Body-centered Cubic Phase Research Articles

Phase constitution, surface chemistry and corrosion behavior of electrodeposited MnFeCoNiCu high entropy alloy-graphene oxide composite coatings

MnFeCoNiCu high entropy alloy (HEA) coatings with and without graphene oxide (GO) incorporation was deposited onto a mild steel substrate by the electrodeposition method. Coating morphology, phase constitution, corrosion properties, and coatings surface chemistry before and after corrosion in a 3.5 wt% NaCl solution was investigated. A predominantly body-centered cubic (BCC) phase with a minor fraction of face-centered cubic (FCC) phase was obtained in the pristine coating. Whereas a nearly equal volume fraction of FCC and BCC phases formed in coatings with increasing GO content. Corrosion characteristics of the MnFeCoNiCu HEA coatings with different GO content were studied. All the GO containing coatings exhibited higher corrosion resistance than the coating without GO in 3.5 wt% NaCl solution. The corrosion resistance of the coatings increased with increasing GO content. The surface chemistry of the coatings before and after exposure was studied by X-ray photoelectron spectroscopy (XPS). These results showed that the GO incorporation into the MnFeCoNiCu matrix enhanced the formation of stable oxides, which can reduce ionic diffusion, thus improving the corrosion resistance of the MnFeCoNiCu HEA-GO coatings when compared to the MnFeCoNiCu HEA coating without GO. • MnFeCoNiCu high entropy alloy-Graphene oxide (GO) composite coatings were electrodeposited. • Phase fraction, microstructure, and corrosion behavior of the coatings changed with changes in the GO volume fraction. • GO incorporation enhanced stable oxide formation which enhanced corrosion resistance.

Effect of superheating on microstructure and wear resistance of Al1.8CrCuFeNi2 high-entropy alloy

In this study, the effects of different superheating temperatures on the microstructure development and wear resistance of the Al1.8CrCuFeNi2 high-entropy alloy by heating the alloy samples in a high-frequency furnace were analyzed. It was observed that the as-cast alloy contains a simple BCC (body-centered cubic) phase and an ordered Al-Ni phase with a flower-like dendritic crystal structure. After the heat treatment, the dendrites grow in a large-angle direction, improving wear resistance.

Decoupling the roles of constituent phases in the strengthening of hydrogenated nanocrystalline dual-phase high-entropy alloys

Nanocrystalline (NC) dual-phase Al0.7CoCrFeNi HEAs containing face-centered cubic (FCC) and body-centered cubic (BCC) microstructural phases were fabricated by high-pressure torsion (HPT). The influences of hydrogen on the thermal desorption and nanoindentation responses of NC HEA were compared with the coarse-grained alloy. The plastic zone size and indentation size effects were carefully considered to identify the distinct contributions of the constituent phases to the hardness and its variation with hydrogen charging. Results show that the FCC phase is susceptible to a larger degree of hydrogen-induced hardening than the BCC phase. Such difference is negated in the NC samples. These results are discussed in terms of the distinct responses of FCC and BCC HEA phases to hydrogen and the governing deformation mechanisms in coarse grained and NC samples.

Phase-field modeling of the interactions between an edge dislocation and an array of obstacles

Obstacles, such as voids and precipitates, are prevalent in crystalline materials. They strengthen crystals by serving as barriers to dislocation glide. In this work, we develop a phase-field dislocation dynamics (PFDD) technique for investigating the interactions between dislocations and second-phase obstacles, which can be either voids or precipitates. The PFDD technique is constructed to account for elastic heterogeneity, elastic anisotropy, dissociation of the dislocation, and dislocation transmission across bicrystalline interfaces. Within the framework, we present a model for “pseudo-voids”, which are voids shearable by dislocations, in contrast to unphysical, unshearable voids in conventional phase-field dislocation formulations. We employ the PFDD technique to investigate the in-plane interactions between an edge dislocation and an array of nano-scale obstacles with different spacings. In this application, the interactions take place in glide planes of either a face-centered cubic (FCC) Cu or a body-centered cubic (BCC) Nb matrix, while the precipitates have a Cu1−xNbx composition, with x varying from 0.1 to 0.9. Our atomistic simulations find that the alloy precipitates can have an FCC, an amorphous, or a BCC phase, depending on the compositional ratio between Cu and Nb, i.e., value of x. Among all types of obstacles, the critical stresses for dislocation bypass are the highest for unshearable amorphous precipitates, followed by shearable crystalline precipitates, and then the pseudo-voids.

Compositionally graded AlxCoCrFeNi high-entropy alloy manufactured by laser powder bed fusion

A high entropy alloy AlxCrCoFeNi (x = 0.07–0.88) coupon with a stepwise compositional gradation was fabricated by laser powder bed fusion (L-PBF) via in situ alloying of the blended metal powders. The chemical compositions, microstructures and mechanical properties of different regions of the coupon were examined to investigate the effect of Al content on the formation of different constituent phases. The maximum Al that can enable crack free printing was observed to be x = 0.5. Microstructural characterization indicates that grain morphologies, textures and phase variations in the graded alloys are closely related to x. Three phases were found across the graded coupon, i.e., face centered cubic (FCC), body centered cubic (BCC) solid solutions, and intermetallic B2. The phase variations can be attributed to the Al-induced lattice distortion and valence electron concentration. Further analysis suggests that the range of duplex FCC+BCC phases region is insensitive to cooling rates and could be mediated by formation of B2 phase. Nanoindentation studies show a steep increase in hardness along with the occurrence of BCC phase.

Formation and beneficial effects of the amorphous/nanocrystalline phase in laser remelted (FeCoCrNi)75Nb10B8Si7 high-entropy alloy coatings fabricated by plasma cladding

High-entropy amorphous composite coatings have high potential application value in the industrial field due to their excellent performance. (FeCoCrNi)75Nb10B8Si7 high-entropy alloy coatings with amorphous/nanocrystalline phases were prepared by plasma cladding and subsequent laser remelting. The phase, microstructure, mechanical properties, and wear resistance of the coatings were studied. Detailed characterization indicated that the microstructure of the plasma cladding coating consisted of the body-centered cubic (BCC) phase, whereas that of the laser remelting coatings consisted of the Fe and Ni-rich BCC phase in the dendritic area and the Nb, B, and Si-rich nanocrystalline-face-centered cubic (nano-FCC) + amorphous phases in the interdendritic area. Excellent mechanical properties were observed, including high microhardness, high nanohardness (H) to elastic modulus (E) ratio (H/E), high H3/E2, and a high elastic recovery rate (η). Furthermore, the coefficient of friction (COF) was lower in the laser remelting coatings (0.6) than in the plasma cladding coating (0.7), which was attributed to severe adhesive wear of the plasma cladding coating and abrasive and slightly adhesive wear of the laser remelting coatings. The microstructure evolution and strengthening contribution of the interdendritic structure in the laser remelting coatings and the relationship between the microstructure and mechanical properties and wear resistance were analyzed.

Decreasing Zr content to improve tensile properties of non-equiatomic TiZrHfNb medium entropy alloys with transformation-induced plasticity

Compositional tuning of phase stability of the body-centered cubic (BCC) matrix is an effective way to improve mechanical properties of refractory medium/high entropy alloys, which are generally brittle or lack of strain hardening capability. Here, reducing Zr content in Ti(80-x)ZrxHf10Nb10 (at.%) alloys was demonstrated to destabilize the BCC phase and trigger the BCC to hexagonal close-packed (HCP) phase transformation. Two metastable Ti50Zr30Hf10Nb10 and Ti60Zr20Hf10Nb10 medium entropy alloys were designed, which exhibit superior strength-ductility combination and evident strain hardening due to the transformation induced plasticity effect.

Dynamic tensile mechanisms and constitutive relationship in CrFeNi medium entropy alloys at room and cryogenic temperatures

Harsh service conditions in aerospace, defense and military, and other fields are calling for materials with excellent mechanical properties to undergo extreme deformation (such as elevated and cryogenic temperatures and high strain rates) without sustaining damage while retaining high strength. Outstanding mechanical properties of high or medium entropy alloys (MEAs) render them potential candidates. In this paper, as the temperature decreases, a breakthrough of the strength-ductility tradeoff is achieved in a CrFeNi MEA with partially recrystallized face-centered cubic phases, together with body-centered cubic (BCC) precipitates. At a strain rate of $3000\phantom{\rule{0.16em}{0ex}}{\mathrm{s}}^{\ensuremath{-}1}$, the yield strength (YS) is increased from 920 MPa at 298 K to 1320 MPa at 77 K, while the uniform elongation (UE) increases by 28.5%. This phenomenon also occurs at quasistatic tension. Under low-temperature loading, nanotwins are popularly activated due to the decreased stacking fault energy with decreasing temperatures. A positive strain-rate dependent YS arises owing to the contributions from the short-range dislocation obstacles, BCC phases, and refined grains. High-density dislocations and BCC phases result in the degeneration of UE, especially with the increase of strain rates. Strain-rate- and temperature-dependent constitutive models were successfully established to predict the deformation behaviors of CrFeNi MEAs under such a wide range of strain rates at room and cryogenic temperatures.

Effects of La on Thermal Stability, Phase Formation and Magnetic Properties of Fe–Co–Ni–Si–B–La High Entropy Alloys

The microstructure, phase formation, thermal stability and soft magnetic properties of melt-spun high entropy alloys (HEAs) Fe27Co27Ni27Si10−xB9Lax with various La substitutions for Si (x = 0, 0.2, 0.4, 0.6, 0.8, and 1) were investigated in this work. The Fe27Co27Ni27Si10−xB9La0.6 alloy shows superior soft magnetic properties with low coercivity Hc of ~7.1 A/m and high saturation magnetization Bs of 1.07 T. The content of La has an important effect on the primary crystallization temperature (Tx1) and the secondary crystallization temperature (Tx2) of the alloys. After annealing at relatively low temperature, the saturation magnetization of the alloy increases and the microstructure with a small amount of body-centered cubic (BCC) phase embedded in amorphous matrix is observed. Increasing the annealing temperature reduces the magnetization due to the transformation of BCC phase into face-centered cubic (FCC) phase.

A High Stability B2-Containing Refractory Multi-Principal Element Alloy

An equiatomic HfNbTaRuZr refractory multi-principal alloy was cast via arc melting and characterized in both ascast and vacuum annealed conditions. (Nb, Ta)-rich body-centered cubic (BCC), (Hf, Ru)-rich B2, and (Hf, Zr)-rich hexagonal close-packed (HCP) phases in annealed materials were identified by both x-ray diffraction and transmission electron microscopy, with all phases stable from 1350 ◦ C to 1575 ◦ C. At 1575 ◦ C the alloy contained 35.9% of a morphologically stable B2 phase. Preferential segregation of oxygen and nitrogen to the HCP phase was observed. The volume fractions of the HCP and B2 phases increased and decreased, respectively, with increasing annealing temperature. Lamellar microstructures present along grain boundaries were determined to result from a discontinuous transformation from a disordered BCC phase to B2 and BCC. Refinement of the grain size and microstructure was achieved by splat quenching, resulting in exceptional hardness due to nano-scale precipitates.

Effect of Mo addition on structures and properties of FeCoNiCrMn high entropy alloy film by direct current magnetron sputtering

FeCoNiCrMnMox high-entropy alloy (HEA) films were prepared by direct current (DC) magnetron co-sputtering. The influence of the Mo content on the microstructures and mechanical properties of the HEA films were systematically studied by X-ray diffraction (XRD), field emission scanning electron microscope (FESEM), Vickers hardness test and tribological test. The addition of Mo results in the formation of a denser film with refined grain size, and promotes the transformation from a face-centered cubic (FCC) phase to a mixture of FCC and body-centered cubic (BCC) phases. The hardness of the films increases from 8.5 GPa (x = 0) to 12 GPa (x = 1) and the friction coefficient decreases from 0.5 (x = 0) to 0.3 (x = 1), which greatly enhance the damage tolerance of the film. The improved mechanical and tribological properties of the HEA films are attributed to the formation of the hard BCC phase and grain refinement at high contents of Mo. The FeCoNiCrMnMox HEA films are suitable candidates for structural application as protective coatings.

Achieving dual-phase structure and improved mechanical properties in AlCoCrFeTi0.5 high-entropy alloys by addition of Ni

High-entropy alloys (HEAs) have attracted increasing attention as structural materials due to their excellent mechanical properties; however, most single-phase HEAs still encounter the conflict between strength and plasticity, which greatly limits their engineering applications. Here we reported the use of composition design, that is the addition of Ni element into a base alloy of AlCoCrFeTi0.5 HEA, to obtain the body-centered cubic (BCC)/face-centered cubic (FCC) dual-phase structure, for avoiding the strength-plasticity trade-off. With the Ni addition increasing from 0 to 3 mol ratio, the initial ordered BCC (B2 phase) structure first transformed to a duplex BCC (A2 + B2) structure, and then to an FCC (L12)/BCC (A2 + B2) dual-phase structure. Meanwhile, the Ni-containing HEAs displayed greatly improved mechanical properties. More specifically, dual-phase AlCoCrFeTi0.5Ni2.5 HEA showed the most promising combination of strength and plasticity with a yield strength of 1.41 GPa and compressive strain of 31.8%, as well as a fracture toughness of 64.3 MPa m1/2. Post-deformation structural analyses of the AlCoCrFeTi0.5Ni2.5 HEA revealed that the activation of dislocations slip and stacking fault in the FCC (L12) phase, precipitation strengthening in BCC phase (B2), together with the synergic deformation between FCC and BCC phases, contribute to the desirable capability of strain hardening. Our work shows that the use of dual-phase architecture can be an effective microstructure-design strategy for developing HEAs with a favorable strength-ductility synergy.

Deformation-induced phase transformation and twinning in Fe and Fe–C alloys

The phase transformations and twinning in a nano-sized face-centred cubic (FCC) crystal of iron with different carbon contents are studied by molecular dynamic (MD) simulations. The simulation cells are stretched uniaxially along the direction. It is found that the FCC matrix transforms into the body-centred cubic (BCC) phase following the Nishiyama–Wassermann orientation relationship (OR). Then, the ‘BCChexagonal close-packed (HCP)’ and ‘BCCFCC’ transformations occur following different variants of the Burgers OR and the Pitsch OR, respectively. The twins form between two neighbouring BCC grains. Additionally, the carbon atoms can destabilise the BCC phase, inhibit the phase transformation and pin phase interfaces, leading to tiny BCC twins.

Modeling and Analysis of Yielding and Strain Hardening in Metastable High‐Entropy Alloys

Metastable dual‐phase high‐entropy alloys (HEAs) have become an attractive paradigm as structural materials due to their outstanding mechanical properties compared with single‐phase HEAs, which originate from the multiple strengthening mechanisms. Herein, a theoretical model is established by integrating the effects of lattice distortion, dislocation, grain boundary, and phase transformation, to study the mechanical responses of metastable HEAs during uniaxial tensile deformation. The results show the contribution of various mechanisms to the strength, and strain hardening can be determined quantitatively. In particular, the face‐centered cubic (FCC) to body‐centered cubic (BCC) phase transformation would dominate the flow stress with plastic strain to exceed 6%, due to the strong phase interface strengthening and the formation of hard BCC phase. This work provides a microstructure‐based constitutive model for predicting the flow stress and strain hardening properties of metastable HEAs.

Effects of heat treatment on the microstructure, wear behavior and corrosion resistance of AlCoCrFeNiSi high-entropy alloy

The effects of heat treatment on the microstructure, wear behavior and corrosion resistance of AlCoCrFeNiSi high-entropy alloy were investigated. Results indicate that the phase composition of as-cast alloy includes disordered body-centered cubic (BCC) solid solution phase, ordered BCC phase (B2), Cr3Si and (Cr, Ni, Si)-phase. The system exhibits a good thermal stability and no phase transition occurs after heat treatment except the change of diffraction peak intensity and the appearance of weak peak. After heat treatment, the dendrites of B2 phase turns into discontinuous spherical particles, the BCC interdendrite region narrows, and the short rod-like precipitation Cr3Si phase is transformed into long needle-like microstructure. The highest compression fracture strength and strain are 1752 MPa and 11.8 % at 1150-2 h, which are 36 % and 19 % higher than the as-cast alloy, respectively. At 1150-2 h, the highest microhardness (1004.3 HV0.5) and lowest friction coefficient (0.115) are obtained, which are 1.2 times and 3.4 times better than that of as-cast alloy. A mixed mechanism of abrasive and oxidative wear are occurred during friction and deformation. Heat treatment of 1100–2 h possesses the optimal corrosion resistance with the Ecorr and Icorr of −330.776 mV and 637.587 nA due to the appearance of weak passivation phenomenon.

Influences of strain rate, Al concentration and grain heterogeneity on mechanical behavior of CoNiFeAlxCu1-x high-entropy alloys: a molecular dynamics simulation

High-entropy alloys (HEAs) with a heterogeneous grain structure have been revealed to possess excellent combination of strength and toughness. However, the atomic-level deformation mechanisms of the heterogeneous HEAs were not reported yet. In this work, physical models were constructed based on the experimental observation and atomic simulations are performed to investigate the tensile behavior of face centered cubic (FCC) heterogeneous CoNiFeAl x Cu 1-x HEAs at different strain rates (5 × 10 7 –1 × 10 10 s −1 ), Al concentration (x = 0.1, 0.2, 0.3 and 0.4) and degrees of grain heterogeneity. Result analysis reveals the multiple deformation mechanisms including dislocation motion, diffusion from grain interior to grain boundary and stacking faults (SFs) as well as their interaction. The strain rates seriously influence the body centered cubic (BCC) transformation from FCC in the large grains. Besides, with the reduction of Al concentration, the value of stable stacking fault energy (SFE) raises, while the tensile yield stress increases. Finally, increasing the large grain size (D G ) of the heterogeneous grain structure improved the plasticity due to the combination of enhanced FCC to BCC phase transformation and high uniform ductility of large grains. This work provides a micromechanical understanding for designing the excellent mechanical property of HEAs by optimizing material structure parameters of heterogeneous grain structure HEAs.

Tailoring Compressive Strength and Absorption Energy of Lightweight Multi-Phase AlCuSiFeX (X = Cr, Mn, Zn, Sn) High-Entropy Alloys Processed via Powder Metallurgy.

The development of lightweight HEAs with high strength and low cost is an urgent requirement. In this study, equimolar AlCuSiFeX (X = Cr, Mn, Zn, Sn) lightweight HEAs were fabricated by advanced powder metallurgy. The mechanical alloying was performed for 45 h, and the powder compacts were densified at 650 °C. The final results revealed that AlCuSiFeSn lightweight HEA was composed of a single face-centered cubic (FCC) and Cu81Sn22, whereas AlCuSiFeZn showed a dual FCC and body-centered cubic (BCC) structures. Similarly, AlCuSiFeMn alloy contained a BCC + FCC phase with a µ-phase, whereas a σ-phase was present in AlCuSiFeCr in addition to FCC + BCC phases. We also calculated various thermodynamic parameters to predict the solid-solution phase stability of each of the above lightweight HEAs. It was found that lightweight HEAs with additive elements Sn and Zn tend to predominant FCC phases, whereas those with Cr and Mn result in major BCC with hard µ and σ phases, which further improve their mechanical strength. A maximum fracture strain of 23% was obtained for AlCuSiFeSn followed by 19% for AlCuSiFeZn HEA. The compressive fracture mechanisms of these lightweight HEAs are also discussed and reported here.

Characterization of cosputtered NbTaMoW films

In this study, NbTaMoW medium-entropy alloy films were prepared using a cosputtering apparatus with four sputter guns. Pure-element targets were used for sputtering. The rotational speeds of the substrate holder (RS) were 1–30 rpm. Lower RS values resulted in the formed films exhibiting a multilayer stacked structure with a cyclical gradient concentration. The crystalline phases, mechanical properties, and oxidation behavior of the prepared NbTaMoW films were investigated. The uniformity of the prepared multilayered alloy films increased with the RS value. The monolithic NbTaMoW alloy films prepared at 30 rpm exhibited a single body-centered cubic (BCC) phase, hardness of 12.9 GPa, and Young's modulus of 278 GPa. The BCC phase evolved into a combination of multiple BCC phases when RS was <10 rpm. The multilayered NbTaMoW alloy films prepared at 10 rpm exhibited high hardness of 14.1 GPa and a comparable Young's modulus of 274 GPa. The prepared NbTaMoW films exhibited activation energy of 104 kJ/mol at 350–600 °C in air, and MoO3 was volatilized at temperatures of ≥550 °C. The monolithic and multilayered NbTaMoW films with a thickness of approximately 1240 nm were completely oxidized after annealing at 600 °C for 2 h.

Microstructure and tensile properties of Co-free Fe4CrNi(AlTi)x high-entropy alloys

According to parameters of atomic size difference (δ), valence electron concentration (VEC) and the cost effectiveness, a series of high Fe-content and Co-free Fe4CrNi(AlTi)x (x = 0.2, 0.6 and 1.0) high-entropy alloys (HEAs) were designed. Effects of the content of Al and Ti on the microstructure and tensile properties were investigated. It is found that the constituent phases change from the single face-centered cubic (FCC) phase to FCC + body-centered cubic (BCC) + L21 phases and BCC + L21 phases as the Al and Ti content increases from 0.2 to 0.6 and 1.0, respectively. With increasing the Al and Ti content, the strength increases but the tensile plasticity decreases. Specifically, Fe4CrNi(AlTi)0.6 exhibits a good combination of a high strength of 858 MPa and a large tensile plasticity of 12.8%. The excellent tensile properties of this alloy are attributed to the synergetic effects of a soft FCC phase and hard BCC/L21 phases, and moreover, the uniform distribution of high-density L21 nanoparticles in the BCC phase also benefits the tensile properties. In addition, the phase-evolution principles in the current HEAs and in many previously-reported HEAs are well discussed within the framework of δ and VEC parameters. These findings are believed to promote the development of low-cost HEAs with high performance for practical applications.

Direct drive friction welding of a multi-phase Al13Cr23.5Fe20Co20Ni23.5 high-entropy alloy

Al13Cr23.5Fe20Co20Ni23.5 high-entropy alloy with three phases (face-centered cubic [FCC], A2 and B2) was joined successfully by direct drive friction welding. The microhardness of the welded samples gradually increases as approaching the weld interface, which was related to the microstructural evolution across the weld interface. The weld joint can be divided into four regions: dynamic recrystallisation zone, thermal-mechanically affected zone, heat affected zone and base metal. It is found that the volume fraction ratio of FCC to body-centered cubic (BCC) (A2+B2) phases slightly decreases as approaching the weld interface. The hardening of the joint is attributed to the refined structure, large deformation and the increased volume fraction of the hard BCC phases in the weld joint.