Formation Of Bcc Phase Research Articles

Microstructural evolution and mechanical properties of FeNiVAlx medium-entropy alloy

FeNiV alloys exhibit potential applications in magnetic, radiation-resistant, mechanical, and superconducting fields, however, the formation of the hard yet brittle sigma phase enriched with Fe and V hindered their application in extreme environments. Therefore, improving the mechanical properties of FeNiV alloys has been a crucial problem. In this study, a series of as-cast FeNiVAlx (values of x is 0, 0.1, 0.2, 0.3) medium-entropy alloys (MEAs) were fabricated by vacuum arc melting, and the evolution of microstructure and mechanical properties with different Al microalloying were investigated systematically. As the Al content increased, FeNiVAlx MEAs underwent a structural transformation from a dual-phase structure consisting of a FCC phase (46.4 %; phase content) and a sigma phase (53.6 %) at x=0 to a single BCC phase in FeNiVAl0.3, while the mechanical properties of the alloy show a decrease in strength and a significant increase in elongation. Notably, the FeNiV alloy exhibits the highest yield strength of 1851 MPa with 7.8 % ductility. In contrast, the FeNiVAl0.2 alloy demonstrated superior comprehensive compressive properties, with a high yield strength of 1255 MPa and an exceptional ductility exceeding 50 % without fracturing. The elimination of the harmful sigma phase, coupled with the formation of BCC phase, contributes to the observed decrement in strength and increment in ductility. This study elucidates the intrinsic relationship between microstructure and mechanical properties with Al contents in FeNiVAlx alloys, providing insights for the design of MEAs with excellent strength-ductility balance through microalloying.

Effect of Milling Time on the Structure Stability of FeMnNiCrAl Non-equiatomic High-Entropy Alloy

A non-equiatomic Fe34.9Ni30.2Mn18.6Cr9.3Al7 high-entropy alloy (HEA) was synthesized by mechanical alloying using different milling times. To study the effect of milling time on the structure stability, X-ray diffraction (XRD), scanning electron microscopy and energy-dispersive X-ray spectroscopy were conducted. For comparison, an Al-free alloy (Fe37.5Ni32.5Mn20Cr10) was produced at 25 h milling. XRD indicated a single FCC phase alloy after 5 h milling time and a dual FCC and BCC phase at 25 h milling time. Clearly, it is found that Al addition is not necessarily the main factor leading to BCC phase formation as reported for similar HEAs produced by casting route. Increasing the milling time, the lattice strain increased reaching an average maximum value of 0.8% with an increase in d-spacing while the crystallite size was reduced to about 5.7 nm. A dual-phase structure formation was related to a decrease in the accumulated strain (ca.32%) confirming a strain-induced transformation.

Microstructural evolution and phase selection during solidification of AlCrCuFeNi high entropy alloy

An equiatomic AlCrCuFeNi high entropy alloy (HEA) was synthesized by arc melting. The as-cast material exhibits the formation of the two-phase structure consisting of BCC and FCC phases. Different heat-treatment schedules were adopted in this investigation for the systematic understanding of the elemental distribution, microstructural evolution and solidification behavior of the alloy. Light microscope and scanning electron microscope (SEM) with EDS analysis was extensively used to characterize the alloy. The results indicate the as-cast material has the primary phase of dendritic structure. The heat-treated sample at 1300 °C, followed by water quenching as well as furnace cooling, primarily showed the Fe-Cr rich BCC phase formation in the dendritic region. The interdendritic space is enriched in Al-Ni rich B2 phase and the Cu rich phase. These outcomes have proximity to the equilibrium phase predictions performed based on the CALPHAD approach. The element Cu plays a crucial role in affecting the solidification behavior of the alloy in the temperature range of 900–1100 °C. This work has shown why understanding the microstructure evolution and solidification behavior is more important in HEAs for their potential future applications.

Effect of B-Si ratio and laser remelting on the microstructure and properties of Fe10Co10Ni10Cr4Mo6BxSi10-x coating by laser cladding

High-entropy alloy coating has huge potential application values in improving the service life of the friction parts. The effects of the B-Si ratio and laser remelting on the microstructure, phases, mechanical and wear properties of the laser cladding Fe10Co10Ni10Cr4Mo6BxSi10-x coatings were investigated. When x equals 10, the coating crystalizes in a FCC structure together with lots of MxB phases. Si element adding promotes the formation of BCC phase of Fe10Co10Ni10Cr4Mo6BxSi10-x coatings with the appearance of new MxSi and MxBSiy phases, but prevents the formation of MxB and MxBSiy with the decrease of B-Si ratio. When x drops to zero, the microstructure of the coating transforms into a BCC with tiny amounts of FCC and MxSi phase. With the increase of Si element content, Mx(B, Si) standing for any combination of MxB, MxSi and MxBSiy phases decrease. The lowest friction coefficient (0.3786) and wear volume loss (2.46 × 106 μm3) show that Fe10Co10Ni10Cr4Mo6B5Si5 coating has the best wear resistance because of a higher toughness and values of H/E and H3/E2. Laser remelting once reduces the component segregation of the coating, and promotes the formation of BCC and Mx(B, Si) phases. The wear resistance of the coating increases with the improvement of its wear characteristics (H/E, H3/E2 and η). However, when the coating is remelted twice, the wear resistance decreases due to the drop of the hardness and wear characteristics attributed to the dominant position of FCC structure. Fine and uniform microstructure, and proper proportion of hard phases (BCC and Mx(B, Si)) and soft phase (FCC) can increase the mechanical properties and wear resistance of Fe10Co10Ni10Cr4Mo6BxSi10-x coatings.

Microstructure and mechanical properties of CoCrNiCuX high-entropy alloys fabricated by spark plasma sintering

Abstract CoCrNiCuX (X = Mn, MnFe, FeAl) high-entropy alloys were prepared by mechanical alloying and spark plasma sintering. The relationship between microstructural and mechanical properties of CoCrNiCuX powders and bulks was investigated. The effects of the addition and substitution of elements on crystal structure transition, microstructural evolution, and mechanical properties were assessed. The results indicated that, with the addition of Fe, the crystal structure of the alloys was transformed from fcc phase to a mix of fcc and bcc phases. By substituting Mn with Fe and Al, the formation of bcc phase was promoted, which caused precipitation hardening that significantly improved the hardness and compression strength of the investigated high-entropy alloys.

Fabrication of FexCoCrAlNi medium and high entropy layers on a carbon steel through TIG cladding

FexCoCrAlNi medium and high entropy layers were fabricated on a carbon steel using multilayer TIG cladding. The Fe content of the deposited layers decreased by moving away from the substrate due to the dilution effect. FexCoCrAlNi (with x>1) medium entropy alloys (MEAs) with a major Fe-rich BCC and minor Fe–Al–Ni-rich FCC phases developed in the inner parts of the fabricated layers. No intermetallic compounds appeared in MEAs. Microstructural assessments revealed that a FeCoCrAlNi high entropy alloy (HEA) containing Al–Ni-rich BCC and Fe–Cr-rich FCC crystal structures was achieved in the upper part of the deposited layers. It was found that Fe–Cr and Al–Ni pairs had contradictory effects on stabilizing crystal structure in MEAs and HEA. The Al–Ni pair was FCC-phase stabilizer in MEA while it was responsible for the formation of BCC phase in HEA. The microhardness of the deposited layers increased successively to reach 430 HV in the HEA due to the solid solution strengthening and formation of BCC phase with high-entropy characteristics.

Corrosion behaviors related to the microstructural evolutions of as-cast Al0.3CoCrFeNi high entropy alloy with addition of Si and Ti elements

In this work, the electrochemical potentiodynamic polarization and impedance spectroscopy analysis were used to investigate the corrosion behaviors related to microstructural evolutions of Al0.3CoCrFeNiSix and Al0.3CoCrFeNiTix HEAs in the NaCl solution. The results presented that addition of Si promoted the formation of BCC phase, which was regarded as a micro-anode to prevent FCC phase as a micro-cathode from being corroded. While the volume fraction of BCC phase increased to enhance the corrosion resistance of Six alloys with increasing Si content. Furthermore, the reduction in the grain sizes of Six alloys with increasing Si content could also improve corrosion resistance of the alloys. Therefore, the corrosion resistance of Six alloys was increased with increasing Si content. Addition of Ti with a high corrosion resistance of Clˉ caused the reduction in the grain sizes of Tix alloys, which can improve the corrosion resistance of alloys. However, the high Ti content can destroy the corrosion resistance of the alloys due to the formation of a large amount of precipitated phases. Therefore, as Ti with atomic percentage be equal to that of Al can effectively eliminate the negative influence of precipitated phases and microstructure on the corrosion resistance of alloys, meaning the Ti0.3 alloy had the best corrosion resistance in Tix alloys. The analysis of X-ray photo-electron spectroscopy (XPS) identified that the contents of oxidized Cr3+ and Fe3+ in the passive film are the main contribution to improve the corrosion resistance of Six alloy. And the contents of oxidized Cr3+ and Ti4+ in the passive film are the main contribution to improve the corrosion resistance of Tix alloy.

(AlxTi1−x)-(FeCoNi)12](AlxTi1−x)0.5Cr2.5 High-Entropy Alloy Coating by Laser Cladding

To prolong the service life of the stirrer impeller made by SUS 904L austenitic super-stainless steel, a series of [(AlxTi1−x)-(FeCoNi)12](AlxTi1−x)0.5Cr2.5 high-entropy alloy (HEA) compositions were designed based on the cluster-plus-glue-atom model. The HEAs’ coatings were successfully fabricated by laser cladding technology. The microstructure, microhardness, wear resistance and corrosion resistance were measured by a scanning electron microscope, transmission electron microscope, microhardness tester, wear machine and electrochemical workstation, respectively. The experimental results indicate that the phase structures of the [(AlxTi1−x)-(FeCoNi)12](AlxTi1−x)0.5Cr2.5 (x = 0, 0.5, 1) HEA coatings mainly consist of a single face-centered-cubic solid solution and the coatings produce BCC phase with the increase of Ti content. When x = 0, the coating has the highest hardness (402.3 HV0.2) which is 1.92 times that of 904L austenitic super-stainless steel (209.0 HV0.2), the lowest wear volume (0.866 mm3) and the best corrosion resistance. The addition of Ti refined the microstructure of the coatings and promoted the formation of BCC phase, which improved the hardness and wear resistance of the coatings. Considering the wide sources of Ti, Fe and Co elements and the convenience of laser cladding, the coating can provide a cheap protective layer for 904L stainless steel.

Enhancing bending fatigue resistance of the CoCrFeMnNi high-entropy alloy thin foils by Al addition

Fatigue behaviors of the CoCrFeMnNi-based high-entropy alloy thin foils with different Al addition were investigated under dynamical bending fatigue loading. The results reveal that the Al addition can effectively decrease the cyclic strain localization and improve the fatigue resistance, which is associated with the reduced cyclic slip irreversibility. The physical reasons for such reduction can be attributed to the enhanced planar-slip ability induced by Al addition. Furthermore, the formation of BCC phases caused by 10 at.% Al addition also leads to a delay in the fatigue crack propagation.

Microstructure evolution and properties of NiTiCrNbTax refractory high-entropy alloy coatings with variable Ta content

In this work, the novel NiTiCrNbTax (x = 0.1, 0.3, 0.5, 1) refractory high-entropy alloy (RHEA) coatings were successfully synthesized on Ti6Al4V by laser cladding technology. The effect of Ta content on microstructure, thermal stability, micro-hardness and corrosion resistance of RHEA coatings was investigated in detail. The results showed that NiTiCrNbTax RHEA coatings were composed of BCC, NiTi2-Laves and Cr2Nb-Laves phases. With Ta content increasing, the formation of BCC phase was promoted while the precipitate of Cr2Nb-Laves phase was affected. The average micro-hardness values of Ta0.1, Ta0.3, Ta0.5 and Ta1.0 RHEA coatings were calculated as ~922.8 HV0.3, ~985.1 HV0.3, ~851.3 HV0.3 and ~825.4 HV0.3, respectively. The decrease of micro-hardness value might be ascribed to the reduction of hard Laves phase. After annealing at 900 °C for 3 h, the phase compositions of all RHEA coatings remained unchanged but the grains of Cr2Nb-Laves phase were refined obviously. Besides, RHEA coatings exhibited excellent corrosion resistance and the minimum corrosion current density was 1.08 × 10−7 A/cm2. Moreover, the secondary passivation phenomenon emerged in Ta0.5 and Ta1.0 coatings, and the passive film mainly consisted of the oxidation state of Ti, Cr, Nb, Ta and the sub-oxide state of Nb, Ta.

Microstructure and mechanical properties of W-Ta-V-Cr high-entropy alloy coatings by double glow plasma surface alloying

A W-Ta-V-Cr high entropy alloy (HEA) coating has been successfully synthesized on the tungsten-based surface by Double glow plasma surface alloying (DGPSA) technology. The effects of W-Ta-V-Cr on the microstructure and properties of the coating were systematically investigated. The results illustrated that a compact and homogeneous alloy coating with a thickness of 8.8 μm was formed on the surfaces of pure tungsten. The addition of four elements promoted the formation of BCC phase in the coating, refined the grain size and formed nanocrystalline materials. Thereby the microhardness and abrasion rate of the coating increased and decreased, respectively. The hardness value of the coating reaches 1130 HV, which is 2 times that of the substrate (∼551 HV) and the specific abrasion rate of the coating was only 1/8.5 of that of the W substrate, showing extremely excellent abrasion resistance performance. The coating effectively improves the mechanical of pure tungsten. Such high hardness and abrasion resistant can be attributed to the combined contribution from solid-solution hardening and grain refinement.

Microstructure and hydrogen storage properties of Ti–V–Cr based BCC-type high entropy alloys

In this work, the crystal structure and hydrogen storage properties of V35Ti30Cr25Fe10, V35Ti30Cr25Mn10, V30Ti30Cr25Fe10Nb5 and V35Ti30Cr25Fe5Mn5 BCC-type high entropy alloys have been investigated. It was found that high entropy promotes the formation of BCC phase while large atomic difference (δ) has the opposite effect. Among the four alloys, the V35Ti30Cr25Mn10 alloy shows the highest hydrogen absorption capacity while the V35Ti30Cr26Fe5Mn5 alloy exhibits the highest reversible capacity. The cause of the loss of desorption capacity is mainly due to the high stability of the hydrides. The higher room-temperature desorption capacity of the V35Ti30Cr25Fe5Mn5 alloy is due to higher hydrogen desorption pressure. After pumping at 400 °C, the hydrides can return to the original BCC structure with only a small expansion in the cell volume.

Effect of Si element on phase transformation and mechanical properties for FeCoCrNiSix high entropy alloys

In present research, the phase evolution and mechanical properties of FeCoCrNiSix high-entropy alloys were investigated by using different testing techniques. The results showed that Si element can promote the formation of BCC phase and had a significant effect on the micro-hardness and wear resistance of alloys. The micro-hardness of HEAs increased from 89.52 HV to 653.71 HV and the depth of the wear track decreased from 22.139 μm to 5.292 μm with the addition of Si element. When x was equal to 1 (FeCoCrNiSix), the alloy possessed the highest micro-hardness and best wear resistance. Moreover, the wear mechanism was transformed into abrasive wear according to the wear tracks measurements due to the addition of Si elements.

Microstructural Evolution and Wear Behavior of AlCoCrCuFeNi High Entropy Alloy on Ti–6Al–4V Through Laser Surface Alloying

AlCoCrCuFeNi high entropy alloy particles were laser surface alloyed on Ti–6Al–4V substrate to improve the tribological properties. The microstructure, phase formation and hardness improvement of the laser alloyed surface were examined. The wear resistance of the laser alloyed specimen were evaluated through pin-on-disc apparatus and compared with substrate specimen. The wear mechanism of the worn-out surface and roughness were studied. The laser alloyed specimen exhibits dual solid solution along with the BCC phase. The alloyed region shows dendrite and interdendrite structure with equiaxed grain formation. The hardness of laser alloyed region is 3 times higher than the substrate material due to dominant BCC phase formation. The laser alloyed specimen shows higher wear resistance compared to substrate due to solid solution strengthening and intermetallic formation. The wear resistance of the laser alloyed specimen was 2.62 times than the substrate at 50 N load and 0.9 m/s sliding velocity. Abrasive, adhesive wear and severe plastic deformation were observed in the substrate specimen, whereas in the laser alloyed specimen mild abrasive wear was observed. The laser alloyed specimen has 0.56 times the coefficient of friction of the Ti–6Al–4V substrate at 50 N load and 0.9 m/s sliding velocity due to self-lubrication property of HEA elements. Surface roughness of worn-out laser alloyed specimen was 0.44 times that of the Ti–6Al–4V substrate.

Irradiation-induced BCC-phase formation and magnetism in a 316 austenitic stainless steel

Abstract Specimens of austenitic stainless steel were irradiated with 6 MeV Xe ions to two doses of 7 and 15 dpa at room temperature and 300 °C respectively. Then partial irradiated specimens were subsequently thermally annealed at 550 °C. Irradiation-induced BCC-phase formation and magnetism were analyzed by grazing incidence X-ray diffraction (GIXRD) and vibrating sample magnetometer (VSM). It has been shown that irradiation damage level, irradiation temperature and annealing temperature have significant effect on BCC-phase formation. This BCC-phase changes the magnetic behavior of austenitic stainless steel. The stress relief and compositional changes in matrix are the driving forces for BCC-phase formation in austenitic stainless steel during ion irradiation.

Corrosion behavior of additively manufactured Ti-6Al-4V parts and the effect of post annealing

This paper evaluates the corrosion behavior of Ti-6Al-4V alloy parts produced by laser-based powder bed fusion additive manufacturing (AM). The effect of post annealing heat treatment on the corrosion resistance of AM parts is studied by comparing the heat treated samples with cold rolled commercial titanium alloy samples. The results obtained via corrosion tests show that the corrosion rate of as-fabricated AM parts is almost sixteen times worse than the commercial grade samples. The accelerated rate was due to the presence of non-equilibrium phases and can be ameliorated by a proper post heat treatment process at 800°C for 2 h. The proposed heat treatment makes the corrosion behavior of AM parts comparable to the commercial grade samples, due to the stress relief of the martensitic phase and formation of BCC phase of β Ti-6Al-4V which has a higher corrosion resistance. CALculation of PHAse Diagrams (CALPHAD) method was used to identify the equilibrium phases.

Correlation between the magnetic properties and phase constitution of FeCoNi(CuAl)0.8Gax (0 ≤ x ≤ 0.08) high-entropy alloys

The as-cast FeCoNi(CuAl)0.8Gax (0 ≤ x ≤ 0.08) high-entropy alloys (HEAs) were produced by vacuum arc melting. The correlation between the magnetic properties and phase constitution of the HEAs was thoroughly investigated. The results reveal that all samples contain BCC plus FCC duplex phases and the addition of Ga can promote the formation of BCC phases. Energy dispersive spectroscopy (EDS) results revealed that Cu and Ga elements are enriched in the BCC phases and they are more seriously in the phase boundary regions. Furthermore, the magnetic properties are closely related to the phase constitution of the as-cast FeCoNi(CuAl)0.8Gax (0 ≤ x ≤ 0.08) HEAs. The values of saturated magnetization (Ms), maximum magnetic flux density (Bm), remanence (Br), coercivity (Hc) and hysteresis losses (Pu) increase with the increase of the volume fraction of BCC phases, while the values of initial permeability (μi) and maximum permeability (μmax) increase with the increase of the volume fraction of FCC phases. In addition, the mechanical property of FeCoNi(CuAl)0.8Gax (0 ≤ x ≤ 0.08) HEAs was also discussed in the work.

Microstructure evolution and strengthening mechanism of Al0.4CoCu0.6NiSix (x=0–0.2) high entropy alloys prepared by vacuum arc melting and copper injection fast solidification

The work prepared Al0.4CoCu0.6NiSix (x = 0, 0.05, 0.1, 0.2) high entropy alloys by vacuum arc melting (Mx) and copper injection fast solidification (Ix), discussed the influence of trace Si and rapid cooling on the alloys' microstructures and mechanical properties, analyzed the alloys' strengthening mechanism and evaluated the proportion of different strengthening effects on the improved mechanical properties. The results showed that Mx had a fcc + L12 structure, but it transformed to fcc + L12 + minor bcc when x = 0.1 and 0.2. Ix had a fcc + L12 structure except I0.2 (fcc + L12 + minor bcc). With the increase of silicon, Mx's hardness increased from 193 HV to 334 HV and Ix's hardness increased from 216 HV to 395 HV. M0.2's yield strength increased to 588 MPa, which was 2 times than that of M0, the fracture strength and plastic deformation were 3074 MPa, 40.5%. Ix's yield strength increased to 690 MPa, which was 2.3 times than that of M0, the fracture strength and plastic deformation reached 3407 MPa, 33.1%. The alloys' strengthening mechanism was the formation of bcc phase, solid solution strengthening and fine grain strengthening. And the strengthening effect of bcc phase was greater than fine grain strengthening and the fine grain strengthening effect was greater than solid solution strengthening.

Submicrometer-scale molecular dynamics simulation of nucleation and solidification from undercooled melt: Linkage between empirical interpretation and atomistic nature

Nucleation and solidification from the undercooled iron melt are investigated from the atomistic point of view by large-scale molecular dynamics (MD) simulations up to 12 million atoms in systems of the submicrometer-scale. There exist some amount of atoms with icosahedral configuration in the undercooled iron melt and these atoms increase with decreasing temperature. It is expected that accumulation of atoms with icosahedral configuration in the initial β-relaxation regime of nucleation is the key to initiate the formation of bcc phase. On the other hand, mobility of atoms in the undercooled melt decreases drastically with decreasing temperature. These two competing factors in the atomistic scale are considered to derive a critical temperature at which nucleation rate becomes maximum, which agrees with a classical theory for homogeneous nucleation. Moreover, the Avrami exponents during solidification are approximately estimated to be close to 3 and 4 in two- and three-dimensional grain growths, respectively, which also agrees with empirical interpretation. Our novel approach utilizing the high parallel efficiency of the GPU supercomputer successfully links the empirical interpretation in metallurgy with the atomistic behavior of nucleation and solidification, which enlarges the application range of MD simulations for the study of structural materials.

Metastable bcc phase formation in 3d ferromagnetic transition metal thin films sputter-deposited on GaAs(100) substrates

Co100−xFex and Ni100−yFey (at. %, x = 0–30, y = 0–60) films of 10 nm thickness are prepared on GaAs(100) substrates at room temperature by using a radio-frequency magnetron sputtering system. The detailed growth behavior is investigated by in-situ reflection high-energy electron diffraction. (100)-oriented Co and Ni single-crystals with metastable bcc structure are formed in the early stage of film growth, where the metastable structure is stabilized through hetero-epitaxial growth. With increasing the thickness up to 2 nm, the Co and the Ni films start to transform into more stable hcp and fcc structures through atomic displacements parallel to bcc{110} slide planes, respectively. The stability of bcc phase is improved by adding a small volume of Fe atoms into a Co film. The critical thickness of bcc phase formation is thicker than 10 nm for Co100−xFex films with x ≥ 10. On the contrary, the stability of bcc phase for Ni-Fe system is less than that for Co-Fe system. The critical thicknesses for Ni100−yFey films with y = 20, 40, and 60 are 1, 3, and 5 nm, respectively. The Co100−xFex single-crystal films with metastable bcc structure formed on GaAs(100) substrates show in-plane uniaxial magnetic anisotropies with the easy direction along GaAs[011], similar to the case of Fe film epitaxially grown on GaAs(100) substrate. A Co100−xFex film with higher Fe content shows a higher saturation magnetization and a lower coercivity.