Bcc Solid Solution Phase Research Articles

Influence of mutual substitution between Cu and Ti on phase components, microstructures, and compressive properties of AlFeNiV–Cu/Ti alloys

Two 3d transition metal high-entropy alloys (3d TM HEAs), AlFeNiVCu and AlFeNiVTi, were suction cast under identical conditions into a cylinder copper mould for investigating the compositional dependence of phase components, microstructures, and compressive properties. The phase components of the AlFeNiVCu were composed of a BCC FeV-rich dendritic phase, a netlike FCC Cu-rich interdendritic phase, and a uniformly distributed B2 AlNi-rich phase, when Cu substituted by Ti the phase components transformed to two BCC solid solution phases of the AlFeNiVTi. Moreover, an outstanding synergy in strength and ductility (ultimate compressive strength: 1936 MPa, plastic strain: 10.0%) was acquired for the AlFeNiVCu, which was sharply contrast with the typically brittle behaviour at the ultimate strength of 108 MPa for the alloy AlFeNiVTi. The distinctions of phase components and morphology were analysed to uncover the conflict in mechanical performance by Cu and Ti mutual substitution in AlFeNiV-base alloys.

Microstructure and hardness of NbTiZr and NbTaTiZr refractory medium-entropy alloy coatings on Zr alloy by laser cladding

Two refractory medium-entropy alloy (RMEA) coatings (NbTiZr and NbTaTiZr) with BCC solid-solution phase as their main constitutional phase were successfully fabricated on Zr702 alloy by utilizing pulsed laser cladding. Phase constitutions, chemical compositions, crystallographic orientations and hardnesses of the coatings were carefully characterized and analyzed. Results show that both the RMEA coatings correspond to cladding zones (CZs) that are composed of irregular-shaped bulk grains (BCC solid-solution phase) with scattered orientations and 370–380 μm in thickness. The addition of Ta is able to promote grain refinement and produce higher micro strains in the coatings. Theoretical analyses show that the phase constitution in the RMEA coatings is consistent with that predicted according to several thermodynamic or structural parameters. Below the CZs, there exist heat-affected zones (HAZs) consisting of a mixture of bulk α-Zr grains and fine martensite plates. Average thicknesses of the HAZs in both the specimens are 180–190 μm. Hardnesses of the NbTiZr and the NbTaTiZr coatings are 510 ± 22 HV and 550 ± 26 HV, respectively, significantly higher than that of the substrate (196 ± 4 HV). Such high hardnesses are attributed to combined effects of solid-solution strengthening, grain refinement strengthening and increased contents of low angle boundaries.

The Effect of Remelting on Microstructure and Hydrogen Storage Properties of Ti–Mn–V Alloy

In this research, the effect of multiple remelting on microstructure and hydrogen storage properties of (TiZr)1(VFeCrMn)2.1 alloy was studied. The alloy was melted using a VIM furnace. This process was repeated three times. The microstructure was evaluated by SEM and XRD analysis, and the hydrogen storage properties were measured by Sieverts method. The results indicated that the microstructure contains predominant C14 Laves, BCC solid solution and titanium oxide phases. The homogenous microstructure and fine BCC phase distribution was obtained by remelting. The volume fraction of BCC phase was decreased from 26 to 18% during remelting. PCT curves revealed that increasing remelting times results in decreasing slope and hysteresis factors. The values of alloys slope factor for single, double and triple melting are 0.57, 0.26 and 0.21, respectively. Therefore, hydrogen storage properties of the alloy are improved as a result of homogenous microstructure and appropriate content of phases during the remelting process.

In-situ development of MB2 and their effect on microstructure and mechanical properties of refractory Hf0.5Mo0.5NbTiZr high entropy alloy matrix composites

To improve the room temperature comprehensive mechanical properties of the refractory high entropy alloy Hf0.5Mo0.5NbTiZr, boron element is added to form the boride phase as the strengthening phase. The Hf0.5Mo0.5NbTiZrBx (x = 0.1, 0.3, 0.5, 0.7, and 0.9) were produced by vacuum arc melting and effect of boron element on the microstructure and room temperature mechanical properties of Hf0.5Mo0.5NbTiZr alloy was investigated. The boride MB2 phase form and the volume fraction of boride phase increases with the addition of boron element. The microstructure exhibits the evolution from the hypoeutectic structure to the eutectic structure, and then to the hypereutectic structure with the increase of boron element. Because of the difference of the formation path, the morphology and chemical compositions of MB2 phase is different. The lamellar or needlelike MB2 phase forms through the eutectic reaction and is rich in Zr element but poor in Mo and Nb elements. The agglomerated or block sexangle MB2 phase forms as the proeutectic phase and is enriched with Nb element and are depleted with Mo and Zr elements. When the content of the boron is x = 0.1 and 0.3, both the strength and ductility are improved due to the refined structure, the clean interface between the BCC solid solution phase and MB2 phase and the precipitated strengthening of boride phase. And the strength increases dramatically at the expense of the decrease of the ductility with more addition of boron element, which is attributed to the formation of more brittle boride phase

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.

Phase constitution, microstructure and properties of pulsed laser-clad ternary CrNiTi medium-entropy alloy coating on pure titanium

A ternary CrNiTi medium-entropy alloy (MEA) coating with excellent surface performance (hardness and wear resistance) was successfully prepared on pure Ti sheet by pulsed laser cladding. Microstructural characteristics of the MEA coating were probed by combined use of multiple characterization techniques and reasons for the formation mechanisms of various phases in the coating were well explored. Results show that fine cellular grains are formed in the MEA coating during the ultrafast non-equilibrium solidification process induced by pulsed laser cladding. These grains have an average size less than 1 μm and correspond to a BCC solid-solution phase. There appears irregular-shaped Cr2Ti Laves phase (C14-type) inside most of the cellular grains, while intergranular structures are demonstrated to be NiTi intermetallics. Hardness tests reveal that the CrNiTi MEA coating has a hardness of 940 ± 35 HV which is ~8 times that of the pure Ti substrate (119 ± 9 HV). Also compared to the pure Ti substrate, a much lower wear rate is noted for the coating demonstrating greatly improved wear resistance. Comprehensive analyses show that the excellent surface performance of the CrNiTi MEA coating can be ascribed to combined contributions from the solid-solution hardening and grain refinement hardening of the BCC phase, as well as second phase hardening produced by Cr2Ti Laves phase and NiTi intermetallics.

Microstructure characteristics and mechanical properties of Hf0.5Mo0.5NbTiZr refractory high entropy alloy with Cr addition

This paper reported the effect of Cr addition on microstructure, phase constitutes and mechanical properties of the Hf0.5Mo0.5NbTiZr refractory high-entropy alloy. The CrxHf0.5Mo0.5NbTiZr (x = 0.1, 0.2, 0.3, 0.4 and 0.5) alloys were composed of one BCC solid solution phase and one CrMoZr-typed Laves phase. This alloy system exhibited the typical dendritic structure after casting. The dendritic structure was refined with the Cr addition. The strength of CrxHf0.5Mo0.5NbTiZr alloys increased and the plasticity also was improved with small amount addition of Cr elements, the strength continued to increase while the plasticity decreased as the concentration of Cr element increased. The homogenization treatment promoted more formation of Laves phase, which significantly increased the strength.

Effect of nitrogen content on microstructure and corrosion resistance of sputter-deposited multicomponent (TiNbZrTa)Nx films

Multicomponent (TiNbZrTa)Nx films were deposited on Si(100) substrates at room temperature using magnetron sputtering with a nitrogen flow ratio fN [fN = N2/(Ar + N2)], which was varied from 0 to 30.8%. The nitrogen content in the films varied between 0 and 45.2 at.%, i.e., x = 0 to 0.83. The microstructure was characterized by X-ray diffraction and electron microscopy. The metallic TiNbZrTa film comprised a dominant bcc solid-solution phase, whereas a single NaCl-type face-centred cubic structure was observed in all nitrogen-containing films (TiNbZrTa)Nx. The mechanical, electrical, and electrochemical properties of these films varied with nitrogen content. The maximum hardness was achieved at 22.1 ± 0.3 GPa when N = 43.0 at.%. The resistivities increased from 95 to 424 μΩcm with increasing nitrogen content. A detailed study of the variation of morphology and chemical bonding with nitrogen content was performed and the corrosion resistance of the TiNbZrTa nitride films was explored in 0.1 M H2SO4. While all the films had excellent corrosion resistances at potentials up to 2.0 V vs. Ag/AgCl, the metallic film and the films with low nitrogen contents (x < 0.60) exhibited an almost stable current plateau up to 4.0 V vs. Ag/AgCl. For the films with higher nitrogen contents (x ≥ 0.68), the current plateau was retained up to 2.0 V vs. Ag/AgCl, above which a higher nitrogen content resulted in a higher current. The decrease in the corrosion resistance at these high potentials indicate the presence of a potential-dependent activation effect resulting in an increased oxidation rate of the nitrides (present under the passive oxide film) yielding a release of nitrogen from the films. TEM results indicate that the oxide layer formed after this corrosion measurement was thick and porous for the film with x = 0.76, in very good agreement with the increased corrosion rate for this film. The results demonstrate that an increased nitrogen content in (TiNbZrTa)Nx system improves their mechanical properties with retained high corrosion resistance at potentials up to 2.0 V vs. Ag/AgCl in 0.1 M H2SO4. At even higher potentials, however, the corrosion resistance decreases with increasing nitrogen concentration for films with sufficiently high nitrogen contents (i.e. x ≥ 0.68).

Novel reduced-activation TiVCrFe based high entropy alloys

The conditions inside next-generation nuclear reactors are likely to be extreme. High-performance materials will be required, and there is still great scope for developing new and improved alloys. High-entropy alloys (HEAs) are potential candidates, and it is likely that such alloys will need to be based around low-activation elements that tend to form body-centred cubic structures. Whilst there have been a number of investigations assessing the capability of CALPHAD databases to predict the phases formed in face-centred cubic HEAs, their applicability to less studied systems is not well known. Here, two low-activation HEAs are produced; TiVCrMnFe and Si0.1TiVCr0.5Fe, and their microstructures assessed and compared to CALPHAD predictions. The microstructures of both alloys comprised a C14 Laves phase and a B2 phase following casting, and a C14 Laves phase and a BCC solid solution (A2) phase following holding at 1200 °C, with high proportions of both constituent phases found in each case. It was found experimentally that the Laves phases in both alloys were quaternary intermetallics of Fe, V, Cr and Ti, with Fe, V and Cr likely disordered across the Fe site. These observations were compared with predictions made using four CALPHAD databases. The accuracy of the predictions varied markedly between the databases. It was found that the Laves phase was underpredicted, often severely, by the databases that did not account for the quinary nature of phase. Removing Ti from TiVCrMnFe resulted in an equiatomic VCrMnFe alloy that formed a single BCC phase at 1200 °C.

Microstructural characteristics and hardness of CoNiTi medium-entropy alloy coating on pure Ti substrate prepared by pulsed laser cladding

A ternary CoNiTi medium-entropy (MEA) alloy coating was successfully fabricated on pure Ti substrate by utilizing pulsed laser cladding. Phase constitutions and microstructural characteristics of the coating were analyzed by combined use of X-ray diffraction, electron channeling contrast imaging, secondary electron imaging, energy dispersive spectroscopy and electron backscatter diffraction techniques. Results show that three zones with distinct microstructures are formed after the pulsed laser cladding treatment: cladding zone (CZ), bonding zone (BZ) and heat-affected zone (HAZ). Dendritic (BCC solid-solution phase)-interdendritic (Ti2Ni type phase) structures are formed in the CZ with many nanoparticles (Ti2Co type phase) dispersed inside the interdendritic structures. The BZ consists of fine acicular grains while the HAZ below the BZ is composed of irregular-shaped bulk grains. Hardness measurements reveal that hardness of the MEA coating reaches 571 ± 46 H V, which is ∼5 times that of the substrate (114 ± 4 H V). Comprehensive analyses show that such high hardness can be attributed to the combined contributions from solid-solution hardening of the BCC phase and second-phase hardening from the Ti2Ni and Ti2Co type intermetallic compounds.

A multiple nonmetallic atoms co-doped CrMoNbWTi refractory high-entropy alloy with ultra-high strength and hardness

A multiple nonmetallic atoms co-doped CrMoNbWTi refractory high-entropy alloy (RHEA) was prepared by mechanical alloying (MA) and spark plasma sintering (SPS). The as-sintered RHEA co-doped with C, N, and O exhibited ultra-high strength and hardness of 4345 MPa and 11.88 GPa, respectively. The microstructural observations indicated that both N and O were completely reacted with the refractory metallic elements to in-situ generate massive dispersed refractory compounds of (Nb,Ti)N and Ti2O3, while C was mainly dissolved in the BCC solid-solution phase. The strengthening mechanisms of the multiple nonmetallic atoms (C, N, and O) were discussed in this paper. The results demonstrated that the excellent mechanical properties were not only attributed to the traditional solid solution, grain boundary, Orowan strengthening mechanisms, but also to the additional C interstitial strengthening and dispersed refractory compounds of (Nb,Ti)N and Ti2O3.

Design of a low density refractory high entropy alloy in non-equiatomic W–Mo–Cr–Ti–Al system

A new refractory high entropy alloy (RHEA) with non-equiatomic composition of W10Mo27Cr21Ti22Al20 was designed based on empirical rules and CALPHAD approach. Calculated results showed that W10Mo27Cr21Ti22Al20 RHEA had lower density and higher melting temperature than that of equiatomic alloy. The designed alloy was prepared by vacuum arc melting then the crystal structure, microstructure, chemical composition, density, hardness and compressive strength had been evaluated. The alloy exhibited a BCC solid solution as major phase with typical dendritic microstructures. The experimental results confirmed calculated results, suggesting a BCC solid solution phase. The density of the alloy was 7.48 g/cm3, which was approximately close to the calculated value, 7.61 g/cm3. However, the measured hardness value (511 kg/mm2) was significantly higher than the calculated value (125 kg/mm2), indicating the effect of solid solution strengthening. This alloy had compressive yield strengths of 1245 MPa at room temperature, 510 MPa at 600 °C, 201 MPa at 1000 °C and 105 MPa at 1200 °C. Compressive strain of the alloy was limited at room temperature, but it increased significantly to above 60% at 1000 °C and 1200 °C. Comparing with four conventional superalloys Inconel 718, Waspaloy, Hastaloy X and Haynes 230, the studied RHEA shows lower density and superior high-temperature yield strength.

Orientation relationship between bcc precipitates and Ti5Si3 matrix in Mo–Si–B–Ti–C quinary alloys

In this study, the crystallographic orientation relationship between bcc precipitates and a Ti5Si3 matrix in Mo–50Ti–14Si–6C–6B (at.%) alloy was established. The alloy microstructure consisted of the bcc solid solution, Ti5Si3, Mo5SiB2, and TiC phases. The metal-rich Ti5Si3 phase was formed during a solidification stage, followed by the precipitation of the bcc phase within the Ti5Si3 matrix in the subsequent annealing process. The orientation relationship between the two phases was described by the expression: 112bcc//011‾0Ti5Si3 and 3‾11bcc//0001Ti5Si3. The formation of a low-energy interface between the two phases was explained by the edge-to-edge matching of their corresponding lattice planes at the faceted interface.

Thermodynamic reassessment of the Mo–Hf and Mo–Zr systems supported by first-principles calculations

Based on the experimental phase equilibria and thermodynamic data available in the literature and enthalpies of formation computed from first-principles calculations, the thermodynamic reassessment of the Mo–Hf and Mo–Zr systems was carried out by means of the CALPHAD (CALculation of PHAse Diagram) method. The enthalpies of formation for stable and metastable Laves (C15, C36, C14) phases and enthalpy of mixing for the β(bcc) solid solution phase in dilute solution were predicted via first-principles calculations to supply the necessary thermodynamic data for the modeling in order to obtain the thermodynamic parameters with physical sound meaning. The relative stability of Laves C14, C15 and C36 in the systems was discussed. The solution phases, i.e. liquid, β(bcc) and α(hcp) were described by the substitutional solution model, and all the Laves phases in the systems were described using two sublattice model. A set of self-consistent thermodynamic parameters were obtained for these binary systems, which agrees well with the experimental data in the literature. Based on these results, the trend of the site occupancy fraction of Laves phase changing with temperature was predicted. The isothermal section and the liquidus projection of Mo-Hf-Zr system were also predicted by combing with the Zr–Hf system reported in the literature.

Microstructural characteristics and properties of CoCrFeNiNbx high-entropy alloy coatings on pure titanium substrate by pulsed laser cladding

CoCrFeNiNbx (x = 0 or 1 at.%) high-entropy alloy (HEA) coatings with good metallurgical bonding were successfully prepared on surfaces of pure titanium sheets by pulsed laser cladding. Phase constitutions and microstructural characteristics of the HEA coatings were characterized and analyzed by combined use of X-ray diffraction, electron channeling contrast imaging, energy dispersive spectroscopy and electron backscatter diffraction techniques. Results show that the HEA coating without Nb is consisted of BCC solid-solution phase with equiaxed bulk grain morphology and Cr2Ti Laves phase (C14-type hexagonal structure) with fine interdendritic lamellar morphology. After adding 1 at.% Nb, in addition to the BCC solid-solution phase and the Cr2Ti Laves phase, Cr2Nb Laves phase with C15-type cubic structure also appears in the interdendritic region in the HEA coating. Hardness tests reveal that the CoCrFeNiNbx HEA coatings are significantly harder than the pure titanium substrate (~122 HV). The hardness value of the HEA coating with 1 at.% Nb reaches 1008 HV, which is ~ 8.3 times that of the substrate and considerably higher than that of bulk HEAs with similar compositions. Such high hardness can be attributed to the combined contribution from solid-solution hardening, and hardening from grain refinement and Laves phases with fine lamellar morphologies.

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 Properties of CeO2-Modified FeCoCrAlNiTi High-Entropy Alloy Coatings by Laser Surface Alloying

The FeCoCrAlNiTi high-entropy alloy (HEA) coatings modified by CeO2 were synthesized on 304 stainless steel substrate with Co-Cr-Al-Ni-Ti-xCeO2 powders by laser high-entropy alloying. The phase composition, microstructure and nano-mechanical and wear properties of FeCoCrAlNiTi-based HEAs were investigated by performing XRD, SEM, EBSD, nanoindentation testing and wear resistance tests. The results displayed that the FeCoCrAlNiTi-xCeO2 HEA coating only had simple FCC and BCC solid solution phases. The microstructure of the FeCoCrAlNiTi-xCeO2 exhibited typical dendrite and interdendritic structures, and the addition of CeO2 resulted in the refined microstructure of the alloyed coating. It was found that BCC phase structure predominantly distributed at the grain boundaries in the HEAs. For the 1 wt.% CeO2 adding HEA, the ratios of the hardness (H) and elastic modulus (E) were much higher than that of FeCoCrAlNiTi HEA coating, indicating that it had the satisfactory property for resistance to scoring and plastic deformation. The wear mechanisms were abrasive, oxidation and adhesive. Among of the three samples, FeCoCrAlNiTi-1 wt.% CeO2 HEA coating displayed the best wear resistance, and the specific wear rate and wear volume loss were 2.748 × 10–5 mm3/N m and 3.71 × 106 μm3, respectively.

Microstructure exploration and an artificial neural network approach for hardness prediction in AlCrFeMnNiWx High-Entropy Alloys

The phase evolution of AlCrFeMnNiWx (X= 0, 0.05, 0.1, 0.5) High-Entropy Alloys (HEAs) during the solidification is understood by both thermodynamic simulation and experimental approach. The detailed structural and microstructural characterization of studied HEAs reveals the presence of BCC Fe–Cr–Mn rich (β1) primary phase and BCC Ni–Al-rich (β2) secondary dendritic phase. It is found that both primary and secondary BCC solid solution phases undergo spinodal decomposition, forming BCC_B2 (α1) and σ phases as well as BCC_B2 (α2) and BCC (γ) phases respectively. Interestingly, the hardness of the HEAs varies in the range 461–552.7 HV with alloying of W. The present investigation also reports the prediction of the hardness of AlCrFeMnNiWx (x = 0, 0.05, 0.1, 0.5) HEAs with the composition variation of tungsten by applying artificial neural network using various experimental data as input parameters. A back-propagation artificial neural network (ANN) model is used by taking the experimental data to understand the effect of alloying elements on the hardness. The ANN modeling results match well with experimental data with the accuracy of prediction 93.54% and error of the predicted value of 6.46%.

Evaluation of microstructural and mechanical properties of AlxCrFeMnNi high entropy alloys

The aim of this work is to investigate microstructural and mechanical properties of the AlxFeNiCrMn HEAs fabricated by arc-melting method followed by annealing at 1000 °C in air. Microstructural features and phase formation of the developed alloy were examined using scanning electron microscope (SEM) and X-ray diffraction (XRD) respectively. The microhardness and compressive strength measurement were performed with a diamond base indenter and Instron compression machine. It was noticed that Al addition encouraged the formation of dendrites in the alloys with dendritic cores and interdendritic regions. The microhardness decreased from 495 HV to 397 HV with increasing Al concentration. Adjusting the Al content above 10 at% increased the ultimate strength from 1200 MPa to 1429 MPa. However, ductility was reduced which significantly lowered the total elongation by around 5%. The alloy with x = 15, displayed a good combination of high strength and ductility due to a reasonable balance in the volume fraction of BCC and FCC solid solution phases. The addition of Al is beneficial to the mechanical performance of the AlxFeNiCrMn alloy, but inadequate amounts will be counterproductive.

A body-centered cubic Zr50Ti35Nb15 medium-entropy alloy with unique properties

The center of phase diagram, relatively high-disordered region, often has few alloy systems available, especially for traditional ternary alloy systems. Here, we develop a novel Zr50Ti35Nb15 alloy near the center of phase diagram, as a body-centered cubic (bcc) solid solution phase, which displays a unique combination of Young's modulus of 62 GPa, yield strength of 657 MPa, tensile ductility of 21.9%, and excellent corrosion resistance (Ecorr=422 mV, Ipit=0.336 µA/cm2). The excellent mechanical properties of this alloy are related to dislocation slip and deformation twins. This result also proves that the disordered structure is valuable for traditional alloy systems.