF Sprayed High Entropy Alloy Coatings

Surface and sub-surface related degradation of steels can be minimized using suitable surface coatings. High entropy alloys (HEA) are prominent and emerging materials among many coating materials. The current study investigates the effect of heat treatment of HEA coating on mechanical, metallurgical, and corrosion properties. The HEA coatings on SS304 steel were deposited using a High-Velocity Oxy-Fuel (HVOF) thermal spray process. The developed coatings were furnace heat treated at 700 °C, 900 °C, and 1100 °C, respectively, and their performance was benchmarked with the as-sprayed coatings. The metallurgical, mechanical, and microstructural analyses were performed using X-ray diffraction (XRD), Nanoindentation, Scratch test, and Field Emission Scanning Electron Microscope (FESEM) techniques. The corrosion response of the as sprayed and heat-treated coatings were recorded using a Potentiostat. The results indicated that as-sprayed coatings consisted of a single-phase BCC solid solution; however, the single-phase changed to a dual dual-phase system after heat treatment (BCC+FCC). The 900 °C heat-treated HEA coating exhibited superior mechanical and corrosion properties. But those characteristics started diminishing when the heat treatment temperature exceeded 900 °C. The introduction of the new FCC phase softened the coating, thereby leading to the evolution of microcracks in the coating. These micro-cracks acted as channels for electrolyte diffusion and further corroded the coatings. The current study surmised that HVOF-sprayed HEA coating should not be heat treated at above 900 °C.

The Microstructures and Wear Resistance of CoCrFeNi2Mox High-Entropy Alloy Coatings

The CoCrFeNi2Mox (x = 0, 0.4, 0.5, 1.0, x values in atomic ratio) high-entropy alloy coatings were designed and prepared on the Ti-6Al-4V substrate by laser cladding technology, their microstructures, and dry sliding wear resistance were studied in detail. When x < 0.4, the coatings were mainly composed of BCC solid solution phase, (Ni, Co)Ti2 phase, and α-Ti phase. When x ≥ 0.4, the new σ phase appeared in the coatings. As the Mo content increases from 0 to 1.0, the hardness showed a trend of first increasing and then decreasing, especially when x = 0.5, the coating hardness reached its maximum (882 HV), which was 2.65 times the hardness of the Ti-6Al-4V substrate. The CoCrFeNi2Mox high-entropy alloy coatings significantly improved the wear resistance of Ti-6Al-4V substrate, and with the increase in Mo content, the friction coefficient, widths/depths of worn tracks and wear rates of the coatings showed a trend of first decreasing and then increasing. In particular, when x = 0.5, the CoCrFeNi2Mo0.5 high-entropy alloy coating has the lowest friction coefficient (0.63), widths/depths of worn tracks (width: 803.690 μm; depth: 20.630 μm) and wear rate (5.136 × 10−5 mm3/(N·m)), which is one order of magnitude smaller than that of the substrate (3.694 × 10−4 mm3/(N·m)), demonstrating the best wear resistance. This is mainly because the appropriate proportion of hard α-Ti and σ phases effectively played a supporting role in resisting wear, while the relatively soft and dispersed BCC and (Ni, Co)Ti2 phases could effectively prevent the occurrence of brittle fracture during wear test process.

Preparation of a hard AlTiVCr compositionally complex alloy by self-propagating high-temperature synthesis

In this study, an AlTiVCr compositionally complex alloy with a high hardness of 865 ± 34 HV was prepared by a one-step self-propagating high-temperature synthesis (SHS) method using TiO2, V2O5, Cr2O3, and Al as starting materials. Experimental results confirmed the formation of a two-phase alloy having BCC solid solution dendritic grains reinforced by a Ti-rich intermetallic phase. Elemental analysis results confirmed that the Ti at.% in the synthesized alloy was lower than other elements. The addition of extra TiO2 to the reactants, pre-milling of the TiO2-Al mixture, and using the Rutile polymorph of TiO2 instead of Anatase as the starting material increased the atomic percent of Ti. The composition of the proper synthesized alloy was formulated as Al27.3Ti17.2V28.7Cr26.8 which is close to the desired AlTiVCr alloy. To clarify the reaction sequence during the SHS, the binary mixtures were analyzed by thermal analysis. It was postulated that the melting of Al facilitated the reduction reaction of Cr2O3 and VxOy compounds, while TiO2 was the last oxide compound reduced by Al. This hindered the completion of the TiO2 reduction reaction and as a consequence, the unreacted TiO2 entered the slag which lowered the concentration of Ti in the composition of the final alloy.