Fcc Solid Solution Research Articles

Nanostructured AlCoFeCrVNi and AlCoFeCrVTi high-entropy alloys resulted from mechanical alloying and sintering

This study reports the structural evolution of AlCoFeCrVNi and AlCoFeCrVTi high-entropy alloys from elemental materials to solid solution phases during mechanical alloying (MA), and further, to equilibrium phases during pressure sintering at 800 °C. The phase and structural transformations in equiatomic powder compositions of the Al–Co–Fe–Cr–V–Ni(Ti) systems during MA and subsequent sintering under high pressure of 5 GPa were studied by X-ray diffraction analysis, scanning and transmission electron microscopy. It was established that the AlCoFeCrVNi(Ti) alloys synthesized by MA consists of a supersaturated bcc solid solutions with nanocrystalline structure and a small amount of WC contaminant was present in both alloys. The bcc solid solutions formed during MA are metastable in nature and transform to a mixture of more stable fcc and bcc solid solutions on consolidation by pressure sintering, and in addition a small amount of (Fe, Cr)23C6 carbide in AlCoFeCrVNi and TiC in AlCoFeCrVTi alloy are formed. Also the WC carbide particles remain in the alloys after sintering. Sintering under high pressure at a relatively low temperature was found to contribute to the maintenance of the nanocrystalline state obtained during MA in compact specimens of both AlCoFeCrVNi and AlCoFeCrVTi alloys exhibited extremely high microhardness of 12.6 and 14.7 GPa and yield strength of 3450 and 4100 MPa, respectively.

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.

Magneto-optical properties of two – layer film systems based on Fe and Pt

The paper shows the experimental results of the study of magneto - optical properties and phase composition of thin films and multilayers based on Fe and Pt. Samples were obtained in a high-vacuum chamber (10–8 Pa) by layer-by-layer deposition from pots at room temperature. It has been shown that FCC solid solution of FePt is formed already in the process of deposition on the substrate (sital plates). Depending on the concentration of the atoms of the components in the unannealed samples, three phases can be formed: (i) s.s. Fe(Pt); (ii) Fe3Pt; (iii) FePt. The first sign of the beginning of ordering in the FCC phase of FePt should be considered the appearance of superreflections in the form of lines (001) and (002) during heat treatment. Depending on the total thickness of the multilayer or individual layers, the annealing temperature at which extrareflexes appear can vary in the range of 300–570 K. All the samples obtained have a low coercivity (0.25 - 0.4 mT). Magneto-optical studies have shown that an increase in the content of the non-magnetic component decreases the main magnetic characteristics. However, In [Fe(3)/Pt(3)]n/S multilayers with the number of repeatable elements from 2 to 8 an increase in the main magnetic parameters is observed compared with bilayer films at the same effective thickness.

MgAlSiCrFeNi low-density high entropy alloy processed by mechanical alloying and spark plasma sintering: Effect on phase evolution and thermal stability

Equiatomic MgAlSiCrFeNi low-density high-entropy alloy (LDHEA) was prepared by mechanical alloying followed by spark plasma sintering (SPS). The phase evolution, chemical composition, and thermal stability of the alloy were analyzed by X-Ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM), and differential scanning calorimetry (DSC) techniques. Formation of a BCC phase having a lattice parameter of 0.2876±0.03 nm and undissolved Si (~3 at%) was observed after milling of 60 h. DSC experiment up to 1200 °C (1473 K) shows five exothermic heating events corresponding to various phase transformations, which were matched with the results obtained from XRD experiments. It has been found that the alloy after high temperature annealing led to the evolution of a major B2 phase (a=0.289 nm) and also BCC phase along with small amounts of FCC Al-Mg solid solution phase, FCC 1 (a= 0.4082 nm) and FCC 2 (a= 0.4215 nm), monoclinic Al13Fe4 (a= 1.549 nm, b= 0.808 nm, c= 1.248 nm, α = β = 90˚), Mg2Si (a= 0.6351 nm), Cr5Si3 (a=b= 0.9165 nm, c= 0.4638 nm). The spark plasma sintered (SPS) sample also exhibited BCC and B2 phases coexisting with minor amounts of other phases observed for 800 °C (1073 K) annealed sample. The co-exsistence of minor phases with parent BCC phase in SPSed alloy (having relative density of ~99.40%) has led to significantly high hardness and modulus of elasticity of ~9.98 ± 0.3 GPa and 229 ± 0.3 GPa respectively. Various thermodynamics parameters were calculated in order to correlate with the experimental findings of phase evolution and stability of the annealed powder and spark plasma sintered alloy.

Deformation Mechanism in Fe61Mn18Si11Cr10 Medium Entropy Alloy Under Different Strain Rates

We introduce a non-equiatomic Fe61Mn18Si11Cr10 medium entropy alloy designed by subjecting it to transformation-induced plasticity upon deformation at room temperature. Microstructure characterization carried out using scanning electron microscopy (SEM), electron backscatter diffraction (EBSD), transmission electron microscopy (TEM) and X-ray diffraction (XRD) shows a homogeneous solid solution FCC + BCC structured dual phase. Investigations on the deformation substructures at specific strain levels via EBSD reveal the deformation-induced transformations of γ → α′ and γ → ɛ. The strengths, particularly yield strength, of the designed alloy are found to be higher than these of the well-studied five component FeMnNiCoCr system for the introduction of the hard phase (α′-martensite). When tensile tests are performed at different strain rates of 10–4 s−1, 10–3 s−1, 10–2 s−1, the tested material exhibits a slightly negative strain rate sensitivity and work hardening rate sensitivity.

Effect of Mo Element on the Mechanical Properties and Tribological Responses of CoCrFeNiMox High-Entropy Alloys

Certain amounts of precipitate in CoCrFeNiMox (simplified as Mox) is beneficial to the wear resistance; however, the optimal chemical content of Mo and the anti-wear mechanism behind it remains unclear. The Mox (x = 0, 0.3, 0.5, 1, 1.5 in molar ratio) high entropy alloys (HEAs) were manufactured, the evolution of their microstructure, mechanical, friction, and wear properties with Mo content was studied. The results displayed that the mechanical properties of the FCC solid solution were enhanced from Mo0 to Mo0.3, then kept unchanged till x = 1.5. The volume fraction of the precipitates increased with Mo content. The Mo1 presents the lower average friction coefficient and wear rate, attributed to the desired types, amount, size, distribution of the hard σ and μ phases in the ductile FCC solid solution. The detailed mechanism behind their tribological behaviors were discussed in the manuscript.

Interdiffusion in CMSX-4 Related Ni-Base Alloy System at a Supersolvus Temperature

Interdiffusion in Ni-base superalloy CMSX-4 and alloys related to CMSX-4 was investigated at the temperature 1288 °C, which is 8 °C above the γ’-solvus temperature of this superalloy, 1280 °C. This temperature is of a special interest because it is a temperature of hot isostatic pressing applied to CMSX-4 and modeling of this process needs verified diffusion data for this specific temperature. Various diffusion couples were assembled from the investigated alloys, annealed at 1288 °C and studied by electron probe microanalysis. So far as the annealing temperature was higher than the γ’-solvus temperature of CMSX-4 and other investigated alloys have no strengthening γ’-phase, interdiffusion occurred in the fcc solid solutions of nickel. It was found that in the case when the γ’-forming and γ-stabilizing elements diffuse in the same direction (towards nickel) the diffusion rate accelerates, but when they diffuse in the opposite directions (counter diffusion) it slows down. Such an interdiffusion behavior is in agreement with the results predicted with diffusion simulation software Dictra.

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.

A mechanistic perspective on the kinetics of plastic deformation in FCC High Entropy Alloys: Effect of strain, strain rate and temperature

Strain rate sensitivity and activation volume that dictate the kinetics of plastic deformation along with strain hardening response determine the plastic deformation behaviour of novel high entropy alloys. Although, the deformation behaviour of FCC single-phase Cantor alloy exhibits some similarity to that of conventional FCC solid solutions, the deformation behaviour over a wide range of temperature, strain rate, strain and grain size is unexplored. More importantly, theoretical understanding of solid solution strengthening, stacking fault energy and dislocation activation in 100 percent solute high entropy alloys (HEAs) and their flow kinetics is not yet established. This necessitates development of combined experimental and computational approaches to realise the mechanistic design of new single-phase and multi-phase transformative FCC HEAs to achieve optimum combination of strength and ductility in the novel complex concentrated alloys.

Synthesis and characterization of Co–B–Fe–Ti nanosized alloyed powders

AbstractIn this study, novel Co60Fe18Ti18B4alloy powders have been synthesized with high compositional homogeneity using a high-energy ball milling technique. The structural, morphological and mechanical properties of the nanosized alloyed powders were examined using different analytical techniques, including scanning electron microscopy, transmission electron microscopy, energy-dispersive X-ray spectrometry and X-ray diffraction. According to the X-ray diffraction analysis for both Co powder and Co60Fe18Ti18B4alloy powders, with increasing milling time, the content of Co-based (hcp) solid solution decreased and Co-based (fcc) solid solution increased. The mechanical properties of the material were also investigated by Vickers micro-hardness testing. The micro-hardness value of the Co60Fe18Ti18B4alloy was found as 120.08 HV. After sintering (1 h– 1000 °C), the hardness improved remarkably (536.32 HV). Furthermore, results indicate that the synthesized Co-based alloy powder has both glassy and nanocrystalline phase forms.

Elemental segregation to lattice defects in the CrMnFeCoNi high-entropy alloy during high temperature exposures

The influence of small plastic pre-strains on the elevated-temperature stability and microstructure of the equiatomic CrMnFeCoNi FCC solid solution is investigated. Particular attention is given to whether any of the alloy elements segregate to individual dislocations. To that end, CrMnFeCoNi samples were first deformed in tension at room temperature to plastic strains of 0.2 and 2.3%, and subsequently annealed at 973 K for 800 hours. The pre-strains activated planar slip of 1/2<110>-type dislocations on {111}-type glide planes. Interactions of this planar slip with special Σ3 grain boundaries formed a large number of dislocation segments with a <110>-type crystallographic orientation suitable for a credible end-on analysis of dislocation cores in HR-STEM. The cores of the 1/2<110> dislocations pushed up against the investigated grain boundaries were found to be close to the compact configuration. Within the sensitivity of the Super-X EDS mapping, no concentration gradient was detected near dislocations that would indicate enrichment at dislocation cores of any of the elemental constituents of the alloy after the pre-deformation and annealing. However, a Cr-rich tetragonal sigma phase nucleated and grew at grain boundary triple junctions during this anneal, processes that were not accelerated by the enhanced dislocation density present after pre-strain. A clear chromium gradient was observed in the Cr-depleted zones near grain boundaries suggesting that Cr transport occurred by relatively slow diffusion from the bulk to the grain boundaries and then by relatively fast diffusion along the grain boundaries to the precipitates. Accompanying the Cr depletion near grain boundaries is a simultaneous Ni and Mn enrichment, which promotes formation of the L10 NiMn phase that is observed on the grain boundaries after prolonged annealing.

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 Interstitial Elements on the Cryogenic Mechanical Behavior of FCC High Entropy Alloys

High entropy alloys (HEAs) with face-centered cubic (fcc) structure, namely equiatomic CoCrFeMnNi alloy, have attracted considerable attention because of impressive cryogenic mechanical properties – strength, ductility, and fracture toughness. Further increase of the properties can be achieved, for example, by proper alloying. A particularly attractive option is the addition of interstitial elements like carbon or nitrogen. In present work, a series of CoCrFeMnNi-based alloys with different amounts of C and N (0-2 at.%) was prepared by induction melting. The alloys doped with C had lower Cr content to increase the solubility of carbon in the fcc solid solution. It was revealed that the solid solution strengthening effect of both C and N is significantly increased when the testing temperature decreases from 293K to 77K. The effect of thermomechanical processing on the structure and mechanical properties of the alloys is analyzed.

Analysis of Elements Non-Uniform Distribution of FeCoCrNi High-Entropy Alloy Coatings on Ti–6Al–4V Surface by Laser Cladding

The evolution of element distribution during laser cladding involves two dynamic behaviors, i.e., liquid molten pool flow and FeCoCrNi high-entropy alloy (HEA) coatings solidification. However, it is quite difficult to characterize element distribution during the flow of the liquid molten pool rigorously. The current investigation conducted the optical microscopy, scanning electron microscopy, X-ray diffraction analysis and energy dispersive spectrometer to study the dilution, phase composition, microstructure of the FeCoCrNi coatings. The flow field was simulated to uncover the dynamic change mechanism of the molten pool and explain the experimental results. The results indicated that the coating is substantially composed of FCC and BCC solid solution with a typical dendrite microstructure. Gray Laves phase-(Ni, Co)2Ti and a small number of white dot particles, Fe–Cr phase, are dispersed in the inter-dendritic region. The HEA atoms (Fe, Co, Cr, Ni) gradually aggregate from the center to the side at the coating boundary region, while the Ti atom is the opposite. The Marangoni flow inflection point at the molten pool boundary will cause HEA atoms to aggregate. On the contrary, Ti atom enters the molten pool from the bottom with the heat buoyance flow and then migrates to the boundary along with the Marangoni flow. Therefore, the content of Ti in the coating boundary decreases. The Marangoni flow, heat buoyance flow, and recoil pressure flow are interwoven in the middle region of the coating, resulting in a more uniform element distribution than the boundary region.

Phase Stability of Rapidly Solidified (Fe1−xNix)88Zr7B4Cu1 Ribbons

In this study, the glass-forming ability (GFA), structure and microstructure of (Fe1−xNix)88Zr7B4Cu1 (x = 0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8 and 1.0) alloys, in both the as-melt spun and heat-treated conditions, have been investigated. Almost complete amorphization was observed only for x = 0, 0.4 and 0.5 alloys at the highest melt spun wheel speed of 47 m/s. At lower wheel speeds all alloys are partially crystalline. The crystalline phases are bcc solid solution (bcc-SS) up to a composition of x = 0.2 and fcc solid solution (fcc-SS) for x = 0.6 and above. At a melt spun wheel speed of 47 m/s, the alloy with the composition of x = 0.3 is almost amorphous, along with traces of both bcc and fcc phases. Ribbons annealed at 450 °C/500 °C (after the first exothermic peak observed in DSC result) exhibit bcc-SS at low Ni concentration (up to x = 0.4) and fcc-SS at higher Ni concentration (x = 0.6 and beyond), whereas x = 0.5 ribbon shows the formation of both bcc-SS and fcc-SS. Ribbons annealed at 620 °C/750 °C (after the second exothermic peak observed in DSC results) exhibit Fe3Zr at low Ni concentration up to x = 0.3 and Ni5Zr from x = 0.4. The crystalline phases form during melt spinning are in the form of dendrites of submicron size, whereas the precipitated phases formed during annealing are nanocrystalline in nature. Thermodynamic modeling was carried out to understand the GFA and phase formation, which are in conformity with the experimental observations.

Microstructure and mechanical properties of AlNiCuCoX high-entropy Alloy

AlNiCuCoX (X=Cr, CrTi, CrFe, FeTi and CrFeTi) series high-entropy alloys were prepared by vacuum arc furnace. The microstructures and properties of the alloys were analyzed. The results show that the microstructures of AlNiCuCoX alloys are composed of FCC solid solution and BCC solid solution, and the number of phases formed in the alloys is far lower than that predicted by equilibrium phase law. AlNiCuCoCrTi shows chrysanthemum like microstructure, while AlNiCuCoFeTi and AlNiCuCoCrFeTi alloys shows petal-like, chrysanthemum like and reticular (intercrystalline) white microstructure. The principal element is the key factor affecting the solidification mode (hypoeutectic, eutectic and hypereutectic solidification) of AlNiCuCoX series alloys. If the interdendritic phase is not considered, AlNiCuCoCr and AlNiCuCoCrFe high-entropy alloys are solidified in the end-solid solution solidification mode. When Ti is added, the alloy is solidified in the hypoeutectic solidification mode, and the equilibrium microstructure is composed of primary phase and eutectic. The microhardness of the most alloys increased with the increase of the number of principal elements, and the hardness of AlNiCuCoCrFeTi alloys is the highest (548HV). The compressive strength and strain of most AlNiCuCoX alloys are low (especially AlNiCuCoCrFeTi alloy). Both AlNiCuCoCr and AlNiCuCoCrFe alloys have better comprehensive mechanical properties.

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.

A New Criterion for Prediction of Phase Stability in Al‐Containing High Entropy Alloys

The reliable phase formation rule is demanded for high entropy alloys’ (HEAs) design and property regulation. Previous studies reveal the significant roles of thermodynamic and geometrical parameters in predicting phase structures of HEAs, whereas an accurate criterion is still not available. Herein, taking the most extensively studied Al‐containing HEAs as research object, two alloying parameters, i.e., d‐orbital energy level (Md) and bond order (Bo), combining with valence electron concentration (VEC) and atomic size difference (δ), are introduced to explore the phase stability of Al‐containing HEAs. The results indicate that single FCC solid solution tends to form in the range of δ below 4.5%, ordered phases (such as B2 and intermetallics) are more likely to be produced when the Md value of the alloy is beyond 1.06, and the FCC + BCC dual‐phase structures are mainly concentrated in the region of δ &gt; 4.5% and Md &lt; 1.06. The Md–δ criterion provides a reliable criterion of phase stability and a good guidance in alloy design for Al‐containing HEAs.

Structural State of High-Entropy Fe40–xNiCoCrAlx Alloys in High-Temperature Oxidation

The evolution of phase composition and mechanical properties and the formation of oxide layers on Fe40–xNiCoCrAlx (x = 5 and 10 at.%) alloys in long-term oxidation at 900 and 1000°C were studied. In the initial cast state, depending on the aluminum content and valence electron concentration, the alloys contain only an fcc solid solution (VEC = 8 e/a) or a mixture of fcc and bcc phases (VEC = = 7.75 e/a). Thin continuous oxide scales containing Cr2O3 and NiCr2O4 spinel formed on the surface of both alloys oxidized at 900°C for 50 h. A further increase in the annealing time to 100 h leads to the formation of aluminum oxide Al2O3 in the scale on the Fe30Ni25Co15Cr20Al10 alloy, having high protective properties. An increase in the oxidation temperature to 1000°C results in partial failure of the protective layer on the alloy with 10 at.% Al. Long-term holding at 900°C (100 h) + 1000°C (50 h) does not change the phase composition of the Fe35Ni25Co15Cr20Al5 alloy matrix, being indicative of its high thermal stability. In the two-phase Fe30Ni25Co15Cr20Al10 alloy, the quantitative ratio of solid solutions sharply changes: the amount of the bcc phase increases from 4 to 54 wt.% and its B2-type ordering is observed. The mechanical characteristics of the starting alloys and those after long-term high-temperature annealing were determined by automated indentation. The hardness (HIT) and elastic modulus (E) of the cast Fe35Ni25Co15Cr20Al5 alloy are equal to 2 and 147 GPa, respectively, and decrease to 1.8 and 106 GPa after a series of long-term annealing operations. The Fe30Ni25Co15Cr20Al10 alloy shows the opposite dependence: HIT increases from 2.5 in the initial state to 3.1 GPa after annealing and E decreases from 152 to 134 GPa. This indicates that the Fe30Ni25Co15Cr20Al10 alloy is promising as a high-temperature oxidation-resistant and creep-resistant material.

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.