Bcc Solid Solution Research Articles

Zr diffusion in BCC refractory high entropy alloys: A case of ‘non-sluggish’ diffusion behavior

Single phase (BCC solid solution) HfTiZrNbTa and HfTiZrNbV refractory high entropy alloys are synthesized and characterized. For the first time for BCC refractory high entropy alloys, tracer diffusion is measured using the short-living radioactive isotope 89Zr in the temperature range of 1123 to 1423 K. A ‘non-sluggish’ diffusion behavior is clearly observed, highlighted by an enhancement of the measured diffusion rates with respect to the geometric mean of the diffusivities in the corresponding pure elements. The factors influencing self-diffusion in BCC multi-principal element alloys are discussed, including the chemical complexity, atomic size mismatch and the phonon broadening.

Refractory Metal Intermetallic Composites, High-Entropy Alloys, and Complex Concentrated Alloys: A Route to Selecting Substrate Alloys and Bond Coat Alloys for Environmental Coatings.

This paper considers metallic ultrahigh-temperature materials (UHTMs) and the alloying behaviour and properties of alloys and their phases by using maps of the parameters δ (based on atomic size), Δχ (based on electronegativity), and valence electron concentration (VEC), and discusses what connects and what differentiates material groups in the maps. The formation of high-entropy or complex concentrated intermetallics, namely 5-3 silicides, C14 Laves and A15 compounds, and bcc solid solutions and eutectics in metallic UHTMs and their co-existence with “conventional” phases is discussed. The practicality of maps for the design/selection of substrate alloys is deliberated upon. The need for environmental coatings for metallic UHTMs was considered and the design of bond coat alloys is discussed by using relevant maps.

Theoretical critical metastability temperature to interpret phase formation in a lamellar-like-structured high entropy alloy

Control over a lamellar-like-structured high entropy alloy (HEA) system is found to be possible by replacement of aluminium with boron into the FeCoNi(Bx Al1-x)0.1Si0.1 composition in the range of x = 0 to 1.0. The BCC/B2 microstructure of FeCoNi(Al0.1Si0.1) HEA is changed into a multiple phase system comprising of BCC/B2, FCC and Fe2B-type intermetallic phases. The microstructures of as-cast alloys were seen to be a lamellar-like structure comprised of nanostructured lamellae with alternating FCC and Fe2B phases. The concept of critical metastability temperature from nucleation theory is employed phenomenologically, and found to correlate with the presence of Fe2B in this alloy at different boron additions. With the substitution of boron, the stability of the disordered BCC solid solution is reduced, promoting the formation of secondary phases. We show a link between the interfacial energy of the phases present, the interlamellar spacing, and alloy metastability as a function of boron addition, the key relationship being that Fe2B phase formation correlates with a drastic reduction in the interfacial energy. These correlations bear further investigation and may be useful in the design of lamellar-like-structured multi-component systems.

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.

Microstructure characterization of high temperature mechanisms in a Nb–Ti–Si alloy

The deformation mechanisms of an Nb–20Si–25Ti–6Al–3Cr–3Mo alloy deformed in compression at 1000 °C and at a strain rate of 10−4 s−1 have been investigated, using focused ion beam and transmission electron microscopy techniques. The initial microstructure of the alloy was constituted of a solid solution bcc Nbss matrix and of Nb5Si3 phases of β (tetragonal) and γ (hexagonal) crystallographic structures, with also hcp-Ti precipitates. As expected, the only phase which exhibited plastic deformation at 1000 °C was the Nbss matrix. Glide of b→=1/2111 dislocations has been observed, in planes compatible with {112} and {123} planes, or with cross-slip in {110} planes, as classically reported in bcc structures. Conversely, climb contribution has not been evidenced, probably because of rearrangement of the climbing dislocations in sub-boundaries. Fine δ-Nb11Si4 precipitates have been detected in the samples compressed at 1000 °C. Interactions of the δ-Nb11Si4 precipitates with the dislocations have been observed, which can potentially lead to strengthening of the Nbss phase.

Laser-clad Nb(Ta)TiZr medium-entropy alloy coatings on pure Zr sheet: Microstructural characteristics, hardness and wear resistance

In this work, two Nb(Ta)TiZr medium-entropy alloy (MEA) coatings were fabricated successfully on a commercially pure Zr sheet by pulsed laser cladding. Their phase constitutions, microstructural characteristics, chemical compositions and grain orientations were well characterized and analyzed by using X-ray diffraction (XRD), electron channeling contrast (ECC) imaging, energy dispersive spectrometry (EDS) and electron backscatter diffraction (EBSD) techniques, with hardness and wear resistance measured and correlated with the microstructural characteristics as well. Results show that irregular-shaped bulk grains (BCC solid-solution phase) are formed in the two Nb(Ta)TiZr MEA coatings (cladding zones), both of which have a thickness of ∼800 μm with good metallurgical bonding to the substrate. After adding Ta, grains in the MEA coating are slightly refined. The phase constitutions in both the coatings agree with calculations based on several theoretical parameters. Hardness tests show that the NbTiZr and the NbTaTiZr coatings have hardnesses of 360 ± 18 HV and 430 ± 28 HV, respectively, ∼1.8 times and ∼2.2 times that of the substrate (196 ± 4 HV). This can be ascribed to combined effects of grain refinement strengthening, solid solution strengthening with denser low angle boundaries in the MEA coatings. Specific wear rates of the NbTiZr and the NbTaTiZr coatings are only ∼17% and ∼6% of that of the substrate, respectively, indicating markedly improved wear resistance. This is demonstrated to be closely related to complete oxide films easily formed on the coatings with oxidation and abrasive wear mechanisms.

Microstructure and wear behavior of AlCrTiNbMo high-entropy alloy coating prepared by electron beam cladding on Ti600 substrate

High-entropy alloys have been widely studied as a new and excellent material in the field of surface protection. To improve the low surface hardness and wear resistance of Ti600 alloy, AlCrTiNbMo high-entropy alloy (HEA) coating was prepared by electron beam cladding. The phase structure and microstructure showed the presence of two BCC solid solution phases A2 and B2 within HEA coating. Laves phase Ti3Al and intermetallic compound Mo3Al8 were also present in the coating. The results of hardness and wear resistance tests showed that the microhardness of coating is 813.5 HV0.2，while substrate is 370.7 HV0.2. The mass loss of coating is only 1.02 mg after wear test, while the mass loss of Ti600 is 38.81 mg. There are cracks and delaminations on the worn surface of Ti600, and wear mechanism is a mixture of micro-ploughing and micro-cutting. The worn surface of HEA coating contains only shear surface traces, and the wear mechanism is micro-cutting. The width and depth of grooves on the worn surface of HEA coating are nearly 10 times that of Ti600 substrate. AlCrTiNbMo HEA coating can be used as an excellent wear-resistant coating with wide application prospect.

Al2O3/MC particles reinforced MoFeCrTiWNb x high-entropy-alloy coatings prepared by laser cladding

ABSTRACT The Al2O3/MC particles reinforced MoFeCrTiW(Al2O3)0.5Nb x (x = 1, 1.5, 2, 2.5, and 3) high-entropy alloy coatings were successfully fabricated on the M2 high-speed steel by laser cladding. An investigation was carried out to assess the effect of the Nb on the microstructure and properties of coatings. The coatings are all composed of the BCC solid-solution phase, C14-Laves phase, and (Nb, Ti)C carbide plus a small amount of Al2O3. An increasing in Nb content results in the microstructural change from hypoeutectic to eutectic and the improvement in the hardness and wear resistance of the coating. When x = 3, the coating possesses the highest hardness (∼782 HV0.2) and lowest wear volume loss (∼0.52 mm3). The dispersion strengthening and lamellar boundary strengthening play a dominating role in enhancing the hardness and wear resistance of coatings.

Microstructure and elevated temperature wear behavior of laser-cladded AlCrFeMnNi high-entropy alloy coating

The AlCrFeMnNi high-entropy alloy (HEA) coatings were deposited on 316L stainless steel through laser cladding technology that employs pre-alloyed HEA powders. The microstructure, phase composition and hardness of the laser-cladded coatings prepared at different laser scanning speeds were investigated. The wear behavior and properties of the HEA coatings at different temperatures (from room temperature to 600 °C) was comprehensively evaluated using a pin-on-disc test. It was demonstrated that the laser-cladded AlCrFeMnNi HEA coatings exhibited dense and uniform microstructures consisting of a single BCC solid solution phase. A columnar-to-equiaxed transition structure feature was observed in the molten pool of the laser-cladded coating. The coatings possessed better high-temperature wear performance due to the protective effect of oxide films on the wear track. The coating at 400 °C exhibited the best wear resistance, which had the lowest friction coefficient and wear rate of 0.48 and 1.246 × 10−4 mm3/N · m, respectively. At room temperature, the main wear mechanism of the coating was abrasive wear. However, the predominant wear mechanism at high temperatures was oxidation and adhesive wear.

Effect of the Addition of 4 wt% Zr to BCC Solid Solution Ti52V12Cr36 at Melting/Milling on Hydrogen Sorption Properties

The addition of 4 wt% Zr to Ti52V12Cr36 alloy was carried out in two different ways: arc-melting or ball-milling. The cast alloy showed rapid hydrogen absorption up to 3.6 wt% of hydrogen capacity within 15 min. Ball milling this sample worsened the kinetics, and no hydrogen absorption was registered when milling was carried out for 30 or 60 min. When zirconium is added by ball-milling, the kinetic is slower than that when addition is by arc-melting. This is due to the fact that when added by milling, zirconium does not form a ternary phase with Ti, V, and Cr but instead is just dispersed on the particles’ surface.

Analytical models of short-range order in FCC and BCC alloys

A statistical-mechanics analysis is used to create analytic estimates for the short-range order (SRO) parameters in FCC and BCC solid solution alloys as a function of composition and temperature. As in the classic quasi-chemical method, the analysis assumes pair interactions among atoms at different distances and partitions the entire crystal into a set of identical independent clusters that are then treated like molecules in a gas. This enables analytic development of the probabilities that govern SRO. For binary and ternary alloys, increasingly complex clusters are considered that systematically extend the range of accuracy of the analytic model relative to reference Monte Carlo simulations. The results enable fast assessment of likely SRO using estimated or computed inputs for atom-atom interaction energies.

Hydrogen absorption/desorption reactions of the (TiVNb)85Cr15 multicomponent alloy

Ti-V-Nb-Cr alloys have been reported as potential candidates for hydrogen storage applications. Investigating the processes of absorption and desorption is paramount to understand the hydrogen storage properties of these novel alloys and to develop more efficient hydrogen storage materials. In this work, we investigated the hydrogen absorption/desorption reactions of the (TiVNb)85Cr15 BCC multicomponent alloy by laboratory and synchrotron X-ray diffraction, Pair Distribution Function (PDF) analyses, Transmission Electron Microscopy (TEM) and thermo-desorption analyses (TDS). Hydrogen absorption behavior was studied by pressure-composition-isotherm (PCI) at room temperature, which demonstrated levels of absorption around 2 H/M (H/M = hydrogen-to-metal ratio) with low equilibrium pressure. The results showed a multi-step hydrogenation process: alloy ↔ BCC solid solution ↔ BCC intermediate hydride ↔ FCC dihydride. PDF analyses showed that the partially hydrogenated alloy at different levels of H/M and the fully hydrogenated alloy can be reasonably described by models with phases presenting random atomic distribution in the metal crystallographic sites. Moreover, the partially hydrogenated sample prior to the complete formation of the intermediate hydride showed evidence of two co-existing BCC phases.

A Fast and Robust Method for Predicting the Phase Stability of Refractory Complex Concentrated Alloys Using Pairwise Mixing Enthalpy

The ability to predict the composition- and temperature-dependent stability of refractory complex concentrated alloys (RCCAs) is vital to the design of high-temperature structural alloys. Here, we present a model based on first-principles calculations to predict the thermodynamic stability of multicomponent equimolar solid solutions in a high-throughput manner and apply it to screen over 20,000 compositions. We develop a database that contains pairwise mixing enthalpy of 17 refractory metals using density-functional theory (DFT)-based total energy calculations. To these, we fit thermodynamic solution models that can accurately capture the mixing enthalpy of multicomponent BCC solid solutions. By comparing their energy with DFT-calculated enthalpy of intermetallics from the Materials Project database and using convex hull analyses, we identify the stable phase of any RCCA as a function of temperature. The predicted stability of NbTiZr, NbTiZrV, and NbTiZrVM (M = Mo,Ta,Cr) systems as a function of temperature agree well with prior experimental observations. We apply our model to predict the phase evolution in NbVZr-Tix (0 < x < 1), which is confirmed experimentally using a high-throughput, laser deposition-based synthesis technique. This method provides a fast and accurate way to estimate the phase stability of new RCCAs to expedite their experimental discovery.

Structure and properties of powder alloys Fe–(45-15)%Ni–(10-5)%Cu, obtained via mechanical alloying

The article investigates the structure and properties of Fe&ndash;(45-15)%Ni&ndash;(10-5)%Cu powder alloys. The production method included low- and high-energy (mechanical alloying, МА) treatment of Fe, Ni, and Cu powders in a planetary ball mills followed by hot-pressing of mixes at 950 &deg;С. After MA, the mixtures consist of composite granules with a lamellar structure and particle size of 10&ndash;100 &mu;m. After low-energy treatment, three phases were detected by XRD in hot-pressed samples: &alpha;-Fe-based solid solution BCC-(Fe, Ni, Cu), Ni-based solid solution FCC-(Ni, Fe, Cu), and FCC-FeO. The appearance of FeO is caused by the partial oxidation of iron during mixing and hot pressing. The total oxide content does not exceed 1.3 wt%. For Fe-X% Ni-5% Cu alloys the ultimate bending strength depends on the nickel content (X, %) linearly according to the equation &sigma;ben = &minus;19&sdot;X + 1995 [MPa], where 15 % &le; X &le;45 %. Alloys obtained by MA have a homogeneous structure and depending on the composition can be either two- or three-phase. As a result of MA the hardness of the alloys increased by 20&ndash;21 HRB, and the ultimate bending strength increases by 300&ndash;540 MPa. The alloy with the composition 80%Fe-15%Ni-5%Cu has the maximum bending strength &sigma;ben = 2135 &plusmn; 60 MPa. The wear of MA alloys is (4.1&ndash;4.4)&sdot;10&minus;5 mm3 /(N&sdot;m) which is more than two times lower than the wear of alloys obtained by low-energy treatment. This work was carried out with financial support from&nbsp;the Russian Science Foundation (Project No. 17-79-20384).

Preparation and microstructure and properties of AlCuFeMnTiV lightweight high entropy alloy

A new type of AlCuFeMnTiV lightweight high entropy alloy (LWHEA) was prepared by mechanical alloying (MA) and spark plasma sintering (SPS). The effects of milling time on the phase evolution of alloy powders and sintering temperature on the microstructure and mechanical properties of alloy blocks were investigated. The results show that the powders can be alloyed after 36 h high-energy ball milling, forming a BCC solid solution, and the alloying sequence is related to the melting point of the alloy elements. After being sintered by SPS, the alloy block is mainly composed of the BCC matrix, and the B2 phase (ordered BCC phase) and the HCP phases are precipitated from the matrix during the sintering process. Besides that, the Cu-rich FCC phase is distributed at grain boundaries. Nano twins are found in some FCC phases, which have a good strengthening effect on the grain boundary so that the alloy maintains high strength and good plasticity. The block sintered at 1050 °C exhibits the best comprehensive mechanical properties, with the density of 6.28 g⋅cm−3, hardness of 618.44 HV, compressive strength and yield strength of 2630 MPa and 2060 MPa respectively, the plastic strain of 15.83% and specific strength of 418.79 MPa⋅cm3⋅g−1, which is superior to most LWHEAs and traditional lightweight alloys.

On the Microstructure and Properties of Nb-Ti-Cr-Al-B-Si-X (X = Hf, Sn, Ta) Refractory Complex Concentrated Alloys.

We studied the effect of the addition of Hf, Sn, or Ta on the density, macrosegregation, microstructure, hardness and oxidation of three refractory metal intermetallic composites based on Nb (RM(Nb)ICs) that were also complex concentrated alloys (i.e., RM(Nb)ICs/RCCAs), namely, the alloys TT5, TT6, and TT7, which had the nominal compositions (at.%) Nb-24Ti-18Si-5Al-5B-5Cr-6Ta, Nb-24Ti-18Si-4Al-6B-5Cr-4Sn and Nb-24Ti-17Si-5Al-6B-5Cr-5Hf, respectively. The alloys were compared with B containing and B free RM(Nb)ICs. The macrosegregation of B, Ti, and Si was reduced with the addition, respectively of Hf, Sn or Ta, Sn or Ta, and Hf or Sn. All three alloys had densities less than 7 g/cm3. The alloy TT6 had the highest specific strength in the as cast and heat-treated conditions, which was also higher than that of RCCAs and refractory metal high entropy alloys (RHEAs). The bcc solid solution Nbss and the tetragonal T2 and hexagonal D88 silicides were stable in the alloys TT5 and TT7, whereas in TT6 the stable phases were the A15-Nb3Sn and the T2 and D88 silicides. All three alloys did not pest at 800 °C, where only the scale that was formed on TT5 spalled off. At 1200 °C, the scale of TT5 spalled off, but not the scales of TT6 and TT7. Compared with the B free alloys, the synergy of B with Ta was the least effective regarding oxidation at 800 and 1200 °C. Macrosegregation of solutes, the chemical composition of phases, the hardness of the Nbss and the alloys, and the oxidation of the alloys at 800 and 1200 °C were considered from the perspective of the Niobium Intermetallic Composite Elaboration (NICE) alloy design methodology. Relationships between properties and the parameters VEC, δ, and Δχ of alloy or phase and between parameters were discussed. The trends of parameters and the location of alloys and phases in parameter maps were in agreement with NICE.

High temperature oxidation resistance of W-containing high entropy alloys

Tungsten containing refractory high entropy alloys (HEAs) are potential candidate for high temperature applications owing to their remarkable mechanical properties. However, the oxidation resistance of W containing HEAs at high temperatures is problematic, and it received less attention. In this work, several equiatomic W containing WTaTiCr, WMoTaTiCr, WTaAlCr, and WMoTaAlCr refectory HEAs were fabricated by arc melting technique and their oxidation behavior at 1000 °C for 10 h was compared with MoTaTiCr medium entropy alloy (MEA). The as cast microstructure of the MoTaTiCr, WTaTiCr, WMoTaTiCr, and WMoTaAlCr alloys exhibited a single BCC solid solution, while the as cast WTaAlCr MEA showed some extent of intermetallic compounds along with the BCC phase. The best oxidation resistance was obtained for MoTaTiCr MEA, which was protected by a continuous CrTaO4 scale layer. The formation of tungsten oxides (mainly WO3) prohibited the evolution of a protective oxide layer of AlTaO4, CrTaO4 or Al2O3 on the surface of W containing alloys. WMoTaAlCr HEA exhibited the best oxidation resistance among all the W containing alloys, which was protected by a nearly continuous rutile type (Al,Cr)TaO4 oxide layer.

Mössbauer spectroscopy investigation of the effect of a high-dose neutron irradiation on the atomic redistribution in the industrial steel EP823

The authors studied radiation-induced structural-phase transformations that had occurred in EP823 steel of industrial application under conditions of its high-dose neutron irradiation in a fast fission reactor. The method of transmission Mössbauer spectroscopy with resonance detection and transmission electron microscopy was utilized. It has been observed that in ferritic-martensitic steel, under the neutron irradiation with doses above 50 dpa at temperatures of 570–660 °C, the decomposition of a BCC solid solution together with the release of the alloying elements chromium, molybdenum, and carbon from the metal matrix occurs. It is accompanied by the formation of an intermetallic χ phase and carbides of the Me23C6 type. The authors revealed the formation of the vacancy-clustered and supposedly gas-filled pores in the structure after irradiation.

Microstructure and mechanical properties of multi-phase reinforced Hf-Mo-Nb-Ti-Zr refractory high-entropy alloys

In this paper, Hf-Mo-Nb-Ti-Zr alloy system with single BCC solid solution phase was selected as the matrix alloy. In order to design a new high temperature structural material with excellent comprehensive properties, the multiphase reinforced refractory high entropy alloys was prepared by adding metallic elements of Al or Cr and nonmetallic elements of B, C or Si into Hf-Mo-Nb-Ti-Zr alloys at the same time. The results show that the type and content of newly formed phases are closely related to the mixing enthalpy between alloying elements and the constituent elements of the matrix alloy. This provides a valuable reference for the prediction of phase formation when adding alloying elements to improve the properties of high entropy alloys.

Effect of Al Content on Phase Compositions of FeNiCoCrMo0.5Alx High Entropy Alloy

The FeCoNiCrMo0.5Alx system with x up to 2.13 was analyzed from the point of view of evolution of the phase composition and microstructure. Cast samples were synthesized by induction melting and analyzed by X-ray diffraction, energy dispersive spectroscopy, scanning electron microscopy, and Vickers microhardness test methods. Phase compositions of these alloys in dependance on Al concentration consist of FCC solid solution, σ-phase, NiAl-based B2 phase, and BCC solid solution enriched with Mo and Cr. Phase formation principles were studied. Al dissolves in a FeCoNiCrMo0.5 FCC solid solution up to 8 at.%.; at higher concentrations, Al attracts Ni, removing it from FCC solid solution and forming the B2 phase. Despite Al not participating in σ-phase formation, an increase in Al concentration to about 20 at.% leads to a growth in the σ-phase fraction. The increase in the σ-phase was caused by an increase in the amount of B2 because the solubility of σ-forming Mo and Cr in B2 was less than that in the FCC solution. A further increase in Al concentration led to an excess of Mo and Cr in the solution, which formed a disordered BCC solid solution. The hardness of the alloys attained the maximum of 630 HV at 22 and 32 at.% Al.