Microstructure and mechanical properties in the TLP joint of FeCoNiTiAl and Inconel 718 alloys using BNi2 filler

High entropy alloy(HEA) of Fe Co Ni Ti Al and Inconel 718 superalloy were firstly transient liquid phase(TLP) bonded by BNi2 filler due to the diffusion of Si and B in the filler to the base metals. The effects of bonding time on microstructure evolution and mechanical properties of the TLP joints were investigated.Owing to the complete isothermal solidification of the joints bonded for 30 min 120 min at 1100°C,no athermally solidified zones(ASZs) formed by eutectic phases were observed in the welded zone. Thus the TLP joints were only composed by the isothermally solidified zone(ISZ) and two diffusion affected zone(DAZ) adjacent to the dissimilar base metals and the negative effect of the ASZ on joint properties can be avoided. In addition, the increase of the bonding time can also make the Ti B2 borides precipitated in the DAZ near HEA and the brittle borides or carbides in the DAZ near IN718 alloy decrease and reduce the possibility of the stress concentration happened in the joints under loading. Therefore, the highest shear strength(632.1 MPa) of the TLP joints was obtained at 1100°C for 120 min, which was higher than that of the joint bonded for 30 min, 404.2 MPa. Furthermore, the extension of the bonding time made the fracture mechanism of the joint be transformed from the intergranular fracture to the transgranular fracture. However, as the brittle borides in the DAZ near IN718 can not be eliminated completely and refining of grains also happened in such region, all the TLP joints fractured inner the DAZ near IN718 alloy.     

Constitutive model and failure criterion for orthotropic ceramic matrix composites under macroscopic plane stress

To predict the nonlinear stress-strain behavior and the rupture strength of orthotropic ceramic matrix composites (CMCs) under macroscopic plane stress, a concise damage-based mechanical theory including a new constitutive model and two kinds of failure criteria was developed in the framework of continuum damage mechanics (CDM). The damage constitutive model was established using strain partitioning and damage decoupling methods. Meanwhile, the failure criteria were formulated in terms of damage energy release rate (DERR) in order to correlate the failure property of CMCs with damage driving forces, and the maximum DERR criterion and the interactive DERR criterion were suggested simultaneously. For the sake of model evaluation, the theory was applied to a typical CMC with damageable and nonlinear behavior, that is, 2D-C/SiC. The damage evolution law, strain response and rupture strength under incremental cyclic tension along both on-axis and off-axis directions were completely investigated. Comparison between theoretical predictions and experimental data illustrates that the newly developed mechanical theory is potential to give reasonable and accurate results of both stress-strain response and failure property for orthotropic CMCs.     

Plasma electrolytic deposition of α-Al2O3 on TiNb fibres and their mechanical properties

A novel TiNb fibre with an α-Al₂O₃ coating was fabricated by cathodic plasma electrolytic deposition (CPED), which has enormous potential for use in intermetallic matrix composites (IMMCs). This study aims to clarify the microstructural evolution of α-Al₂O₃ coatings on TiNb fibres and to systematically evaluate the mechanical properties of such modified fibres. The results revealed that the CPED process can be divided into three stages as voltage and deposition time increased: gas film formation, spark discharge, and spark fading, where the coating successively underwent local nucleation, uniform deposition, micropore self-sealing, and loose structure formation. The optimum deposition parameters of the deposition voltage of 300–400 V and deposition time of 3–4 min were determined, under which the α-Al₂O₃ coating combined tightly with the TiNb fibre matrix, micropores were completely self-sealed, and the loose structure and detrimental phase transitions in TiNb were effectively avoided. The fracture strength calculated by the Weibull method suggested that the fracture strength of the modified Al₂O₃/TiNb fibre was enhanced by more than 30%; this improved strength maintained high stability, benefiting from the intact α-Al₂O₃ ceramic coating. In particular, the fibre coated at 300 V for 4 min had the highest strength reaching 1620 MPa. The fracture morphology presented marked necking and shear lip characteristics, indicating excellent plasticity.     

Interactions of Li2O volatilized from ternary lithium-ion battery cathode materials with mullite saggar materials during calcination

In recent years, the rapid development of Li(Ni_xCo_yMn_1-x-y)O₂ (LNCM) materials for application in ternary lithium-ion batteries has led to an increased demand for refractory kiln saggars in industries. However, saggars used for firing ternary Li-ion battery cathode materials are often subjected to severe corrosion and spalling. To investigate the damage mechanism of the saggar materials, non-contact corrosion experiments were designed to study the effects of the precursor additions, calcination temperature, and number of calcinations during the interaction between mullite saggar and LNCM materials. The phase composition and microstructure of the mullite saggar specimens before and after corrosion were characterized using X-ray diffraction and scanning electron microscopy, respectively, to obtain a comprehensive understanding of the causes of the deterioration of mullite saggar materials during corrosion.     

Microstructure and mechanical properties of α/β-Si3N4 composite ceramics with novel ternary additives prepared via spark plasma sintering

In this study, α/β-Si₃N₄ composite ceramics with high hardness and toughness were fabricated by adopting two different novel ternary additives, ZrN–AlN–Al₂O₃/Y₂O₃, and spark plasma sintering at 1550 °C under 40 MPa. The phase composition, microstructure, grain distribution, crack propagation process and mechanical properties of sintered bulk were investigated. Results demonstrated that the sintered α/β-Si₃N₄ composite ceramics with ZrN–AlN–Al₂O₃ contained the most α phase, which resulted in a maximum Vickers hardness of 18.41 ± 0.31 GPa. In the α/β-Si₃N₄ composite ceramics with ZrN–AlN–Y₂O₃ additives, Zr₃AlN MAX-phase and ZrO phase were found and their formation mechanisms were explained. The fracture appearance presented coarser elongated β-Si₃N₄ grains and denser microstructure when 20 wt% TiC particles were mixed into Si₃N₄ matrix, meanwhile, exhibited maximum mean grain diameter of 0.98 ± 0.24 μm. As a result, the compact α/β-Si₃N₄ composite ceramics containing ZrN–AlN–Y₂O₃ additives and TiC particles displayed the optimal bending strength and fracture toughness of 822.63 ± 28.75 MPa and 8.53 ± 0.21 MPa?m^1/2, respectively. Moreover, the synergistic toughening of rod-like β-Si₃N₄ grains and TiC reinforced particles revealed the beneficial effect on the enhanced fracture toughness of Si₃N₄ ceramic matrix.     

Effect of interface types on the heat-resistance of Cansas-II SiCf /SiC composites

The heat-resistance of the Cansas-II SiC/CVI-SiC mini-composites with a PyC and BN interface was studied in detail. The interfacial shear strength of the SiC/PyC/SiC mini-composites decreased from 15 MPa to 3 MPa after the heat treatment at 1500 °C for 50 h, while that of the SiC/BN/SiC mini-composites decreased from 248 MPa to 1 MPa, which could be mainly attributed to the improvement of the crystallization degree of the interface and the decomposition of the matrix. Aside from the above reasons, the larger declined fraction of the interfacial shear strength of the SiC/BN/SiC mini-composites might also be related to the gaps in the BN interface induced by the volatilization of B₂O₃·SiO₂ phase, leading to a significant larger declined fraction of the tensile strength of the SiC/BN/SiC mini-composites due to the obvious expansion of the critical flaws on the fiber surface. Therefore, compared with the CVI BN interface, the CVI PyC interface has better heat-resistance at high temperatures up to 1500 °C due to the fewer impurities in PyC.     

Coexistence of antiferro- and ferrimagnetism in the spinel ZnFe2O4 with an inversion degree δ lower than 0.3

Samples with inversion parameter values (δ) ranging from 0.27 to 0.14 while maintaining the crystallite size value have been successfully fabricated from commercially available powders by mechanical grinding and thermal annealing treatments at temperatures ranging between 400 and 600 °C. Detailed characterization studies of these samples using X-ray, neutron diffraction and magnetic measurements have confirmed for the first time the simultaneous coexistence at 2 K of short range antiferromagnetic and ferrimagnetic ordering for a wide range of the inversion parameter. The magnetic phase diagram obtained is different from the one previously reported, which shows at 2 K the coexistence of long range antiferromagnetic order and short range order for values of inversion parameters less than 0.1 and the presence of a ferrimagnetic order only for values of δ > 0.2. At room temperature, the Rietveld analysis of NPD patterns and the magnetization curves showed a paramagnetic behavior in the samples with δ ≤ 0.1. For the samples with higher cationic inversion, typical hysteresis curves of ferrimagnetic materials were observed and the saturation magnetization values obtained agree quite well with the net magnetic moment obtained from the Rietveld refinement of the neutron diffraction patterns.     

Development and performance analysis of novel in situ Cu–Ni/Al2O3 nanocomposites

Cu–Ni/Al₂O₃ nanocomposite powders were manufactured using an in situ chemical reaction technique. This technique provides improved wettability and adhesion between the matrix and reinforcement phases. Aluminum nitrate, copper nitrate and nickel nitrate were used as start materials for the production of the composites. The powders were sintered in a hydrogen environment at 900 °C for 2 h after being cold pressed at 700 MPa. To determine the effect of Al₂O₃ on electrical and thermal conductivities and thermal expansion behaviors, the Cu–Ni matrix was supplemented with 3, 5, and 8 wt% Al₂O₃. The findings revealed that Al₂O₃ nanoparticles (20 nm) were dispersed uniformly throughout the copper-nickel matrix. Microhardness was improved from 53.3 HV for Cu–Ni matrix to 92.7 HV for Cu–Ni/8%Al₂O₃ nanocomposites. The electrical and thermal conductivities and thermal expansion coefficient were reduced as the amount of Al₂O₃ in the Cu–Ni matrix increased. The electrical conductivity was reduced by 38.7% by addition 5% Al₂O₃ nanoparticles to Cu–Ni matrix. The high interfacial bonding between Cu–Ni and Al₂O₃ nanoparticles was the main reason of the hardness improvement and maintaining relatively good electrical and thermal properties.     

Effects of andalusite and kyanite addition on the microstructure,thermo-mechanical properties and corrosion resistance of ultralow-cement bauxite-corundum castables

Ultralow-cement bauxite-corundum refractory castables are increasingly attracting attention because of the advantages in performance and cost. To improve its volume stability and high-temperature performance, andalusite and kyanite were incorporated into this type of refractory, respectively, and a comparative investigation of their effects on microstructure, thermo-mechanical properties, and slag corrosion resistance was carried out in this work. The expansion effects induced by the mullitization of these additives not only suppressed the sintering shrinkage but also made the matrix structure more compact and uniform, mainly owing to the generation of many new and strong mullite bonds. This improvement resulted in a significant increase in the refractoriness under load and creep resistance. This effect was far more pronounced for the andalusite-added castables, for which the RUL increased by 62 °C and the creep rate at the stationary stage decreased by 26%. Besides, adverse effects on microstructure and properties appeared when adding a higher amount of kyanite due to the excessive expansion. Moreover, the corrosion mechanisms of these refractory castables by refining slag were discussed, and the active role of andalusite addition in the corrosion resistance was also proposed.     

Corrosion resistance and electrical conductivity of plasma nitrided titanium

The continuous and dense Ti–N compound layers with a thickness ranging from 0.7 to 2.1 μm were formed on the titanium by plasma nitriding at 700 °C for different times with hollow cathode discharge assistance. Scanning electron microscope (SEM), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) were used to characterize the nitrided layer. XRD and XPS results showed that the compound layer was mainly composed of Ti₂N phase. The corrosion current density of 4 h nitrided titanium was 0.016 μA/cm² (cathode) and 0.03 μA/cm² (anode), respectively. The electrical conductivity of samples was evaluated by means of the interfacial contact resistance (ICR). The value of 4 h nitrided titanium was 4.94 mΩ-cm² which was much lower than that of original titanium 26.25 mΩ-cm² under applied force of 150 Ncm^?2 after corrosion test. The results showed that the electrical conductivity and corrosion resistance of the titanium bipolar plates (BPs) were apparently improved with the formation of Ti₂N compound layer.