Single Bcc Phase Research Articles

Magnetic properties and electronic structure of CeFe2−xMnx and CeFe2Mnx compounds

The effect of manganese alloying on the structure and magnetic properties of CeFe2 has been studied on two isostructural series: quasibinary CeFe2−xMnx and nonstoichiometric CeFe2Mnx alloys. The single-phase bcc MgCu2-type structure is formed at x≤0.5 in both systems. At x=0.5, the lattice parameter is increased by ∼0.3%. The Mn alloying leads to a nonmonotonic variation of magnetic moment and gradual decrease in the Curie temperature from 230 K to 150 K and 167 K for CeFe1.5Mn0.5 and CeFe2Mn0.5, respectively. For x≥0.3, the magnetization data indicate the formation of noncollinear magnetic structure. The binary CeFe2 and nonstoichiometric CeFe2Mn0.15 have been studied using Mössbauer effect and X-ray photoelectron spectroscopy. The Ce valence state remains unchanged upon the Mn alloying. The parameters of hyperfine interactions have been determined in paramagnetic state at room temperature and in magnetically ordered state at 78 K. The Mössbauer data revealed a difference in the quadrupole coupling constants of CeFe2 and CeFe2Mn0.15, which is associated with the difference in the local distortions of the lattice.

Effect of Ti content on microstructure and properties of TixZrVNb refractory high-entropy alloys

This study aimed to investigate the microstructure and mechanical properties of TixZrVNb (x = 1, 1.5, 2) refractory high-entropy alloys at room and elevated temperatures. The TiZrVNb alloy consisted of the body-centered cubic (bcc) matrix with a small amount of V2Zr phase. The Ti1.5ZrVNb and Ti2ZrVNb alloys exhibited a single-phase bcc structure. At room temperature, the tensile ductility of the as-cast alloys increased from 3.5% to 12.3% with the increase in the Ti content. The TixZrVNb alloys exhibited high yield strength at 600°C, and the ultimate yield strength was more than 900 MPa. Softening occurred at 800°C, but the ultimate yield strength could still exceed 200 MPa. Moreover, the TixZrVNb alloys displayed low densities but high specific yield strengths (SYSs). The lowest density of TixZrVNb alloys was only 6.12 g/cm3, but the SYS could reach about 180 MPa·cm3·g−1, which is better than those of most reported high-entropy alloys (HEAs).

Correlation between microstructure and soft magnetic parameters of Fe-Co-Ni-Al medium-entropy alloys with FCC phase and BCC phase

The magnetic property and microstructure of as-cast FeCoNiAlx (x = 0.2, 0.4, 0.6, 0.8, 1.0) MEAs prepared by copper mold suction were investigated. For x ≤ 0.6 alloys, the soft magnetic properties monotonously decrease with increasing Al content, while a turning point of improving magnetic property occurs at x = 0.8. XRD result shows that Al addition changes the phase constitution. The alloys (x ≤ 0.4) consist of single FCC phase. FeCoNiAl0.6 (x = 0.6) and FeCoNiAl0.8 (x = 0.8) alloys are composed of FCC phase and BCC phase. FeCoNiAl1.0 (x = 1.0) alloy consists of single BCC phase. Furthermore, more serious lattice distortion is present in FCC phase than that in BCC phase. It results in the deterioration of soft magnetic property by increasing the coercivity (Hc) and the hysteresis loss (Pu), and decreasing the initial permeability (μi) and the maximum permeability (μm). The grain-boundary character distribution of FeCoNiAl0.6 (x = 0.6) alloy and FeCoNiAl0.8 (x = 0.8) alloy was studied by electron backscatter diffraction (EBSD). Less strain concentration is observed in x = 0.8 alloy than that in x = 0.6 alloy because of containing much smaller number of low angle boundaries (LAB) with a misorientation angle between 2° and 5° (5.71%) than 36.6% in x = 0.6 alloy. It leads to an improvement of soft magnetic property by decreasing the Hc, the Pu, and increasing the μi and the μm of x = 0.8 alloy. Energy dispersive spectroscopy (EDS) result shows that, in both FeCoNiAl0.6 (x = 0.6) and FeCoNiAl0.8 (x = 0.8) alloys, the enrichment of Fe and Co elements and the depletion of Ni and Al elements are observed in FCC phase, while Fe and Co elements are depleted and Ni and Al elements are enriched in BCC phase. Moreover, a close value of ferromagnetic elements, (Fe + Co + Ni)%, is obtained in FCC phase (≈85%) and BCC phase (≈78%) in both FeCoNiAl0.6 (x = 0.6) and FeCoNiAl0.8 (x = 0.8) alloys. Combined with results of EDS and magnetic properties of x = 0.6 and x = 0.8 alloys, it can conclude that the saturation magnetization (Ms) of BCC phase is larger than that of FCC phase.

Influence of processing route on the alloying behavior, microstructural evolution and thermal stability of CrMoNbTiW refractory high-entropy alloy

Abstract

Preparation of MoNbTaW refractory high entropy alloy powders by pressureless spark plasma sintering: Crystal structure and phase evolution

This paper presents a novel method for preparing MoNbTaW refractory high entropy alloy (RHEA) powders. A porous MoNbTaW RHEA was firstly prepared by pressureless spark plasma sintering from pure elemental powders of Mo, Nb, Ta, and W, and then it was ball milled to achieve the RHEA powders. The sintering behavior, crystal structure, and phase evolution of the RHEA were studied by SEM, XRD, and TEM. The MoNbTaW RHEA with a single BCC phase was obtained at a sintering temperature far below melting points of the elements. The formation of MoNbTaW RHEA was a dynamic process through which a continuous solid solution formed due to atomic diffusion. The elemental atoms migrated and dissolved in a sequence of Mo→Nb→Ta→W. At the same temperature, the element with low cohesive energy diffused and dissolved prior to those with high cohesive energy. A TFDC model was also proposed to calculate the lattice constants of MoNbTaW RHAEs. In comparison, the lattice constants of MoNbTaW RHEAs calculated by the TFDC model were more accurate than those were calculated by Vegard's law. The results provide an efficient method for the preparation of RHEA powders and an accurate method to calculate lattice constants of HEAs.

Hydrogen storage properties of the refractory Ti–V–Zr–Nb–Ta multi-principal element alloy

Abstract A new quinary multi-principal element alloy Ti0.30V0.25Zr0.10Nb0.25Ta0.10 was prepared by high temperature melting technique and the physicochemical as well as hydrogen sorption properties have been determined. The as-cast alloy crystallizes into a single-phase bcc lattice and can very quickly absorb hydrogen at room temperature forming a fcc hydride phase with capacity of 2 H/M (2.5 wt%). The absorption/desorption of hydrogen is reversible and occurs within one step, as proven by in-situ neutron diffraction for the desorption reaction. The capacity is slightly fading during the first 10 cycles and then stabilizes at around 2.2 wt% for the next 10 cycles.

Effect of Ti Addition on the Microstructure and High-Temperature Oxidation Property of AlCoCrFeNi High-Entropy Alloy

This study investigated the effect of Ti addition on microstructure and high-temperature oxidation property of AlCoCrFeNi high-entropy alloy. Ti content was controlled at 0 at% and 1 at%. The two alloys were found to have BCC single phase, and the average grain sizes of Ti0.0 and Ti1.0 were 47.3 µm and 49.7 µm, respectively, showing similarity. The EDS mapping of the inside of grains found that both alloys were characterized to be divided into Al-Ni element rich region and Cr–Fe element rich region. As a result of high-temperature oxidation test at 1100 °C, oxidation weight gains were measured at Ti0.0: 0.75 mg/cm2 and Ti1.0: 0.17 mg/cm2, respectively. The results indicate that Ti addition largely improved high temperature oxidation resistance of AlCoCrFeNi HEA. Ti1.0 alloy, in particular, showed remarkably more excellent 1100 °C high-temperature oxidation resistance than other previously reported major ones such as NiCrAl and FeCrAl. In the surface and cross-section observations after oxidation tests, both alloys were found to have Al2O3 oxides mostly. While the Ti0.0 material was observed to have Al2O3 spallation macroscopically, the Ti1.0 alloy showed Al2O3 spallation only in some local areas. In addition, a unique result was found in AlCoCrFeNiTix(x=0,1) alloy that BCC → FCC phase transformation was accelerated, and FCC phase layer was formed in the surficial area where Al element had been depleted due to high-temperature oxidation. Moreover, as Ti was added, the thickness of FCC layer induced by high-temperature oxidation decreased. Based on the results, it was also discussed on how to improve the high-temperature oxidation resistance of AlCoCrFeNiTix HEA.

Phase evolution and mechanical properties of non-equiatomic Fe–Mn–Ni–Cr–Al–Si–C high entropy steel

Non-equiatomic FeMnNiCrAlSi–C high entropy steel was synthesized by mechanical alloying (MA) followed by spark plasma sintering (SPS). The effect of MA with milling time on phase formation and microstructural evolution was analyzed by X-Ray diffraction (XRD) and scanning electron microscopy (SEM). The dissolution of the elemental peaks with milling time and formation of the solid solution phase was followed systematically. The formation of a metastable BCC phase (a = 2.91 ± 0.02 Å) was observed after 40 h of milling. The average particle size of the 40 h milled powder was varying in the range of ∼ 8–10 μm. SPS of the 40 h milled powder at 1000 °C showed the phase transformation. The single-phase BCC structure was transformed into the FCC (austenite type) (cF4) (a = 3.58 ± 0.01 Å), B2 (cP2) (a = 2.87 ± 0.02 Å), Cr3Si (cP8) (a = 4.54 ± 0.04 Å) and Cr23C6 (cF116) (a = 10.62 ± 0.03 Å) phases. The spark plasma sintered sample had shown the density of 6.8 g/cm3. Hardness and wear properties of non-equiatomic FeMnNiCrAlSi–C high entropy alloy was determined by Vickers hardness and pin-on-disk system respectively. The alloy had shown the hardness of 596 HV with wear loss (rate) of 1.7 × 10−8 mm3/m. This high entropy alloy steel requires more investigation to classify them as a wear-resistant material.

Linear patterning of high entropy alloy by additive manufacturing

An equimolar AlCoCrFeNi high-entropy alloy (HEA) was synthesized using selective laser melting (SLM). XRD patterns show peaks of a single-phase BCC structure typical for a HEA. The process parameters were optimized carefully in such a way that a layered microstructure with gaps of controllable size (average size ~67 ± 1 µm) is obtained. The size of the gaps can be controlled using the process parameters and the hatch distance. SLM may be the only process that can process such unique microstructures (linear patterns), where their unique properties may be used as filters in extreme environments like corrosion, temperature, nuclear, etc.

Phase Evolution and Thermal Stability of Mechanically Alloyed AlCrFeCoNiZn High-Entropy Alloy

In the present investigation, a newly designed composition of equiatomic AlCrFeCoNiZn high-entropy alloy (HEA) has been synthesized by mechanical alloying. The milled powder after 30 h exhibited the formation of a single solid solution phase of BCC crystal structure with lattice parameter, a = 2.87 ± 0.02 A. Decomposition of the single-phase BCC structure into the two-phase, tetragonal (Cr–Co)-based σ phase (a = 8.81 A, c = 4.56 A)- and L12 (a = 3.59 ± 0.02 A)-type intermetallics was observed at temperature of ~ 800 °C (1073 K). However, after heat treatment of the 30 h milled powder at the temperatures of 300 °C (573 K) and 600 °C (873 K), similar type of phases was also noticed to coexist along with B2 (a = 2.87 ± 0.03 A)-type phase. This behaviour of the alloy confirms the diffusive nature of the phase transformation. The consolidated bulk alloy exhibited similar type of phases after sintering at 950 °C (1223 K).

Ab Initio Calculations on Elastic Properties of IF Steel Matrix Phase at High Temperature Based on Lattice Expansion Theory

Elucidating the evolution law of the elastic properties of the matrix phase is of great significance for the control of steel properties and quality during continuous casting and subsequent heat treatment. In this paper, thermal expansion experiments and ab initio calculations are used to study the elastic properties of the interstitial free (IF) steel matrix phase in different magnetic states and crystal structures. The results show that the bulk modulus B and the tetragonal shear elastic constant C’ for the entire temperature range decrease with increasing temperature, but C44 is the opposite. While from paramagnetic (PM) to ferromagnetic (FM) state, C’(C44) have changed ~188% (~27%), B increases by ~55% during the crystal structure change (fcc→bcc). With the FM to PM state, the Zener anisotropy parameter increases sharply, and Young’s modulus decreases significantly in the [001] direction; the maximum difference is ~76 GPa. The evolution rate of average Young’s modulus in single bcc-phase FM (fcc-phase PM) range reaches ~5.5(~5.6) × 10−2 GPa K−1. The research provides an effective method for ab initio calculation of the elastic properties of interstitial free and ultra-low carbon steels at high temperature, also furnishing a basis for the application of ab initio calculations to the high temperature performance of steel materials.

Radiation damage tolerance of a novel metastable refractory high entropy alloy V2.5Cr1.2WMoCo0.04

A novel multicomponent alloy, V2.5Cr1.2WMoCo0.04, produced from elements expected to favour a BCC crystal structure, and to be suitable for high temperature environments, was fabricated by arc melting and found to exhibit a multiphase dendritic microstructure with W-rich dendrites and V–Cr segregated to the inter-dendritic cores. The as-cast alloy displayed an apparent single-phase XRD pattern. Following heat treatment at 1187 °C for 500 h the alloy transformed into three different distinct phases - BCC, orthorhombic, and tetragonal in crystal structure. This attests to the BCC crystal structure observed in the as-cast state being metastable. The radiation damage response was investigated through room temperature 5 MeV Au+ ion irradiation studies. Metastable as-cast V2.5Cr1.2WMoCo0.04 shows good resistance to radiation induced damage up to 40 displacements per atom (dpa). 96 wt% of the as-cast single-phase BCC crystal structure remained intact, as exhibited by grazing incidence X-ray diffraction (GI-XRD) patterns, whilst the remainder of the alloy transformed into an additional BCC crystal structure with a similar lattice parameter. The exceptional phase stability seen here is attributed to a combination of self-healing processes and the BCC structure, rather than a high configurational entropy, as has been suggested for some of these multicomponent “High Entropy Alloy” types. The importance of the stability of metastable high entropy alloy phases for behaviour under irradiation is for the first time highlighted and the findings thus challenge the current understanding of phase stability after irradiation of systems like the HEAs.

Development of a novel light weight Al35Cr14Mg6Ti35V10 high entropy alloy using mechanical alloying and spark plasma sintering

A new light-weight high entropy alloy (LWHEA) composition Al35Cr14Mg6Ti35V10 is designed using the thermodynamic parameters of stability with a density of 4.05 g cm−3. The alloy was prepared by mechanical alloying (MA) using a high-energy planetary ball milling and compacted by using spark plasma sintering (SPS). X-ray diffraction (XRD), scanning electron microscopy (SEM) with energy dispersive spectroscopy (EDS), and hardness testing were used for analysis of the ball-milled powders and bulk sample. After 30h of milling a single-phase BCC was obtained with some minor impurities of WC. After SPS, the final microstructure was having one HCP and two BCC phases with minor contamination of WC embedded into it. The hardness of a light-weight Al35Cr14Mg6Ti35V10 alloy was found to be 460 HV which is superior to other Al, Mg, and Ti-based conventional alloys.

Gum-like mechanical behavior of a partially ordered Al5Nb24Ti40V5Zr26 high entropy alloy

Microstructure and mechanical behavior of a new bcc Al5Nb24Ti40V5Zr26 high-entropy alloy in the as-cast, cold-rolled and recrystallized conditions were studied. In the as-cast condition, the Al5Nb24Ti40V5Zr26 alloy had a single bcc phase coarse-grained (~80 μm) structure. After cold rolling to a total thickness strain 80% the alloy demonstrated typical of deformed metals tensile behavior with a short hardening stage and early necking. Annealing at 800 °C resulted in recrystallization of the matrix and formation of a small fraction of C14 type Laves phase particles. After annealing at 1100 °C, a partial B2 ordering of the matrix phase was found. In the annealed condition, the alloy showed some typical of gum metals features associated with low work hardening and large elongation. The manifestation of a gum-like behavior can be ascribed to the formation of dislocation channels due to the local disordering of the bcc matrix in shear planes. In addition, an unusual (inverse) dependence of yield strength on the recrystallized grain size was observed, most possibly due to a different degree of ordering of the matrix phase.

Microstructure evolution of a novel low-density Ti–Cr–Nb–V refractory high entropy alloy during cold rolling and subsequent annealing

Refractory high entropy alloys represent a new class of metallic alloys attractive for high-temperature applications. However, most of the developed alloys have either low ductility at room temperature or high density. In this work, we report structure and mechanical properties of a novel non-equiatomic Ti1.89CrNbV0.56 alloy produced by vacuum arc melting. The density of the alloy was 6.17 g/cm3. In the as-cast condition, the alloy had a single-phase bcc structure enabling room temperature deformation in compression to ε>50% or cold-rolling to a thickness strain of 80%. Rolling resulted in the formation of a dislocation substructure and development of kink and shear bands. Meanwhile, microhardness measurements have revealed only a moderate increase from 396 HV in the as-cast condition to 454–469 HV after 40–80% rolling. After 80% cold rolling the alloy had yield strength of 1020 MPa, ultimate tensile strength of 1535 MPa, and elongation to fracture of 3.5%. The cold rolled alloy was annealed at 800, 1000 or 1200 °C for 1–100 h. Microstructural response to the annealing strongly depended on temperature. Annealing at 800 °C mostly resulted in Cr-rich fcc (C15) Laves phase particles precipitation. Annealing at 1000 °C led to the bcc matrix recrystallization along with the precipitation of the Laves phase particles, thereby producing a fine duplex microstructure. Finally, annealing at 1200 °C resulted in a coarse-grained recrystallized single-phase bcc microstructure. Microhardness of the alloy lowered with an increase in the annealing temperature while the annealing time had a small effect on hardness.

Precipitation behavior and mechanical properties of a hot-worked TiNbTa0.5ZrAl0.5 refractory high entropy alloy

A TiNbTa0.5ZrAl0.5 refractory high entropy alloy was prepared by powder metallurgy and subsequently subjected to hot forging. The precipitation behavior and its effects on compression properties were studied. Results show that the as-sintered TiNbTa0.5ZrAl0.5 alloy has a single BCC phase. The primary BCC phase is thermodynamically instable, and decomposes into BCC (Ti, Nb, Ta)-rich matrix and HCP (Zr, Al)-rich precipitates after hot forging. The precipitates are much harder than the matrix, and have significant strengthening effect. The hot-worked TiNbTa0.5ZrAl0.5 alloy has a high compressive yield strength of 1740 MPa with a considerable compressive ductility of 12% at room temperature. The precipitation strengthening contributes about 51% while the grain-boundary strengthening contributes 35% to the increment of the yield strength.

Investigation on microstructure and properties of AlxCoCrFeMnNi high entropy alloys by ultrasonic impact treatment

In the present study, ultrasonic impact treatment was applied to promote the as-cast AlxCoCrFeMnNi (x = 0, 0.5, 1.0 and 1.5) high entropy alloys, and the microstructures, phase composition and surface properties were investigated. The increase in Al mole fraction from 0 to 0.5, 1.0 and 1.5 led to the different phase compositions from single FCC, duplex FCC + BCC to single BCC phase as well as the increased microhardness and reduced corrosion resistance. Ultrasonic impact treatment did not change the phase compositions of AlxCoCrFeMnNi alloys, while the successive network structures were broken and the short rod-like structures formed rather than long strip-like structures in the impacted hardening layer. The microhardness of AlxCoCrFeMnNi alloys (x = 0, 0.5 and 1.0) were improved by 92% (approximately 308.5 HV, 312.5 HV and 450.8 HV, respectively) after ultrasonic impact treatment due to the refine microstructures and precipitation strengthening mechanisms. Owing to the application of ultrasonic impact treatment the surface roughness of AlxCoCrFeMnNi alloys (Ra = 0.54–0.57 μm) decreased. Additionally, the impacted hardening AlxCoCrFeMnNi alloys exhibited higher corrosion resistance than the as-cast alloys resulted from the lower surface roughness.

Synthesis, characterisation and erosion behaviour of AlCoCrMoNi high entropy alloy coating

AlCoCrMoNi equiatomic high entropy alloy (HEA) powders were prepared by mechanical alloying. High energy planetary ball-mill was employed to mill the powders till single solid solution formation. X-Ray Diffraction (XRD) analysis of as prepared powder was done at 5, 10, 15 and 20 hours of milling. A single BCC phase was formed after 20 h. Field emission scanning electron microscopy (FESEM) was used to study the microstructure of the prepared powder. After 20 hours of milling, a homogenous mixture formed, which was further confirmed by EDAX mapping of as prepared powders. The alloy was coated onto a 316 steel substrate using High-velocity Oxy-fuel (HVOF) coating. Cross-section of the coating was analysed using scanning electron microscope (SEM). The slurry erosion resistance exhibited by the coating was tested using Water-jet Erosion tester with 20 m s−1, 25 m s−1 and 30 m s−1 jet velocity at 30°, 60° and 90° angular impingement. The coating exhibited high ductility along with superior erosion resistance compared to conventional wear resistant coatings.

Microstructure transition and mechanical property variation of Al0.6CoCrFeNi high-entropy alloys prepared under different cooling rates

Cooling rate is attracting considerable attention because of its significant effect on the phase stability, switchable properties, and elements’ diffusion mechanism in alloys, especially in high-entropy alloys (HEAs) with metastable structure. In this study, the microstructure transition and mechanical property variation of Al0.6CoCrFeNi HEAs with a change in the cooling rate were investigated in detail. Five cooling rates of ∼2.5 K s−1 (I0.6 alloy), 40 K s−1 (C0.6–10 alloy), 2.5 × 102 K s−1 (C0.6–4 alloy), 103 K s−1 (C0.6–2 alloy) and ∼105 K s−1 (R0.6 alloy) were obtained by adjusting the prepared methods and alloy sizes. The structure of Al0.6CoCrFeNi HEAs changed from an FCC + BCC dual-phase (I0.6 alloy) to a single BCC phase (R0.6 alloy) with an increase in the cooling rate, and the microstructure of alloys evolved from a columnar dendrite (I0.6 alloy) to a lamellar morphology (C0.6–2 alloy), and finally to an equiaxed grain microstructure (R0.6 alloy). Both the hardness and yield strength of alloys exhibited first a minor and then a considerable increase with an increase in the cooling rate, which is mainly ascribed to microstructure and/or phase structure transitions. The R0.6 alloy displayed the highest hardness (501 HV) and yield strength (1530 MPa) in five alloys and had the least plasticity. Our findings may help to explore HEAs with excellent mechanical properties and provide support for HEAs’ industrialization.

Tribo-corrosion behavior of VAlTiCrCu high-entropy alloy film

In recent years, high-entropy alloy (HEA) films, with excellent comprehensive performance due to their unique structure, have received wide attention from scientists and engineers all over the world. In this work, the VAlTiCrCu HEA films with different deposition temperatures (100 °C–400 °C) were synthesized successfully on 304 stainless steel using a facile splicing target technology by magnetron sputtering. The microstructure, mechanical properties and tribo-corrosion behavior were studied in detail. It was found that the VAlTiCrCu HEA films with different deposition temperatures were all composed of a single BCC phase, which was in good agreement with the thermodynamic calculation. The VAlTiCrCu HEA films deposited at 300 °C have the best mechanical properties and corrosion-resistance. In the tribo-corrosion test, the HEA films behaved differently in NaCl, H2SO4 and NaOH solutions. In NaCl solution, the HEA films were seriously corroded due to the strong pitting corrosion of Cl− ions despite the formation of tribo-corrosion products, while in NaOH solution, the HEA films suffered the most serious wear due to the oxidation film and corrosion products were washed away. The lowest friction coefficient and wear area were found in H2SO4, showing the best tribo-corrosion resistance, which was attributed to the hindering effect of SO42− on pitting nucleation and the protection provided by the formation of tribo-corrosion products.