Sluggish diffusion in Co–Cr–Fe–Mn–Ni high-entropy alloys

Sluggish diffusion kinetics is an important contributor to the outstanding properties of high-entropy alloys. However, the diffusion kinetics in high-entropy alloys has never been probed directly. Here, the diffusion couple method was used to measure the diffusion parameters of Co, Cr, Fe, Mn and Ni in ideal-solution-like Co–Cr–Fe–Mn–Ni alloys. These parameters were compared with those in various conventional face-centered cubic metals. The results show that the diffusion coefficients in the Co–Cr–Fe–Mn–Ni alloys are indeed lower than those in the reference metals. Correspondingly, the activation energies in the high-entropy alloys are higher than those in the reference metals. Moreover, the trend of the normalized activation energy is positively related to the number of composing elements in the matrix. A quasi-chemical model is proposed to analyze the fluctuation of lattice potential energy in different matrices and to explain the observed trend in activation energies. Greater fluctuation of lattice potential energy produces more significant atomic traps and blocks, leading to higher activation energies, and thus accounts for the sluggish diffusion in high-entropy alloys.     

Corrosion behavior of Zr–Nb–Cr cladding alloys

Zr-Nb-Cr alloys were used to evaluate the effects of alloying elements Nb and Cr on corrosion behavior of zirconium alloys. The microstructures of both Zr substrates and oxide films formed on zirconium alloys were characterized. Corrosion tests reveal that the corro- sion resistance of ZrxNb0.1Cr （x = 0.2, 0.5, 0.8, 1.1; wt%） alloys is first improved and then decreased with the increase of the Nb content. The best corrosion resistance can be obtained when the Nb concentration in the Zr matrix is nearly at the equilibrium solution, which is closely responsible for the formation of columnar oxide grains with protective characteristics. The Cr addition degrades the corrosion resistance of the Zrl.lNb alloy, which is ascribed to Zr（Cr,Fe,Nb）2 precipitates with a much larger size than β-Nb.     

Nanoindentation study on solid solution softening of Fe-rich Fe2Nb Laves phase by Ni in Fe–Nb–Ni ternary alloys

In an attempt to understand the solid solution softening by Ni of an Fe-rich Fe₂Nb Laves phase that is in equilibrium with γ-Fe, we have examined the defect structure in the Laves phase in Fe–Nb–Ni alloys by transmission electron microscopy, and its hardness as a function of orientation and solute (Ni) content by nanoindentation. The binary Fe-rich Laves phase and the ternary Laves phases with lower Ni content (18%) exhibit a featureless morphology with low dislocation density, whereas the ternary Laves phase with higher Ni content (33%) includes basal planar faults extending to several micrometers, producing a local change in the stacking sequence of the three 3⁶-nets (triple layer) of the C14 structure. The hardness of the binary and the ternary Laves phases with 18% Ni in solution is almost constant and independent of orientation, whereas the ternary Laves phase with 33% Ni in solution exhibits a substantial dependence of hardness on orientation and this dependence appears associated with the ease of basal slips evidenced by slip traces around nanoindents. The relative ease in activating basal slip in the presence of a large amount of Ni in solution is thought to be the dominant contributor to the observed solid solution softening.     

Precipitate Behavior in Fe–20Cr–30Ni–2Nb Austenitic Heat-Resistant Steel

In this study,the precipitation behavior of a new austenitic heat-resistant steel(Fe–20Cr–30Ni–2Nb,in at%)was investigated.The effects of alloying addition of boron(B) and lanthanum(La) on the microstructure of the austenitic steel were scrutinized using SEM,EPMA,TEM,and XRD.The results showed that the addition of B enhanced the precipitation of bar-type Laves phase.A small precipitate with high La concentration was observed at the grain boundary in the alloy without aging;similar precipitates without La also presented in region adjacent to the La single phase.This result indicates that La can exist independently and does not contribute to the formation of new compounds.However,in both Band La-modified alloy,B appeared in the precipitate free zone.In the alloy containing both B and La,only Fe2 Nb Lavesphase precipitates,as indicated by the XRD result.     

The influence of Cr alloying on microstructures of Fe–Al–Mn–Cr alloys

The effects of increasing chromium content on the phase transformations in Fe–Al–Mn–Cr alloys have been investigated by means of transmission electron microscopy and energy-dispersive X-ray spectrometry. The experimental results revealed that increasing the chromium addition would expand both the A12α-Mn and DO₃ phase-field regions.     

Structure of strengthening particles of niobium carbide in Fe–Cr–Ni cast refractory alloys

Methods of optical and electron microscopies were used to study the structure of particles of niobium carbide in a cast refractory Fe–Cr–Ni–C alloy modified by Nb and Ti. Particles of niobium carbide in the structure of the cast alloy are predominantly multiphase polycrystalline clusters that are inhomogeneous in the chemical composition and crystal structure. The misorientation angle between individual crystals that compose the carbide particles is 30°–60°. The polycrystalline character of carbides is probably associated with significant thermal stresses that arise at the interphase boundaries in the structure of the alloy upon the primary cooling of the ingot. To explain the polymorphism of the cluster of niobium carbide, a further analysis of the structural and geometrical crystallography is required.     

Strengthening due to coherent Cr precipitates in Fe–Cr alloys: Atomistic simulations and theoretical models

Atomistic simulations were carried out to study the interaction of dislocations with Cr precipitates in body-centered cubic Fe and Fe–10%Cr at 0 K. The results are compared with predictions of theoretical models accounting for precipitate strengthening based on different mechanisms. It is shown that solute hardening (in Fe–Cr matrix) and precipitate hardening can be considered, as a good approximation, to be additive effects.     

Two-phase growth in peritectic Fe–Ni alloys

Bridgman crystal growth experiments were carried out to investigate the solidification behavior of Fe–Ni alloys containing nominally between 4 and 4.5 at.% Ni. Due to macrosegregation, a radial concentration gradient was established across the cylindrical specimens. Due to this gradient, a series of solid/liquid interface morphologies was observed. Oriented two-phase microstructures, which formed either lamellar or fibrous δ-ferrite in an austenite (γ) matrix, were found in the central region of specimens with a composition of some 4.2 at.% Ni and a G/V ratio close to the critical ratio for solid/liquid interface breakdown. At slightly smaller concentrations, oscillatory two-phase structures formed which were similar to the 2-λ instabilities of off-eutectic alloys. The observations confirm that at low solidification rates the stable growth morphology in peritectic alloys cannot be selected by the highest growth temperature criterion. A recently developed nucleation and constitutional undercooling criterion (NCU) was applied to establish a solidification microstructure selection map. Reasonable agreement was obtained between calculated and experimental results. Based on eutectic growth theory the possibility of simultaneous two-phase growth in peritectic alloys is discussed.     

Crystallographic structure of Al3Nb in laser-processed Al–Nb alloys

The aim of the present work is to study the influence of the solidification rate of Al₃Nb on the crystallographic structure and on the order parameter of Al₃Nb in Al–Nb coatings produced by laser-alloying, using a CO₂ laser. The laser treatments were performed using a laser power density of 1.7×10³ W/mm² and scanning speeds of 5, 10, 20 and 40 mm/s, corresponding to interaction times in the range 0.03–0.24 s. The microstructure of all the surface coatings consists only of the phases predicted by the phase diagram, i.e. α-Al and Al₃Nb. X-ray diffraction on polished bulk samples revealed a strong (001) texture, which increases with increasing scanning speed. After filing, the particles of Al₃Nb become randomly oriented, leading to a better agreement between the relative intensities the experimental and calculated diffraction patterns. The long-range order parameter of Al₃Nb varies in the range 0.8–0.9 and does not depend on the solidification rate, within the limits of experimental error.     

Heterogeneous eutectic structure in Ti–Fe–Sn alloys

Systematic microstructural investigations on ultrafine eutectic Ti–Fe–Sn alloys reveal that formation of the unique bimodal eutectic structure upon solidification is originated by quasi-peritectic reaction cooperated with two distinct univariant reactions. Based on this understanding, it is possible to control the volume fraction and morphology of the bimodal eutectic structures thus influencing both strength and plasticity of the samples at room temperature compression.     

Wear characteristics of Fe–25Cr–C–B eutectic cast alloys

Abstract

To clarify the characteristics of Fe–25Cr–C–B cast alloys, a pin on disc friction and wear test was conducted on Fe–25Cr–0C–2.2B, Fe–25Cr–2.2C–1.0B and Fe–25Cr–3.5C–0B eutectic alloys, at various sliding velocities ranging from 0.125 to 1.99 m s^-1. The effects of sliding velocity on the wear resistance of these alloys were studied by the pin on disc friction and wear test, SEM and an X-ray diffraction method. The results show that the effects of sliding velocity on the increase in wear loss were different due to the differences in structure among the alloys. The X-ray diffraction method shows the presence of Fe₂ O₃ and Fe₃ O₄ in the alloys after conducting wear tests for almost all of the wear conditions. From the sliding velocity dependence of wear loss, worn surface observation after the wear tests and X-ray diffraction results, the relationships between the type of oxide and wear loss for Fe–25Cr– 0C–2.2B and Fe–25Cr–2.2C–0B alloys are not clear. However, the wear loss of Fe–25Cr–3.5C–0B alloy decreases at a sliding velocity of 0.5 m s^-1 or lower, due to the presence of red Fe₂ O₃ oxide on the worn surface. The wear loss peaks at a sliding velocity of 0.95 m s^-1, and decreases again at a sliding velocity of 1.99 m ^-1 due to the presence of black Fe₃ O₄ oxide.     

Precipitation strengthening of stress-aged Al–xCu alloys

Effects of stress aging on yield strength and yield anisotropy of single crystal and cube-textured polycrystalline Al–xCu alloys were investigated. The resulting microstructures were correlated with the yield stress and analyzed with respect to continuum mechanic models and computer simulations. The yield stresses of the stress-aged specimens, were found to be lower than those of the stress-free-aged specimens whether the test direction was along or perpendicular to the aging-stress direction. This was attributed to the effect of stress-induced preferential orientation of the θ′{100}-plates as well as to their “detrimentally” different volume fraction and/or morphology in the stress-aged specimen subjected to the same thermal treatment as the stress-free-aged specimen. When compared with the continuum models, both of which account for only effects of the orientation and volume fraction of the θ′{100}-plates, the computer simulation of interaction between slip dislocation and unshearable plates, which additionally incorporates the effect of plate morphology, yields the closest results to those obtained experimentally.     

Reduction of Fe concentration in Al–4Si–1Fe–1Cu–0.5Zn–0.5Mn alloys with S

In this article,the effect of sulfur on the reduction of Fe concentration in aluminum alloy scraps was investigated.The iron content decreases from1.224 wt% to \0.854 wt% and achieves an optimal ratio of30 % when the sulfur addition is 3 %.Thermodynamic calculations,the X-ray diffraction(XRD),scanning electron microscope(SEM),and differential scanning calorimetry(DSC)analyses of the sample indicate that the formation of Fe S can occur spontaneously in molten aluminum with the addition of sulfur.The mechanism of Feremoving purification process was also discussed.     

Liquid phase separation in immiscible Cu–Fe alloys

Cu–Fe binary alloys containing 20–50wt. % Fe were studied by the combination of the melt fluxing and cyclic superheating technique. The microstructural evolution of Cu–Fe alloys was investigated with scanning electron microscopy. When the undercooling was larger than the critical undercooling, the Fe-rich spheroids were embedded into a Cu-rich matrix and the metastable phase separation was observed in microstructures. The size of separated particles in the Cu-35wt.% Fe alloy was larger than that of other Cu–Fe alloys with different compositions and the size of separated droplets was related to the ΔT_S, which was equal to the undercooling (ΔT) minus the critical undercooling (ΔT_C). Moreover, a large undercooling tended to promote the coagulation of the separated droplets, so the size of the separated Fe-rich spheroids in the microstructure of the immiscible Cu–Fe alloys increased with the increase in the undercooling.     

Deformation-intensified atomic separation in bcc Fe–Mn alloys

The deformation-intensified atomic Mn-related separation of the bcc solid solution has been found in Fe_100–xMn_x alloys (x = 4.5–9.9) subjected to ball milling using Mössbauer spectroscopy. In the near surrounding of iron atoms, the atomic separation is similar to that observed upon the annealing of the alloys in a temperature range of 400–500°С. It has been found that the deformation-intensified atomic separation leads to the stabilization of the bcc phase with regard to the α → γ transformation, as well as to the expansion of the field of the existence of the bcc phase during heating.     

Phase selection in undercooled peritectic Fe–Mo alloys

The change of the primary solidification mode of undercooled peritectic Fe–Mo melts has been studied by in situ observation of recalescence events during electromagnetic levitation. A maximum melt undercooling up to 380 K has been achieved. Levitated drops of controlled undercooling were quenched onto chill substrates and subjected to phase and microstructure analysis. A transition from the primary bcc-Mo to the peritectic σ-phase solidification mode beyond a critical undercooling of 345 K was revealed for the Fe₄₇Mo₅₃ alloy and in a similar way for other compositions between Fe₄₅Mo₅₅ and Fe₅₄Mo₄₆. The suppression of the properitectic bcc-Mo phase was also achieved for subcritical undercooling in substrate-quenched Fe₄₅Mo₅₅ samples. In Fe₆₁Mo₃₉ a transition from the primary σ- to the peritectic R-phase solidification mode beyond a critical undercooling of 150 K was inferred from recalescence processes and X-ray investigation of as-quenched undercooled samples.