Single BCC Solid Solution Research Articles

First principle calculation of the effect of Cr, Ti content on the properties of VMoNbTaWMx (M = Cr, Ti) refractory high entropy alloy

The influence of Cr and Ti content on the phase structure, elastic constants, elastic properties and anisotropy of VMoNbTaWMx (M = Cr, Ti) refractory high entropy alloy was studied by the first principle method in this paper. Based on the plane wave pseudo-potential and density functional theory, the structure model of solid solution was established by virtual crystal approximation (VCA). By calculating the atom size difference (δ), valence electron concentration (VEC) and atom distribution (λ), it was indicated that the VMoNbTaWMx (M = Cr, Ti, 0 < x < 2) maintain a single BCC solid solution structure. The calculation results showed that with the increase of Cr content, the strength of the alloy increased, the plasticity presented a certain fluctuation, and the anisotropy first decreases and then increases. The anisotropy of VMoNbTaWCr was the lowest, VMoNbTaWCr1.75 had the highest plasticity and VMoNbTaWCr2 had the highest strength. With the increase of Ti element, the strength of the alloy decreased, the plasticity first decreased and then increased, and the anisotropy decreased first and then increased. VMoNbTaW had the best strength and plasticity and VMoNbTaWTi1.75 had the lowest anisotropy.

Significantly strengthening the Ti70Nb10Mo10Zr10 alloy via architecting two-scale silicide reinforcements

Abstract To further strengthen the Ti70Nb10Mo10Zr10 alloy (denoted as TI70 alloy) with a single BCC solid solution microstructure, two-scale silicide reinforcements were successfully tailored by adding 1 at.%, 2 at.% and 4 at.% Si element during powder metallurgy process (denoted as TI70-Six alloys). The nano-scale S1-type (Ti,Zr)5Si3 silicides were precipitated and distributed dispersively at grain interior to provide dispersion strengthening effect, while the nano-scale and micro-scale S2-type (Ti,Zr)6Si3 silicides were distributed at the grain boundaries and formed a novel network microstructure which could prohibit grain growth and enhance grain boundary strengthening effect. The TI70-Six alloys strengthened by the two-scale silicides exhibited remarkable strength enhancement not only at ambient temperature but also at high temperatures. The compressive yield and fracture strengths of the TI70-Si4 alloy were increased to 1550.4 MPa and 2002.5 MPa at ambient temperature. Moreover, the TI70-Si4 alloy exhibited ultrahigh yield strengths of 1048 MPa and 437 MPa at 600 °C and 800 °C, which were increased by 61.1% and 111.2% compared with TI70 alloy, respectively. The superior strengths can be mainly attributed to the coordinated strengthening mechanisms of two-scale silicide reinforcements and novel network microstructure.

Microstructure and Mechanical Properties of TaNbVTiAlx Refractory High-Entropy Alloys.

A series of TaNbVTiAlx (x = 0, 0.2, 0.4, 0.6, 0.8, and 1.0) refractory high-entropy alloys (RHEAs) with high specific strength and reasonable plasticity were prepared using powder metallurgy (P/M) technology. This paper studied their microstructure and compression properties. The results show that all the TaNbVTiAlx RHEAs exhibited a single BCC solid solution microstructure with no elemental segregation. The P/M TaNbVTiAlx RHEAs showed excellent room-temperature specific strength (207.11 MPa*cm3/g) and high-temperature specific strength (88.37 MPa*cm3/g at 900 °C and 16.03 MPa*cm3/g at 1200 °C), with reasonable plasticity, suggesting that these RHEAs have potential to be applied at temperatures >1200 °C. The reasons for the excellent mechanical properties of P/M TaNbVTiAl0.2 RHEA were the uniform microstructure and solid solution strengthening effect.

Microstructure and mechanical properties of low activation Fe–Ti–Cr–V–W multi-principal element alloys

Single-phase multi-principal element alloys can have low activation and rapid induced radioactive decay properties that make them ideal structural materials for the first wall/cladding structure of nuclear fusion reactors. The present work proposes the design of multi-principal element alloys consisting of low activation elements Fe, Ti, Cr, V, and W with both equiatomic and optimized compositions. The optimal composition of the multiple components is based on the formation of a single-phase solid solution with a minimal Gibbs free energy. The microstructures and mechanical properties of the alloys are investigated. The equiatomic alloy is observed to be composed of dendritic crystals consisting of a body-centered cubic (bcc) solid solution phase and a complex intermetallic compound, while the optimized alloy is composed of cellular dendritic crystals consisting of a single bcc solid solution phase in its as-cast state. Homogenization treatment is observed to reduce the fracture strength of the equiatomic alloy due to grain coarsening, although it has little effect on the hardness and other mechanical properties. However, homogenization increased the hardness and compressive yield strength of the optimized alloy to different degrees. The optimized alloy has better plastic toughness than the equiatomic alloy, which increases its workability and radiation resistance.

Microstructure and mechanical properties of NbZrTi and NbHfZrTi alloys

Two medium-entropy alloys, NbZrTi and NbHfZrTi, were prepared by arc melting. Both NbZrTi and NbHfZrTi alloys are composed of simple body-centered cubic (bcc) solid solution phase and exhibit dendritic structure. After being homogenized, both NbZrTi and NbHfZrTi alloys are still composed of the single bcc solid solution phase, but the microstructure of the two alloys transforms from the dendritic structure into the polycrystalline structure. Two alloys display significantly work-hardening effect during compression at room temperature and show relatively good deformation plasticity during compressive deformation at room temperature. For NbZrTi and NbHfZrTi alloys, the dynamic recrystallized grains form along the boundary during compression at the temperatures of 1073 and 1273 K.

ICME approach to explore equiatomic and non-equiatomic single phase BCC refractory high entropy alloys

High entropy alloys made from refractory metals, commonly known as refractory high entropy alloys (RHEAs) are potential candidates for high-temperature applications beyond the temperature regime (>1873 K) of conventional nickel based super alloys. In the present investigation, integrated computational materials engineering (ICME) framework consisting of detailed CALPHAD (CALculation of PHase Diagram) modeling and experimental investigation have been carried out to design both equiatomic and non-equiatomic RHEA composition exhibiting thermodynamically stable single-phase BCC/B2 solid solution. Starting with 126 equiatomic compositions, the CALPHAD calculations reveal only two equiatomic (MoNbTaVW and CrMoReVW) alloys having single phase BCC solid solution at 1000K. Further, low temperature (1000–400 K) CALPHAD assessment on 2902 non-equiatomic alloys reveal the formation of 54 single phase RHEAs based on MoNbTaVW at 400 K and 86 single phase RHEAs based on CrMoReVW at 800 K. In order to validate the CALPHAD predictions, one equiatomic, one non-equiatomic CrMoReVW alloy and six non-equiatomic MoNbTaVW alloys were synthesized and characterized using X-Ray diffraction, scanning electron microscopy and transmission electron microscopy. All of them show single phase solid solution with an excellent combination of microhardness (5.33–17.64 GPa) and elastic modulus (188–474 GPa) thus proving high fidelity of the CALPHAD approach in predicting phase stability of HEAs. To summarize, ICME approach involving CALPHAD modeling aids in accelerated design and development of existing RHEAs and discover new RHEAs showing potential for high-temperature applications.

Experimental and theoretical study of microstructural characteristics and phase stability in equiatomic CrFeMoV alloy

Multi-principal elemental or high entropy alloys are viewed as promising class of materials with enhanced performance in a variety of environments. Study of microstructure of these alloys is essential to understand its influence on various properties. This study is aimed at the synthesis of an equiatomic Cr-Fe-Mo-V alloy by conventional casting method and its microstructural characterization. For the first time, crystal structure of this alloy has been predicted by ab-initio density functional approach and found to be in close agreement with the experimental results. Micro-segregation observed during solidification is also predicted by Scheil cooling model and is attributed to the higher melting point and partition coefficient of Molybdenum. This alloy may be a potential radiation resistant alloy, in view of the possibility of formation of a single BCC solid solution, shown experimentally and theoretically.

Manufacturing and Analysis of High-Performance Refractory High-Entropy Alloy via Selective Laser Melting (SLM).

Refractory high-entropy alloys (HEAs) have excellent mechanical properties, which could make them the substitutes of some superalloys. However, the high melting point of refractory HEAs leads to processing problems when using traditional processing techniques. In this study, a single BCC solid solution of NbMoTaW alloy was formed by selective laser melting (SLM) with a linear energy density of up to 2.83 J/mm. The composition distribution was analyzed, and the element with a lower melting point and lower density showed a negative deviation (no more than 5%) of the molar ratio in the formed alloy. The HEA shows an excellent microstructure, microhardness, and corrosion resistance performance compared with traditional superalloys, making it a new substitute metal with great application prospects in aerospace and energy fields.

Diffraction, microstructure and thermal stability analysis in a double phase nanocrystalline Al20Mg20Ni20Cr20Ti20 high entropy alloy

AbstractAn effort has been made to develop a new composition of AlMgNiCrTi high entropy alloy (HEA) with a distinct properties includes squat density, intense strength and hardness, superior corrosion resistance, better oxidation resistance, high temperature resistance, fatigue load and crack resistance to congregate the necessity of aircraft applications. The equivalent atomic percentage for the above defined composition is established using analytical correlation for molar and atom renovation by trial and error method. The alloy is synthesized by powder metallurgy technique through mechanical alloying. Succeeding to mechanical alloying it is elucidated that the metal powder is primarily composed of single BCC solid solution with crystallite magnitude &lt;10 nm. It is also observed that the alloy is thermally stable at prominent temperature about 800°C as it is retained its nanostructure which was revealed using differential scanning caloriemetry (DSC). This alloy powder was consolidated and sintered using spark plasma sintering at 800°C with 50 Mpa pressure to a density of 98.83%. Subsequent to sintering, Titanium carbide FCC phase evolved along with the BCC phase. The alloying behavior and phase transformation were studied using X-ray diffraction (XRD) and scanning electron microscope (SEM). The homogeneity of the composition is confirmed by energy dispersive spectroscopy (EDS). The hardness of the alloy is found to be 710±20 HV. The evolutions of the phases and hardness imply that this alloy is apposite for both high strength and high temperature applications.

AlCrFeNi High-Entropy Coating Fabricated by Mechanical Alloying and Hot Pressing Sintering

The AlCrFeNi high entropy coating was successfully prepared by mechanical alloying and vacuum hot pressing sintering technique on Q235 steel substrate, and the microstructures and microhardness, wear resistance and corrosion resistance were characterized in detail. Results showed that single body-centered cubic solid solution phase appears when the blended powder is ball milled for 30 h. The vacuum hot pressing sintered AlCrFeNi high entropy coating with single BCC solid solution is about 500 μm in thickness, which metallurgically bonded to substrate. The microhardness of the AlCrFeNi high entropy coating is about 520 HV, more than three times larger than that of the substrate. The wear resistance and corrosion resistance of the coating in 3.5% NaCl solution are greatly improved compared with the substrate, showing a dramatic reduction in average corrosion rate and wider passive range.

Effects of Different Levels of Boron on Microstructure and Hardness of CoCrFeNiAlxCu0.7Si0.1By High-Entropy Alloy Coatings by Laser Cladding

High-entropy alloys (HEAs) are novel solid solution strengthening metallic materials, some of which show attractive mechanical properties. This paper aims to reveal the effect of adding small atomic boron on the interstitial solid solution strengthening ability in the laser cladded CoCrFeNiAlxCu0.7Si0.1By (x = 0.3, x = 2.3, and 0.3 ≤ y ≤ 0.6) HEA coatings. The results show that laser rapid solidification effectively prevents brittle boride precipitation in the designed coatings. The main phase is a simple face-centered cubic (FCC) matrix when the Al content is equal to 0.3. On the other hand, the matrix transforms to single bcc solid solution when x increases to 2.3. Increasing boron content improves the microhardness of the coatings, but leads to a high degree of segregation of Cr and Fe in the interdendritic microstructure. Furthermore, it is worth noting that CoCrFeNiAl0.3Cu0.7Si0.1B0.6 coatings with an FCC matrix and a modulated structure on the nanometer scale exhibit an ultrahigh hardness of 502 HV0.5.

Laser surface alloying of FeCoCrAlNi high-entropy alloy on 304 stainless steel to enhance corrosion and cavitation erosion resistance

FeCoCrAlNi high-entropy alloy coating was synthesized with premixed high-purity Co, Cr, Al and Ni powders on 304 stainless steel by laser surface alloying, aiming at improving corrosion and cavitation erosion resistance. Phase constituents, microstructure and microhardness were investigated using XRD, SEM, and microhardness tester, respectively. The cavitation erosion and electrochemical corrosion behavior of FeCoCrAlNi coating in 3.5% NaCl solution were also evaluated using an ultrasonic vibrator and potentiodynamic polarization measurement. Experimental results showed that with appropriate laser processing parameters, FeCoCrAlNi coating with good metallurgical bonding to the substrate could be achieved. FeCoCrAlNi coating was composed of a single BCC solid solution. The formation of simple solid solutions in HEAs was the combined effect of mixing entropy (ΔSmix), mixing enthalpy (ΔHmix), atom-size difference (δ) and valence electron concentration (VEC), and the effect of ΔSmix was much larger than that of the other factors. The microhardness of the FeCoCrAlNi coating was ~3 times that of the 304 stainless steel. Both the corrosion and cavitation erosion resistance of the coating were improved. The cavitation erosion resistance for FeCoCrAlNi HEA coating was ~7.6 times that of 304 stainless steel. The corrosion resistance was also improved as reflected by a reduction in the current density of one order of magnitude as compared with 304 stainless steel.

Microstructure and Properties of FeAlCrNiMo x High-Entropy Alloys

FeAlCrNiMox high-entropy alloys were prepared. The effect of Mo content on the microstructure and the properties of the alloys were investigated. When the Mo content was 0.1, the alloys were composed of single BCC solid solution; when Mo content reaches 0.25, the alloys were composed of BCC solid solution and ordered B2 solid solution. When Mo content is more than 0.75, some σ phases emerged. The volume fraction of the second phase increases with the increasing Mo content, and the crystal grains became coarsening. The yield strength, fracture strength, and hardness increase with the increasing Mo content and reach 2252, 2612 MPa, and 1006 Hv, respectively. The magnetic transformation undergoes from the ferromagnetism to paramagnetism with the increasing Mo content. The saturation intensity and remnant magnetism are decreased with the increasing Mo content.

Structure and mechanical properties of the AlCrxNbTiV (x = 0, 0.5, 1, 1.5) high entropy alloys

The AlCrxNbTiV (x = 0, 0.5, 1, 1.5) high entropy alloys were produced by vacuum arc melting. The crystal structure, microstructure, density and compression mechanical properties at 22°C–1000 °C of the AlCrxNbTiV alloys after homogenization annealing at 1200 °C for 24 h are reported. After homogenization the AlNbTiV and AlCr0.5NbTiV alloys consist from single bcc solid solution phase, while the AlCrNbTiV and AlCr1.5NbTiV alloys contain bcc phase and respectively 13% and 35% of hexagonal (C14) Laves phase. The density of alloys increases with increase of Cr content from 5590 kg m−3 of the AlNbTiV alloy to 5900 kg m−3 of the AlCr1.5NbTiV alloy. The strength of the AlCrxNbTiV alloys increases with increase of Cr concentration at temperatures from room to 800 °C. The yield strength of the AlNbTiV and AlCr1.5NbTiV alloys is 1000 MPa and 1700 MPa respectively at room temperature and 560 MPa and 970 MPa at 800 °C. Strengthening of the AlCrxNbTiV alloys with increase of Cr content is accompanied by decrease of ductility. Phase composition of the alloys was affected by deformation at 800 °C and 1000 °C. In the AlNbTiV and AlCr0.5NbTiV alloys Nb2Al-type phase precipitated predominantly on grain boundaries, while in the AlCrNbTiV and AlCr1.5NbTiV alloys additional amounts of Laves phase appeared inside grains of bcc matrix. The experimentally observed phase composition of the AlCrxNbTiV alloys is compared with results of thermodynamical modeling of equilibrium phases in the alloys. The effect of phase composition of the alloys on mechanical properties is discussed and possibilities for properties optimization of the Al–Cr–Nb–Ti–V high entropy alloys are suggested.

Ab Initio Predicted Alloying Effects on the Elastic Properties of AlxHf1−xNbTaTiZr High Entropy Alloys

Using ab initio alloy theory, we investigate the equilibrium bulk properties and elastic mechanics of the single bcc solid-solution AlxHf1−xNbTaTiZr (x = 0–0.7, 1.0) high entropy alloys. Ab initio predicted equilibrium volume is consistent with the available experiment. We make a detailed investigation of the alloying effect of Al and Hf on the equilibrium volume, elastic constants and polycrystalline elastic moduli. Results imply that the partial replacement Hf with Al increases the stability of the bcc phase and decreases the ductility of the AlxHf1−xNbTaTiZr HEAs. The inner ductility of Al0.4Hf0.6NbTaTiZr is predicted by the calculations of ideal tensile strength.

Synthesis and characterization of FeCoCrAlCu high-entropy alloy coating by laser surface alloying

FeCoCrAlCu high-entropy alloy coating was synthesized on Q235 steel with premixed high-purity Co, Cr, Al and Cu four-component powders on a Fe-base alloy by laser surface alloying. Constituent phases, microstructure and chemical composition of the coating were investigated using XRD, SEM, and EDS, respectively. Surface mechanical property and wear resistance were also studied. Results showed that the FeCoCrAlCu coating was ~800μm in thickness, and a hard high-entropy alloy coating with good metallurgical bonding to the substrate was obtained. FeCoCrAlCu coating was composed of a single BCC solid solution. The microstructure of the coating exhibited a typical dendrite structure growing from the substrate. The microhardness of the FeCoCrAlCu coating was ~3 times that of the Fe-base alloy substrate. The wear resistance of the FeCoCrAlCu coating was also improved. Both the wear volume and specific wear rate of the coating are an order of magnitude lower than that of the Q235 substrate under a dry sliding condition, which might be attributed to the fact that the wear resistance is proportional to the alloy hardness according to Archard's law.

Structure evolution and mechanical properties of Ta–Nb–Ti–W–N multielement films

In this study, the multielement films (Ta50·5Nb16·7Ti8·8W24·0)N, (Ta25·4Nb13·4Ti20·9W40·3)N and (Ta9·3Nb4·7Ti22·6W63·4)N are prepared by combining magnetron sputtering deposition and nitrogen plasma based ion implantation (N-PBII). The composition, structure and mechanical properties of the films are investigated. X-ray diffraction patterns show that the alloy films Ta50·5Nb16·7Ti8·8W24·0, Ta25·4Nb13·4Ti20·9W40·3 and Ta9·3Nb4·7Ti22·6W63·4 are composed of hcp+bcc, two types of bcc and a single bcc solid solution respectively, whereas all the Ta–Nb–Ti–W films are composed of bcc and fcc structures after N-PBII treatment. The lattice constant increases for both the bcc and fcc phases, while it decreases for the Ta and Nb concentrations in the films. The hardness and modulus of the Ta–Nb–Ti–W–N films are improved with increasing nitrogen implantation dose. The wear rate of the Ta25·4Nb13·4Ti20·9W40·3 with nitrogen implantation for 1 h is decreased two times than the Ta25·4Nb13·4Ti20·9W40·3 alloy films.

Thermodynamic Analysis for Microstructure of High-Entropy Alloys

Abstract The Gibbs free energy of liquid and crystal, intermetallic compounds, and amorphous phases were investigated by thermodynamic analysis for microstructure of high-entropy alloy (HEA). Results show that the value of Δ S r Δ H h(sol.) plays an important role in estimating the microstructure of HEAs. The alloys with lower value of Δ S r Δ H h(sol.) have stronger tendency of forming single fcc or bcc solid solution, and the alloy with complicated microstructure always has larger value of Δ S r Δ H h(sol.) .

Effect of aluminium content of AlxCrFe1·5Ni0·5 multiprincipal alloys on microstructure and alloy hardness

AlxCrFe1·5Ni0·5 alloys (x = 0·15, 0·2, 0·3, 0·4) were prepared by arc melting pure elements under argon atmosphere. All the AlxCrFe1·5Ni0·5 alloys present a single bcc solid solution crystal structure. Because of the common precipitation strengthening effect of lamellar precipitates and equiaxed particles, the hardness of the Al0·15CrFe1·5Ni0·5 alloy is quite high, reaching 443·2 HV. Microstructures of the AlxCrFe1·5Ni0·5 alloys (x = 0·2, 0·3, 0·4) are very simple. The crystal boundary is quite straight. With the content of Al increasing from 0·2 to 0·4, the hardness of the alloy increases from 303·8 to 450·1 HV through the solution strengthening effect of the Al atom.

Microstructures and compressive properties of multicomponent AlCoCrFeNiMo x alloys

Multicomponent AlCoCrFeNiMo x ( x values in molar ratio, x = 0, 0.1, 0.2, 0.3, 0.4 and 0.5) alloys were prepared using a well-developed copper mould casting. The effects of Mo element on the structure and properties of AlCoCrFeNi alloy were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and differential scanning calorimeter (DSC). It was found that Mo addition took significant effects on the structure and properties of AlCoCrFeNi alloy. Mo 0.1 alloy, similar to Mo 0 alloy, was of single BCC solid solution structure. When Mo content was more than 0.1, the alloy exhibited a typical laminar eutectic structure. The alloy strength was improved obviously, but the ductility of alloy was lowered at the same time. The maximum yield strength reached 2757 MPa when Mo content was 0.5, and the maximum compressive fracture strength reached 3208 MPa when Mo content was 0.3. The strengthening effect and mechanism of Mo addition on AlCoCrFeNi alloy were discussed from different aspects.