FCC Solid Solution Phase Research Articles

Microstructure and mechanical properties of as-cast (CuNi)100−xCox medium-entropy alloys

Microstructure and mechanical properties of non-equiatomic (CuNi)100−xCox (x=15, 20, 25 and 30, at.%) medium-entropy alloys (MEAs) prepared by vacuum arc-melting were investigated. Results show that all the as-cast MEAs exhibit dual face-centered cubic (fcc) solid-solution phases with identical lattice constant, showing typical dendrite structure consisting of (Ni, Co)-rich phase in dendrites and Cu-rich phase in inter-dendrites. The positive enthalpy of mixing among Cu and Ni-Co elements is responsible for the segregation of Cu. With the increase of Co content, the volume fraction of (Ni, Co)-rich phase increases while the Cu-rich phase decreases, resulting in an increment of yield strength and a decrement of elongation for the (CuNi)100−xCox MEAs. Nano-indentation test results show a great difference of microhardness between the two fcc phases of the MEAs. The measured microhardness value of the (Ni, Co)-rich phase is almost twofold as compared to that of the Cu-rich phase in all the (CuNi)100−xCox MEAs. During the deformation of the MEAs, the Cu-rich phase bears the main plastic strain, whereas the (Ni, Co)-rich phase provides more pronounced strengthening.

Tailoring oxidation resistance of Fe2CoCrNi0.5 based high entropy alloys by addition of alloying elements (Si, Cu and Si-Cu co-added)

Investigations about how to improve the properties of high entropy alloys (HEAs) play a key role in growing their potential for use in future industrial applications. Accordingly, this study aims to shed light on the effects of alloying elements (Si, Cu and Si-Cu co-added) on the microstructure and oxidation performance of the Fe 2 CoCrNi 0.5 based high entropy alloys. To this end, a series of high entropy alloys containing Fe 2 CoCrNi 0.5 , Fe 2 CoCrNi 0.5 Si 0.25 , Fe 2 CoCrNi 0.5 Cu 0.25 and Fe 2 CoCrNi 0.5 Si 0.25 Cu 0.25 were produced by electric current assisted sintering (ECAS) method, which is founded on the principle of simultaneous application of high-density electric current flowing through the powder mixture and uniaxial pressure within a short span of time. The oxidation test was carried out in air at 1123 K for 1200 h under cyclic conditions. The X-ray diffraction results showed that all HEAs have simple FCC solid solution phases. Among all HEAs, the oxidation resistance of the alloy exhibited an increasing trend with the Si addition and the best oxidation behavior at 1123 K for 1200 h was noted with Fe 2 CoCrNi 0.5 Si 0.25 , which experienced the least mass change of 4.72 mg cm −2 . Cu addition caused the highest mass change of 15.92 mg cm −2 and the thickest oxide scale of 38.2 µm after oxidation for 1200 h. Si addition is observed to enhance the amount of Cr-rich oxide in the scale. The oxide scale formed on HEA with Cu-added and HEA with Si-Cu co-added contained two oxide layers, including an outer layer of Fe-rich oxide and an inner layer of Cr-rich oxide, which showed continuous structure. However, the alloying elements did not play a favorable role in relieving the formation of internal oxidation. Additionally, it has been reported that Fe 2 CoCrNi 0.5 Si 0.25 HEA can exhibit high resistance to cyclic oxidation, which can be reasoned as an optimum material for structural and high-temperature applications. • Alloying element behavior has been shown to have a significant influence on the investigated properties of HEAs. • Si has proved to be effective in enhancing oxidation resistance. • Cu addition produced the thickest oxide scale. • One of the encouraging results of this investigation was the synergic effect of Si-Cu co-added.

Nano-TiC reinforced [Cr–Fe4Co4Ni4]Cr3 high-entropy-alloy composite coating fabricated by laser cladding

The agitator blade made by 904 L stainless steel was subjected to serious corrosion-wear due to its low hardness in phosphoric acid reactors. In this aspect, various nano-TiC reinforced [Cr–Fe4Co4Ni4]Cr3 HEA composite coatings were designed using the cluster-plus-glue-atom model and fabricated on 904 L stainless steel by laser cladding. The phase structure of the composite coatings was composed of FCC solid solution and TiC phase. The microstructure observation detected that tiny TiC particles widely distributed along the inter-dendrites of FCC matrix. Also, the particle dimensions and volumes rapidly enhanced with the addition of TiC. Whereas, excessive TiC contents (≥12.5vol%) led to the generation of microcracks. The TEM results further confirmed that TiC particles did not decompose during laser cladding. With the addition of TiC, the microhardness, wear and corrosion resistance of the composite coatings gradually increased. Especially, the microhardness of [Cr–Fe4Co4Ni4]Cr3-15vol%TiC composite coating reached the peak value of 357.4 HV0.2, approximately twice higher than that of the substrate. It's specific wear rate (3.974 mm3 N−1 m−1) was lower than that of the substrate (5.545 mm3 N−1 m−1). Compared with 904 L stainless steel, the corrosion current density of [Cr–Fe4Co4Ni4]Cr3-15vol%TiC composite coating reduced by nearly an order of magnitude, the impedance increased by 3.5 times.

Phase and Mechanical Properties Response of the Mechanically Alloyed CoCrFeNiAlX High Entropy Alloys

The present work describes the influence of Al content on the CoCrFeNiAl high-entropy alloys pre-pared by the powder metallurgy technique. The preparation procedure consisted of mechanical alloy-ing and subsequent spark plasma sintering. The content of Al varied from 10 – 30 at.% which affected the microstructure and mechanical properties. Using scanning electron microscope (SEM) and X-ray diffraction analysis (XRD) was found the microstructure becomes more refined with increasing con-tent of Al accompanied by the annihilation of the ductile FCC solid solution (Cr0.25Fe0.25Co0.25Ni0.25) phase and growth of the brittle and hard BCC solid solution phase (α-Fe) and formation of Al(Co0.5Ni0.5) phases, improving the mechanical properties. The best combination of the porosity, hardness HV 30, and ultimate compressive strength (UCS) was achieved for the studied high-entropy alloy when it contained 20 at. % Al.

Study on laser cladding system of the high-entropy alloy layer on the AZ91D magnesium

A new type of composite layer material system, Al4Ni/Al2CrCuFeNi2Ti high-entropy alloy, was designed. The Al4Ni transition layer was prepared on the surface of an AZ91D magnesium alloy by laser cladding, which solves the problem of excessive melting point difference between the magnesium alloy and the high-entropy alloy. The Al4Ni/Al2CrCuFeNi2Ti high-entropy alloy composite layer with good surface formation was successfully prepared on the AZ91D magnesium matrix. Optical microscope, scanning electron microscope, and x-ray diffraction were used to characterize the microstructure and properties of the composite layer. It showed that the high-entropy alloy layer was mainly composed of simple BCC and FCC solid solution phases. The Al2CrCuFeNi2Ti high-entropy alloy layer, the Al4Ni layer, and the AZ91D magnesium matrix have excellent metallurgical bonds. The hardness of the high-entropy alloy layer was about 12 times that of the AZ91D magnesium alloy. The corrosion resistance of the high-entropy alloy layer in 3.5 wt. % NaCl was also improved.

Effect of heat-treatment time on microstructure and tribological behavior of (TiVCrAlMo)N high-entropy alloy films

In this work, the effect of heat-treatment time (0 h, 2 h, 4 h, and 8 h) at 300 °C on the microstructure and tribological properties of (TiVCrAlMo)N high-entropy alloy (HEA) films are studied in detail. The results indicate that the HEA films with and without heat-treatment are all composed of a single FCC solid solution phase. With the increase of heat-treatment time, the lattice constant decreases gradually but the surface roughness has a tendency to increase, however, the microhardness and tribological properties don't show a linear relationship. The HEA film with 2 h heat-treatment has a relatively low micro-hardness, but the lowest friction coefficient (~0.22) with the best wear-resistance, while the HEA film with 8 h heat-treatment has the highest micro-hardness, but has the worst wear-resistance. The lowest friction coefficient of HEA film with 2 h heat-treatment is mainly due to the highest content of the MoO3 self-lubricating phase. Moreover, owing to the different micro-hardness and the different surface formation with different heat-treatment times, the wear mechanisms of these HEA films are quite different.

Temperature-dependent enthalpy and entropy stabilization of solid solution phases in non-equiatomic CoCrFeNiTi high entropy alloys: computational phase diagrams and thermodynamics

Research interest in multi-principal element high entropy alloys (HEAs) has increased drastically since the field was first formally introduced in 2004. Since then, HEAs have become important candidate materials for many key applications. However, despite the progress made in this field, there remains much ambiguity surrounding HEA phase stabilities. To that end, the calculation of phase diagrams (CALPHAD) method was used to construct extensive temperature-composition phase diagrams of the CoCrFeNi x Ti2−x , Co x CrFeNiTi2−x , CoCrFe x NiTi2−x , and CoCr x FeNiTi2−x HEA systems. Due to its potentially favorable properties, the current work was focused on the single face-centered cubic (fcc) solid solution phase and an extensive thermodynamic analysis was carried out to examine the underlying thermodynamic factors of its stabilization. The mixing enthalpies and entropies of the alloys in the studied systems were calculated, where it was found that the single fcc solid solution phase can be either enthalpy- or entropy-stabilized depending on the temperature. The deviation of these quantities from the ideal solid solution thermodynamic behavior was considered, and it was found that close to and within the single fcc solid solution regions, the deviation is smallest in all systems. Furthermore, a preliminary exploration of the impact of interstitial nonmetals such as C, N, and O showed noticeable alteration of the phase equilibria of the studied systems. This work emphasizes the importance of exploring non-equiatomic compositions of HEAs as well as the necessity of a comprehensive thermodynamic analysis to understand HEAs phase stabilities.

Microstructure and properties of CoCrFeNi-based multi-principal element alloys containing C and Sc

A series of novel CoCrFeNi-based high-entropy alloys containing Sc and C were designed. These HEAs exhibited hardness, fracture toughness and wear resistance comparable or better than a range of selected benchmark systems. Microstructurally, we observed composite microstructures comprising variously a Sc-rich intermetallic phase, a sigma phase and an FCC solid solution phase within which precipitated M23C6 carbides. The Sc-rich intermetallic phase possessed a stoichiometry close to (Co, M)2(Sc, M), where M = Ni and Fe. As the carbon content increased, the hardness levels ‘softened’ from 799 VHN (C-free) to 674 VHN (C = 4 at.%), though the fracture toughness was increased ~ 112%. At the same time, the specific wear rate was enhanced from 0.33 × 10−3 to 0.14 × 10−3 mm3·(N m)−1. The effectiveness of these microstructures in balancing high hardness, crack deflection and a relatively low wear rate was attributed to the enhanced stability of the FCC solid phase as the C content was increased.Graphical abstract

Microstructure and wear mechanism of CoCrCuFeNiVx high entropy alloy by sintering and electron beam remelting

In this study, electron beam current was used to modify the surface of CoCrCuFeNiVx (x = 0, 0.6, 1.0) prepared by sintering to improve wear resistance. Thermal behavior, microstructure, hardness and wear resistance of the alloy were investigated with conclusions below. CoCrCuFeNiV alloy is composed of three phases (FCC, Cu-rich phase, V-rich phase) after sintering and electron beam remelting. V is beneficial to reduce the enthalpy of CoCrCuFeNiVx and promote the stabilization of FCC solid solution phase. The hardness of CoCrCuFeNiV can reach up to 408.4HV, which is 35.4% higher than CoCrCuFeNi (301.5HV). Moreover, the wear resistance is greatly improved by V due to the formation of alloy hardening layer and oxide film on the surface, and the wear rate is reduced by 80.6%. The wear mechanism of CoCrCuFeNiV is mainly abrasive wear, supplemented by adhesive wear and oxidative wear.

Influence of heat-treated temperature and its rate on structural, mechanical, and corrosion properties of (Ti-Mo-Zr)60CoXCrY high entropy alloys for lightweight applications

ABSTRACT Multi-component (Ti, Mo, Zr, Co, and Cr) high-entropy alloys (HEAs) are prepared are via high energy ball milling technique and followed by spark plasma sintering (SPS) technique for their compaction using the general formula (Ti-Mo-Zr)60CoxCry labeled as (TMZ)60CoxCry where x & y as at.%; equiatomic ( = and non-equiatomic ( = ,); stoichiometric ratios of Ti, Mo, Zr elements). XRD traces recorded the presence of FCC and BCC solid solution phases, at 20 h of milling schedule and stability of as precipitated phases are analyzed using DSC profiles. Mechanical properties include maximum load (19.06 kN), compressive strength (σmax: 205 MPa), Young’s Modulus (E: 57.88 GPa), and Vickers’s hardness (166HV) are achieved to be maximum for equiatomic (TMZ)60Co20Cr20 HEA pellet (rate of heat treatment: 125°C/min), compared with all other compositions under investigation. The superior corrosion properties such as corrosion current density (ic), passive current density (ip), corrosion potential (Ec), and passive potential (Ep) of obtained equiatomic (TMZ)60Co20Cr20 HEA pellet made this family of network a potential candidate for use in lightweight vehicle applications.

Corrosion Resistance of CoCrFeNiMn High Entropy Alloy Coating Prepared through Plasma Transfer Arc Claddings

High entropy alloy attracts great attention for its high thermal stability and corrosion resistance. A CoCrFeNiMn high-entropy alloy coating was deposited on grey cast iron through plasma transfer arc cladding. It formed fine acicular martensite near the grey cast iron, with columnar grains perpendicular to the interface between the grey cast iron substrate and the cladding layer as well as dendrite in the middle part of the coatings. Simple FCC solid solutions present in the coatings which were similar to the powder’s structure. The coating had a microhardness of 300 ± 21.5 HV0.2 when the cladding current was 80 A for the solid solution strengthening. The HEA coating had the highest corrosion potential of −0.253 V when the plasma current was 60 A, which was much higher than the grey cast iron’s corrosion potential of −0.708 V. Meanwhile, the coating had a much lower corrosion current density of 9.075 × 10−7 mA/cm2 than the grey cast iron’s 2.4825 × 10−6 mA/cm2, which reflected that the CoCrFeNiMn HEA coating had much better corrosion resistance and lower corrosion rate than the grey cast iron for single FCC solid solution phase and a relatively higher concentration of Cr in the grain boundaries than in the grains and this could lead to corrosion protection effects.

Microstructure and Properties of a CoCrFeNi-Based High Entropy Alloy Containing C and Sc

A series of novel CoCrFeNi-based high entropy alloys containing Sc and C were designed. These HEAs exhibited hardness, fracture toughness and wear resistance comparable or better than a range of selected benchmark systems. Microstructurally, we observed composite microstructures comprising variously a Sc-rich intermetallic phase, a sigma phase, and an FCC solid solution phase within which precipitated M23C6 carbides. The effectiveness of these microstructures in balancing high hardness, crack deflection and a relatively low wear rate was attributed to the enhanced stability of the FCC solid phase as the C content is increased.

New studies on wear and corrosion behavior of laser cladding FeNiCoCrMox high entropy alloy coating: The role of Mo

In this paper, the wear and corrosion resistance of FeNiCoCrMox (x: atomic ratio, x = 0, 0.15, 0.20, 0.25) high entropy alloy coatings (HEACs) which prepared by laser cladding (LC) technology on 316 stainless steel (SS) was investigated. According to XRD, SEM, MicroXAM-3D, microhardness tester and electrochemical workstation tests results, the HEACs presented typical DR-ID structure with single FCC solid solution phase. The average microhardness of Mo0, Mo0.15, Mo0.20 and Mo0.25 HEACs increased by 70.1%, 77.0%, 84.9% and 90.5% relative to the substrate. Dry sliding friction and wear tests results showed that the average friction coefficient of the HEACs were significantly lower than that of 316 SS substrate. Meanwhile, it can be found that Mo0.25 HEACs had the lowest average friction coefficient which displayed the best wear resistance. Compared with Mo0 HEAC, the specific wear rates of Mo0.15, Mo0.20 and Mo0.25 HEACs decreased by 11.1%, 27.8% and 38.9% respectively. The electrochemical tests results showed that the corrosion type of HEACs was intergranular corrosion and all HEACs had a higher corrosion resistance than that of 304 SS or 316 SS. Moreover, Mo0.20 HEACs showed the best corrosion resistance due to the enhancing effect of MoO3 on Cr2O3 passivation film.

The Effect of Heat Treatment on the Microstructure of Co50Ni25Al8Ti8Nb3Ta3W3 High Entropy Alloy and Its High- temperature Oxidation Resistance

The design based on high-entropy alloys (HEAs) is undergoing a conceptual expansion from equiatomic alloys to non-equiatomic alloys. To provide the experimental basis for non-equiatomic high-entropy alloys and their oxidation resistance at high temperatures, in this study, a Co50Ni2sAl8Ti8Nb3Ta3W3 high-entropy alloy was prepared by a vacuum arc melting method, and then it was subjected to different aging heat treatments. The phase composition, element composition distribution, and mechanical properties of the alloy were characterized by XRD SEM/EDS. In addition, we have systematically studied the high-temperature oxidation resistance of the alloy and analyzed the high-temperature oxidation mechanism of the alloy. The results showed that the phase composition of the Co50Ni25Al8Ti8Nb3Ta3W3 high-entropy alloy is a single fcc solid solution phase, and the grains of the alloy become coarser after secondary aging. Many oxidation products were produced after high-temperature oxidation of the alloy. The oxidation products of the outermost layer of the alloy are mainly CoNiO2, CoTiO3, and M2O3, and the oxidation products of the intermediate layer are mainly Al2O3, TiO2, and Nb2O3.

A tool to predict the evolution of phase and Young’s modulus in high entropy alloys using artificial neural network

We report an artificial neural network (ANN) based tool to predict the evolution of phase(s) in high entropy alloys (HEAs) and their Young’s modulus. Two independent networks as single channel output and four channel binary output methods have been adopted for testing as well as for predicting the evolution of either single-phase BCC, FCC solid solution (SS) phase, or a mixture of SS phase(s) and intermetallic compound (IM). The data of as-solidified HEAs were collected from the literature, and various thermo-physical as well as electronic parameters, such as mixing entropy (ΔSmix), mixing enthalpy (ΔHmix), atomic size difference (δr), Allen electronegativity ΔχA, Pauling electronegativity (ΔχP), and valence electron concentration (VEC) were calculated for each alloy composition, which was used as input features. The Bayesian regularization backpropagation has been adopted as the learning algorithm. Furthermore, the model has been trained and validated by developing eutectic CoCrFeNiNbx (x = 0.45, 0.5, 0.55 and 0.65) and CoCrFeNiTay (y = 0.2, 0.4, 0.455 and 0.5) HEAs experimentally. The single channel output approach and four channel binary output approach generate the matching accuracy of 85.95% and 92.97% for the main dataset, where the same is 70.83% and 91.67% for the deployment dataset, respectively for phase prediction. The predicted values of Young’s modulus have achieved the relative accuracy of 94.96% and 96.44% for the main and deployment dataset, respectively. TheANN suggests that VECbecomes the dominant factor for phase stability when δr < 3%, whereas the higher value of δr promotes the evolution of two phases in HEAs.

Mechanical and tribological performance of CoCrNiHfx eutectic medium-entropy alloys

The excellent properties of the multi-principal element alloys (e.g., the CoCrNi medium-entropy alloy) make them a perfect candidate for structure materials. Their low strength and poor wear-resistance, however, limit considerably their applications. In this study, a lamellar eutectic microstructure was introduced by addition of Hf into CoCrNi alloy to produce a series of CoCrNiHfx (x = 0.1, 0.2, 0.3 and 0.4) eutectic medium-entropy alloys. A homogeneous eutectic microstructure with an alternate array of the soft FCC solid-solution phase and the hard Laves phase was identified for the as-cast CoCrNiHf0.3 alloy. After an investigation of the microstructure, mechanical and tribological properties, it was found that the hardness (plasticity) increases (decreases) with the increasing volume fraction of the Laves phase and the CoCrNiHf0.3 eutectic alloy exhibits both good plasticity and high strength. The wear behavior is strongly dependent on the applied normal load. For a low normal load, its tribological behavior follows the Archard's equation and a higher hardness due to Hf addition can resist plastic deformation and abrasive wear. When the normal load is high enough, the hypoeutectic or hypereutectic alloy, which possessing either high strength or good ductility but not at the same time, exhibit a poor wear resistance. In comparison, the full eutectic CoCrNiHf0.3 alloy with a superior combination of strength and toughness shows the best wear performance, as it can significantly reduce fracture during wear.

Study of (Ni,Cr) pre-milling for synthesis of CoFe(NiCr)Mn high entropy alloy by mechanical alloying

Abstract Cr-rich oxide is the most frequently formed on Cr-containing high entropy alloys by a powder metallurgical processing route. Therefore, in this study, the pre-milling of (Ni,Cr) elements was introduced into the CoFe(CrNi)Mn model HEAs synthesized by mechanical alloying to alter the formation behavior of Cr-rich oxides. The results suggest that the pre-milling method can promote the formation of a partial FCC solid solution phase with nanostructured Ni–Cr particles, which facilitates the nucleation of nano-Cr oxides and induces the uniform distribution of fine Cr-rich oxides dispersed in the model HEAs. Besides, the formation of Cr-rich oxides and powder agglomeration can be obstructed by the subsequent secondary ball milling.

Formation of the CoCrCuFeNi high entropy cladding layer by non-vacuum electron beam treatment

Non-vacuum electron beam cladding was applied to obtain CoCrCuFeNi cladding layers with thicknesses of 0.89 and 1.24 mm on a surface of mild steel. The cladding layers possessed the dendritic structure. Interdendritic space was filled with a Cu-rich phase. It was found by X-ray diffraction analysis that FCC solid solution phases were formed during crystallization. The lattice parameter of the dendritic phase was a=3.59 Å, while that of the interdendritic phase was equal to a=3.60 Å. The average microhardness values of the cladding layers were lower compared to the base material and equaled 156 HV and 190 HV.

Effect of Co on the microstructure and oxidation behavior of CoxCrCuFeMnNi high entropy alloy powders

The microstructure, phase, alloying behavior, and oxidation behavior of the CoxCrCuFeMnNi (x = 0, 0.5, 1.0, 1.5, 2.0, named as Co0, Co0.5, Co1.0, Co1.5, and Co0.5, respectively) high entropy alloy powders (HEAPs) prepared by mechanical alloying were studied. The results indicated that the HEAPs began to be alloyed after ball milling for 5 h, and the particle size is about 40∼60 μm. After 50 h of milling, the Co0 and Co0.5 HEAPs are composed of metastable FCC and minor BCC solid solution phases, while the other HEAPs consist of single metastable FCC solid solution phase. After vacuum annealing or atmospheric oxidation at 700 °C, the metastable FCC and BCC phases decompose into FCC1 and FCC2 and ρ phases, respectively. The oxidation mass gain of CoxCrCuFeMnNi HEAPs shows a parabolic upward trend. With the increase of Co content, the oxidation rate constant decreases, indicating that the oxidation resistance of the HEAPs decreases with the increase of Co content.

Hot deformation behavior and processing map of a Sn0.5CoCrFeMnNi high entropy alloy with dual phases

The hot compressive deformation behavior of a cast Sn0.5CoCrFeMnNi high-entropy alloy composed of dual phases, an FCC solid solution phase and a MnNiSn-rich ordered FCC phase (with a volume fraction of 30%), was investigated in temperature and strain rate ranges of 1023–1248 K and 10-3–10 s−1, respectively. During hot deformation, the MnNiSn-rich phase with a relatively low melting temperature (1297 K) underwent a significantly higher degree of dynamic recrystallization (DRX) than the FCC solid solution phase. Continuous DRX occurred in the FCC solid solution phase, but it was pronounced only at the highest temperature of 1248 K. The Sn0.5CoCrFeMnNi alloy exhibited dislocation climb creep at low strain rates and high temperatures and power-law breakdown at high strain rates and low temperatures. The flow stresses of Sn0.5CoCrFeMnNi were notably lower than those of CoCrFeMnNi, though they shared similar deformation mechanisms, by a factor of 0.37–0.71, and this could be explained by the rule of the mixture model considering the contribution of the FCC solid solution phase and the MnNiSn-rich phase to the total deformation rate under the isostress condition. Compared to the Al element in Al0.5CoCrFeMnNi, the Sn element is anticipated to create a larger atomic size misfit in Sn0.5CoCrFeMnNi but promote less viscous glide (solute drag) creep·. The analysis for explaining this reason was conducted from the perspective of factors that affect solute drag creep and breakaway stresses of dislocations from the solute atmosphere.