Oxidation and interdiffusion behavior of a germanium-modified silicide coating on an Nb-Si-based alloy

To investigate the interdiffusion behavior of Ge-modified silicide coatings on an Nb-Si-based alloy substrate, the coating was oxidized at 1250℃ for 5, 10, 20, 50, or 100 h. The interfacial diffusion between the (Nb,X)(Si,Ge)₂ (X=Ti, Cr, Hf) coating and the Nb-Si based alloy was also examined. The transitional layer is composed of (Ti,Nb)₅(Si,Ge)₄ and a small amount of (Nb,X)₅(Si,Ge)₃. With increasing oxidation time, the thickness of the transitional layer increases because of the diffusion of Si from the outer layer to the substrate, which obeys a parabolic rate law. The parabolic growth rate constant of the transitional layer under oxidation conditions is 2.018 μm·h-1/2. Moreover, the interdiffusion coefficients of Si in the transitional layer were determined from the interdiffusion fluxes calculated directly from experimental concentration profiles.     

Microstructure and growth kinetics of Ce and Y jointly modified silicide coatings for Nb–Ti–Si based ultrahigh temperature alloy

In order to protect Nb-Ti-Si based ultrahigh temperature alloy from oxidation, pack cementation processes were utilized to prepare Ce and Y jointly modified silicide coatings. The Ce and Y jointly modified silicide coating has a double-layer structure: a relatively thick (Nb, X)Si2 (X represents Ti, Cr and Hf elements) outer layer and a thin (Ti, Nb)5Si4 transitional layer. The pack cementation experiments at 1150 ℃ for 8 h proved that the addition of certain amounts of CeO2 and Y2O3 powders in the packs distinctly influenced the coating thickness, the contents of Si, Ce and Y in the (Nb, X)Si2 outer layers, and the density of cavities in the coatings. In order to study the effects of Ce and Y joint modification in the silicide coatings, both only Ce and only Y modified silicide coatings were also prepared for comparison. The mechanisms of the beneficial effects of Ce and Y are discussed. A pack mixture containing 1.5CeO2-0.75Y2O3 (wt%) powders was employed to investigate the growth kinetics of the Ce and Y jointly modified silicide coating at 1050, 1150 and 1250 ℃. It has been found that the growth kinetics obeyed parabolic laws and the parabolic rate constants were 109.20 mm2/h at 1050 ℃, 366.75 mm2/h at 1150 ℃ and 569.78 mm2/h at 1250 ℃, and the activation energy for the growth of the Ce and Y jointly modified silicide coating was 197.53 kJ/mol.     

Effects of alloying and high-temperature heat treatment on the microstructure of Nb–Ti–Si based ultrahigh temperatur e alloys

The phase co mpositions, microstructure and especi ally phase i nterfaces in the as-cast and heat-treated Nb– Ti–Si based ultrahigh temperature alloys have been investigated. It is shown that β(Nb,X)5Si3 and γ(Nb,X)5Si3 are the primary p hase s in the Nb–22Ti–16Si–5Cr–5Al (S1) (at%) and Nb–20Ti–16Si–6C r–4Al–5Hf–2B–0.06Y (S2) (at% ) alloys, respectively. The Nb solid solution (Nbss) is the primary phase in Nb–22Ti–14Si–5Hf–3Al–1. 5B –0.0 6Y (S3) (at%) alloy . An orientation relationship between Nbss and γ(Nb,X)5Si3 was determine d to be (1-10)Nb//(101-0)γ and [111]Nb//[0001]γ in the as-cast S2 and S3 alloys. Some original β(Nb,X)5Si3 transfor med into α(Nb,X)5Si3 because Al and Cr diffused from the β(Nb,X)5Si3 to Nbss during heattreatment at 1500 °C for 50 h in the S1 alloy. Mean while, Ti diffused from Nbss to β(Nb,X)5Si3, which induced a Ti to generate near the interface between Nbss and Ti-rich β(Nb,X)5Si3. The orientation relationship between the newl y-formed a Ti and previous Nbss was (110 )Nb//(1-10-1) αTi and [001]Nb//(12-3-1)αTi. Among the ( Nb,X)5Si3 phases , the contents o f Cr and Al in β(Nb,X)5Si3 are n earl y the same as those in γ(Nb,X)5Si3 but obviously hi gher than those in the α(Nb,X)5Si3, where as the content of Si in α(Nb,X)5Si3 is nearly the same a s that in γ(Nb,X)5Si3 but higher than that in the β(Nb,X)5Si3     

High-throughput synthesis of oxidation-resistant Nb–Si based alloy thin film by magnetron co-sputtering

A high-throughput method was applied to study oxidation behavior of Nb–Si based alloy using composition spread alloy film as combinatorial libraries. An extended range of composition gradients of Nb–Si based alloy film was deposited by (multi)magnetron co-sputtering. The as-deposited film was composed of amorphous phase. Cr₂Nb, Nb₅Si₃ and Nb_ss could be detected after annealing treatment. After oxidation at 1250 ?°C for 10 ?min and 20 ?min, the film composition space was divided into three regions of the distinct oxide scales. The compositions for establishment of the different oxides on Nb–Si based alloy was defined efficiently across the entire composition space by the combinatorial libraries.     

Improvement of oxidation resistance of Nb–Ti–Si based alloys with additions of Al,Cr and B at different temperatures

The effects of Cr, Al and B addition on the microstructure and high-temperature oxidation behaviors (at 1200, 1250 and 1300 ?°C) of Nb–Ti–Si based alloys were investigated. The results showed that the addition of Cr stabilized α-Nb₅Si₃, while Al promoted the formation of β-Nb₅Si₃ and adding B promoted the formation of γ-Nb₅Si₃. Among the three elements, Al and Cr were beneficial to oxidation resistance at 1200 ?°C, and B was favorable to the oxidation resistance at 1300 ?°C. At 1250 ?°C, Al and B had the same effects on the improvement of oxidation resistance. The ratio of these alloying elements might play an important role in enhancing oxidation resistance. The oxidation resistance of the three kinds of silicides was compared, and the sequence was: γ-Nb₅Si₃> β-Nb₅Si₃> α-Nb₅Si₃. To predict the effects of the investigated alloying elements on the oxidation resistance of Nb–Ti–Si based alloys in a wider range of concentration, an artificial neural network (ANN) model was established, showing excellent accuracy and generalization ability. With the instructions of the ANN model, the oxidation resistance can be optimized with less additions of alloying elements.     

Rapid directionally solidified microstructure characteristic and fracture behaviour of laser melting deposited Nb–Si–Ti alloy

Aiming at achieving fine and directionally-solidified microstructure of Nb-Si based alloy,Nb,Si and Ti powder particles were utilized as the raw materials,and laser melting deposition(LMD) experiments were conducted with1500 W and 2000 W laser power,respectively.The microstructure characteristic,micro-hardness,and indentation fracture toughness were examined by scanning electron microscope(SEM),X-ray energy dispersive spectrometer(XEDS) and X-ray diffraction(XRD).The results showed that the two kinds of as-deposited Nb-17 Si-23 Ti alloy samples were mainly composed of Nbss,(Nb,Ti)3 Si and Ti-rich Nbss,and the microstructure presents rapid directionally-solidified characteristic,in which the 100 crystallographic direction of Nbss and 110 crystallographic direction of(Nb,Ti)3 Si tend to be parallel to the LMD building direction.With the laser power increasing from 1500 W to 2000 W,the microstructure became more oriented and homogeneous.For the sample at 2000 W,the dendrite spacing was only about 1-2 μm,and Nbss and(Nb,Ti)3 Si couples grow alternately and they are parallel to the building direction strictly.In the meantime the micro-hardness and indentation fracture toughness reached 1075 HV and 20.3 MPa·m~(1/2),respectively,indicating significant improvement by the directional growth of fine Nbss and(Nb,Ti)3 Si phases,and the dimple fracture mode of the Nbss dendrites.     

Preparation and performance of a cold gas dynamic sprayed high-aluminum bronze coating

By mixing preheated high-aluminum bronze powders with different amounts of Al₂O₃ powder, a low-pressure cold-sprayed coating was prepared and sprayed onto a Cr12MoV steel substrate. The hardness of the coating and the bonding strength between the coating and the substrate were tested with a HV-1000 microhardness tester and a mechanical universal testing machine. The surface microstructure, cross-section and tensile fracture surface of the coating were observed with a scanning electron microscope (SEM). Correspondingly, the influences of the preheat treatment temperature of the bronze powder and the Al₂O₃ content on the coating performance were investigated. The results indicate that the hardness of bronze powders decreased and the coating deposition rate increased after the preheating treatment of the bronze powder. The Al₂O₃ content in the mixed powders contributed to the deformation of bronze powders during the spraying process. This trend resulted in varied performance of the coating.     

Reaction mechanism for in-situ β-SiAlON formation in Fe3Si-Si3N4-Al2O3 composites

In this work, Fe₃Si-Si₃N₄-Al₂O₃ composites were prepared at 1300℃ in an N₂ atmosphere using fused corundum and tabular alumina particles, Al₂O₃ fine powder, and ferrosilicon nitride (Fe₃Si-Si₃N₄) as raw materials and thermosetting phenolic resin as a binder. The effect of ferrosilicon nitride with different concentrations (0wt%, 5wt%, 10wt%, 15wt%, 20wt%, and 25wt%) on the properties of Fe₃Si-Si₃N₄-Al₂O₃ composites was investigated. The results show that the apparent porosity varies between 10.3% and 17.3%, the bulk density varies from 2.94 g/cm3 and 3.30 g/cm3, and the cold crushing strength ranges from 67 MPa to 93 MPa. Under the experimental conditions, ferrosilicon nitride, whose content decreases substantially, is unstable; part of the ferrosilicon nitride is converted into Fe₂C, whereas the remainder is retained, eventually forming the ferrosilicon alloy. Thermodynamic assessment of the Si₅AlON₇ indicated that the ferrosilicon alloy accelerated the reactions between Si₃N₄ and α-Al₂O₃ fine powder and that Si in the ferrosilicon alloy was nitrided directly, forming β-SiAlON simultaneously. In addition, fused corundum did not react directly with Si₃N₄ because of its low reactivity.     

Thermodynamics and kinetics of alumina extraction from fly ash using an ammonium hydrogen sulfate roasting method

A novel method was developed for extracting alumina (Al₂O₃) from fly ash using an ammonium hydrogen sulfate (NH₄HSO₄) roasting process, and the thermodynamics and kinetics of this method were investigated. The thermodynamic results were verified experimentally. Thermodynamic calculations show that mullite present in the fly ash can react with NH₄HSO₄ in the 298–723 K range. Process optimization reveals that the extraction rate can reach up to 90.95% when the fly ash reacts with NH₄HSO₄ at a 1:8 mole ratio of Al₂O₃/NH₄HSO₄ at 673 K for 60 min. Kinetic analysis indicates that the NH₄HSO₄ roasting process follows the shrinking unreacted core model, and inner diffusion through the product layer is the rate-controlling step. The activation energy is calculated to be 16.627 kJ/mol; and the kinetic equation can be expressed as 1 ? (2/3)α ? (1 ? α)2/3 = 0.0374t exp[?16627/(RT)], where α is the extraction rate and t is the roasting temperature.     

Wear properties of NiAl based materials

The NiAl based materials including NiAl-TiC-Al2O3 composite,NiAl-Cr(Mo)-Hf-Ho eutectic alloy and NiAl-Cr(Mo)-CrxSy in situ composite were fabricated and their wear properties were tested at different temperatures.The results revealed that the NiAl-TiC-Al2O3 composite,NiAl-Cr(Mo)-Hf-Ho eutectic alloy and NiAl-Cr(Mo)-CrxSy in situ composite exhibited the excellent wear properties between 700℃ and 900℃.The microstructure observations exhibited that the self-lubricant films formed on the worn surfaces during the dry sliding test at high temperature,which decreased the wear rate and friction coefficient significantly.TEM observation on the self-lubricant film revealed that it was mainly comprised by ceramic amorphous and nanocrystalline.Compared with the NiAl-TiC-Al2O3 composite,the NiAl-Cr(Mo)-CrxSy in situ composite has lower friction coefficient at low temperature.Such phenomena may be ascribed to the addition of sulfide which contributes much to the formation of self-lubricant,and moreover the TiC addition increase the strength of NiAl based material and its wear resistance.     

The strain ampli tude-controlled cyclic fatigue behavior of Al2O3 fiber reinforced Al–Si alloy composite at elevated temperatures

The strain amplitude-controlled fatigue characteristics of an Al–Si casting alloy and itscomposite reinforced with 17 vol% Al₂O₃ fibers (Al–Si/Al₂O₃) are studied at three different temperatures. Both the alloy and the composite showed different degrees of cyclic softening at elevated temperatures. Increasing the temperature, fatigue damage of either the alloy or the composite occurred with varying mode from brittle fracture of silicon particles to their separation from the aluminum matrix. This is explained by the different thermal expansion coefficients of silicon particles and the aluminum matrix. The reinforcement Al₂O₃ fibers in the composite showed a similar damage behavior with those silicon particles despite temperature variation     

Formation mechanism of an aluminium-based chemical conversion coating on AZ91D magnesium alloy

An environmentally clean aluminium-based conversion coating on AZ91D magnesium alloy was studied in aluminium nitrate solutions. The morphology, composition, structure, and formation mechanism of the coating were investigated in detail using scanning electron microscopy/energy dispersion spectrometry, X-ray diffraction, transmission electron microscopy, and electrochemical corrosion tests. The results show that the conversion coating is composed of magnesium, aluminium, and oxygen, and shows an amorphous structure. In the initial stage of coating formation, the grain-like nucleus is composed of Al₁₀O₁₅·xH₂O, (Al₂O₃)_5.333, Al₂O₃, AlO(OH), MgAl₂O₄, (Mg_0.88Al_0.12)(Al_0.94Mg_0.06)₂O₄, and (Mg_0.68Al_0.32)(Al_0.84Mg_0.16)₂O₄. The conversion coating formed in the 0.01 mol/L aluminium nitrate solution for 15 min can improve the corrosion resistance of the magnesium alloy greatly. The discussion reveals that the possible formation mechanism for the aluminium-based conversion coating is the reduction reaction on micro-cathodic sites due to the electrochemically heterogeneous magnesium alloy substrate.     

两步包埋法制备C/C复合材料SiC/Cr-Al-Si涂层研究

采用两步包埋法在碳/碳复合材料表面制备了SiC/Cr-Al-Si涂层.采用XRD、SEM和EDS分析了涂层的物相组成、微观结构及断面元素分布,测试了双涂层碳/碳复合材料试样在1 500℃静态空气中的抗氧化性能.结果表明:SiC/Cr-Al-Si涂层主要由SiC、AlCr2Si及Al4Si2C三相组成,厚度约为120μm,无穿透性裂纹;与一步包埋法所得SiC涂层相比,SiC/Cr-Al-Si涂层碳/碳复合材料试样的抗氧化性能有所提高,该涂层试样氧化12 h后的失重不超过5%.两步包埋法所得SiC/Cr-Al-Si涂层表面存在微裂纹,并且包埋过程易于使Cr-Al-Si合金成分扩散到SiC涂层内部,从而无法形成内SiC涂层、外Cr-Al-Si涂层的双层涂层结构,降低了Cr-Al-Si合金涂层对C/C复合材料基体的高温氧化保护效果.     

Behaviors and interactions of strengthening and constraint for ductile Nbss matrix in Nb–Si based alloys

The ductile matrix in composites could be strengthened by incorporating brittle inclusions, while the constraint effect for matrix would become more obvious and detrimental to material toughening. The balance between strengthening and toughening was crucial to achieve excellent comprehensive mechanical properties. Upon the basis of microstructure of Nb–Si in situ composites, a unit rectangular cell containing elastic and isotropic Nb₅Si₃ particles was modeled, the Niobium solid solution (Nb_ss) matrix according with the Ramberg-Osgood constitutive relation was recognized as elastic-plastic. For Nb–Si based alloys, the effects of volume fractions of Nb₅Si₃ reinforcements, stress states and interfacial characteristics on the stress-strain field, flow localization and damage evolution were investigated by the finite element method (FEM). Further, the normalized work of rupture (χ) was calculated quantitatively as a function of volume fractions of Nb₅Si₃ phase as well as yield strength of Nb_ss matrix. These results would contribute to acquire more fundamental understanding of the strengthening and constraint effects which were exerted by Nb₅Si₃ reinforcements to ductile Nb_ss matrix, also an instructive guidance for the optimum design of Nb–Si based alloys.     

Electrochemical corrosion behavior of the cobalt modified aluminide coating in 3.5 wt% NaCl solutions

In this paper, we achieved the Co modified aluminide coating on nickel base superalloys by pack cementation process. The corrosion currents(i_(corr)) of coated and uncoated substrate after 1 h of immersion tested by potentiodynamic polarization in 3.5 wt% NaCl solutions are 9.73 b × 10~(-7) A/cm~2 and 1.31 × 10~(-6) A/cm~2, respectively. With the immersion time extension, the icorrof the coating decreases and the values of |Z| by EIS increase, indicating that the electrochemical reaction rate of the coating becomes slower. The coating presents good corrosion resistance due to the Al_2O_3 formed on the surface.     

Microstructure and wear behavior of Mo–60Si–5B coating doped with 0.5 ​at% La by spark plasma sintering

Mo–60Si–5B coating doped with 0.5 ?at% La was prepared on niobium silicon based alloy by spark plasma sintering. The microstructure and wear behavior of the 0.5La–Mo–60Si–5B (0.5La-MSB) coating were investigated. The results show that the mean grain sizes of the Mo–60Si–5B (0La-MSB) and 0.5La-MSB coatings are calculated to be 3.27 ?μm and 2.85 ?μm, respectively. The addition of La plays a role of refining the grains of 0.5La-MSB coating. The specific wear rate of 0.5La-MSB coating is decreased by up to 26.8% at the oscillation frequency of 20 ?Hz and the sliding load of 11 ?N compared to 0La-MSB coating. The anti-friction performance of Mo–60Si–5B coating is modified by the addition of 0.5 ?at.% La. The improvement of anti-friction performance of the Mo–60Si–5B coating is due to the increased hardness and the provided lubrication function by La₂O₃.     

Mineral-phase evolution and sintering behavior of MO-SiO2-Al2O3-B2O3 (M=Ca,Ba) glass-ceramics by low-temperature liquid-phase sintering

In this work, network former SiO₂ and network intermediate Al₂O₃ were introduced into typical low-melting binary compositions CaO·B₂O₃, CaO·2B₂O₃, and BaO·B₂O₃ via an aqueous solid-state suspension milling route. Accordingly, multiple-phase aluminosilicate glass-ceramics were directly obtained via liquid-phase sintering at temperatures below 950℃. On the basis of liquid-phase sintering theory, mineral-phase evolutions and glass-phase formations were systematically investigated in a wide MO-SiO₂-Al₂O₃-B₂O₃ (M=Ca, Ba) composition range. The results indicate that major mineral phases of the aluminosilicate glass-ceramics are Al₂₀B₄O₃₆, CaAl₂Si₂O₈, and BaAl₂Si₂O₈ and that the glass-ceramic materials are characterized by dense microstructures and excellent dielectric properties.     

Alkaline digestion behavior and alumina extraction from sodium aluminosilicate generated in pyrometallurgical process

In pyrometallurgical process, Al-and Si-bearing minerals in iron and aluminum ores are easily transformed into sodium aluminosilicates in the presence of Na₂O constituents, which alters the leaching behaviors of Al₂O₃ and SiO₂. It was confirmed that sodium aluminosilicates with different phase compositions synthesized at various roasting conditions were effectively digested in the alkaline digestion process. Under the optimum conditions at temperature of 100-120℃, liquid-to-solid ratio (L/S) of 10:2 mL/g, caustic ratio of 4, and Na₂O concentration of 240 g/L, the actual and relative digestion ratio of Al₂O₃ from the synthesized sodium aluminosilicates reached maximums of about 65% and 95%, respectively, while SiO₂ was barely leached out. To validate the superior digestion property of sodium aluminosilicate generated via an actual process, the Bayer digestion of an Al₂O₃-rich material derived from reductive roasting of bauxite and comprising Na_1.75Al_1.75Si_0.25O₄ was conducted; the relative digestion ratio of Al₂O₃ attained 90% at 200℃.     

A comparative study of four modified Al coatings on Ni3Al-based single crystal superalloy

Four modified Al diffusion coatings (Al, Cr–Al, Al–Si and Cr–Al–Si coatings) were prepared on Ni₃Al based single crystal superalloy IC20. The oxidation tests were carried out at 1 150 °C for up to 100 h. Cyclic hot corrosion tests were carried out at 950 °C for 50 h. The results indicate that the oxidation and corrosion resistances of IC20 alloy are improved significantly by the coatings, and both oxidation and hot corrosion resistances of the four coatings are rated in the order (from worst to best) of Cr–Al, Al, Cr–Al–Si, Al–Si coatings. It is found that the degradations of Al and Cr–Al coatings are very quick due to the serious inter-diffusion between the coatings and substrates. The inter-diffusion between Si-containing coatings and substrates is reduced since Si effectively retards the outward diffusion of Mo. The weak effects of Cr and benefit effects of Si to the oxidation and hot corrosion resistance were discussed. The hot corrosion degradation mechanism of superalloy IC20 was analyzed.     

Composition design of superhigh strength maraging stainless steels using a cluster model

The composition characteristics of maraging stainless steels were studied in the present work investigation using a cluster-plus-glue-atom model. The least solubility limit of high-temperature austenite to form martensite in basic Fe–Ni–Cr corresponds to the cluster formula [NiFe12]Cr3,where NiFe12is a cuboctahedron centered by Ni and surrounded by 12 Fe atoms in FCC structure and Cr serves as glue atoms. A cluster formula [NiFe12](Cr2Ni) with surplus Ni was then determined to ensure the second phase(Ni3M) precipitation,based on which new multicomponent alloys [(Ni,Cu)16Fe192](Cr32(Ni,Mo,Ti,Nb,Al,V)16) were designed. These alloys were prepared by copper mould suction casting method,then solid-solution treated at 1273 K for 1 h followed by water-quenching,and finally aged at 783 K for 3 h. The experimental results showed that the multi-element alloying results in Ni3M precipitation on the martensite,which enhances the strengths of alloys sharply after ageing treatment. Among them,the aged [(Cu4Ni12)Fe192](Cr32(Ni8.5Mo2Ti2Nb0.5Al1V1)) alloy(Fe74.91Ni8.82Cr11.62Mo1.34Ti0.67Nb0.32Al0.19V0.36Cu1.78wt%) has higher tensile strengths with YS?1456 MPa and UTS?1494 MPa. It also exhibits good corrosion-resistance in 3.5 wt% NaCl solution.