Cyclic Oxidation Behavior and Microstructure of Nanocrystalline Ni–20Cr–4Al Coating

The cyclic oxidation behavior and microstructure of a nanocrystalline Ni–20Cr–4Al coating have been investigated. Cyclic oxidation tests were conducted on uncoated and coated samples at peak temperatures of 750 and 1010 °C for up to 2070 thermal cycles between the peak and room temperatures. The results showed that a dense Al₂O₃ scale formed on the coated samples during thermal cycling to both peak temperatures. Evidence of internal oxidation of the coating was observed only on the samples exposed to 1472 one-hour thermal cycles at the peak temperature of 1010 °C. The external scale exhibited good spallation resistance during cyclic oxidation testing at both temperatures. Thermal exposure led to depletion of Al from the coating and grain coarsening within the coating. The improvement in oxide scale spallation resistance and accelerated depletion of aluminum are believed to be related to the fine-grained structure of the coating.     

The long-term thermal stability of thermo-span alloy

In order to evaluate the long-term thermal stability of Thermo-Span® alloy—an age-hardenable, Fe-Co-Ni-based superalloy—heat-treated samples were exposed for 1,500 hours at temperatures ranging from 480°C to 675°C. Evaluations included tensile tests at room temperature, 595°C, and 650°C; stress-rupture tests at 675°C; and post-creep tensile tests at 620°C. The results showed that Thermo-Span alloy exhibits stable tensile properties and microstructure after 1,500 hour exposures at temperatures within the range of 480°C to 675°C. No embrittlement occurred at any temperature. Exposure at 675°C did not degrade stress-rupture properties. Neither stressed nor unstressed exposures at 620°C had a significant effect on tensile properties at 620°C.     

Investigation of Isothermal and Cyclic Oxidation of Plasma-Nitrided Hot-Work Tool Steel at Elevated Temperatures

The oxidation of a plasma-nitrided, hot-work tool steel at temperatures that cover a range of operations from post-plasma-nitriding oxidation to steel thixoforging processing was investigated. Thermal exposure at 500 °C led to the formation of a thin Fe?CCr spinel layer and an even thinner outermost layer of hematite. The former is the only oxide that grew on samples exposed to oxygen-lean conditions at 750 °C. A thick, multi-layered oxide scale formed on the surface when the plasma nitrided hot-work tool steel was held at 750 °C under atmospheric conditions. In this scale, the outermost hematite layer and the inner Fe?CCr spinel were separated by a magnetite layer. The oxide scale produced during thermal cycling at 750 °C was also multi-layered with an identical oxide scale configuration to that formed during isothermal exposure at 750 °C. The hematite layer, which retained its integrity during isothermal exposure at 750 °C, suffered small cracks that were instrumental in its fracture and spallation during thermal cycling. The distinct feature resulting from cyclic oxidation, however, was the wide gap that formed along the magnetite?Cspinel interface. Thermal expansion mismatch produced compressive stresses which in turn led to buckling of the magnetite layer and to its detachment; while, the spinel layer adhered to the tool steel substrate and survived throughout thermal cycling. Enrichment of nitrogen and the subsequent precipitation of N₂ gas were also believed to have contributed to the gap formation. Formation of such a gap poses a serious threat to the integrity of the oxide scale and was shown to be responsible for the spallation of the magnetite layer upon thermal cycling.     

Tensile behavior and cyclic creep of continuous fiber-reinforced glass matrix composites at room and elevated temperatures

In this study we investigated the stress-strain behavior at room and elevated temperatures and the tensile creep and cyclic creep response of a unidirectional SiC fiber-reinforced aluminosilicate glass matrix composite. The interfacial condition of the as-received material was measured by a push-out indentation technique. The stress-strain behavior was that expected for this kind of composite, i.e. “pseudoductile” behavior with extensive fiber “pull-out” at room temperature and brittle failure at intermediate temperatures (750 °C) due to oxidation embrittlement. The stiffness of the composite at 750°C was analyzed for different loading rates, highlighing the influence of the loading rate on apparent composite stiffness, due to matrix softening. The creep studies were conducted at temperatures above and below the softening temperature of the glass (T _g, 745 °C) in air. The cyclic creep experiments showed the existence of extensive viscous strain recovery during the unloading period. The creep strain recovery was quantified using strain recovery ratios. These ratios showed a slight dependence on the temperatures investigated (700 and 750 °C). The crept composites retained their “graceful” fracture behavior only partially after testing, indicating that oxidation of the fiber/matrix interface due to oxygen diffusion through the matrix occurred in the peripheral area of the samples. Dr. Boccaccini is presently at the Institute for Mechanics and Materials, University of California, San Diego, CA 92093, USA.     

Inconel 617 and Stellite 6 alloys for tooling in thixoforming of steels

Thermal fatigue and high temperature wear are the two principle failure mechanisms for thixoforming dies. Samples of Inconel 617 and Stellite 6 alloys were submitted to thermal cycling under conditions which approximate thixoforming of steels and to sliding wear tests at 750 °C. The experimental results thus obtained were compared with those of the X32CrMoV33 hot work tool steel. The Inconel 617 and Stellite 6 samples are much more resistant to oxidation and to softening than the hot work tool steel, providing a superior resistance to thermal fatigue cracking. The wear resistance of the Inconel 617 and Stellite 6 alloys at 750 °C is also markedly superior. The adhesive oxides growing slowly on Inconel 617 and Stellite 6 alloys sustain the wear action without spalling and are claimed to be responsible for the superior wear resistance of these alloys at 750 °C.     

Effect of Temperature on Microstructure and Fracture Mechanisms in Friction Stir Welded Al6061 Joints

Aluminum and its alloys are widely used in different industries due to such attractive properties as adequate strength, ductility, and low density. It is desirable to characterize welds of aluminum alloys obtained using “friction stir welding” at high temperatures. Al-to-Al (both 6061-T6) butt joints are produced by friction stir welding at tool rotation speed of 1600 rpm and four levels of tool advancing speeds: 250, 500, 750, and 1000 mm/min. Microstructural properties of the different welds are investigated. Observed are noticeable differences in microstructure characteristics between the various weld zones. Mechanical properties of these welded joints are characterized under tensile tests at temperatures of 25, 100, 200, and 300 °C, at a constant strain rate of 10^?3/s. The optimum microstructural and mechanical properties were obtained for the samples FS welded with 1600 rpm tool rotation speed at 1000 mm/min tool advancing speed. The studied welds exhibited yield strength, ultimate tensile strength, and strain to failure with values inferior of those of the base material. Observations of postmortem samples revealed that in the temperature range of 25-200 °C the locus of failure originates at the region between the thermo-mechanically affected zone and the heat-affected zones. However, at higher temperatures (300 °C), the failure occurs in the stir zone. A change in the crack initiation mechanism with temperature is suggested to explain this observation.     

Determination of acid-base properties of hcl acid activated palygorskite by potentiometric titration

The surface acidity of raw and acid activated palygorskite clay was studied by acid-base potentiometric titration. The Gran plot method was applied for the hydroxide titration, and the total surface sites (Hs) and the average number of protons reacted per surface site (Z) of the palygorskite samples at a given ionic strength were calculated. Acid treatment increases the clay acidity and modifies its surface charge. The point of zero charge value, which was determined by the common crossing point of Z vs. the pH curves performed at different ionic strengths, decreased from 8.8 to 3.5 with the acid treatment period. For illustrating the acidic characteristics of the treated and untreated palygorskite surface, three surface protonation models were tested: (a) the one site one pK_a model, ≡SOH?≡SO^?+H⁺; (b) the two sites two pK_as model,≡S_IOH?≡S_IO^?+H⁺ and S_IIOH?≡S_IIO^?+ H⁺; and (c) the one site two pK_a model,≡SOH?≡SO^? + H⁺ and SOH + H⁺?≡SOH ₂ ⁺ . The three surface protonation models sufficiently describe the surface properties and their evolution with the acid treatment.     

High pressure experiments on the Mg2Ni and Mg2NiH4–H systems

Electrical resistance of the Mg₂Ni and Mg₂NiH₄ powders was measured under compressing up to 6 gigapascals (GPa), using the four-point probe method at 20°C. Effects induced by the high-pressure treatment and heating at 400°C with or without hydrogen source LiAlH₄ were also studied. The structure and hydrogenation properties of the samples recovered after quenching from high pressure and high temperature were investigated employing the techniques of X-ray diffraction and thermal analyses such as differential scanning calorimetry, thermogravimetry and differential thermal analysis. Decreased absorption/desorption temperatures were observed for the treated samples owing to the changes of microstructure and kinetic factor.     

Niobium carbo-nitride precipitation behavior in a high nitrogen 15Cr-15Ni heat resistant austenitic stainless steel

A high nitrogen 15Cr-15Ni niobium-stabilized austenitic alloy has been produced and subjected to a special heat treatment consisting of 5 hours of solution treatment at 1270 °C followed by hot rolling, quenching and subsequent aging at temperatures of 700 °C to 800 °C. It was found that fine dispersion of nano-sized thermally stable primary Nb(C,N) precipitates had already formed in the as-cast condition. The particles were presented at all examined stages of the TMT process (as-homogenized, as-solution treated and as-aged conditions). Secondary precipitates Nb(C,N) were densely formed during subsequent aging; these precipitates had sizes of 4 nm to 5 nm. Both the primary and secondary Nb(C,N) particles showed excellent thermal stability within the temperature range of 700 °C to 800 °C. The creep properties of the studied alloy at 750 °C were superior when compared to those of commercial type 347 stainless steel.     

Creep Properties of Directionally Solidified Nb-Modified Ni-Base Superalloy,Mar-M247

The Ni-base superalloy Mar-M247 is widely used for elevated-temperature applications in both equiaxed and directionally solidified forms. The alloy contains about 3 wt.% Ta. Due to the cost and density of Ta, an atom-for-atom substitution of Nb for Ta was investigated. The creep properties in the temperature range of 750-950?°C and stresses ranging from 200 to 1000 MPa were determined. In general, the creep properties of the Nb-modified Mar-M247 material were very similar to those of Mar-M247 samples at 750 and 850?°C. However, the Mar-M247 samples exhibited superior creep properties at 950?°C. The lower creep properties of the Nb-modified Mar-M247 are likely due to the finer starting microstructure, more rapid coarsening rate of the γ′ and reduced γ′ strengthening effect (i.e., APB energy) of Nb, compared to Ta. No evidence of TCP phases was observed in either alloy.     

The Effect of Surface Chemistry and Morphology on the Properties of HVAF PEEK Single Splats

Thermal spray of polymers has had limited investigation due to the narrow processing windows that are inherent to polymer powders, especially their low temperatures of thermal degradation. The polymer poly aryl ether ether ketone (PEEK) has a continuous use temperature of 260 °C, does not suffer significant thermal degradation below 500 °C (Lu et al., Polymer, 37(14):2999-3009, 1996), and has high resistance to alkaline and acidic attack. These properties led to PEEK being selected for investigation. To minimize thermal degradation of the particles, the high velocity air fuel technique was used. To investigate the effect of substrate pretreatment on single splat properties, single splats were collected on aluminum 5052 substrates with six different pretreatments. The single splats collected were imaged by scanning electron microscopy and image analysis was performed with ImageJ, an open source scientific graphics package. On substrates held at 323 °C, it was found that substrate pretreatment had a significant effect on the circularity and area of single splats, and also on the number of splats deposited on the substrates. Increases in splat circularity, area, and the number of splats deposited on the surface were linked to the decrease in chemisorbed water on the substrate surface and the decrease of surface roughness. This proved that surface chemistry and roughness are crucial to forming single splats with good properties, which will lead to coatings of good properties.     

Thermochemical material removal of diamond by solid iron and mischmetal

Solid iron and mischmetal (Ce, La, Fe, etc. alloy) were used to remove material from single crystal diamond. Contact experiments with diamond and mischmetal were carried out at 600°C and 750°C with hold times from 3 to 24 h. Contact experiments with diamond and iron were performed at 850°C with hold times of 3, 4 and 5 h. In the case of mischmetal, material removal was only observed for temperatures of 750°C with hold times of 16 h or more. In these experiments it was found that material was removed only at certain areas of the contact surface whereas for the iron experiments material was removed over the entire contact surface.     

Effect of Heat Treatment Parameters on the Microstructure and Properties of Inconel-625 Superalloy

Inconel-625 is a solid solution-strengthened alloy used for long-duration applications at high temperatures and moderate stresses. Different heat treatment cycles (temperatures of 625-1025 °C and time of 2-6 h) have been studied to obtain optimum mechanical properties suitable for a specific application. It has been observed that room temperature strength and, hardness decreased and ductility increased with increase in heat treatment temperature. The rate of change of these properties is found to be moderate for the samples heat-treated up to 850 °C, and thereafter, it increases rapidly. It is attributed to the microstructural changes like dissolution of carbides, recrystallization and grain growth. Microstructures are found to be predominantly single-phase austenitic with the presence of fine alloy carbides. The presence of twins is observed in samples heat-treated at lower temperature, which act as nucleation sites for recrystallization at 775 °C. Beyond 850 °C, the role of carbides present in the matrix is subsided by the coarsening of recrystallized grains and finally at 1025 °C, significant dissolution of carbide results in substantial reduction in strength and increase in ductility. Elongation to an extent of >71% has been obtained in sample heat-treated at 1025 °C indicating excellent tendency for cold workability. Failure of heat-treated specimens is found to be mainly due to carbide particle-matrix decohesion which acts as locations for crack initiation.     

Plasma Spraying of Silica-Rich Calcined Clay Shale

Silica-rich clay shale is a viable candidate for replacement of mullite in many applications, especially when outstanding refractoriness and chemical resistance to various agents are desirable. In this contribution, instead of the commonly used synthetic mullite feedstock, the thermal stability of inexpensive calcined natural raw clay shale sprayed using water stabilized plasma system is reviewed. Phase stability and phase changes at elevated temperatures up to 1500 °C were studied by an array of experimental techniques ranging from measurements of thermal conductivity and the heat flow as functions of temperature, scanning electron microscopy, x-ray diffraction (XRD) of the annealed samples, and in situ high temperature XRD. The mostly amorphous as-sprayed coatings with less than 10 wt.% of mullite are temperature stable up to 800 °C and rapid crystallization occurs between 920 and 940 °C. Performed analyses gave evidence about the increase of mullite grain sizes for temperatures higher than 1200 °C and, moreover, certain saturation of crystallinity, not surpassing the threshold of 60 wt.% even for 1500 °C, is observed. The microstructure after annealing at 1500 °C is notable by clusters of fine needle-like mullite crystallites with sizes within the range of tens of nanometers in Si-rich amorphous matrix.     

Adsorption of quinalizarin from non aqueous solution onto acid activated palygorskite

In this study, the adsorption of quinalizarin by natural and mesoporous acid activated palygorskite has been examinated in order to remove coloured textile dyes from non aqueous solutions. The adsorption of quanilizarin from ethanol solution onto acid activated palygorskite was followed by UV-Visible measurement. The adsorption capacity was largely improved when the clay was activated by HCl. The high adsorption extents were observed from 2M8h to 2M35h acid treated palygorskite samples. Adsorption isotherms of quinalizarin on acid activated palygorskite were determined and correlated with common isotherm equations such as Langmiur and Freundlich models. It was found that i/the adsorption isotherm exhibits two plateaus ill no model fit the isotherm data. In order to explain the adsorption behaviour of quinalizarin from ethanol onto palygorskite, the presence of stronger and weaker adsorption sites was suggested and checked by using the modified Langmuir model.     

Agglomeration during reduction of MoO3

Molybdenum powder is manufactured in a two step process starting from MoO₃. The first step reduction of MoO₃ to MoO₂ is carried out in rotary calciners. Agglomeration of powder occurs during this reduction stage resulting in several manufacturing issues. The evolution of agglomeration during the reduction of MoO₃ was investigated in the current study. As-received MoO₃ and MoO₃ milled for 0.5 h were used as the starting powders. The powders were reduced at 550 °C, 650 °C and 750 °C in a hydrogen atmosphere. The starting and reduced powders at various temperatures were analyzed using BET surface area, XRD, and SEM techniques. The surface area of the reduced powders was monitored for quantifying the degree of agglomeration. The surface area was found to be minimum for the samples reduced at 650 °C. SEM observations confirmed the agglomeration of powders during reduction process. XRD analysis showed complete reduction of MoO₃ to MoO₂ at 650 °C and 750 °C. The agglomeration of the powders was either due to melting of eutectic formed between MoO₃ and Mo₄O₁₁ or due to partial melting of MoO₃. The reduction of MoO₃ is recommended to be completed at a low temperature to prevent agglomeration of the oxide powders.     

Thermal stability of polyaniline networks in conducting polymer blends

The thermal stability of polyaniline-camphor sulfonic acid (PANI-CSA) networks in blends with poly (methyl methacrylate) (PMMA) and a low molecular weight polyester (PES ) has been studied by transmission electron microscopy (TEM) supplemented with measurements of the optical and electrical properties. The tenuous interpenetrating fibrillar network of PANI-CSA is robust and remains intact at temperatures above the glass transition temperature (T_g) and the melting point (T_m) of the host polymers (T_g~90 °C for PMMA; T_m~60 °C for PES), indicating that the phase-separated network morphology is a thermodynamically stable phase. Although deprotonation of the PANI gradually starts around 100 °C, the network morphology persists (with crystallites of CSA, released by the deprotonation, distributed within the network). After exposure to increasingly higher temperatures, especially above 200 °C, the fibrils of the network coarsen, and the structure becomes more open. Complete deprotonation and degradation of the PANI are observed at temperatures above 200 °C. The conclusions from the TEM micrographs are consistent with thermal gravimetric analysis, spectroscopic data and electrical conductivity measurements.     

The oxidation behaviour of uniaxial hot pressed MoSi2 in air from 400 to 1400 °C

MoSi₂ samples were prepared by hot uniaxial pressing from a 2 μm grain-size powder of commercially available MoSi₂. The oxidation behaviour of MoSi₂ was systematically studied from 400 °C to 1400 °C, which includes the pest-oxidation temperature range. It was observed that the rate and mechanism for oxidation of MoSi₂ change significantly with increasing temperature. Five temperature regimes have to be considered regarding both kinetic results and cross-sections: i) 400 < T < 550 °C; ii) 550 ≤ T < 750 °C; iii) 750 ≤ T < 1000 °C; iv) 1000 ≤ T < 1400 °C; v) T ≥ 1400 °C. In the first range, pesting did not occur in samples that were free of cracks and residual stresses and the oxidation kinetics were governed by surface or phase boundary reactions. Above 550 °C, there was a change in the physical properties of the oxidation products due to the evaporation of MoO₃. The formation of Mo₅Si₃ was observed above 800 °C showing that the thermodynamic previsions were satisfied above this temperature. At higher temperatures (>1000 °C), the oxide scale became very protective and transport in the silica scale (amorphous and β cristobalite) governed the oxidation kinetics. The Mo₅Si₃ phase did not appear anymore at 1400 °C, indicating that another oxidation mechanism has to be proposed.     

The effects of ultrasonic nanocrystal surface modification (UNSM) on pack aluminizing for the fabrication of Pt-modified aluminide coatings at low temperatures

An 8–9 μm thick Pt layer was coated on a superalloy and transformed to a Ni–Pt alloy layer by the interdiffusion of Ni and Pt at 1050 °C for 3 h. The surface of the Ni–Pt alloy layer was pack aluminized to form a Pt-modified aluminide coating. Ultrasonic nanocrystal surface modification (UNSM) was applied to the alloy layer prior to pack aluminizing. The effects of UNSM on Pt-modified aluminide coatings fabricated at 750, 850, 950, and 1050 °C were studied. The treated Ni–Pt alloy layers had finer grain sizes than the untreated specimens. In addition, UNSM made the grain size of the Ni–Pt alloy finer and reduced the surface roughness. During pack aluminizing, the Pt-modified aluminide coatings fabricated following UNSM uptook more Al and were thicker than the untreated Pt-modified aluminide coatings at the various temperatures (750, 850, 950, and 1050 °C). The untreated Pt-modified aluminide coatings with pack aluminizing performed at 750 and 850 °C were composed of only a two-phase (NiAl + PtAl₂) layer, due to insufficient diffusion of Pt at the lower temperatures. However, two-phase and one-phase (NiAl) layers were obtained in the treated Pt-modified aluminide coatings which were pack-aluminized at 750, 850, 950, and 1050 °C, due to the diffusion of Pt through the greater amount of grain boundaries and increased volume generated by UNSM before the pack aluminizing. Additionally, the treated coatings had smoother surfaces even after the pack aluminizing. During cyclic oxidation at 1150 °C for 1000 h, the treated Pt-modified aluminide coatings aluminized at relatively low temperatures (750 and 850 °C) showed better cyclic oxidation resistance than the untreated Pt-modified aluminide coating aluminized at 1050 °C.     

水热晶化法制备RhCeZrLaNdOx催化剂及其性能研究

采用水热晶化法一步制备了RhCeZrLaNdOx系列催化剂,测定其比表面积、储氧量和还原温度等性能参数,并进行结构和形貌表征,研究了制备过程中水热晶化温度和时间、表面活性剂等条件对催化剂性能、结构和形貌的影响。结果表明,以月桂酸作为表面活性剂,在160℃水热晶化4 h制备的催化剂具有较大的比表面积、较高的储氧量和较低的还原温度,微观结构为四方相的固溶体纳米颗粒。与传统浸渍法制备的Rh/CeZrLaNdOx催化剂比较,水热晶化法制备的催化剂对CO还原NO具有更好的低温催化活性。