Search

Tantalum (Ta) alloys are important ultra-high-temperature structural materials owing to their excellent high-temperature mechanical properties and processability. However, they exhibit poor high-temperature oxidation resistance. In this study, a dense MoSi2 ceramic coating was prepared on a Ta substrate using an innovative multi-arc ion plating process and halide activated pack cementation in order to improve its ultra-high-temperature oxidation resistance. This ceramic coating exhibited a low roughness space arithmetic (287.1 ± 26.3 nm) and a dense structure. The relationship between the thickness of the coating and the duration of pack-cementation at 1250 ℃ was parabolic. The coating had a service life of more than 12 h at 1750 ℃, and showed excellent high-temperature oxidation resistance because of the uniform and dense structure of the coating and the rapid formation of a dense SiO2 layer with low O2 permeability during high-temperature oxidation.

To protect refractory metal against oxidation at ultra-high temperatures, a MoSi2-TaSi2 ceramic coating was prepared on a pure tantalum (Ta) substrate using a novel three-step process, which included dip-coating with a molybdenum slurry, vacuum sintering, and halide-activated pack cementation (HAPC). The original coating had a MoSi2-TaSi2 double-layer structure from the surface to the substrate. After oxidation at 1700°C for 8 h in air, the coating exhibited a complex multi-layer structure composed of SiO2-Mo5Si3-MoSi2-(Mo,Ta)5Si3-TaSi2-Ta5Si3 from the outer layer to the inner layer, due to the high-temperature phase transition and diffusion of Si and O. The coating effectively protected the Ta substrate at 1700°C for 12 h without failure, thereby demonstrating great improvement to its service life in an ultra-high-temperature aerobic environment. The protective effect was attributed to the integrity of the ceramic coating and the formation of a dense, uniform SiO2 film that effectively lowered the inward oxygen diffusion rate.

The dispersion of WC particles is critical to the microstructures and properties of WC-based alloys. The ultra-fine WC powders were uniformly dispersed in aqueous media via sodium dodecyl sulfate (SDS) with physical means. The influence of dispersion conditions such as concentration of dispersant, dispersion time and solid-liquid ratio on the powders dispersion was investigated by sedimentation test in present work. The particle size and distribution of WC powders, phase analysis and microstructure were characterized by the laser particle size analyzer, X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR) and Transmission electron microscope (TEM) and Zeta potential respectively. The results show that SDS was adsorbed and scattered on the WC powders, which could enhance the stability and dispersibility of suspension by the electrostatic effect. The diameter of dispersed WC powders ranged from 0.2 to 0.8 μm. The corresponding dispersive mechanisms of SDS were electrostatic repulsion and static hindrance, which kept the dispersibility and stability of the suspension. Furthermore, the optical dispersing time, solid-liquid ratio and the concentration of SDS for the dispersed WC powders in this work is 3 h, 1:50 and 1.44 g/L, respectively.

A new YSZ (yttria stabilized zirconia) modified silicide coating was prepared on Mo-based alloy through a two-step process, including spraying of Si-Mo-YSZ slurry followed by reaction sintering. Microstructure and oxidation behavior of the coatings with different YSZ content (0, 5, 10 and 15%) were studied. The coating consists of a MoSi2-ZrSi2-SiO2 outer layer and a MoSi2 inner layer. The 10% YSZ modified coating possessed the best oxidation protection at ultra-high temperatures and, for the coating, the mass gain was 0.96 mg/cm2 after being exposed at 1725 °C in air for 22 h, whereas the uncoated Mo-based alloy completely ablated for 0.35 h. The excellent oxidation resistance of the silicide coating was attributed to the compact and self-healing SiO2 protective scale, which contained the dispersed ZrO2 and ZrSiO4. Keywords: Molybdenum-based alloy, Silicide coating, Oxidation, YSZ, MoSi2, Ultra-high temperature

The cobalt coating with an average thickness of 15–20 nm was uniformly plated on the surface of ultra-fine WC powders by electroless plating in the present work. The effects of electroless plating condition parameters on the deposition rate were studied. It was found that the deposition rate increased with the increase of CoSO4·7H2O content, pH value, complexing agent concentration, and bath temperature, while decreased with the increase of reducing agent (NaH2PO2·H2O) concentration. Furthermore, the cobalt deposition rate empirical equation and activation energy were determined. The results show that the activation energy (Ea) for the electroless cobalt on WC substrates is approximately 54.65 kJ/mol.

The microstructures and mechanical properties of powder metallurgy Inconel 718 alloy were investigated in the temperatures range between 25 and − 253 °C. Tensile strength increased with the decrease in temperature, while the ductility first increased and then decreased. There was no significant change in impact toughness. When the temperature was − 253 °C, a zigzag stress–strain curve was observed for the alloy, owing to the interaction of dislocation glide and twinning, which effectively maintained the relatively good ductility.

A new type MoSi 2 /(Mo, Ti)Si 2 dual-phase composite coating for oxidation protection was designed and prepared on molybdenum alloy by a method of fused slurry. High temperature oxidation resistance of the coating has been investigated by isothermal oxidation test. Microstructure evolution and phase transformation of the as-prepared and oxidized coating were characterized. The results show that the composite coating is composed of primary C11 b MoSi 2 and C40 (Mo, Ti)Si 2 phases. The substitution of molybdenum with titanium in MoSi 2 gave (Mo,Ti)Si 2 of the C40 structure, resulting the formation of a C11 b MoSi 2 /C40 (Mo, Ti)Si 2 dual-phase composite coating. The composite coating could provide long term oxidation resistant protection for Mo alloy at high temperature, attributing to the good protection of a Si-Ti-O compound glass formation during oxidation.

A high-temperature oxidation-protective Si-Mo coating on a molybdenum alloy substrate was prepared by slurry sintering method. The structure, composition, phase distribution, and oxidation behavior of the coating were studied. The results show that the coating has a double-layer microstructure with an outer MoSi 2 layer and an inner Mo 5 Si 3 layer. After oxidation, the coating is composed of a SiO 2 outer layer, a MoSi 2 middle layer, and a Mo 5 Si 3 inner layer. The mechanism of the phase distribution and the oxidation kinetic equation were also studied. The oxidation rate K p determined over 12 h at 1650 °C, was found to be 0.061 mg 2 cm − 4 h − 1 . The excellent oxidation protective ability of the coating at ultra-high temperatures is attributable to the formation of self-healing SiO 2 on the coating surface after the oxidation of MoSi 2 .

The stress-aging method is proposed to improve the dimensional stability of Al-Cu-Mg (2024 aluminum) alloy. Both the reduction of residual stress and the acceleration of precipitates are investigated during the stress-aging process. It is observed that some S′ particles precipitate in the alloy during stress-aging. With the external stress increase to 100 MPa, the residual stress and thermal strain (ɛ c ) decrease, while the micro-yield strength increases significantly. This is attributed to that the dislocations are effectively pinned by the uniformly distributed precipitates and the stored elastic energy diminishes remarkably, which leads to a considerable improvement of the dimensional stability of 2024 aluminum alloy. However, when further increases the external stress to 150 MPa, due to the coarsened precipitates, the residual stress and thermal strain increase, while the micro-yield strength decreases slightly.

Ultra-fine WC/Co composite powders were prepared by electroless plating in this study. The Co layers with an average thickness of 50–100 nm were uniformly and completely covered on raw WC particles with a diameter of 0.3–0.5 µm. The influences of electroless plating conditions on the composites were investigated. The Co layer covering on WC particles was controlled by the content of CoSO 4 ·7H 2 O, pH value and bath temperature. With increasing the content of CoSO 4 ·7H 2 O up to 25 g/L, both the weight gain and the plating rate markedly increased. However, when the content of CoSO 4 ·7H 2 O further increased to 30 g/L, while the weight gain displayed a slight decrease, the plating rate continued to increase moderately. Moreover, with the increase of pH value from 9.5 to 11, both the weight gain and the reaction time decreased. Furthermore, the plating rate exhibited an exponential relationship with the bath temperature. When the pH value, bath temperature and the concentration of CoSO 4 ·7H 2 O are 10, 80 °C and 25 g/L respectively, the weight gain of Co-coated WC composite powders is 185.1 mg/g.

Due to the excellent structural and functional properties, such as low density, high fracture toughness and fatigue strength, Al-Cu-Mg (2024 aluminum) alloy has become an attractive light weight material for inertial instruments and aerospace applications. However, it has relatively low dimensional stability. The aim of this investigation is to enhance the dimensional stability of 2024 aluminum alloy via two-stage aging method, and identify the precipitation behavior and the underlying stabilizing mechanisms of 2024 aluminum alloy. When the second aging time was 24 h, the uniformly distributed precipitates could effectively pin the dislocations and impede their movement. This leaded to a significant improvement of the micro-yield strength and the thermal strain in the 2024 aluminum alloy, and thus the dimensional stability of the alloy increased.

The Nb-30Ti-18Al alloys doped with the amounts of 0 at%, 1 at% and 5 at% carbon were prepared by arc melting and annealing. The effects of carbon addition on the microstructure and mechanical properties of Nb ss /Nb 3 Al in situ composites as well as the strengthening mechanisms were investigated. The results showed that during annealing at 1200 °C, supersaturated Nb ss decomposed and Nb 3 Al phase precipitated in C-free alloy, while carbide decomposed with the precipitation of Nb 3 Al in C-doped alloys. Furthermore, the orientation relationship of Nb 3 Al phase and the surrounding (Nb, Ti)C was observed in Nb-30Ti-18Al-5C (NA5) alloy. Additionally, with increasing the carbon content, the compressive strength of alloys at high temperature were improved significantly ascribed to the occurrence of solid solution and volume fraction increase of strengthen phase, while the fracture toughness decreased due to the volume fraction decrease of Nb ss .

Due to excellent chemical stability, high rigidity, superior corrosion and wear resistance, aluminum oxynitride (AlON) has been considered as one of most promising candidate ceramic materials in high-performance structural, advanced abrasives and refractory fields. However, it usually exhibited relatively low flexural strength and poor fracture toughness. The study is aimed to develop silicon carbide (SiC) and zirconium nitride (ZrN) nano-particulate reinforced AlON composites with improved mechanical properties and fracture resistance via a hot-press sintering process. It was found that the addition of ZrO 2 nanoparticles would be transformed into ZrN during sintering. Due to the pinning effect of SiC and ZrN nano-particles positioned at grain boundaries of micro-sized AlON particles, the presence of SiC and ZrN nano-particles resulted in the reduction of both porosity and grain size, and a change of fracture mode from intergranular cracking in AlON to intragranular cracking in composites. With presence of 8 wt% SiC and 5.2 wt% ZrN nano-particles, the relative density, microhardness, Young’s modulus, flexural strength and fracture toughness increased. Different toughening mechanisms including crack bridging, crack branching and crack deflection were observed, thus effectively increasing the crack propagation resistance and leading to a considerable improvement in flexural strength and fracture toughness.

The in-situ laminated Nb/Nb5Si3 composite was prepared by spark plasma sintering. The microstructures and mechanical properties of the laminated Nb/Nb5Si3 composite were investigated in present work. It could be found that, alternant distribution microstructures of Nbss and Nb5Si3 layers were observed after sintering. The Nb/Nb5Si3 composite exhibited fracture toughness of 11.2 MPa·m1/2, and the predicted fracture toughness was estimated by the Ashby model. Moreover, the deviation of the experimental value from the predicted value was discussed. In addition, the compressive strength of the in-situ laminated Nb/Nb5Si3 composite was measured at 1400 °C and compared with that of the unidirectionally solidified Nb-17.5Si alloy.

The microstructure and mechanical properties of columnar-grained copper under casting and drawing conditions were investigated in the present work. It was found that the as-casted copper mainly consisted of continuous columnar grains with orientation preference along and exhibited excellent plastic deformation ability. Moreover, the texture of columnar-grained copper transferred from to during the drawing process. A typical microstructural morphology as multangular-cup feature of as-casted copper as well as drawn copper was observed on the fracture surfaces. With increasing the drawn strain, the texture evolution and the grains broken occurred, leading to a significant increase in the work hardening rate and tensile strength.

We investigate an unconventional resistive switching (RS) behavior in La0.67Sr0.33MnO3/BaTiO3/metal (LSMO/BTO) ferroelectric tunnel junctions (FTJs), which is dominated by the variation of the barrier potential profile modulated by the migration of oxygen vacancies in the p-LSMO/n-BTO junction. The LSMO/BTO/Co junction exhibits a remarkable self-rectifying effect ascribed to the high-density interface state at the BTO/Co interface, in contrast to the symmetric conductivity when the top metal electrode is inert Pt. The effects of ferroelectric polarization on the RS behavior are also emphasized. Our work builds a bridge between FTJs and resistive random access memory devices.

To improve the service reliability of components prepared by TiC-reinforced steel matrix composite material, tempering treatment was carried out to tailor the microstructure and properties of the composite. The aim of this study was to investigate the microstructure, properties, and dimensional stability of a TiC-reinforced steel matrix composite during tempering and to identify the underlying stabilizing mechanisms. We observed that the metastable retained austenite transformed into martensite and that residual stress released during tempering. The microstructure and properties of the composite reached an optimal state for a tempering time of 10 h. Furthermore, the relative dimensional change, Vickers hardness, thermal strain, and saturation magnetization of the composite were 0.0219%, 960.8 HV, 3.16 × 10−4, and 109.7 emu/g, respectively. Various stabilizing mechanisms, including retained austenite-martensite transformation and residual stress release, were observed during tempering, effectively improving the dimensional stability of TiC-reinforced steel matrix composites.

The 8% (mass fraction) yttrium-partially-stabilized zirconia (8YSZ) ceramic was fabricated via liquid phase sintering at 1 200–1 400 °C by adding different mass ratios of CuO-16.7%TiO2 (molar fraction) as sintering aid. Relative density, microstructure, Vickers hardness and bending strength as a function of sintering temperature and additive content were investigated. The experiment results show that liquid phase sintering at low temperature can be realized through adding CuO-16.7% TiO2 to 8YSZ. The Vickers hardness and bending strength of samples with sintering aid are generally much higher than those of samples without sintering aid for all sintering temperatures, and increase with the increase of sintering temperature. When the addition content of CuO-16.7% TiO2 is beyond 0.5%, the relative density, Vickers hardness and bending strength decrease with the increase of the mass ratio of sintering aid. Low additions of sintering aid are beneficial to aiding densification; high additions of sintering aid are detrimental to the sintered properties mainly due to greater amounts of pores generated by the volatilization of oxygen with the eutectic reaction between copper oxide and titanium dioxide. It is found that the fine grain size and high relative density are two main reasons of the high bending strength and Vickers hardness of the materials.

The sinterablility of 8 wt. % yttria-stabilized-zirconia (8YSZ) containing different weight ratios of MnO2 as sintering aid was investigated over the temperature range of 1200-1400 °C Relative density, microstructure, microhardness and bending strength as a function of sintering temperature and sintering aid content were investigated. The experiment results show that the addition of MnO2 is effective in aiding densification, and the relative density, microhardness and bending strength of doped samples are generally much higher than those of the undoped samples; the relative density, grain size and microhardness increase with the increasing of sintering temperature and addition content of MnO2; when the additive content of MnO2 is below 3 wt. %, the bending strength increases with the increasing of sintering temperature and addition content of MnO2, sintered at 1300 °Cwith 3 wt. % MnO2 addition, the sample has the maximum bending strength 235.5 MPa, fine grain size and high relative density are two main reasons of the high bending strength and hardness of the materials.

Lanthanum zirconate (La 2 Zr 2 O 7 , LZ) and CeO 2 doped La 2 Zr 2 O 7 (LCZ) ceramic were synthesized by the coprecipitation–calcination method. The chemical compositions, phase compositions, thermophysical properties of these materials were investigated. Inductively coupled plasma-atomic emission spectrometry (ICP-AES) and XRD results revealed that the compositions of all prepared ceramic materials were in the range of the synthesis of single La 2 Zr 2 O 7 with pyrochlore structure. After the prepared ceramic powders were mechanically ball-milled, vacuum dried, molded by cold pressure and densified by sinter at 1650 °C for 10 h, the bulk density of the sample was measured by the Archimedes method with an immersion medium of deionized water, a high-temperature dilatometer, DSC and laser thermal diffusivity method were used to analyze its thermal expansion coefficient, specific heat and thermal diffusivity. The results showed that, with the temperature increasing, the thermal expansion coefficient (TEC) of these ceramics increased, while the thermal conductivity decreased. The thermal expansion coefficient of La 2 Zr 2 O 7 was enhanced clearly by adding CeO 2 , and its thermal diffusivity and specific heat were all changed slightly. The thermophysical results also indicated that TEC of LCZ was slightly higher than that of conventional Y 2 O 3 –8 wt.% ZrO 2 (8YSZ), but its thermal conductivity was lower than that of 8YSZ. The lower thermal conductivity of LCZ was mainly attributed to more oxygen vacancies and larger atomic weight of substitutional atom (La, Ce). These results imply that LCZ can be explored as the candidate material for the ceramic layer in TBCs system.

Aluminum doped zinc oxide (AZO) polycrystalline thin films were prepared by sol–gel dip-coating process on optical glass substrates. Zinc acetate solutions of 0.5 M in isopropanol stabilized by diethanolamine and doped with a concentrated solution of aluminum nitrate in ethanol were used. The content of aluminum in the sol was varied from 1 to 3 at.%. Crystalline ZnO thin films were obtained following an annealing process at temperatures between 300 °C and 500 °C for 1 h. The coatings have been characterized by X-ray diffraction, UV–Visible spectrophotometry, scanning electron microscopy, and electrical resistance measurement. The ZnO:Al thin films are transparent (∼ 90%) in near ultraviolet and visible regions. With the annealing temperature increasing from 300 °C to 500 °C, the film was oriented more preferentially along the (0 0 2) direction, the grain size of the film increased, the transmittance also became higher and the electrical resistivity decreased. The X-ray diffraction analysis revealed single-phase ZnO hexagonal wurtzite structure. The best conductors were obtained for the AZO films containing 1 at.% of Al, annealed at 500 °C, 780 nm film thickness.

We report a large tunnel electroresistance (TER) effect up to ∼104% in La0.67Sr0.33MnO3/BaTiO3/Co (LSMO/BTO/Co) multiferroic tunnel junctions (MFTJs), which couples with well-defined tunnel magnetoresistance. The large TER is related to (LaAlO3)0.3(LaSrTaO6)0.7 substrates which guarantee a high-quality LSMO/BTO interface and robust ferroelectricity in BTO. The insert of 0.5 nm-thick Pt between the Co electrode and BTO barrier further enhances the TER value to 105% and improves the endurance of the MFTJs, ascribed to the shortened screening length and reduced oxidation of BTO/Co interface. Their use would advance the process towards practical MFTJs with four resistance states.