Mechanical properties of polytypoidally joined Si3N4–Al2O3

Crack-free joining of alumina and silicon nitride has been achieved by a unique approach introducing sialon polytypoids as a functionally graded materials (FGM) bonding layer. The polytypoid compositions are identified in the phase diagram of the Si₃N₄–Al₂O₃ system. The thermal stresses of this FGM junction were analyzed using a finite element analysis program taking into account both coefficient of thermal expansion and modulus variations. From this analysis, the result showed a dramatic decrease in radial, axial and hoop stresses as the FGM changes from three layers to 20 graded layers. Scaling was considered, showing that the graded transition layer should constitute about 75% or more of the total sample thickness to reach a minimal residual stress. Oriented Vickers indentation testing was used to qualitatively characterize the strengths of the joint and the various interfaces. The indentation cracks were minimally or not deflected at the sialon layers, implying strong interfaces. Finally, flexural testing was conducted at room temperature and at high temperature. The average strength at room temperature was found to be 581 MPa and the average strength at high temperature (1200°C) was found to be 262 MPa. Scanning electron microscope observation of fracture surfaces at a different loading rates indicated that the strength loss at higher temperatures was consistent with a softening of glassy materials present at grain junctions.     

Effect of predeformation on globularization of Ti–5Al–2Sn–2Zr–4Mo–4Cr during annealing

The microstructure evolution during annealing of Ti–5Al–2Sn–2Zr–4Mo–4Cr alloy was investigated. The results show that for the alloy compressed at 810 °C and 1.0 s^?1 deformation amount (height reduction) 20% and 50% and annealed at 810 °C, thermal grooving by penetration of β phase is sufficient during the first 20 min annealing, resulting in a sharp increase in globularization fraction. The globularization fraction continuously increases with the increase of annealing time, and a height reduction of 50% leads to a near globular microstructure after annealing for 4 h. For the alloy with deformation amount of 50% by compressing at 810 °C, 0.01 s^?1 and then annealed at 810 °C, thermal grooving is limited during the first 20 min of annealing and large quantities of high-angle grain boundaries (HABs) remain. With long time annealing, the chain-like α grains are developed due to the HABs, termination migration and Ostwald ripening. The present results suggest that a higher strain rate and a larger height reduction are necessary before annealing to achieve a globular microstructure of Ti–5Al–2Sn–2Zr–4Mo–4Cr.     

Electromagnetic attenuation property of multiphase Fe–Fe_3O_4–Al_2O_3 cermets near percolation threshold

Cermets are widely applied as attenuating materials due to high electromagnetic loss and better mechanical properties. In this paper, multiphase cermets composed of iron, iron oxide and alumina were successfully prepared by a two-step in situ synthesis process,which includes pressureless sintering and a selective reduction in hydrogen atmosphere. The phase composition and microstructure of cermets were analyzed by X-ray diffraction(XRD) and scanning electron microscopy(SEM), respectively. It is shown that nanosized cuboid Fe particles and octahedral Fe_3O_4 particles are distributed in alumina matrix. The permittivity and permeability of composites were tested with radio frequency impedance analyzer(0.01–1.00 GHz). The results show that permittivity presents obvious frequency dispersion. Furthermore,dielectric constants of multiphase cermets get enlarged due to the enhancement of interfacial polarization. On the other hand, there is a magnetic loss peak in permeability spectra,which indicates typical relaxation behavior. It is possible to achieve better electromagnetic attenuation property by adjusting process parameters.     

Reaction between Ti–6Al–4V and Y_2O_3–SiO_2 based face shell for investment casting

The pH value and viscosity of Y_2O_3–SiO_2 (Y–Si) slurry made by Y_2O_3 powders and silica sol for the face coat of Ti–6Al–4V investment casting were measured. The thermal behavior of the shell made by the Y–Si face coat system was investigated by differential scanning calorimeter(DSC), thermal gravimetric(TG) analysis combined with mass spectrometry(MS), and the phase transformations were determined by X-ray diffraction(XRD). Hot strength, residual strength, linear expansion coefficient, and wearing resistance performance of the shell were also tested. The microstructure and elements distribution of the interaction layer were studied by scanning electron microscope(SEM) and energy-dispersive spectrometer(EDS), respectively. The microhardness tester was applied for the microhardness. The results showed that the slurry was stable for at least 60 h. A very small amount of YZrO_3 was formed below 1050 °C and Y_2 SiO_5was formed around 1450 °C. The shell made by Y–Si system had good mechanical property which could reduce cracks during the procedure of dewaxing and inclusions during pouring.Some Al volatilized from the melt, permeated the surface of the face coat shell, and formed the black reaction layer,which blocked the permeation of O so that O penetration was limited to 5 lm. The depth of Si penetration was about 60 lm. The hard layer was also around 60 lm.     

Investigating mechanochemical behavior of Cr2O3–B2O3–Mg–C quaternary system

Fundamental aspects of reaction behavior and formation path in the Cr₂O₃–B₂O₃–Mg–C quaternary system have been studied to synthesize chromium boride–chromium carbide nanocomposite. In order to find the influence of simultaneous presence of magnesium and carbon on final products, various powder mixtures were chosen according to following reaction: B₂O₃ + Cr₂O₃ + (9 − x) Mg + x C. The value of x varied from 0 to 4. In the absence of carbon (x = 0), CrB₂ was synthesize through mechanically induced self-propagating reaction (MSR). In the presence of 8 mol Mg and 1 mol C (x = 1), the dominant boride phase was CrB while no chromium carbide was detected. By increasing C content (x = 2), the magnesiothermic reduction occurred in MSR mode; whereas, the synthesis of Cr₃C₂ initiated after combustion reaction and completed gradually during milling for 6 h. Further increase in C amount (x = 3) resulted in formation of Mg₃(BO₃)₂ as unwanted phases as well as CrB and Cr₃C₂. In the presence of 6 mol Mg and 4 mol (x = 4), no mechanical reaction was observed even after 8 h of milling. Optimum value of x for the formation of CrB–Cr₃C₂ nanocomposite was 2. Based on the morphological evolutions, it is evident that the mechanosynthesized powder is made up of nanometric particles.     

Synthesis and characterization of novel CoFe2O4–BaTiO3 multiferroic core–shell-type nanostructures

Novel multiferroic nanostructures of cobalt ferrite–barium titanate were synthesized by a two-step wet chemical procedure, combining co-precipitation and sol–gel techniques. The fraction of cobalt ferrite in the nanostructures was varied from 20 to 60 wt.%. X-ray diffraction confirmed the presence of both the spinel and the perovskite phases, with average crystallite sizes in the range of 15–28 nm. Both the degree of tetragonality of barium titanate and the lattice parameter of cobalt ferrite significantly increased with the content of ferrite in the nanostructures, revealing a crystallographic distortion related to the shell thickness. Transmission electron microscopy data showed two-phase composite nanostructures consisting of a cobalt ferrite core surrounded by a barium titanate shell-like coating. Magnetization data showed expected ferromagnetic behavior. The multiferroic nanostructures are proposed as building blocks for higher-order multiferroic inorganic and hybrid inorganic–organic nanocomposites.     

Hydrothermal conversion of Ti-containing minerals in system of Na_2O–Al_2O_3–SiO_2–CaO–TiO_2–H_2O

Titanium has a great effect on the digestion of bauxite in the Bayer process because it reacts readily at high temperatures in alkaline sodium aluminate solution.Under this consideration, the hydrothermal conversion of Ti-containing minerals in the system of Na_2O–Al_2O_3–Si O_2–Ca O–Ti O_2–H_2O with increased temperatures was studied based on the thermodynamic analysis and systematic experiments. The results show that anatase converts to Al_4Ti_2 SiO_(12) at low temperatures(60–120 °C), which is similar to anatase in crystal structure. As the temperature continues to rise, Al4Ti2 Si O12decomposes gradually and converts to Ca_3 Ti Si_2(Al_2Si_(0.5)Ti_(0.5)O_(14) at 200 °C. When the temperature reaches 260 °C, Ca Ti O_3 forms as the most stable titanate species for its hexagonal closest packing with O_2-and Ca_2?. The findings enhance the understanding of titanate scaling in the Bayer process and clarify the mechanism of how additive lime improves the digestion of diaspore.     

Crystal morphology engineering in SiO2–Al2O3–MgO–K2O–Na2O–F− mica glass-ceramics

The morphology of mica crystals formed in extruded glass-ceramics of the system SiO₂–Al₂O₃–MgO–K₂O–Na₂O–F₂ was characterized using transmission electron microscopy. Detailed information on the crystal shape could be obtained since mica crystals in the extruded material exhibit an extraordinary degree of orientation. Highly defective mica plates start growing in coexistence with norbergite from a Mg²⁺- and F⁻-depleted glass. Upon the dissolution of norbergite at elevated temperatures, defective plates are sandwiched between 1M polytype phlogopite slabs epitaxially formed on {001} faces of the defective mica crystals. As shown by means of several examples, depending on the schedule of heat treatment the thickness of the defective slab as well as the diameter ratio of inner and outer plates can be adjusted. Linear growth rates are given for defective and non-defective mica plates at different temperatures enabling the controlled engineering of the mica crystal morphology.     

Hardening precipitation in a Mg–4Y–3RE alloy

Early stages of precipitation in a Mg–Y–Nd based alloy aged at 150 °C have been studied using TEM and small angle X-ray scattering (SAXS). The former brings information concerning nature, morphology and size of precipitates, and the latter adds qualitative and quantitative information concerning populations of precipitates in terms of size and volume fraction. Precipitation at 150 °C involves formation of DO₁₉ monoplanar precipitates, which further develop into the β″ and β′ phases having platelet and globular morphologies, respectively. TEM observations on samples aged at 150 °C reveal the formation mechanism of the bco-β′ structure by the ordering of monoplanar DO₁₉-β″ precipitates. Additional examinations at 250 °C revealed the DO₁₉-β″→bco-β′ transformation, as well as β₁ precipitates. Estimation of the volume fraction deduced from SAXS is discussed on the basis of the TEM results.     

A Comparative Study of High Temperature Corrosion of Al2O3, SiO2 and Al2O3–SiO2 Forming Alloys in a Na2SO4–NaCl Atmosphere

The present paper is designed to provide a summary of our study on the high temperature corrosion of Al₂O₃, SiO₂ and Al₂O₃–SiO₂ forming alloys in the gas phase and liquid phase of Na₂SO₄–NaCl system by comparing their corrosion resistance at 1000 °C. The obtained results show that the alumina-forming alloy experiences severe internal corrosion in the gas phase compared to in the liquid phase due to oxide cracking. This results in an increase in the inward diffusion and/or penetration of constituents of the salts and oxygen to form internal Al-oxide and Cr-sulfides. In the liquid phase, however, the formation of yttrium sulfide beneath a continuous double oxides layer of Al₅Y₃O₁₂ and Al₂O₃ may be related to the high affinity of yttrium for sulfur. On the other hand, it is apparent from the cross-sectional observations that a SiO₂ and Al₂O₃–SiO₂ forming alloys form a continuous and dense oxides layer, and demonstrate a high resistance against internal oxidation and corrosion in both corrosive environments.     

Microstructure and tensile properties of isothermally forged Ni–43Ti–4Al–2Nb–2Hf alloy

NiTi-Al-based alloys are promising high-tem- perature structural materials for aerospace and astronautics applications. A new NiTi-Al-based alloy Ni--43Ti-4AI- 2Nb-2Hf （at%） was processed via isothermal forging. The microstructure and mechanical properties at room temperature and high temperature were investigated through scanning electron microscope （SEM）, X-ray diffraction （XRD）, and tensile tests. Results show that the micro- structure of as-forged Ni-43Ti--4AI-2Nb-2Hf alloy con- sists of NiTi matrix, Ti2Ni phase, and Hf-rich phase. The simultaneous addition of Nb and Hf, which have strong affinities for Ti sites, promotes the precipitation of Hf-rich phases along the grain boundaries. The tensile strengths of Ni-43Ti-4A1-2Nb-2Hf alloy are dramatically increased compared with the ternary Ni-46Ti-4A1 alloy. At room temperature and 650℃, the yield stress of Ni--43Ti-4Al- 2Nb-2Hf alloy reaches 1,070 and 610 MPa, respectively, which are 30 % and 150 % higher than those of Ni--46Ti- 4Al alloy. The improved tensile property results from the solid solution strengthening by Nb and Hf, as well as the dispersion hardening of the Ti2Ni and Hf-rich phases.     

Na_2SO_4-Fe_2(SO_4)_3、K_2SO_4-Na_2SO_4-Fe_2(SO_4)_3熔体的生成热力学

通过使Fe_2O_3-Na_2SO_4与不同分压的SO_3建立平衡,对低熔点Na_2SO_4-Fe_2(SO_4)_3二元熔融硫酸盐的形成进行热力学研究。实验确定了在熔融Na_2SO_4-Fe_2(SO_4)_3中平衡Fe_2(SO_4)_3的摩尔含量。用Temkin随机混合模型和规则溶液近似并加以适当修正,计算了反应Fe_2O_3 3SO_3=Fe_2(SO_4)_3(1,溶于液相Na_2SO_4)的自由焓变化ΔG°(T)=—116830 141.15T cal/mol(943～1003K) 用这一结果并借助于Na_2SO_4-Fe_2(SO_4)_3系相图估算了形成液相Na_2SO_4-Fe_2(SO_4)_3所需临界Pso_3,并用实验加以检验。由于缺少必要的热力学数据,形成液相K_2SO_4-Na_2SO_4-Fe_2(SO_4)_3的临界Pso_3值则是用实验确定的。     

Oxidation behavior of Nb–24Ti–18Si–2Al–2Hf–4Cr and Nb–24Ti–18Si–2Al–2Hf–8Cr hypereutectic alloys at 1250°C

Nb-24Ti-18Si-2Al-2Hf-4Cr and Nb-24Ti-18Si-2Al-2Hf-8Cr alloys were prepared by arc melting in a water-cooled crucible under argon atmosphere.Microstructural characteristics and oxidation resistance of the alloys at 1250 ℃ were investigated.The results show that,when the Cr content is 4 at%,the microstructures consist of(Nb,Ti)_(ss) and Nb_5Si_3;as Cr content increases to8 at%,C14 Laves phase Cr_2Nb is formed.The isothermal oxidation tests show that the oxidation kinetics of the two alloys follow similar features.The weight gains of the two alloys after oxidation at 1250℃ for 100 h are 235.61 and198.50 mg·cm~(-2),respectively.During oxidation,SiO_2,TiO_2,Nb_2O_5 and CrNbO_4 are formed at first.Then,Ti_2Nb_(10)O_(29) is formed after oxidation for 20 min and begins to change into TiNb_2O_7 as the oxidation proceeds.SiO_2 is formed as solid state at first but later evolves into glassy state to improve the cohesion of the scale.After oxidation for 100 h,oxidation products consist of SiO_2,TiNb_2O_7,Nb_2O_5 and CrNbO_4.     

α-Fe_2O_3、Fe_3O_4和Cr_2O_3在熔融Na_2SO_4中的溶解度

作者于1200K在1大气压氧和几个低氧压下测定了α-Fe_2O_3及Fe_3O_4及Cr_2O_3在熔融Na_2SO_4中的溶解度与Na_2SO_4熔盐碱度的关系,结果与热力学预示是一致的。在测量溶解度的基础上,确定了这几种金属氧化物在熔融Na_2SO_4中碱性和酸性溶解的溶质形式,计算了它们在该熔盐中的活度系数。金属氧化物溶解行为的研究将有助于了解热腐蚀的盐溶机理和电化学机理。     

Cytotoxicity of Ti–6Al–4V–5Cu Alloy to MC3T3-E1 Cells

The aim of this work was to study the cytotoxicity of Ti–6 Al–4 V–5 Cu(TC4–Cu) alloy for dental applications and evaluate the cell viability of diff erent fabricated TC4–Cu alloys in contact with MC3 T3-E1 cells in vitro. Ti–6 Al–4 V(TC4) alloy was used as a negative control to evaluate the cytotoxicity level of TC4–Cu alloy so as to provide basic support for the dental clinical application. Control group TC4 and experimental group TC4–Cu with diff erent fabrications were incubated in the cell culture medium. The absorbance value of mouse osteoblast MC3 T3-E1 cells was detected by CCK-8 assay after 1, 3 and 7 days, respectively. The relative proliferation rate of cells was calculated, and then the toxicity level was valued for each group. Cell morphology on the surface was also studied by observing the cytoskeleton through F-actin filament staining. The experimental results showed that the absorbance values for the experimental and the negative control groups were not same at diff erent time points. Compared with diff erent fabricated TC4–Cu alloys, the annealing-TC4–Cu alloy showed much better biocompatibility. The mouse osteoblast MC3 T3-E1 cells cultured with annealing-TC4–Cu, rolling-TC4–Cu and solution + aging-TC4–Cu have no toxic eff ects, and these alloys could promote the proliferation of mouse osteoblasts.     

Preparation of Al2O3–ZrO2–Y2O3 Composite Coatings by a Modified Sol–Gel Technique for Thermal Barrier Application

Spatial-network Al₂O₃–ZrO₂–Y₂O₃ composite coatings were prepared by a modified sol–gel technique, so-called thermal pressure and filtration of sol–gel paint. The composite coatings were derived from a composite paint of yttria partially stabilized zirconia (YSZ) particles, Al₂O₃ particles and Al₂O₃–Y₂O₃ sol. Their microstructure showed that YSZ particles were covered with spatial-network Al₂O₃–Y₂O₃ blanket. Cyclic oxidation at 1,050 °C in air for 200 h demonstrates that the oxygen diffusion rate in the coatings could be effectively inhibited. Meanwhile, suitable coefficients of thermal expansion (CTE) gave the composite coatings better spallation resistance than that of Al₂O₃–Y₂O₃ or ZrO₂–Y₂O₃ coatings. The positive results of cyclic oxidation indicated that the composite coating can be used as an interlayer between the bond coat and the top ceramic layer in traditional TBCs. Not only the depletion rate of aluminum-rich phase in MCrAlY alloy could be slowed down by spatial-network Al₂O₃–Y₂O₃, but also different thermal expansion between thermally grown oxides layer and top layer could be relieved by suitable CTE. In this paper, the mechanisms of the inhibition of oxygen diffusion and thermal match between ceramic coating and alloy are also discussed.     

Microstructure evolution of eutectic Nb–24Ti–15Si–4Cr–2Al–2Hf alloy processed by directional solidification

In this work, the near-eutectic Nb–24Ti–15Si–4Cr–2Al–2Hf(at%) alloy was directionally solidified at 1900 ℃ with withdrawal rates of 6, 18, 36, 50 mm·min~(-1) and then heat-treated at 1450 ℃ for 12 h. The microstructure evolution was investigated. The results show that the microstructure of the directionally solidified(DS) alloy is composed of Nb_(ss)+Nb_5Si_3 eutectics within the whole withdrawal rate range, while the variation of rates makes a great difference on the solidification routes,the morphology and size of Nb_(ss)+Nb_5Si_3 eutectic cells.With the increase in withdrawal rates, the petaloid Nb_(ss)+Nb_5Si_3 eutectic cells transform into granular morphology. After the heat treatment, a mesh structure Nbssis formed gradually which isolates the Nb_5Si_3, and the phase boundaries become smoother in order to reduce the interfacial energy. Moreover, two kinds of Nb_5Si_3 exist in the heat-treated(HT) samples identified by crystal form and element composition, which are supposed as α-Nb_5Si_3 and γ-Nb_5Si_3, respectively. This study exhibits significant merits in guiding the optimization of Nb–Si-based alloys' mechanical properties.     

Determination of the habit plane characteristics in the β–α′ phase transformation induced by stress in Ti–5Al–2Sn–4Zr–4Mo–2Cr–1Fe

The characteristics of the habit planes of the α″ orthorhombic martensite plates, induced by stress within a commercial sheet of Ti–5Al–2Sn–4Zr–4Mo–2Cr–1Fe have been investigated. First, the orientations of several grains in BCC phase which presented at least one α″ orthorhombic martensite plate, were determined by EBSD. The Miller indexes of habit planes within a parent grain were deduced from the intersection lines of the martensite plates on two perpendicular sides of the sample and from the orientation of the parent grain. These data are compared to the characteristics of the habit planes, calculated from the phenomenological theory of the martensitic transformation proposed by Wechsler, Liebermann and Read. The different results and their dispersions are analysed in this contribution.