Resistance characteristics to oxidation of the metal-catalyzer for Cat-CVD

In order to obtain oxide films using catalytic chemical vapor deposition (Cat-CVD), oxidation experiments for catalyzers of Alumel, Al Chrom-O, Chromel, Hastelloy C-276, Kanthal, Kovar, Inconel-600, Inconel-601, Inconel X-750, Iron Chrome 30, Moleculoy, Monel, Ni, Nickel Chrome, Pt, SUS-304, SUS-316, Super Invar, and Ti have been carried out. The electric resistance measured for each metal heated at 900 °C, exposed for 30 min in O₂ atmosphere, have revealed that Alumel, Al Chrom-O, Chromel, Hastelloy C-276, Kanthal, Inconel-600, Inconel-601, Inconel X-750, Iron Chrome 30, Moleculoy, Ni, Nickel Chrome, Pt, SUS-304, and SUS-316 are in resistance to oxidation.     

An oxidation study of Ni-Cr-Fe alloys in carbon dioxide at 800 to 1000° C

Nickel-based alloys containing chromium and iron are alternative high-temperature materials to austenitic stainless steels at temperatures in excess of 800° C in gas-cooled reactor systems. In particular, some of the commercial superalloys may find application as cladding for ceramic fuel elements. These complex alloys contain a number of minor constituents that appear to play an ill-defined but essential role in regulating mechanical properties and oxidation resistance.This paper describes a structural evaluation of thin, oxide films formed on two such alloys, Hastelloy X and Inconel 600, under representative reactor conditions, with emphasis on the application of electron and X-ray diffraction techniques for the identification of multicomponent, surface oxide phases. The difficulties and limitations of these techniques in relation to the complexity of the oxide under examination are apparent, but nevertheless the large amount of useful information obtained has enabled some understanding of the oxidation mechanism. As in the case of highly alloyed, chromiumnickel steels, the rate-determining step in the growth of surface oxide on both alloys appears to be cation diffusion through an initially formed, chromic oxide phase, resulting in the subsequent development of an outer spinel oxide of variable composition.     

Thermal conductivity of Inconel 718 and 304 stainless steel

The results of thermal conductivity measurements on Inconel 718 and 304 stainless steel by the comparative and flash diffusivity techniques are reported for the temperature range 0–700°C. For 304 stainless steel, excellent agreement with published data is found for the specific heat, thermal diffusivity, and thermal conductivity. In the case of Inconel 718, the measurements show that the conductivity depends critically on the sample thermal history and the metallurgical condition of the alloy. Measurements on a solution-treated sample indicated a conductivity function close to that reported previously, while precipitated samples showed a higher conductivity, similar to the conductivityvs-temperature function used for reduction of comparative thermal conductivity data with Inconel 718 references. These results indicate that Inconel 718 is not a suitable reference for high-accuracy comparative thermal conductivity measurements unless its thermal history and associated conductivity function are known.     

Intercrystalline corrosion of surface-welds of nickel alloys on carbon steel

Surface-welds of Hastelloy C4 and some mixed welds of Inconel 600 and Hastelloy C4 were prepared by the plasma-hot-wire-surface-welding method. Intercrystalline corrosion tests were performed in a diluted Streicher-solution. On-line recording of the corrosion potential uses the stability of the redox-pair, Fe(III)/Fe(II) (called the chemical potentiostat). Microscopic observations of specimens after corrosion accompanied the potential recording and gave valuable information about the morphology changes that occur due to intercrystalline corrosion. Potential measurements in a 1/3-diluted Streicher-Test solution showed that 2-layer surface-welds of Hastelloy C4, prepared by the hot-wire-surface-welding method with arc-deflection, were the most resistant to intercrystalline corrosion. A mixed 2-layer surface-weld of Inconel 600 inner-layer and Hastelloy C4 outer-layer was not effective due to the heterogeneous interface formed between both layers.     

The interaction between silicon nitride and several iron,nickel and molybdenum-based alloys

Solid-solid reactions have been studied between silicon nitride and AlSl 316 and 20/25/Nb austenitic stainless steels, Fecralloy ferritic stainless steels (with and without yttrium), PE 16, Nimonic 75, Hastelloy X nickel-based alloys and a TZM molybdenum alloy. The reactant couples were heat-treated, in gettered inert gas, for up to 5161 h, at 800 to 1100° C. The temperature for the onset of measurable reaction with the iron and nickel-based alloys was between 825 and 900° C. Interaction was appreciable at 1000° C, being greatest with 20/25/Nb and least with the Fecralloy steel. The overall pattern of these reactions was similar, in that selected alloy constituents (chromium, together with iron and/or nickel where appropriate) reacted with the silicon nitride to form an adherent product, which was basically a silicide, although it also contained nitrogen. Some of the silicon and/or nitrogen released by subsequent decomposition of the primary reaction product was taken up by the alloys. In PE 16 and Hastelloy X alloys silicon was associated with molybdenum. There were several types of nitrogen pick-up: in the Hastelloy X alloy it followed a diffusion profile, while with other alloys it reacted with the constituents Ti, Al or Y to form nitrides. The surface layers on the austenitic stainless steel were denitrided, with nitrogen being transferred, via the gas phase, to a tantalum getter. With the TZM alloy no constituent was transferred to the silicon nitride. However, a silicon layer built up at the alloy surface and nitrogen was picked up, with its penetration following a diffusion profile.Trade Mark of the United Kingdom Atomic Energy Authority.Trade Mark of Henry Wiggin and Co. Ltd.Trade Mark of Union Carbide Corporation.     

Effect of temperature on the oxidation of Fe-7.5A1-o.65C alloy

An alloy with the chemical composition Fe-7.5A1-0.65C was employed to investigate the effect of temperature on oxidation between 600 and 900° C in dry air. Kinetic curves were determined by thermogravimetry analyses (TGA). Optical metallography and electron probe microanalysis (EPMA) were used to examine the oxide scales. At 600°C, the initial stage of oxidation followed a parabolic rate law, and oxidation subsequently, increased dramatically. Internal oxidation occurred beneath the nodule formed on the present alloy at 600°C. In contrast, no internal oxidation could be found in specimens of the Fe-7.5A1-0.65C alloy after oxidizing at 700, 800 and 900°C. The kinetic results have two distinct parabolic rates for the present alloy.     

Thermo-physical properties of Hastelloy X and Haynes 214 close to the melting range

ABSTRACT

The nickel-based superalloys Hastelloy X and Haynes 214 are widely used for honeycomb liners; however, thermo-physical properties at temperatures close to the melting range are poorly described in the literature. Based on the thermo-physical properties, endothermic effects between 550°C and 650°C have been observed that develop due to the formation of the short-range order in both alloys and in Haynes 214, in addition to the short-range order, an increase in the specific heat capacity due to the γ′-precipitation up to 950° was detected. At low and intermediate temperatures, the Hastelloy X is significantly better suited than the Haynes 214. At high temperatures the Haynes 214 is superior to the Hastelloy X due to the dissolution of the γ′-phase.     

Effect of Cu dopant on microstructure and phase transformation of ZnTiO3 thin films prepared by radio frequency magnetron sputtering

Cu doped zinc titanate (ZnTiO₃) films were prepared using radio frequency magnetron sputtering. Subsequent annealing of the as-deposited films was performed at temperatures ranging from 600 to 900 °C. It was found that the as-deposited films were amorphous and contained 0.84 at.% Cu. This was further confirmed by the onset of crystallization that took place at annealing temperatures 600 °C. The phase transformation for the as-deposited films and annealed films was investigated in this study. The results showed that Zn₂Ti₃O₈, ZnTiO₃, and TiO₂ can coexist at 600 °C. When annealed at 700 °C, the results revealed that mainly the hexagonal ZnTiO₃ phase formed, accompanied by minority amounts of TiO₂ and Zn₂Ti₃O₈. Unlike pure zinc titanate films, this result showed that the Zn₂Ti₃O₈ phase can be stable at temperatures above 700 °C. Moreover, Cu addition in zinc titanate thin film could result in the decomposition of hexagonal (Zn,Cu) TiO₃ phase at 800 °C. When the Cu content was increased in zinc titanate thin films from 0.84 at.% to 2.12 at.%, there were only two phases; Zn₂Ti₃O₈ and ZnTiO₃, coexisting at temperatures between 700 and 800 °C. This result indicated that a greater presence of Cu dopants in zinc titanate thin films leads to the existence of the Zn₂Ti₃O₈ phase at higher temperatures.     

Creep rupture strength and grain-boundary sliding in austenitic 21 Cr-4Ni-9Mn steels with serrated grain boundaries

The effect of grain-boundary strengthening on the creep-rupture strength by modification of the grain-boundary configuration is studied using austenitic 21 Cr-4Ni-9Mn steel in the temperature range from 600 to 1000° C in air. Grain-boundary sliding is also examined on a steel with serrated grain boundaries during creep at 700° C. The improvement of creep-rupture strength by the strengthening of grain boundaries is observed at high temperatures above 600° C. The 1000 h rupture strength of steels with serrated grain boundaries is considerably higher than that of steels with straight grain boundaries, especially at 700 and 800° C. The strengthening by serrated grain boundaries is effective in retarding both the crack initiation and the crack propagation at 700° C, while it does not improve the life to crack initiation at 900° C. Grain-boundary sliding is considerably inhibited by the strengthening of grain boundaries at 700° C. The amount of it in steels with serrated grain boundaries is less than about one-third of that of steels with straight grain boundaries at the same creep strain. The stress dependence of grain-boundary sliding rate in the steady-state regime is also examined from the steels with these two types of grain-boundary configuration.     

High-temperature mechanical properties of nickel-based superalloys manufactured by additive manufacturing

Mechanical behaviour of Haynes 230 and Inconel 625 manufactured by selective laser melting (SLM) has been experimentally investigated in 870^oC. Significant embrittlement of SLM Inconel 625 was found, accompanied by hardening behaviour during the test. Moreover, due to inhomogeneous grain deformation, the hardness dispersion of the two SLM alloys increased significantly. Employing a systematic research with scanning electron microscopy and energy-dispersive spectrometer, intergranular cracking of two SLM alloys can be observed from the surface cracks and fracture appearances, respectively. In addition, carbides along grain boundaries of the two nickel-based superalloys were found and affected the strength of the grain boundary at 870^oC. Therefore, it resulted in different fracture mechanisms.     

Magnetic and structural properties of Co ion-implanted GaN

A GaN epilayer was grown on Al/sub 2/O/sub 3/ substrate by metal-organic chemical vapor deposition, and Co/sup -/ ions with a dose of 3/spl times/10/sup 16/ cm/sup -2/ were implanted into GaN at 350/spl deg/C. The implanted samples were postannealed at 700/spl deg/C-900/spl deg/C to recrystallize the samples and to remove implantation damage. We have investigated the magnetic and structural properties of Co ion-implanted GaN by using X-ray diffraction (XRD), superconducting quantum interference device (SQUID) magnetometer, and X-ray photoelectron spectroscopy (XPS). XRD results did not show any peaks associated with the second phase formation, and only the diffraction from the GaN layer and substrate structure were observed. The temperature dependence of magnetization taken in zero-field-cooling and field-cooling conditions showed the features of superparamagnetic system in films annealed at 700/spl deg/C-900/spl deg/C. The magnetization curves at 5 K for samples annealed at 700/spl deg/C-900/spl deg/C exhibits ferromagnetic hysteresis loops, and the highest residual magnetization (M/sub R/) and coercivity (H/sub c/) of M/sub R/=1.5/spl times/10/sup -4/ emu/g and H/sub c/=107 Oe were found in the 800/spl deg/C annealed sample. XPS measurement showed the metallic Co 2p core levels and the metallic valence band spectra for as-implanted and 700/spl deg/C-900/spl deg/C annealed samples. From these, it could be explained that the magnetic property of our films originated from Co and CoGa magnetic clusters.     

Temperature dependence of dynamic Young's modulus and internal friction in LPPS NiCrAlY

The piezoelectric ultrasonic composite oscillator technique (PUCOT), operating near 80 kHz, was used to measure the temperature dependence, in the range 23–1000 °C, of dynamic Young's modulus,E, and internal friction,Q ^–1 in three compositions of low-pressure plasma-sprayed NiCrAlY: Ni-15.6Cr-5.2Al-0.20Y (16-5), Ni-17.2Cr-11.6Al-0.98Y (17–12), and Ni-33Cr-6.2 Al-0.95 Y (33–6). Ambient temperature (23 °C) dynamic Young's moduli for the three alloys were 205.0, 199.8, and 231.0 GPa, respectively. In each case, dE/dT was found to be — 0.06 GPa °C^–1 over temperature ranges 23–800, 23–400 and 600–900, and 23–700 °C, respectively. Internal friction was essentially independent of temperature to about 600 °C (700 °C for the 16–5 alloy), at which point a temperature dependence of the formQ ^–1 =A exp (C/RT) was observed. The constantA for the three alloys was determined to be 62.7, 555, and 2.01 × 10⁶, respectively. The constantC for the three alloys was determined to be 82.8, 111, and 170 kJ/mol^–1, respectively. While the physical mechanism is not fully understood, both the pre-exponential constantA and the activation energyC correlate with durability in thermal barrier coatings (TBCs) wherein these alloys are used as bond coats.     

Elevated temperature tensile properties of powder metallurgy Ni3Al alloyed with chromium and zirconium

The tensile properties of hot extruded powders of Ni-24.1Al, Ni-19.1Al-8.5Cr, and Ni-17.4Al-7.9Cr-0.5Zr have been evaluated from room temperature to 1000° C. These powder metallurgy materials have a fine grain size that results in relatively little increase in yield stress with increasing temperature compared to coarse-grained or single-crystal materials. The alloy containing chromium and zirconium shows greatly reduced dynamic embrittlement in the temperature range 600 to 800° C where the unalloyed aluminide exhibits brittle behaviour. The Cr- and Cr + Zr-containing alloys deform superplastically above 900° C. The mechanism of superplastic deformation appears to be predominantly grain-boundary sliding.     

Joint strength and interfacial microstructure in silicon nitride/nickel-based Inconel 718 alloy bonding

Joining of Inconel 718 alloys to silicon nitrides using Ag–27Cu–3Ti alloys was performed to investigate the microstructural features of interfacial phases and their effect on joint strength. The Si3N4/Inconel 718 alloy joints had a low shear strength in the range 70.4–46.1 MPa on average, depending on joining temperature and time. When the joining time was held for 1.26 ks at 1063 K, shear, tension, and four-point bending strength were 70.4, 129.7, and 326.5 MPa on average. The microstructures of the joints typically consisted of six types of phases. They were TiN and Ti5Si4 between silicon nitride and filler metal, a copper- and silver-rich phase, island-shaped Ti–Cu phase, a Ti–Cu–Ni alloy layer between filler and base metal, and diffusion of titanium into the Inconel 718 alloys. With increasing joining temperature, the thickness increase of the Ti–Cu–Ni alloy layer was much greater than that of the reaction layer. Thus the diffusion rate of titanium into the base metal was much greater than the reaction rate with silicon nitride. This behaviour of titanium results in the formation of a Ti–Cu–Ni alloy layer in all the joints. The formation of these layers was the cause of the strength degradation of the Si3N4/Inconel 718 alloy joints. This fact was supported by the analyses of fracture path after four-point bending strength tests.     

Formation and transformation of ZnTiO3 prepared by sputtering process

Zinc titanate (ZnTiO₃) films were prepared using RF magnetron sputtering at substrate temperatures ranging from 30 to 400 °C. Subsequent annealing of the as-deposited films was performed at temperatures ranging from 600 to 900 °C. It was found that all as-deposited films were amorphous, as confirmed by XRD. This was further confirmed by the onset of crystallization that took place at annealing temperatures 600 °C. The phase transformation for the as-deposited films and annealed films were investigated in this study. The results revealed that pure ZnTiO₃ (hexagonal phase) can exist, and was obtained at temperatures between 700 and 800 °C. However, it was found that decomposition from hexagonal ZnTiO₃ to cubic Zn₂TiO₄ and rutile TiO₂ took place with a further increase in temperature to 900 °C.     

Crystallization of alkaline earth zirconates and niobates from compositionally flexible aqueous solution-gel syntheses

Barium and strontium zirconates and niobates were prepared by means of an economic and ecologic aqueous solution-gel route with high compositional flexibility. The synthesis of precursor solutions and gels, and thermal analysis of the gels are presented. The crystallization of the different oxides was studied in situ using high-temperature X-ray diffraction measurements (HT-XRD). Zirconate crystallization was observed between 600 °C and 700 °C. Small amounts of Ba(II)- and Sr(II)-carbonate crystallized before the oxide did, and decomposed between 700 °C and 800 °C. More sensitive Fourier transform infrared spectra, showed that very small traces of carbonate remain at 900 °C. The crystallization behavior of strontium niobates was strongly dependent on the Sr:Nb ratios, as observed in HT-XRD. Phase pure SrNb₂O₆ was obtained at low temperature (600 °C). Sr₂Nb₂O₇ and Sr₅Nb₄O₁₅ were formed at 900 °C from an intermediate phase, with a minor secondary phase. Nb₂Sr₄O₉ crystallized at 600 °C, but transformed to a mixture of phases at high temperature (1000 °C). By off-line furnace annealing, phase pure strontium niobates of all four compositions were obtained. The presented work demonstrates the high compositional flexibility of the aqueous solution-gel route, allowing fast synthesis of different multimetal oxides with high purity.     

Effect of manganese on the oxidation of Fe-Mn-Al-C alloys

The effects of manganese on the oxidation of alloys with the chemical composition (wt%) Fe-5AAl-1.5Mn-0.58C and Fe-5.3Al-3.5Mn-0.53C at 600, 800 and 1000° C in dry air were investigated. Kinetic curves were determined by thermogravimetric analyses. Optical metallography and electron probe microanalysis were used to examine the oxide scales. The kinetic curves of Fe-5.4Al-1.5Mn-0.58C alloy oxidized at 600, 800 and 1000° C had simple, three- and two-stage parabolic rate laws, respectively. On the other hand, two stages of linear rate law were observed in Fe-53Al-3ZMn-0.53C alloy when oxidized at 600° C, while two distinct parabolic rate laws were found in the same alloy oxidized at 800 and 1000° C. Oxidation behaviours and the oxide formation mechanisms of the alloys at different temperatures are discussed in this paper.     

Microstructure of sol-gel derived corundum containing coatings

Up to ∼ 3 μm thick alumina coatings on corundum ceramic, soda-lime-silica glass and Inconel718™ were produced from mixtures of boehmite sols and corundum suspensions.Transmission electron micrographs in combination with electron diffraction and energy dispersive X-ray spectroscopy served to identify crystallographic phases and to characterize the microstructure of the coatings. Using corundum ceramic as substrate the initially deposited boehmite transforms via transition aluminas to corundum while heating to 1200 °C. In the cases of glass and Inconel718™ thermal treatments up to 520 °C and 1100 °C, respectively, cause diffusion of ions from the substrates into the coatings. Thus additional oxide phases were formed.All coatings are free of cracks or delaminations and do not show any directed crystal growth.     

The microstructural observation and wettability study of brazing Ti-6Al-4V and 304 stainless steel using three braze alloys

Both Ti-6Al-4V and 304 stainless steels (304SS) are good engineering alloys and widely used in industry due to their excellent mechanical properties as well as corrosion resistance. Well-developed joining process can not only promote the application of these alloys, but also can provide designers versatile choices of alloys. Brazing is one of the most popular methods in joining dissimilar alloys. In this study, three-selected silver base filler alloys, including Braze 580, BAg-8 and Ticusil®, are used in vacuum brazing of 304SS and Ti-6Al-4V. Based upon dynamic sessile drop test, Braze 580 has the lowest brazing temperature of 840°C, in contrast to 870°C for BAg-8 and 900°C for Ticusil® braze alloy. No phase separation is observed for all brazes on 304SS substrate. However, phase separation is observed for all specimens brazed above 860°C on Ti-6Al-4V substrate. The continuous reaction layer between Braze 580 and 304SS is mainly comprised of Ti, Fe and Cu. The thickness of reaction layer at Braze 580/Ti-6Al-4V interface is much larger than that at Braze 580/304SS interface. Meanwhile, a continuous Cu-Sn-Ti ternary intermetallic compound is found at the Braze 580/Ti-6Al-4V interface. Both Ticusil® and BAg-8 brazed joint have similar interfacial microstructures. Different from the Braze 580 specimen, there is a thick Cu-Ti-Fe reaction layer in both BAg-8/304SS and Ticusil®/304SS interfaces. The formation of Cu-Ti-Fe interfacial layer can prohibit wetting of BAg-8 and Ticusil® molten brazes on 304SS substrate. Meanwhile, continuous Ti₂Cu and TiCu layers are observed in Ti-6Al-4V/BAg-8 and Ti-6Al-4V/Ticusil® interfaces.     

Oxidation behaviour and strength of B4C-30 wt% SiC composite materials

The oxidation behaviour and the effect of oxidation on the room-temperature flexural strength of B₄C-30 wt% SiC composite material were investigated. The weight changes of the samples exposed to air at temperatures between 500 and 1000 °C were continuously monitored with a microbalance. At temperatures below 800 °C, the weight change of the specimen was negligible. As the temperature was increased to 800 °C, parabolic weight gain was observed. The rate of the weight gain increased with exposure temperature. The oxidation product formed on the surface was found to be a crystalline boric oxide (B₂O₃) by X-ray analyses. The oxide layer was severely cracked due to the thermal expansion mismatch between the oxide layer and the substrate. However, the room-temperature flexural strength was increased when the samples were exposed at temperature between 700 and 900 °C, apparently due to the blunting of strength-limiting defects at the surface. When the temperature was higher than 1000 °C, a severe reduction in strength was observed. The reliability of the composite material was also improved significantly by such exposures.