B2 Structure Research Articles

Mechanical properties of (HfCo)100−x(NbMo)x refractory high-entropy alloys with a dual-phase bcc-B2 structure

The common concept for the development of the high-performance metallic materials is associated with the combination of a soft disordered matrix and hard ordered reinforcements. However, unusual combinations of a hard disordered phase and a soft(er) ordered phase are also possible. In this work, we explored structure and mechanical properties of (HfCo)100−x(NbMo)x (x = 0; 10; 25; 60; 75; 100 at%) refractory high-entropy alloys. The binary HfCo and NbMo alloys had ordered B2 and disordered bcc structures, respectively. The (HfCo)100−x(NbMo)x (10 ≤ x ≤ 75) alloys had mostly a dual-phase structure of disordered (Nb, Mo)-rich bcc and ordered (Hf, Co)-rich B2 phase. Almost fully a eutectic-like structure was found in the (HfCo)75(NbMo)25 alloy. "Cube-on-cube" orientation relationships between the bcc and B2 phases ((110)bcc||(110)B2, [001]bcc||[001]B2) were detected. In addition, a small amount of Nb-rich bcc and/or Hf-rich fcc oxide phases were detected in some alloys. The B2 HfСo alloy was ductile but extremely soft, while, the NbMo alloy was hard and brittle. Simultaneous improvement of strength and ductility in the dual-phase (HfCo)100−x(NbMo)x (10 ≤ x ≤ 75) was revealed. The (HfCo)90(NbMo)10 alloy with a close to single-phase B2 structure exhibited a yield stress anomaly (YSA) at 500 °C. The YSA was associated with a change of dominant slip mode from a<100>to a<110>. Relationships between the chemical composition, microstructure, and the mechanical properties of the (HfCo)100−x(NbMo)x alloys were discussed.

Microstructure and wear resistance of AlCrFeNiMo0.5Six high-entropy alloy coatings prepared by laser cladding

In recent years, the coating prepared by laser cladding has attracted much attention in the field of wear research. In this work, AlCrFeNiMo0.5Six (x=0, 0.5, 1.0, 1.5, 2.0) high-entropy alloy coatings were designed and prepared on Q235 steel by laser cladding. The effect of Si content on microstructure, microhardness and wear resistance of the coatings was studied in detail. The results indicate that the AlCrFeNiMo0.5Six high-entropy alloy coatings show an excellent bonding between substrate and the cladding layer. The AlCrFeNiMo0.5Six coatings are composed of nano-precipitated phase with BCC structure and matrix with ordered B2 structure. With the addition of Si, the white phase (Cr, Mo)3Si with cubic structure appears in the interdendritic, and the morphology of the coating (x=2.0) transforms into lamellar eutectic-like structures. The addition of Si enhances the microhardness and significantly improves the wear resistance of the coatings. As x increases from 0 to 2.0, the average hardness of the cladding zone increases from 632 HV to 835 HV, and the wear rate decreases from 1.64×10−5 mm3·(N·m)−1 to 5.13×10−6 mm3·(N·m)−1. When x≥1.5, the decreasing trend of the wear rate gradually slows down. The wear rates of Si1.5 and Si2.0 coatings are 5.85×10−6 mm3·(N·m)−1 and 5.13×10−6 mm3·(N·m)−1, respectively, which is an order of magnitude lower than that of Q235 steel.

Pressure effects on structural, electronic and anisotopic elastic properties of Si doped RuGe compound with different concentrations by first-principles calculations

In this study, we have performed first-principles density functional theory (DFT) calculations to investigate pressure and composition effects on the structural, elastic, and electronic properties of silicon doped RuGe ternary compounds (RuSixGe1-x) for an increasing molar fraction of Si atom from 0.0 to 1.0 by 0.1. For each x composition, we have investigated formation energies of different compositions under three different pressure to study the alloying effects on the stability of RuGe in the B2 structure. It was determined that our calculated lattice parameters were in good agreement with the experimental results and decreased with Si content. The band structures and partial density of states (PDOS) have been investigated as electronic property. Using calculated second-order elastic constants, mechanical properties have been obtained for all x compositions. Among the different compositions for RuSixGe1-x under pressure it has been found that the most stable alloys have been obtained for x = 0.9, 0.7, and 0.6 under 0 GPa, 30 GPa, and 60 GPa, respectively. Elastic and dynamical stabilities were confirmed by Born Stability Criteria and positive phonon dispersion curves. Also, the elastic anisotropy has been visualized in detail by plotting the directional dependence of compressibility, Poisson ratio, Young's, and Shear module.

Theoretical prediction of high entropy intermetallic compound phase via first principles calculations, artificial neuron network and empirical models: A case of equimolar AlTiCuCo

Understanding the role of high entropy intermetallic compounds is essential for the fundamental description of high entropy alloys. Here, we present a theoretical study to predict the structure of equimolar AlTiCuCo using ab-initio calculations, agricultural neuron network and empirical models. Based on Miedema's theory, the potential reasonable configurations of the compound with L12 and B2 structure are predicted by ab-initio calculations. The mixing energy result indicates that the L12-type intermetallic compound is more likely to form. The calculated elastic moduli and phonon dispersion show that the predicated structures are mechanical and dynamical stable. The correlation coefficient between the elastic moduli and the input features for phase structure prediction is obtained. It reveals that average bulk modulus related features of the components have a strong effect on the elastic properties of the Al–Ti–Cu–Co intermetallic compound system. Finally, thermo-physical properties, electronic density of states and Bader's net charge are computed and compared.

The hyperfine magnetic fields and the effect of high pressure on the magnetic transition temperatures of the noncentrosymmetric FeRhGe2 compound (B20)

Hyperfine parameters and the pressure dependence of the magnetic transition temperatures of FeRhGe2 have been investigated. Sample has been prepared using high pressure—high temperature synthesis technique. FeRhGe2 consists of two B20 structure phases with close lattice constants. The phase separation stays constant in the temperature range 4–300 K. The magnetic transition temperatures = 213 K and = 135 K of FeRhGe2 slightly increases with pressure in the range 0–4.5 GPa. We have compared this pressure dependence with some others compounds in the family FeRh x Ge. The two phases in FeRhGe2 have slightly different values of the hyperfine magnetic fields.

First-Principles Calculation of Transport Properties of Heusler Alloy Co2MnAl at Finite Temperatures

We develop a framework for first-principles calculations of the transport properties of magnetic materials at finite temperatures within the coherent potential approximation. First, we calculate the grand potential by the tight-binding linearized muffin-tin orbital method and apply it to the Kubo formula to obtain the temperature dependence of the conductivities. Next, we demonstrate the computation for the magnetic moments M(T), longitudinal and transverse electrical conductivities, σxx(T), σxy(T), Seebeck coefficients Sxx(T), and anomalous Nernst coefficients Nxy(T) of Co2MnAl of both L21 and B2 structures. It is found that σxx(T) of L21 and B2 structures exhibit slow relaxation with temperature compared with most transition metal alloys, which reflects a transition of the system from a half-metallic to metallic system with increasing temperature. The σxy(T = 0) of the B2 structure is much smaller than that of the L21 structure, while σxx(T) of the B2 structure exhibits slower relaxation with temperature than the L21 structure. The Sxx(T) and Nxy(T) exhibit nonmonotonic variation in the low temperature region and the values above room temperature are in agreement with the experimental data.

Tuning skyrmions in B20 compounds by 4d and 5d doping

Skyrmion stabilization in novel magnetic systems with the B20 crystal structure is reported here, primarily based on theoretical results. The focus is on the effect of alloying on the $3d$ sublattice of the B20 structure by substitution of heavier $4d$ and $5d$ elements, with the ambition to tune the spin-orbit coupling and its influence on magnetic interactions. State-of-the-art methods based on density functional theory are used to calculate both isotropic and anisotropic exchange interactions. Significant enhancement of the Dzyaloshinskii-Moriya interaction is reported for $5d$-doped FeSi and CoSi, accompanied by a large modification of the spin stiffness and spiralization. Micromagnetic simulations coupled to atomistic spin-dynamics and ab initio magnetic interactions reveal the spin-spiral nature of the magnetic ground state and field-induced skyrmions for all these systems. Especially small skyrmions $\ensuremath{\sim}50\phantom{\rule{0.16em}{0ex}}\mathrm{nm}$ are predicted for ${\mathrm{Co}}_{0.75}{\mathrm{Os}}_{0.25}\mathrm{Si}$, compared to $\ensuremath{\sim}148\phantom{\rule{0.16em}{0ex}}\mathrm{nm}$ for ${\mathrm{Fe}}_{0.75}{\mathrm{Co}}_{0.25}\mathrm{Si}$. Convex-hull analysis suggests that all B20 compounds considered here are structurally stable at elevated temperatures and should be possible to synthesize. This prediction is confirmed experimentally by synthesis and structural analysis of the Ru-doped CoSi systems discussed here, both in powder and in single-crystal forms.

Experimental investigation and thermodynamic description of the Co–Nb–Ti system

The solidification microstructures of as-cast alloys and the equilibria microstructure of annealed alloys at 1273 K in the Co–Nb–Ti system were determined by scanning electron microscopy with energy dispersive spectroscopy (SEM/EDS) and X-ray diffraction (XRD), and the liquidus surface projection and partial isothermal section at 1273 K were constructed. Nine primary solidification regions, bcc(Nb,Ti), fcc(Co), Co3Ti, CoTi, CoTi2, λ1, λ2, λ3, and μ and five ternary invariant reactions, liq. → λ1 + λ2 + μ, liq. → CoTi + λ2 + μ, liq. → bcc(Nb,Ti) + CoTi + μ, liq. + fcc(Co) + λ3 → Co3Ti, and liq. + CoTi → bcc(Nb,Ti) + CoTi2 were experimentally determined or deduced in the liquidus surface projection. Three three-phase regions, CoTi + λ2 + μ, bcc(Nb,Ti) + CoTi + μ, and bcc(Nb,Ti) + CoTi + CoTi2 and six two-phase regions, Co3Ti + λ3, λ2 + μ, CoTi + λ2, CoTi + μ, bcc(Nb,Ti) + μ, and bcc(Nb,Ti) + CoTi at 1273 K were experimentally determined. The continuous compound λ2 with a C15 structure from the Co–Nb side to Co–Ti side was formed at 1273 K. The solubilities of Ti in μ and Nb in CoTi were determined to be ~11.1 at% and ~18.4 at% at 1273 K, respectively. And no new phases were found. According to the available experimental data from the present work and literatures, the Co–Nb–Ti system was optimized by CALPHAD (CALculatiuon of PHAse Diagram) method. Four solution phases (liquid, bcc, fcc, and hcp) were described using the substitutional solution model. A two-sublattice model (Co,Nb,Ti)2(Co,Nb,Ti)1 was used to described three compounds λ1, λ2, and λ3 according to their crystal structures and homogeneity ranges. Co7Nb2 and CoTi2 were treated as Co7(Nb,Ti)2 and Co(Nb,Ti)2 by a two-sublattice model, respectively. μ was treated as (Co,Nb,Ti)1(Nb,Ti)4(Co,Nb,Ti)2(Co,Nb,Ti)6 by a four-sublattice model. Co3Ti with L12 structure and CoTi with B2 structure were treated as the ordered phases of fcc and bcc solutions using the thermodynamic models (Co,Nb,Ti)0.75(Co,Nb,Ti)0.25Va1 and (Co,Nb,Ti,Va)0.5(Co,Nb,Ti,Va)0.5Va3, respectively. A set of self-consistent thermodynamic parameters of the Co–Nb–Ti system was obtained.

Predicting the Effect of Mo Addition on Metastable Phase Equilibria and Diffusion Path of Fe in NiAl Laser-Clad Coatings Using First-Principle Calculations and CALPHAD Simulations

This study used first-principle calculations and CALPHAD simulations to investigate the effects of adding Mo to NiAl laser-clad coatings in terms of metastable phase equilibria and Fe diffusion path with a focus on thermodynamic phase stability and element diffusion behavior. First-principle calculations were performed using 3 × 3 × 3 supercells to determine the formation energies of NiAl and Mo-rich phases within a Mo-doped NiAl cladding layer. The findings of this analysis are consistent with the d-orbital energy and bond order results obtained using DV-Xa molecular orbital calculations and phase diagrams obtained using Thermo-Calc simulations. The results also revealed that the substitution of Ni and Al atoms for Fe and Mo in the NiAl matrix decreased the stability of the B2 structure, thereby reducing phase formation energy. DICTRA simulations were also performed to characterize the diffusion behavior of Fe from the substrate to the surface of the coating. This analysis revealed that the rate of Fe diffusion was slower in the Mo phase than in the NiAl phase. Furthermore, the rate of Fe diffusion in molten material was inversely proportional to the Mo content. These results are consistent with the substitution mechanism used to describe diffusion, wherein diffusivity is inversely proportional to Mo content, due to its high melting point and the fact that un-paired electrons in the outer shell of Mo atoms increase the bonding strength, thereby hindering the diffusion of Fe. Due to the high cooling rates involved in the laser-cladding process, DICTRA simulations tend to overestimate the Fe diffusion distance. Nonetheless, the theoretical results obtained in this study were in good agreement with experiment observations (EPMA line scans). These results confirm the feasibility of using quantum modeling techniques and first-principle calculations to predict the effects of Mo addition on phase formation and element diffusion behavior in the NiAl laser-cladding process.

Structural transition and re-emergence of iron's total electron spin in (Mg,Fe)O at ultrahigh pressure

Fe-bearing MgO [(Mg1−xFex)O] is considered a major constituent of terrestrial exoplanets. Crystallizing in the B1 structure in the Earth’s lower mantle, (Mg1−xFex)O undergoes a high-spin (S = 2) to low-spin (S = 0) transition at ∼45 GPa, accompanied by anomalous changes of this mineral’s physical properties, while the intermediate-spin (S = 1) state has not been observed. In this work, we investigate (Mg1−xFex)O (x ≤ 0.25) up to 1.8 TPa via first-principles calculations. Our calculations indicate that (Mg1−xFex)O undergoes a simultaneous structural and spin transition at ∼0.6 TPa, from the B1 phase low-spin state to the B2 phase intermediate-spin state, with Fe’s total electron spin S re-emerging from 0 to 1 at ultrahigh pressure. Upon further compression, an intermediate-to-low spin transition occurs in the B2 phase. Depending on the Fe concentration (x), metal–insulator transition and rhombohedral distortions can also occur in the B2 phase. These results suggest that Fe and spin transition may affect planetary interiors over a vast pressure range.

High-entropy intermetallics with striking high strength and thermal stability

In this work, we report on a serious of high-entropy intermetallics (HEIs) based on CoHf-like B2 crystal structure. The microstructure evolution and mechanical properties of the targeted TiZrHfCoxNixCux alloys were detailedly investigated. In particular, the equal atomic ratio TiZrHfCoNiCu intermetallic with single-phase B2 structure shows ultrahigh compressive yield strength of 1.58 GPa and fracture strength of 2.48 GPa. It is striking that TiZrHfCoNiCu intermetallic exhibits a certain toughness and very high elastic strain limit of approximately 1.5%. Furthermore, it can maintain single-phase B2 structure and high strength after annealing at 900 °C, indicating its exceptional elevated temperature phase stability. This work not only broadens the family of high-entropy materials, but also provides a new perspective for the design of superalloys based on multi-principal element intermetallics.

Structural and magnetic properties of Fe–Al alloys: Ab initio studies

The structural and magnetic properties of Fe100−xAlx (5≤x≤25 at.%) alloys with different structural order are investigated within the framework of density functional theory. Using the Korringa–Kohn–Rostoker Green’s function method with a coherent potential approximation, the equilibrium lattice parameters, ground state energy and shear moduli for D03, B2 and A2 structures are calculated. For all structures, the optimized lattice constant increases and the shear modulus shows a decrease with increasing Al content. The tetragonal magnetostriction constants are estimated by the torque method. The A2 and B2 structures provide a positive contribution to the tetragonal magnetostriction. Using Monte Carlo simulations of the three-dimensional Heisenberg model, Curie temperatures are obtained over a wide concentration range.

Role of the M point phonons for the dynamical stability of B2 compounds

Although many binary compounds have the B2 (CsCl-type) structure in the thermodynamic phase diagram, an origin of the dynamical stability is not understood well. Here, we focus on 416 compounds in the B2 structure extracted from the Materials Project, and study the dynamical stability of those compounds from first principles. We demonstrate that the dynamical stability of the B2 compounds lies in whether the lowest frequency phonons around the M point in the Brillouin zone are endowed with a positive frequency, except for VRu. We show that the interatomic interactions up to the fourth nearest neighbor atoms are necessary for stabilizing such phonon modes, which should determine the minimum cutoff radius for constructing the interatomic potentials of binary compounds with guaranteed accuracy.

DFT Study of ScP and AlP in B1, B2, and B3 Phases at Ambient Pressure

Structural, electronic, elastic, and dynamic characteristics of scandium phosphide and aluminium phosphide compounds are studied theoretically. The study has been performed for rock salt (B1), CsCl (B2), and zinc blend (B3). We use the density functional theory, which is based on the plane wave basis and the pseudopotential approach, as implemented in the Quantum Espresso Software package. ScP and AlP exhibits metallic behavior in the B1 and B2 phases. The B3 phase of ScP displays a semiconductor characteristic with a broad and straight band gap at X of (1.617). In AlP, the B3 phase exhibits semiconductor characteristics with an indirect band gap at Г-X of (1.60). Both compounds ScP and AlP on B1, B2, and B3 structures report computed ground state parameters. The zinc blend phase is the most stable of the three phases, according to the findings. Our computed results are in good agreement with earlier investigations. The work presented in this article is related to sustainable development goals (SDGs) to transform our world regarding industry, innovation, and infrastructure.

Microstructure and atomic order analyses in CoFeCrAl Heusler alloy thin films: Interpretation of spin gapless semiconductor-like transport properties

We report a systematic analysis of transport properties, microstructure and atomic order in the thin films of the CoFeCrAl (CFCA) quaternary Heusler alloy that was predicted to be a spin-gapless semiconductor (SGS). Scanning transmission electron microscopy (STEM) observations revealed that the CFCA films in the as-deposited state and the one annealed at 400°C show single phase, but distinct phase separation occurs in the films annealed above 550°C. Quantitative atomic order analysis performed by the combination of laboratory XRD and anomalous X-ray diffraction (AXRD) shows the CFCA film annealed at 400°C had the B2 structure with large disorders with respect to the Y structure. Based on the site occupation of the elements obtained by the present structural analysis, we predict the electronic structure of the CFCA film by first-principles calculations, which clearly shows the disappearance of the SGS-like band structure. Nevertheless, the CFCA film still exhibits SGS-like transport properties such as negative linear temperature coefficient of resistivity, the change of dominant carrier type with temperature, and positive linear magnetoresistance. The origins of these transport properties are discussed based on the obtained non-SGS-like electronic structure to give a reasonable interpretation other than SGS electronic structure.

Study of the Fabrication of Gold Nanoparticle–Graphene-Arrayed Micro/Nanocavities as SERS Substrates Compared to Two Different Angles of Triangular Pyramid Tips

Surface-enhanced Raman scattering (SERS) has attracted attention because of its enormous potential to detect molecules with low concentrations. The method of fabricating SERS substrates is of great importance for improving the detection resolution. However, SERS substrates with different triangular pyramid tips fabricated by using the tip-based nanoindentation method has not been reported. Here, we prepared arrayed micro/nanocavities on copper-based graphene using the continuous indentation method with a Berkovich tip and a cube-corner tip, which have different face angles. Gold nanoparticles were then sputtered onto the graphene-copper micro/nanocavities to form the Au@GR@Cu micro/nanocavities SERS substrates. The substrates formed using the Berkovich tip and cube-corner tip were labeled B2-B9 and C2-C9, respectively, in which the numbers indicate the machining feed. Rhodamine 6G (R6G) was employed, and the Raman intensities of R6G on the differently arrayed Au@GR@Cu micro/nanocavities were measured. The Raman intensities of R6G were stronger on the pile-ups than on the inverted triangular pyramid cavities. The Raman intensities of R6G were highest on the C2 and B2 structures and lowest on the C9 and B9 structures. The Raman intensities of R6G on the arrayed Au@GR@Cu micro/nanocavities fabricated by the cube-corner tip were stronger than those on the arrayed Au@GR@Cu micro/nanocavities fabricated using the Berkovich tip with the same machining feed. In addition, the electric field intensity and distribution of the B9 and C9 arrayed Au@GR@Cu were simulated using Comsol software. Au@GR@Cu structures fabricated by the cube-corner tip were generated with higher electric field intensities. Furthermore, the relative standard deviations at 1362 cm-1 of R6G were 6.19 and 6.62% on the C2 and C4 surfaces, respectively, showing good homogeneity. The SERS spectra of 10-9 mol/L malachite green solution and 10-6 mol/L carbaryl solution were recognized on the C1, C2, and C4 surfaces on day 1 and after 3 months, respectively. After storage at room temperature for 3 months, the reductions in the Raman intensities were less than 10%, indicating excellent stability. The results showed that the arrayed Au@GR@Cu micro/nanocavities fabricated using the cube-corner tip performed better than those fabricated using the Berkovich tip and exhibited excellent uniformity, availability, and stability, providing great potential for detecting pesticides at low concentrations.

Effect of Ni addition on magnetism and corrosion resistivity of Ti/Co/Ni nanocomposites by sol–gel methods

In this study, nanocomposites of Ti/Co with varying Ni content were synthesized via the sol–gel process and calcination at 500 °C for 3 h, which were later used as a precursor for film fabrication. In order to fabricate the Ti/Co/Ni films, the prepared nanocomposites of Ti/Co/Ni were initially spin-coated on a glass substrate, and later heated to 250 °C for 4 h. The crystallite phase and size, surface morphology, microstructure, magnetic behavior, and corrosion resistivity were investigated using XRD, TEM, SEM, VSM, and potentiostat, respectively. The synthesized nanoparticles formed crystallite size ranging between 25 to 30 nm in diameter and exhibited the major crystallite phases of B2 structure, depending on the studied conditions. The saturation magnetization (Ms) was significantly increased from 0.07 to 1.27 emu/g and coercivity (Hc) increased from 280 to 400 Oe. Based on the result, the increase of Ni content enhanced not only the magnetic behavior but also the corrosion resistivity. Based upon the results, the synthesis of Ti/Co/Ni via sol–gel method was successfully conducted and used as a precursor for film fabrication.

Evolution of the microstructure and mechanical properties of as-cast Al0.3CoCrFeNi high entropy alloys by adding Si content

In this paper, a set of as-cast Al0.3CoCrFeNiSix (values of x is 0, 0.1, 0.2, 0.3, 0.4, 0.5) HEAs were fabricated by arc vacuum melting. The evolutions of microstructure and mechanical properties of alloys were systematically investigated. The results revealed that addition of Si could promote formation of (Al, Ni, Si)-rich phase with B2 structure (BCC1 phase) and (Fe, Cr)3Si phase with L21 structure (BCC2 phase). The microstructure of alloys changed from a single FCC phase to mixed FCC plus BCC1/BCC2 phases with a typical dendritic structure with increasing Si content. Meanwhile, addition of Si could improve the hardness and strength of alloys, the hardness and yield strength of alloys increased from 141 ± 2.0 to 445 ± 6.6 HV and 167 ± 4.3 to 713 ± 5.7 MPa with increasing Si content. According to the relationship between microstructure and mechanical properties, the strengthening mechanisms of current HEAs were evaluated, demonstrating that precipitation strengthening of ordered BCC phase is the dominant strength mechanism.

Microstructure and Properties of Additively Manufactured AlCoCr0.75Cu0.5FeNi Multicomponent Alloy: Controlling Magnetic Properties by Laser Powder Bed Fusion via Spinodal Decomposition.

A non-equiatomic AlCoCr0.75Cu0.5FeNi alloy has been identified as a potential high strength alloy, whose microstructure and consequently properties can be widely varied. In this research, the phase structure, hardness, and magnetic properties of AlCoCr0.75Cu0.5FeNi alloy fabricated by laser powder bed fusion (LPBF) are investigated. The results demonstrate that laser power, scanning speed, and volumetric energy density (VED) contribute to different aspects in the formation of microstructure thus introducing alterations in the properties. Despite the different input parameters studied, all the as-built specimens exhibit the body-centered cubic (BCC) phase structure, with the homogeneous elemental distribution at the micron scale. A microhardness of up to 604.6 ± 6.8 HV0.05 is achieved owing to the rapidly solidified microstructure. Soft magnetic behavior is determined in all as-printed samples. The saturation magnetization (Ms) is dependent on the degree of spinodal decomposition, i.e., the higher degree of decomposition into A2 and B2 structure results in a larger Ms. The results introduce the possibility to control the degree of spinodal decomposition and thus the degree of magnetization by altering the input parameters of the LPBF process. The disclosed application potentiality of LPBF could benefit the development of new functional materials.

Investigation of Cu-enriched precipitates in thermally aged Fe-Cu and Fe-Cu-Ni model alloys

To better understand the mechanisms of thermally aged and embrittled reactor pressure vessel steels, the hardening behavior and microstructure evolution of different Fe-Cu and Fe-Cu-Ni model alloys were investigated using transmission electron microscopy (TEM) and small-angle neutron scattering (SANS) analysis methods. The effects of different aging conditions and alloy composition on the formation and growth of Cu-enriched precipitates were discussed. The results showed that the hardness of the Fe-Cu model alloys with low copper content continuously increased with aging, increasing to peak hardness and then decreasing for the Fe-Cu model alloys with high copper content. The aging time to reach peak hardness was greatly reduced due to the addition of Ni in the Fe-Cu model alloys. The TEM results showed that with increased aging temperature and copper content, the Cu-enriched precipitates with a B2 structure precipitated in the ferritic matrix following a direct relationship. Therefore, Ni addition led to relatively higher precipitation nucleation and growth, and the size distribution of the Cu-enriched precipitates derived from the SANS was in overall good agreement with the TEM observations for 6–25 nm regions. Therefore, small-angle scattering could provide better details when analyzing smaller, nano-sized precipitates.