Intergranular Phase Research Articles

Effect of multiple thermal cycles on the microstructure evolution of GA151K alloy fabricated by laser-directed energy deposition

The microstructure and mechanical properties of high-performance Mg-RE (Magnesium rare earth) alloys prepared through laser-directed energy deposition (LDED) are worthy of an in-depth study, due to their excellent application perspectives in the aviation industry. In this work, a nominal Mg-15Gd-1Al-0.4Zr wt% (GA151K) alloy was fabricated by LDED, and the solid phase transformation induced by the application of multiple thermal cycles was investigated in detail. Moreover, in the last deposited layer, fine α-Mg grains, particle-like secondary phases, and intergranular island-like phases were observed. In addition, in the second layer from the top, α-Mg grains grew slightly by 1.4 ± 1.0 µm and the fraction of the intergranular island-like phase was significantly decreased by 9.3 ± 2.1%. As the deposition process progressed, the pre-solidified layers experienced additional thermal cycles, and two solid phase transformations occurred. One of the recorded transformations was that of the intergranular phase from Mg 3 Gd to Mg 5 Gd, while the other one was the precipitation of β′-Mg 7 Gd within the α-Mg matrix from the supersaturated solid solution. Here, the thermal cycles acted as an incomplete solution and aging treatment, leading to the unique morphology of the as-built GA151K alloy. The microhardness was improved by 6.9 ± 5.0 HV 0.2 with the application of consecutive thermal cycles. This work established a simplified relationship between the process parameters, the microstructure, and the mechanical properties of LDED Mg-Gd alloys, which is expected to promote the development and application of LDED high-performance Mg-RE alloys. • The transformation of intergranular phase from Mg 3 Gd to Mg 5 Gd and precipitation of β′ in AM Mg-Gd alloys are firstly reported. • The microstructure evolution principle of LDED GA151K under the influence of thermal cycles is studied in detail. • The importance of in situ precipitation strengthening to AM Mg-RE alloy is proposed.

High performance and exchange coupling in magnetization reversal of sintered (Nd,Dy)-Fe-B magnets

Improving the coercivity in sintered Nd-Fe-B magnets would sacrifice the remanence and the energy product. In this work it develops a method of preparing the composite (Nd,Dy)-Fe-B magnets to enhance the coercivity with a slighter decrease of remanence. The high performance (Nd,Dy)-Fe-B magnets were prepared via adding Dy-Fe-B into Nd-Fe-B powders in blending, followed by compacting and sintering. The addition of Dy-Fe-B powders gave rise to the enhancement of coercivity in the sintered magnets. The investigation of thermal activation indicates that the fluctuation field in thermal activation doesn’t obey the law of increasing with the coercivity. The grains of Dy-Fe-B crystal phase coexist in (Nd,Dy)-Fe-B magnets, which could increase the domain wall size by exchange coupling and decrease the fluctuation field in magnetization reversal. Some of Dy atoms diffuse into the grain outer-layer of Nd2Fe14B crystal phase, the grain boundary structure is probably optimized, the anisotropy increases at grain shell, and so the domain wall size would be reduced, leading to an increase of the fluctuation field. Though their effects of the exchange coupling and the anisotropy increasing are opposite on the changes of the domain wall size and fluctuation field, they are beneficial to improve the nucleation field in magnetization reversal, and the coercivity increases significantly in the sintered magnets. Since Dy-Fe-B powders mainly evolve into the main phase of RE2Fe14B crystal instead of the intergranular phase, the mass fraction of ferromagnetic phase doesn’t decrease evidently. Owing to the exchange coupling effect, the core-shell morphology of grains and the no remarkable increase in the amount of intergranular phase, the high performance with high energy product of 46.6 MGOe and high coercivity of 20.5 kOe could be obtained in the sintered (Nd,Dy)-Fe-B composite magnets.

Active Control of Interface Dynamics in NASICON-Based Rechargeable Solid-State Sodium Batteries.

Severe challenges are restraining the practical application of solid-state batteries, such as the dendrite growth and unsatisfactory compatibility between solid electrolyte and electrode. Here, we propose an interface dynamic control (IDC) strategy to ensure the stable operation of NASICON-based solid-state sodium batteries. First, we introduce intergranular phase (CuO) to effectively promote the densification of Na3Zr2Si2PO12 with an optimized ionic conductivity of 1.74 × 10-3 S cm-1 at 25 °C. Moreover, the kinetically formed Na-Cu-O interlayer reveals outstanding conductive capability. The dramatically reduced interfacial area-specific resistance (70 ohm cm-2) boosts the resistance to Na dendrite growth, ensuring the excellent cycling stability of symmetric Na cells at a current density of 0.4 mA cm-2 and room temperature. All-solid-state sodium metal batteries with Na3V1.5Cr0.5(PO4)3 cathode and modified Na3Zr2Si2PO12 ceramic electrolyte reveal a high retention of 87.4% at 100 mA g-1 over 300 cycles. This work opens up a new route for the rational interface design of NASICON-structure solid electrolyte toward the application in the high energy-density and high safety electrochemical energy storage devices.

An insight into the improved microstructure and elevated comprehensive properties of sintered Nd-Fe-B magnets via the infiltration of DyCu alloy

Grain boundary diffusion (GBD) can maximize the utilization of heavy rare earth (HRE) elements, and achieve the purpose of greatly improving the coercivity without reducing the remanence and maximum magnetic energy product. In this work, based on GBD technique, high coercivity Nd-Fe-B magnets are produced by using Dy30Cu70 alloy as the diffusion source. The results show that, with increasing diffusion temperature, the coercivity is enhanced monotonously from 1103 kA/m for as-prepared magnet to 1428 kA/m for diffusion magnet processed at 900 °C, increasing by 29.5%, and the remanence is decreased slightly. Microstructural characterization reveals that the GBD does not change the crystal structure of intergranular phase. Dy has diffused into matrix phase to form the (Nd, Dy)2Fe14B shell with a higher magnetocrystalline anisotropy field in the extended layer of matrix phase, inhibiting the nucleation of reversed magnetic domain. Cu is mainly enriched in RE-rich phase, which is responsible to the formation of thin RE-rich phase and can effectively increase magnetic isolation effect between matrix phases. Thus, the optimization of the intergranular phase and the formation of (Dy, Nd)2Fe14B shell should be responsible to the huge improvement of coercivity. In addition, the diffusion of DyCu alloy is beneficial in enhancing the thermal stability and corrosion resistance.

Effect of cobalt on the mechanical properties of 718 base alloy with 1.2 wt% Al

In this paper, the effect of Co (0–5 wt%) on the mechanical properties of 718 alloy and its strengthening mechanism were studied to clarify the function of Co on the mechanical properties of 718 base alloy. The experimental results showed that the addition of Co refined the grain. After the heat treatment, the addition of Co promoted the precipitation of Laves phase and γ′ phase, but inhibited the precipitation of γ″ phase. With the increasing of the Co content, the tensile strength was increased first and then decreased. It was found that the increase in the tensile strength of the alloy was caused by refining the grains and promoting the intergranular precipitation of the Laves phase. Nevertheless, Co increased the volume fraction of intragranular γ′ phase, while suppressed the precipitation of intragranular precipitation of γ″, thus the intragranular strength of the alloy was reduced. Accordingly, the alloy with 3 wt% Co shows the highest yield strength among the experimental alloys. Besides, the Co content extent 3 wt% will show a yield plateau under room temperature tension, but not in the case of 650 °C tension. It was revealed by analyzing the TEM morphology of the Laves phase in the room-temperature tensile samples and 650 °C tensile samples that the dislocation could shear through the intergranular Laves phase under the 650 °C tensile test, but it was not the case under room temperature tensile test. Therefore, the yield plateau in the stress-strain curves is due to the intergranular Laves phase impeding the dislocation motion.

Evolution of Nanometer-Scale Microstructure within Grains and in the Intergranular Region in Thermoelectric Mg3(Sb, Bi)2 Alloys.

n-type Mg3Sb2-Mg3Bi2 alloys have been investigated as one of the most promising thermoelectric materials. To achieve high performance, a detailed understanding of the microstructure is required. Although Mg3Sb2-Mg3Bi2 is usually considered to be a complete solid solution, nanosized compositional fluctuations were observed within a matrix and in the vicinity of the grain boundary. As an inhomogeneous microstructure can be beneficial or detrimental to thermoelectric performance, it is important to investigate the evolution of compositional variations for the engineering and long-term use of these materials. Using scanning transmission electron microscopy and atom probe tomography, a Bi-rich phase and compositional fluctuations are observed in sintered and annealed samples. After annealing, the broad intergranular phase was sharpened, resulting in a greater compositional change in the intergranular region. Annealing considerably reduces the fluctuations of Bi and Mg content within the grain as observed in atom probe tomography. Weighted mobility and lattice thermal conductivity were both increased as a result of the homogenized matrix phase. The combined microstructure features of intragrain and grain boundary effects resulted in an increased thermoelectric figure-of-merit zT of Mg3Sb0.6Bi1.4. These findings imply that the optimization of thermal and electrical properties can be realized through microstructure tuning.

Effect of low activation rare-earth oxides on sintering of β-SiC

The feasibility of sintering β-SiC at 1850 ℃ with using low-activation rare-earth oxides as sintering additives was studied. Thermodynamic calculation suggested that all five rare-earth oxides (Y2O3, Sc2O3, CeO2, La2O3, and Pr6O11) were suitable sintering additives. Comprehensive microstructure characterization further revealed that Pr6O11 was the most effective sintering additive as it suffered the least influence from the content effect. A mechanism based on liquid phase sintering was proposed based on microstructure observations. The radiation stability of sintered SiC was investigated as well. The intergranular phases (rare-earth oxides and silicates) and SiC grains remained after being irradiated to 17 dpa at 750 ℃.

Microstructure and extrinsic magnetic properties of anisotropic Sm(Fe,Ti,V)12-based sintered magnets

Anisotropic SmFe12-based sintered magnets require an optimum microstructure and alloy composition to have a large coercivity μ0Hc and remanent magnetization μ0Mr. In this study, we investigated the microstructure transformation and extrinsic performance change in the anisotropic Sm(Fe,Ti,V)12-based sintered magnets via reducing the Ti content and partially substituting Fe with Co. Reduction of Ti content in anisotropic sintered Sm8Fe73.5+xTi8-xV8Ga0.5Al2 magnet eliminated the secondary Fe2(Ti,V) phase, resulting in a remanence of μ0Mr = 0.8 T and energy density of (BH)max = 113 kJ/m3 (14.2 MGOe) in x = 3. A relatively high coercivity μ0Hc of 0.6 T is maintained owing to the formation of thin intergranular phase (IGP). Substitution of Co for Fe in the Sm8(Fe0.95Co0.05)73.5Ti8V8Ga0.5Al2 sintered magnet resulted in a low remanence and low coercivity μ0Hc. The electron backscatter diffraction (EBSD) and transmission electron microscope (TEM) observations indicate the formation of twins inside 1:12 grains which deteriorate the texture and remanent magnetization. Magneto-optical Kerr effect (MOKE) microscopy and micromagnetic simulations revealed that although the magnetization reversal starts at the twinned grains, Sm-rich intergranular phase hinders propagation of reversed domains to the neighboring grains, preventing the deterioration of coercivity. However, the formation of ferromagnetic SmFe2-based phase of the MgCu2 type in Co-containing magnet is responsible for a further decrease of coercivity to μ0Hc = 0.55 T. This study provides an insight on the optimum microstructure to realize high-performance anisotropic bulk SmFe12-based sintered magnets, i.e., ones without twins and ferromagnetic SmFe2-based phase of the MgCu2 type, but with refined grain size, and thin non/weak ferromagnetic IGP isolating the SmFe12 grains. For SmFe12-type phase, reducing the nonmagnetic elements and increasing its saturation magnetization remain crucial to realize the potential to compete with the Nd2Fe14B phase.

Unusual surface microstructural evolution of Nd–Ce–Fe–B sintered magnets by (Nd, Pr)Hx grain boundary diffusion

Usually the grain boundary diffusion process (GBDP) provides a facile approach toward high-coercivity Nd–Fe–B magnets by modulating the surface microstructure, i.e. constructing nonferromagnetic grain boundary layers and forming magnetically hardening shells of 2:14:1 main phase. However, by applying (Nd, Pr)H x GBDP to the 25 wt% Ce-substituted Nd–Ce–Fe–B sintered magnet, it is surprisingly discovered the ultimate coercivity below 12.4 kOe over a wide range of diffusion times. The low coercivity increment level of 1.5 kOe after an optimal 10 h diffusion is restrained by the unusual surface layer, i.e. an apparent Ce segregation at the surface region to form REFe 2 (RE = rare earth) intergranular phase at both triple junctions and grain boundaries, rather than the expected Nd/Pr-rich enrichment at the RE 2 Fe 14 B/RE-rich interface or at the GBs. Prolonging diffusion time from 6 to 20 h exacerbates the formation of REFe 2 phase and witnesses a more pronounced surface layer with a limited diffusion depth of ~80 μm, which is much lower than the reported GBDP Nd–Fe–B (millimeters) or Nd–La–Ce–Fe–B (hundreds of micrometers). The limited diffusion depth may be correlated to the blocked diffusion channel by the massive REFe 2 intergranular phase that replaces the conventional RE-rich phase. Above findings demonstrate that the characteristic REFe 2 phase highly affects the surface microstructural evolution of GBDP Nd–Ce–Fe–B magnet, and highlight future work toward sophisticated engineering of REFe 2 phase. • (Nd, Pr)H x GBDP yields unusual surface microstructure evolution of Nd–Ce–Fe–B sintered magnets. • REFe 2 TJ and GB phase forms preferentially at the diffusion surface. • Massive Ce segregates at surface region, rather than the expected Nd/Pr-rich intergranular phase. • Long-time diffusion of 20 h still restrains the diffusion depth to be below 100 μm. • The coercivity increment level of 1.5 kOe is much lower than other reported GBDP magnets.

Mechanically Reinforced Artificial Enamel by Mg2+-Induced Amorphous Intergranular Phases.

Amorphous intergranular phases in mature natural tooth enamel are found to provide better adhesion and could dramatically affect their mechanical performance as a structure reinforcing phase. This study successfully synthesized an amorphous intergranular phase enhanced fluorapatite array controlled by Mg2+ (FAP-M) at room temperature. Furthermore, atom probe tomography (APT) observation presents that Mg2+ is enriched at grain boundaries during the assembly of enamel-like fluorapatite arrays, leading to the formation of intergranular phases of Mg-rich amorphous calcium phosphate (Mg-ACP). APT results also demonstrated that the segregation of Mg2+ caused the chemical gradient in nanocrystalline attachment and realignment under the drive of inherent surface stress. These results indicate that the amorphous intergranular phases served like glue to connect each nanorod to reinforce the enamel-like arrays. Therefore, the as-received FAP-M artificial enamel exhibits excellent mechanical properties, with hardness and Young's modulus of 2.90 ± 0.13 GPa and 67.9 ± 3.4 GPa, which were ∼8.3 and 2.2 times higher than those of FAP arrays without controlled by Mg2+, respectively.

Rapid liquid phase-assisted ultrahigh-temperature sintering of high-entropy ceramic composites.

High-entropy ceramics and their composites display high mechanical strength and attractive high-temperature stabilities. However, properties like strong covalent bond character and low self-diffusion coefficients make them difficult to get sintered, limiting their mass popularity. Here, we present a rapid liquid phase–assisted ultrahigh-temperature sintering strategy and use high-entropy metal diboride/boron carbide composite as a proof of concept. We use a carbon-based heater to fast-heat the composite to around 3000 K, and a small fraction of eutectic liquid was formed at the interface between high-entropy metal diborides and boron carbide. A crystalline dodecaboride intergranular phase was generated upon cooling to ameliorate the adhesion between the components. The as-sintered composite presents a high hardness of 36.4 GPa at a load of 0.49 N and 24.4 GPa at a load of 9.8 N. This liquid phase–assisted rapid ultrahigh-temperature strategy can be widely applicable for other ultrahigh-temperature ceramics as well.

Formation of intergranular phases in precipitation hardening nickel-based alloy 725

From the two possible intergranular phases in nickel alloy 725 (UNS N07725, Inconel 725), i.e., M23C6 and the σ-related F phase, the abundant formation of F phase was found to critically compromise the hydrogen embrittlement resistance of the alloy. Nevertheless, the processing conditions and microstructural features that favor the formation of one phase over the other are yet to be investigated. Herein, intergranular phases in commercial batches in their as-received condition and after selective thermal treatments were studied to better understand the precipitation behavior. The distribution of grain boundary planes of the nickel matrix was assessed using stereological analysis of electron backscatter diffraction data. Results showed a direct correlation between the extent of F phase precipitation and the population of grain boundaries terminated on (111) planes, which provide low-energy nucleation sites for F phase. Our results suggest that while intergranular precipitates formed during aging, the preceding thermomechanical and thermal treatments defined the relative abundance of nucleation sites for each phase by affecting the grain boundary character distribution. Thus, targeted grain boundary engineering approaches could be developed to avoid susceptible microstructures and improve the hydrogen embrittlement resistance of the alloy. Furthermore, the results may help produce samples with different extents of intergranular M23C6 and F phase to evaluate, in future studies, the individual effect of the phases on the properties of the alloy.

Quasi‐Solid Electrolyte Design and In Situ Construction of Dual Electrolyte/Electrode Interphases for High‐Stability Zinc Metal Battery

AbstractInterfacial stability and compatibility in rechargeable metal batteries (RMBs) is still made difficult by deterioration under electrochemical dynamic operation due to the activeness of the metallic anodes and their spontaneous reaction with the liquid electrolyte. Herein, robust quasi‐solid zinc metal batteries enabled by in situ formation of stable dual electrolyte/electrode interphases with an electrochemical stability during cycling are reported. The quasi‐solid electrolyte sufficiently transfers the zinc ions due to the construction of the unobstructed ions transportation network composed of intergranular liquid phase migration and interlayer diffusion. The distinctive in situ formation of dual interphases, specially with heterojunction charge aggregation, cleverly settles the issues of interfacial compatibility, which achieves a stable Zn2+ plating/stripping of more than 3000 h and far exceeding the transient 250 h of its liquid counterpart. This design strategy for quasi‐solid electrolytes is expected to advance the development of the quasi‐solid battery field.

Concurrent improvements of corrosion resistance and coercivity in Nd-Ce-Fe-B sintered magnets through engineering the intergranular phase

Usually the improved coercivity of rare earth (RE) based 2:14:1-type permanent magnets via RE-rich intergranular additives is achieved at the cost of more corrosion channels and deteriorated corrosion resistance, which remains a challenging hurdle in the RE-Fe-B community. Distinctly, here we report the concurrent improvements of corrosion resistance and coercivity in 40 wt.% Ce-substituted Nd-Ce-Fe-B sintered magnets through engineering the intergranular phase using simple (Nd, Pr)Hx additive. The dehydrogenated Nd/Pr changes the RE concentration gradients between 2:14:1 matrix and intergranular phases during sintering and enlarges the fraction of corrosion-resistant REFe2 phase, rather than the conventionally assumed Nd/Pr-rich intergranular phase with high chemical vulnerability. The spontaneous formation of REFe2 intergranular phase after (Nd, Pr)Hx addition generates the uniquely enhanced corrosion resistance against the hot/humid and acidic environments, and counts as one peculiar feature of Nd-Ce-Fe-B magnets at high Ce substitution level, being distinct from previously reported Ce-free/lean RE-Fe-B. Simultaneously, the formation of continuous grain boundaries enhances the coercivity from 8.7 to 12.5 kOe with trace addition of (Nd, Pr)Hx. Above findings may spur progress towards developing a high-performance Nd-Ce-Fe-B permanent magnet.

The anisotropic fatigue properties of extruded Al–Zn–Mg–Cu alloy

The effect of specimen orientation on the fatigue properties of extruded Al–Zn–Mg–Cu alloy was investigated, and the relationship between tensile properties and fatigue strength was established. The results show that the fatigue properties of specimens vary largely with the orientation. The specimens with 0° orientation (relative to extrusion direction) have the highest fatigue strength, i.e. 301.3 MPa, whereas the fatigue strength of specimens with 45°and 90° orientation was reduced by 13.3% and 33.5%, respectively. The fatigue strength is linearly proportional to the elongation. It was found that the crack propagation modes vary with specimen orientation, a result of the grain orientation and intergranular coarse second phase particles, leading to the difference in fatigue lives. In addition, Basquin equations for specimens in the three directions were proposed for fatigue life prediction.

Magnetic properties and coercivity mechanism of high Ce-content CeNdFeB film with Tb diffusion

Magnetic properties and coercivity mechanism of an anisotropic Ce–Nd–Fe–B film with a high Ce content (70 at. % Nd is replaced by Ce) have been investigated. After grain boundary diffusion with Tb layers, an enhancement of coercivity from 4150 to 9250 Oe is observed. Combining the initial magnetization curves, micromagnetic theory, and in-siut observation of magnetic domains in the demagnetization process, it is confirmed that the coercivity mechanism for the high-Ce-content magnets is the mixed type dominated by the pinning mechanism. Moreover, as the thickness of the Tb diffusion layer increases, the pinning center of domain walls changes from narrow planar inhomogeneities to wide planar inhomogeneities. A growing role of pinning plays in determining the coercivity of samples with increasing the thickness of Tb layer due to the increase in magnetocrystalline anisotropy after Tb substitution in the RE2Fe14B phase. In addition, the CeFe2 intergranular phase leads to the enhancement of coercivity due to decoupling the hard magnetic phase grains. Our results provide an insight into the coercivity mechanism of high-Ce-concentration Ce–Nd–Fe–B magnetic materials and promote the comprehension of the effect of Tb diffusion in the magnetization reversal process.

Significant enhancement in the coercivity and thermal stability of bulk hot-deformed Nd-Fe-B magnets by intergranular addition of Nd-Dy/Tb-Cu-Ga alloys

High coercivities of 2.6 T and 2.7 T were achieved in 6-mm-thick hot-deformed Nd-Fe-B magnets through the intergranular addition of Nd35Dy35Cu15Ga15 and Nd35Tb35Cu15Ga15 alloy powders, respectively, consuming only 2.2 wt.% heavy rare-earth elements (HREs). These coercivities are similar to those attained in previous works in which approximately 4.5 wt.% HREs were consumed. The temperature coefficients of coercivity at 20−200 °C were improved from −0.45 %/°C to −0.39 %/°C and −0.38 %/°C for the two doped magnets, respectively, showing the excellent thermal stability of coercivity of the bulk hot-deformed magnets. Microstructure observations revealed that the Dy/Tb-containing nonferromagnetic intergranular phase was formed, the grain size and aspect ratio were decreased, and the high-anisotropy (Nd,Dy/Tb)2Fe14B matrix phase was formed after the doping process, which were responsible for the enhancement in coercivity. Micromagnetic simulations were performed to facilitate discussions of the importance and priority of these factors with regard to their effects on coercivity.

Pushing Lithium Cobalt Oxides to 4.7 V by Lattice‐Matched Interfacial Engineering

AbstractThe utilization of high‐voltage LiCoO2 is imperative to break the bottleneck of the practical energy density of lithium‐ion batteries. However, LiCoO2 suffers from severe structural and interfacial degradation at >4.55 V. Herein, a novel lattice‐matched LiCoPO4 coating is rationally designed for LiCoO2 which works at 4.6 V (vs Li/Li+) or above. This LiCoPO4 coating, derived by an in situ chemical reaction, grows epitaxially on LiCoO2 crystallite with strong bonding and complete coverage to LiCoO2, ensuring a stable cathode–electrolyte interface with fewer side reactions and alleviated intergranular cracking and phase collapse during repeated high‐voltage lithiation/delithiation processes. In addition, the formed strong covalent P–O tetrahedron configuration at the interface effectively decreases the surface oxygen activity of LiCoO2, further suppressing oxygen release and irreversible phase transition. Therefore, the LiCoPO4‐LiCoO2ǁLi cells display excellent capacity retention of 87% after 300 cycles at 4.6 V and stable operation at 4.6 V/55 °C or 4.7 V/30 °C. The strategy of lattice‐matching growth affords a new way to impact the development of high‐voltage LiCoO2 and beyond.

The effects of Nb addition on magnetic property of Nd10.23Pr2.56Fe68.32Co13.08B5.81 ribbons through modification of the intergranular phase and grain refinement

The (Nd10.23Pr2.56Fe68.32Co13.08B5.81)100-xNbx (x = 0, 0.5, 1, 2, 3) ribbons were prepared by melt-spinning at a wheel speed of 15 m/s. The effects of Nb addition on the microstructure and the magnetic properties of the ribbons were investigated. The X-ray diffraction (XRD) patterns of the ribbons show that the Nd2(Fe, Co)14B (2:14:1) phase is observed in all samples indicating that Nb addition does not change the phase constitution. The transmission electron microscopy (TEM) images show that Nb addition can dramatically reduce the average grain size from 52 ± 5 nm of the Nb-free alloy to 12 ± 2 nm of the Nb-containing x = 3 alloy. Based on the atom probe tomography (APT) result, we identify different mechanisms for grain refinement at different Nb content. When x = 0.5, the distribution of Nb is uniform at the intergranular phase and Nb addition promotes the segregation of rare-earth Nd and Pr at the thick ferromagnetic intergranular phase (≈ 10 nm), which leads to decreasing the average grain size to 38 nm. With further increasing Nb addition to x = 2, the existence of Nb-rich precipitations (2–4 nm in width) at the grain boundaries effectively reduces the grain size to 16 nm. The remanence (Br) and the maximum energy product ((BH)max) are obviously increased by Nb addition and the maximum of the Br and the (BH)max is obtained in the (Nd10.23Pr2.56Fe68.32Co13.08B5.81)98Nb2 (x = 2) alloy. The values of the Br and the (BH)max are 0.90 T and 138 kJ/m3, 8.4% and 22.1% higher than that of the Nb-free alloy. It ascribes to the grain refinement, the changes of the distribution and the composition of the ferromagnetic intergranular phase by Nb addition. Our findings provide a new method of designing prospective permanent alloys with increased magnetic properties by reducing the grain size and tuning the distribution and chemical composition of the intergranular phase.

Designing low-carbon MgO–Al2O3–La2O3–C refractories with balanced performance for ladle furnaces

In order to achieve high-quality and stable production of special steel, the performance of low-carbon MgO-C refractories needs to be further optimized. For this purpose, low-carbon MgO–Al2O3–La2O3–C refractories with enhanced thermal shock resistance and slag resistance were designed and successfully prepared by introducing Al2O3 as a reinforcer and La2O3 as a modifier. The results showed that the refractory samples with additives show better overall performance than those without additives. When 10 wt% of Al2O3 and La2O3 were added, the oxidation resistance, thermal shock resistance and slag resistance of the refractory samples coked at 1400 °C are increased by 13.57%, 17.75% and 43.09%, respectively. The analysis found that this can be mainly attributed to the formation of MgAl2O4, Mg2SiO4, and 2CaO·4La2O3·6SiO2 and the consequent volume expansion effect and intergranular phase enhancement effect. Therefore, a low-cost and enforceable reinforcement strategy for low-carbon MgO-C refractories is proposed, which is expected to be applied in steelmaking.