Bcc Phase Research Articles

An inspection into the unexpected gamma precipitation in supersaturated U-13 at.% Nb subjected to aging

Phase separation of the U-Nb (13 at.%) alloy at an aging temperature of 723 K was investigated through transmission electron microscopy. Chemical redistribution initiated from a supersaturated state was identified after aging for half an hour before the discontinuous precipitation (DP) process. Such redistribution was accompanied by a fine two-phase structure and non-lamellar morphology. Among the decomposition products at this early stage, a lath-shaped Nb-rich phase was confirmed to be the bcc (γ) phase, with Nb content in the range of 35∼45 at.%. Meanwhile, discrete regions of much smaller size were detected to bare an Nb content approaching the equilibrium of the α phase. It is envisaged that the γ precipitation was a preceding event of the α. Thermodynamic estimation was carried out to reveal the decomposition mechanism responsible for this unexpected behavior. The results were in favor of a nucleation and growth mechanism over the spinodal decomposition. These results provide a deeper understanding and a new sight into the aging phenomenon of U-13Nb alloys, which are also worth consideration in other binary systems slightly outside the coherent spinodal regions such as Fe-20Cr.

Elemental effect on the phase formation and mechanical properties in the FeNiMnCuAlTi alloy system

The FeNiMnCuAlTi alloys were prepared using the arc-melting method by varying the individual elements. The change in microstructure and mechanical properties of the alloys due to the changes in the elemental composition was carried out by X-ray diffraction, Scanning Electron Microscopy, and transmission electron microscopy. Equiatomic composition exhibited the B2_BCC and C_14 Laves phase, whereas increasing the Fe content in the alloys enhanced the formation of the BCC phase. Cu and Ni enhanced the formation of the FCC phase and the addition of Al and Ti favored the formation of intermetallics. Cu tends to segregate in the inter-dendritic region in all these alloys by forming a Cu-Mn-rich FCC phase. Fe, Ni, Cu and Mn-rich alloys showed good plasticity, whereas Al and Ti-rich samples are extremely brittle. A high compressive yield strength of 2016 MPa, 1526 MPa, and 1255 MPa was observed for the high-entropy alloys Fe30(NiMnCuAlTi)70, Ni30(FeMnCuAlTi)70, and Mn30(NiFeCuAlTi)70 respectively, with plasticity varying between 3% to 10%. Whereas medium entropy alloys showed a lower compressive yield strength but with increased plasticity e.g., a compressive yield strength of 1525 MPa and 27% plasticity for the case of Fe50(NiMnCuAlTi)50 was observed. The good combination of strength and plasticity is due to the presence of the FCC/BCC phase with a fine uniform distribution of intermetallic phases.

Local lattice distortions and the structural instabilities in bcc Nb–Ta–Ti–Hf high-entropy alloys: An ab initio computational study

Local lattice distortions (LLD) and structural stability of body-centered cubic (bcc) Nb–Ta–Ti–Hf high-entropy alloys (HEAs) are studied as functions of composition employing ab initio density-functional theory calculations, with specific focus on the role of the relative concentrations of group IV (Ti and Hf) versus group V (Nb and Ta) elements. Calculated results are presented as a function of composition x in NbxTa0.25Ti(0.75−x)/2Hf(0.75−x)/2 alloys, for elastic moduli, phonon spectral functions, LLD and structural energy differences for the bcc and competing hexagonal close-packed (hcp) and ω phases. The results highlight the important role of group V elements and LLD in stabilizing the bcc structure. They further reveal how composition x can be tuned to alter both the magnitude of the LLD and structural energy differences. Specifically, the magnitude of the structural energy differences, and elastic and dynamic stability of the bcc phase, are enhanced with increasing x, while the LLD increase in magnitude as this concentration is decreased. The results also show evidence of correlated LLD at lower values of x, reflecting local structural distortions towards the ω phase, but not hcp. The degree of ω-collapse is nevertheless partial i.e., transformation towards this phase is not observed to be complete due to the presence of Ta and Nb. At lower values of x we further find an energy landscape characterized by multiple, nearly degenerate local energy minima for different values of the LLD.

New insight into the strengthening mechanism of AlCoCrFeNi2.1 eutectic high-entropy alloy with dual-phase nanolamellar structures achieved via laser powder bed fusion

Recently, the AlCoCrFeNi2.1 eutectic high-entropy alloy (EHEA) prepared by laser powder bed fusion (LPBF) could reach a good balance of strength and ductility. The strengthening effect was only qualitatively clarified, however, quantitative analysis remained unclear. In this study, the quantitative analysis of strengthening mechanisms and the relationship between microstructural characteristics and tensile properties were revealed in depth. The AlCoCrFeNi2.1 EHEA with FCC and BCC dual-phase nanolamellar structures (interlamellar spacings of 200–300 nm) fabricated by LPBF achieved an excellent combination of high yield strength (1116 MPa) and fracture elongation (20.3 %). Compared with as-cast EHEA samples, the FCC/BCC nanolamellar interfaces in the LPBF-printed AlCoCrFeNi2.1 EHEA were still semi-coherent but with a higher degree of coherency, and the Al concentration in the FCC phase increased. Post-deformation analysis showed that there were high-density dislocation pile-up and slip, stacking faults (SFs), and Lomer-Cottrell (L-C) locks in the FCC phase, and there were deformation nano-twins and SFs in the BCC phase. The back stress generated in FCC/BCC phases during tensile deformation could maintain the plastic co-deformation of the two phases. The theoretical calculation revealed that the strengthening mechanisms mainly stemmed from the nanolamellar structures, dislocation strengthening, and the solution strengthening induced by the increased Al concentration in the FCC phase. The calculated result (1096.3 MPa) was consistent with the experimental value, and the nanolamellar structures strengthening (∼720.7 MPa) was the main contributor. This work provides a theoretical quantitative understanding of the strengthening mechanism of the AlCoCrFeNi2.1 EHEA fabricated by LPBF and references for the strengthening mechanism of additively manufactured EHEAs.

A lightweight NbMoZrTi refractory high entropy alloy with high specific strength

Refractory high entropy alloys are an obstacle to their use as engineering applications due to their low room temperature ductility and high density. In the present work, a new low-density Nb-Mo-Zr-Ti RHEA was prepared by vacuum arc melting method. The as-cast alloy exhibits a single BCC phase with lower density and higher hardness of 7.4331 g/cm3 and 522 HV. The compressive yield strength of 1462 MPa at room temperature, peak strength of 2158 MPa, and strain at break of 24.6 %. The specific yield strength of this alloy is the highest among all other reported RHEAs, and the ultra-high specific yield strength and excellent strong plastic synergistic effect provide important experience in designing and preparing lightweight structural materials for practical applications.

A novel Fe-rich Co-free high entropy alloys with low cost and excellent comprehensive mechanical properties

In this paper, a novel strategy for designing high entropy alloys (HEAs) with low cost and high mechanical properties was proposed. Accordingly, a Fe-rich Co-free Fe2CrNiSi0.3Al0.2 HEA was designed and fabricated by arc-melting. The results show that this HEA is mainly composed of FCC dendrites, while BCC phase locates at the inter-dendrite regions. Moreover, this HEA exhibits outstanding comprehensive mechanical properties among dual-phase HEAs produced via various fabrication methods considering its product of strength and plasticity as well as raw material cost. It is interesting that, compared to its Al-free FCC counterpart, this HEA has significant enhancement in yield strength and ultimate tensile strength, while its plasticity is merely slightly reduced. Further investigation reveals that the heterostructure in this HEA plays an important role in this phenomenon. This work provides a novel insight into the development of high-performance HEAs with broad applications.

Understanding the difference in fragility of the bcc phases of highly compressed P, As, and Sb through first-principles investigations

Understanding the difference in fragility of the bcc phases of highly compressed P, As, and Sb through first-principles investigations

Hydrogen induced cracking behavior of the dual-phase Co30Cr10Fe10Al18Ni30Mo2 eutectic high entropy alloy

In this paper, the hydrogen induced cracking behavior of the Co30Cr10Fe10Al18Ni30Mo2 eutectic high entropy alloy was studied. The results showed that the strength and ductility of the alloy decreased after hydrogen pre-charging, and quasi-cleavage fracture features and hydrogen-induced cracks were found. Moreover, the cracks induced by hydrogen pre-charging were found to propagate along the two-phase interface and inside the BCC phase.

Study on the process optimization and wear resistance of electron beam cladding AlCoCrFeNi coating on 253MA austenitic stainless steel surface

This study employs a continuous scanning electron beam to deposit AlCoCrFeNi composite cladding on the surface of 253MA austenitic stainless steel to enhance its surface properties. The characteristics were examined through a scanning electron microscope (SEM), backscattered electron diffraction (EBSD), and white light interferometric three-dimensional surface profile instrument. This study investigated the effect of electron beam process parameters on the surface organization and mechanical properties of 253MA steel. The sample surface was divided into four zones: overlay, bonding, thermal influence, and substrate, according to the results. Coarse columnar and island-shaped crystals composed the overlay zone, while equiaxed austenite grains primarily constituted the substrate. With the increase of the beam current, the microstructure's grain size decreased, the substructures increased, and the BCC phase transformed into the FCC phase. The grain size slightly decreased, substructures changed marginally, and the BCC phase increased as the scanning speed increased. Additionally, the microhardness and mechanical properties of the overlay layer increased significantly, and wear volume and wear rate decreased progressively with the increase in beam current and scanning speed. At a beam current of 18 mA and a scanning speed of 200 mm/min, the surface hardness of 253MA steel reached the maximum value of 520 HV. When the beam current was 24 mA, and the scanning speed was 200 mm/min, the wear volume and wear rate of the overlay sample were minimal, accounting for 37.4% of the original sample's, and its wear resistance was 2.67 times more than that of the substrate.

Introducing a new heterogeneous nanocomposite thin film with superior mechanical properties and thermal stability

High entropy alloy (HEA) films offer excellent mechanical properties due to their random solid solution structure. However, their large grain boundary volume fraction can cause thermal instability, resulting in phase decomposition that affects their high-temperature performance. Nevertheless, it remains an interesting question whether phase decomposition can be used as a processing tool to create new HEA materials. In this study, AlCrFeCoNiCu0.5 HEA thin films were fabricated and annealed at 500 °C for up to 72 h. The 72-hour annealed thin film exhibited a 30 % increase in mechanical properties, exceeding most other HEA thin films. Characterization using X-ray and electron microscopy revealed a decomposition-induced phase transformation, which produced four new phases, including Cu-rich FCC phase, Cr-rich BCC phase, and ordered B2 phase of AlNi and FeCo. The enhanced mechanical properties were due to back stress strengthening and BCC + B2 phase strengthening. The 72-hour annealed thin film also showed excellent thermal stability through a new round of annealing, exhibiting a low variation in microstructure and chemical composition after being annealed at 500 °C for 100 h.

Effect of solidification behaviors on microstructures and properties of high-entropy alloys coatings by laser melting deposition

Laser melting deposition is commonly used to prepare high-entropy alloy (HEAs) coatings. Laser melting deposition with coaxial powder feeding (CPF) and laser melting deposition with powder pre-placed (PPP) lead to differences in the melting and solidification behaviors in non-equilibrium solidification conditions due to the differences in forming principles and energy input. In this paper, the solidification behaviors, microstructures, mechanical properties and strengthening mechanisms of FeCoCrNiAl0.5 high-entropy alloys coatings prepared by CPF and PPP were studied. The results demonstrated that the solidification path, crystal orientation, and average grain size of CPF and PPP HEAs coatings were significantly influenced by the temperature gradient (G) and solidification rate (R). The distinct microstructures between CPF and PPP HEAs coatings resulted in differences in their mechanical properties. Notably, the PPP HEAs coating, characterized by a higher G × R value, exhibited a combination of both FCC and BCC phases, along with a larger kernel average misorientation (0.4592) and a smaller average grain size (41.12 μm). Consequently, in non-equilibrium solidification conditions, the PPP HEAs coating displayed enhanced grain boundary strengthening, dislocation strengthening, BCC hard phase strengthening, and solid solution strengthening effects compared to the CPF HEAs coating. This led to a higher average microhardness (100 HV) and a lower friction coefficient (0.27) for the PPP HEAs coating in comparison to the CPF HEAs coating.

High-entropy alloy TiV2ZrCrMnFeNi for hydrogen storage at room temperature with full reversibility and good activation

The development of alloys that are hydrogenated and dehydrogenated quickly and actively at room temperature is a challenge for the safe and compact storage of hydrogen. In this study, a new high-entropy alloy (HEA) with AB-type configuration (A: hydride-forming elements, B: inert-to-hydrogen elements) was designed by considering valence electron concentration, electronegativity difference and atomic-size mismatch of elements. The alloy TiV2ZrCrMnFeNi had dual C14 Laves and BCC phases, in which C14 stored hydrogen and BCC/C14 interphase boundaries contributed to activation. The alloy absorbed 1.6 wt% of hydrogen at room temperature without any activation treatment and exhibited fast kinetics and full reversibility.

Bulk and grain boundary tracer diffusion in multiphase AlCoCrFeNiTi0.2 compositionally complex alloy

Tracer self-diffusion of Co in a compositionally complex AlCoCrFeNiTi0.2 alloy is measured using the radiotracer technique. The complex multi-phase microstructure (a mixture of BCC, B2, σ, and FCC phases in dependence of the processing conditions) is carefully analyzed to quantify the measured concentration-depth profiles in terms of volume and short-circuit diffusion contributions. The analysis allowed to determine the volume diffusion coefficients in the B2 phase and those along the high-angle B2/B2 grain boundaries (GBs). The absence of any “sluggishness” of diffusion in this alloy is unambiguously verified, both for volume and GB diffusion contributions.

Evolution of M7C3 carbides near the solidus and the influence of Mn element on the formation of M23C6 carbides in a high carbon martensitic stainless steel 90Cr18MoV

The evolution behavior of M7C3 carbides in a 90Cr18MoV martensite stainless steel during high temperature diffusion annealing treatments at temperatures near the solidus was investigated. Results showed that only M7C3 carbides dissolved, and the dissolution of large-sized M7C3 carbides was limited when the temperature was below the solidus. However, when the temperature was above the solidus, a certain amount of liquid phase appeared around the grain boundaries. Meanwhile, two different types of precipitates appeared within the grains. And these two types of precipitates were eutectic products that mainly consisted of M7C3 carbides and bcc phase, and divorced eutectic products that mainly consisted of M23C6 carbides, respectively. Thermodynamic calculations indicated that with the increase of Mn content, the Gibbs free energy of M23C6 carbides would be lower than that of M7C3 carbides. Thus, the difference in Mn content was the main reason for the formation of those two types of precipitates, i.e., M7C3 carbides were formed in regions with low Mn content and M23C6 carbides were observed in areas with high Mn content.

Effect of laser energy densities on the phase composition change in laser-cladded AlCoCrNbMo high-entropy alloy coatings

This work designed a new high-entropy alloy (HEA), namely, AlCoCrNbMo, and prepared it by using laser cladding (LC). The effects of different values of laser energy density (Es) on the phase composition, microstructure, mechanical properties and corrosion resistance of the HEA coating were investigated via X-ray diffractometry; scanning electron microscopy; energy dispersive spectrometry and microhardness, friction wear and electrochemical tests. Results indicate that increasing Es (Es = 35, 55, 75 J/mm2) leads to the formation of different phases, including bcc1, bcc1 +bcc2 and bcc1 +bcc2 +Al3Nb, in the coating. The bcc1 phase is rich in Co and Cr elements, whereas the bcc2 phase is rich in Nb and Mo elements. As Es increases, the HEA coating shows increasing hardness and wear resistance but decreasing corrosion resistance. Moreover, the AlCoCrNbMo alloy prepared through vacuum arc melting does not conform to the form of HEAs but is instead composed of various intermetallic compounds, reflecting the unique advantage of LC. This paper discusses the influence of LC process parameters on the microstructure of HEAs and the unique advantages of LC for HEA preparation.

An excellent synergy in yield strength and plasticity of NbTiZrTa0.25Cr0.4 refractory high entropy alloy through the regulation of cooling rates

The solidification process often determines the microstructure of alloys from the atomic scale to the macroscopic scale, which distinguishes the performance of one alloy from another. Among all modification measures, the cooling rate plays a vital role in the solidification process. Hereinto, to thoroughly investigate the cooling rate effects on phase components, microstructures and mechanical properties, a series of NbTiZrTa0.25Cr0.4 refractory high entropy alloys were prepared under a wide range of cooling rates. The results show that the microstructures with combined dendrites and interdendrites gradually coarsened as the cooling rate decreased, coupled with the transition from a single BCC phase to BCC + Laves dual-phase. During the microstructural evolution, the prominent strengthening mechanism (i.e., solution strengthening) of the alloy prepared under high cooling rates became weak, while the secondary strengthening mechanism (i.e., second phase strengthening associated with Laves phase) was gradually reinforced. The reinforced second phase strengthening compensated for the weakened solution strengthening; as a result, the yield strength first decreased and then increased as the cooling rate decreased without sacrificing the plasticity of the alloys. It was found that the alloy prepared under an appropriate cooling rate of 62.5 K/s exhibited the maximum yield strength of 1452 MPa and maintained a competitive plasticity of approximately 23.2%, which exceeded the NbTiZrTa0.25Crx system and other reported refractory high entropy alloys. This study provides insight into the development of high-temperature alloys and a promising avenue to enhance the performance of an alloy by regulating the cooling rates.

Exploring the mechanical and tribological properties of AlCrFeNiTi high-entropy alloy fabricated by mechanical alloying and spark plasma sintering

The present study explores the mechanical and tribological properties of AlCrFeNiTi high entropy alloy (HEA) processed by mechanical alloying (MA) and spark plasma sintering (SPS). The results show the formation of a single-phase BCC structure after MA at 30 h, and after SPS a single BCC phase decomposed into dual BCC1; AlNi2Ti, and BCC2; CrFe phases. The maximum hardness of ∼960 ± 10 HV and ultimate compressive strength of ∼1650 ± 50 MPa were achieved. The nano hardness of ∼951 ± 21 HVIT and the elastic modulus of ∼256 ± 22 GPa were observed. Further, the ball-on-disc dry sliding tests were performed to study the friction and wear behavior of HEA at different loads and sliding velocities. The average coefficient of friction is about 0.3 for all the applied loads, suggesting no significant difference with load. In contrast, the average coefficient of friction slightly increased from 0.23 to 0.27 with increasing velocity. The specific wear rate increased from 2.66 to 5.06 × 10−6 mm3/Nm and 2.2 to 3.61 × 10−6 mm3/Nm with increasing load from 5 to 15 N and velocity from 0.08 to 0.12 m/s, respectively. Moreover, a transition in wear mechanism was observed from abrasive to oxidative and delamination wear with increasing load and sliding velocity.

Mechanisms underlying the influence of Co and Ti on the microstructure, mechanical and wear properties of A2/B2 typed AlCrFeNi alloy

AlCrFeCoNi is one of widely studied high-entropy alloys, in which Co is considered to be beneficial. Here we demonstrate in detail that Co does not play a positive role. Instead, Co-free AlCrFeNi possesses higher ductility than AlCrFeCoNi while maintaining a similar mechanical strength. It is of interest to observe that removing Co results in a two-fold increase in the wear resistance. The improvements are related to the formation of a soft-hard alternating lamellar structure which is absent in the Co-containing alloy. We further tailor the alloy by replacing Co with Ti, leading to further enhanced wear resistance. AlCrFeCoxNi (x = 0.5 and 1) and Co-free AlCrFeNiTix (x = 0, 0.2, 0.5, 0.7 and 1) all contain a Fe-Cr-rich disordered BCC phase (A2) and a NiAl (Ti)-rich ordered BCC phase (B2). For x = 0.5, a small amount of L21 phase with embedded nanoscale A2 phase is observed. For x ≥ 0.7, an additional CrFeTi Laves phase forms mainly at grain boundaries. The increase in Ti content alters the lamellar eutectic microstructure of AlCrFeNi, forming a cellular eutectic microstructure. Ti considerably increases hardness and compressive strength at the expense of ductility. AlCrFeNiTi0.7 appears to have a proper balance between hardness and ductility with a superior wear resistance comparable to that of a well-known industrial wear-resistant high-Cr cast iron (Cr30C2.85). This HEA has demonstrated itself a promising candidate for the mechanical and anti-wear applications.

Phase prediction, densification, and microstructure of AlCrFeNi(TiO2)x high entropy alloy composite fabricated by spark plasma sintering

This study incorporates the knowledge of empirical calculation and computational concepts using Thermo-Calc as a design guide to develop AlCrFeNi high entropy alloy (HEA). Subsequently, a novel AlCrFeNi HEA composite consisting of varying weight percentages (wt%) of TiO2 reinforcements is synthesized using mechanical alloying (MA) and spark plasma sintering (SPS) techniques. Our work investigated the role of TiO2 reinforcement on the densification behavior, phase changes, microstructure, and properties of AlCrFeNi HEA. The phase diagram was constructed using Thermo-Calc software. Powder densification during SPS was analyzed using obtained sintering data. The samples were characterized using Xray Diffraction (XRD) and Scanning Electron Microscopy (SEM) techniques. The density and micro-hardness were evaluated. The result shows that the densification during SPS is dependent on kinetic mechanisms such as melt diffusion, surface diffusion, and plastic flow. Phases of the disordered BCC phase (rich in FeCr) and ordered B2 phase (rich in NiAl) were found in the AlCrFeNi HEA. The phases were consistent with the prediction using thermo-calc software. The BCC phases in the AlCrFeNi are retained regardless of the addition of TiO2 particles but with the diffusion of Ti with O atoms fitting into the interstitial sites in the BCC and B2 lattice. The microstructure verified the homogeneous distribution of the TiO2-rich phase within the BCC and B2 phases of the AlCrFeNi HEAs. The density decreases with the adding TiO2. The hardness of reinforced AlCrFeNi is enhanced from 537.24 Hv to 752.74 Hv with an increment of TiO2 reinforcement from 0 to 8 wt%. TiO2 particles acted as impediments to the mobility of dislocation, thereby increasing the resistance to deformation of HEA composites during micro indentation.

Effect of C14 Laves/BCC on microstructure and hydrogen storage properties of (Ti32.5V27.5Zr7.5Nb32.5) 1-xFex (x = 0.03, 0.06, 0.09) high entropy hydrogen storage alloys

In order to explore the effect of C14 Laves/BCC on the hydrogen storage properties of high entropy hydrogen storage alloys, (Ti32.5V27.5Zr7.5Nb32.5) 1-xFex (x = 0.03, 0.06, 0.09) high entropy hydrogen storage alloys are prepared. The C14 Laves/BCC rises with the increase of Fe content. The C14 Laves phase provides a path for the diffusion of hydrogen atoms, so that defects with higher hydrogen binding energy capture more hydrogen atoms to ensure the increase of initial hydrogen absorption content in the activation process. The appropriate ratio of C14 Laves/BCC ensures that the main hydrogen absorbing phase BCC phase of high entropy hydrogen storage alloy plays a full role. The (Ti32.5V27.5Zr7.5Nb32.5) 0.94Fe0.06 high entropy hydrogen storage alloy possesses the best hydrogen absorption capacity of 3.19 wt% at 150 °C under 3 MPa hydrogen pressure. The gradual increase of Fe content makes the VEC value of C14 Laves phase rise, so that the hydrogen binding energy decreases. The factor reduces the thermal stability of metal hydrides. The reversible phase transition of BCC phase + metal C14 Laves phase → FCC phase + metal hydride C14 Laves phase → BCC phase + metal C14 Laves phase occurs in the whole cycle.