Stability and Heat Resistance of Silicide Coatings on Refractory Metals. Part 3. Stability of Silicide Coatings on Niobium Heated in Air at 1500-1800°C

The kinetics of phase redistribution in the (Mo, W)Si₂ Nb system at 1500-1800°C was investigated. The kinetic parameters for growth of the lower silicides (Mo, W, Nb) ₅Si₃ + Nb₅Si₃ and decrease in the layer thickness of the higher silicide (Mo, W)Si₂ as function of the oxidation temperature were determined. It was established that the stability of the multiphase and multicomponent system was more than twice that of the system MoSi₂ Nb, and 15-18 times that of MoSi₂ Mo.     

Stability and Heat Resistance of Silicide Coatings on Refractory Metals. Part 1. Effect of Subsurface Layer on Process of Phase Redistribution in System WSi2 ― W

The effect of a subsurface layer of metal silicide on the phase composition stability of high-temperature MeSi₂-type coatings on refractory metals was investigated. Using the system WSi₂ W as a prototype it was experimentally determined that a subsurface layer of the tungsten disilicide, and the distribution profile of silicon in the diffusion zone upon high-temperature heating have a substantial effect on the formation of a barrier layer of lower silicide which determines the stability of the system as a whole. It is proposed that the search for more stable silicide coatings on refractory metals should be directed toward the creation of diffusion barriers on not only the inner interface MeSi₂ Me, but also the external surface of the coating. The process of coating formation must be accompanied by the formation of a metal silicide on the external surface with the highest possible concentration of silicon.     

Role of Nb Addition on Microstructural Stability and Deformation Behaviors of FeMnAlC Lightweight Steels at 400 °C

Metallurgical and Materials Transactions A - As part of efforts to fabricate FeMnAlC lightweight steels suitable for high-temperature applications, the influence of Nb addition on the...     

Oxidation Behavior and Structure Stability at 1250 °C of Chromium-Rich TaC-Containing Cast Alloys Based on Nickel and Cobalt

Metallurgical and Materials Transactions A - In this work, six refractory nickel and cobalt-based cast alloys, rich in chromium and designed to be strengthened by tantalum carbides, were cast in...     

Hot Corrosion Behaviour of Refractory and Rare Earth Oxide Reinforced CoCrAlY APS Coatings at 700 °C

This paper investigates cyclic hot corrosion of plasma sprayed CoCrAlY?+?Al₂O₃?+?YSZ (C1) and CoCrAlY?+?CeO₂ (C2) composite coatings on MDN 321 and Superni 76 substrates in molten salt (Na₂SO₄-60%V₂O₅) environment exposed to 700 °C. Weight change technique is used to evaluate the corrosion performance. Both C1 and C2 coatings showed better corrosion resistance than uncoated alloy. Both the coatings showed linear weight gain during the initial cycles and parabolic weight gain nature with subsequent hot corrosion cycles. The parabolic rate constant (K_p) of C1 and C2 coating was observed to be in the range 0.29–0.32?×?10^?10 g² cm^?4 s^?1 and 1.0–1.13?×?10^?10 g² cm^?4 s^?1 respectively. In C1 coating, the globular and continuously packed structure on the corroded surface having CoO, Cr₂O₃, CoCr₂O₄ and CoAl₂O₄ spinel oxides provided superior hot corrosion resistance. While in case of C2 coating, the outward growth of CeVO₄ irregular crystals as a corrosion product of CeO₂ and V₂O₅ salt deteriorated the oxide scales resulting in higher corrosion rate.     

Interaction of Cerium Oxide with Hafnium,Zirconium, and Yttrium Oxides at 1500°C

Using the methods of x-ray diffraction and petrographic analysis as well as electron microscopy, phase relationships in the ternary systems ZrO₂ Y₂O₃ CeO₂ and HfO₂ Y₂O₃ CeO₂ were investigated over the entire range of concentrations at 1500°C. The accuracy of the phase relationships of the binary systems HfO₂ CeO₂, ZrO₂ CeO₂, and CeO₂ Y₂O₃ at this temperature was improved. Isothermal sections of the ternary phase diagrams at 1500°C were constructed. Specimens of various compositions were prepared from nitrate solutions by evaporating, drying, and heat treating at 1500°C. These systems are characterized by the formation of broad ranges of solid solutions based on C-Y₂O₃, F-CeO₂, F-HfO₂ and ZrO₂, and narrow ranges based on T-ZrO₂ and M-HfO₂. No new phases were detected. The course of isoparametric lines in the solid solution ranges of F-HfO₂ (ZrO₂) leads to the assumption that yttrium and cerium ions substitute for each other, but no appreciable substitution by them for zirconium or hafnium ions occurs.     

A Re-evaluation of the Solubility and Stability Regions of Calcium Arsenites and Calcium Arsenates in Aqueous Solution at 25°C

Abstract

The systems Ca(II)-As(III)-H₂O and Ca(II)-As(V)-H₂O were studied by equilibrating mixtures of calcium oxide and arsenious or arsenic acid solution at 25°C for one month. Where the pH of the final solution was likely to be greater than 8, care was taken to exclude atmospheric carbon dioxide. Chemical analysis of the final solid phases and the associated solution from each experiment for calcium and arsenic, together with X-ray diffraction analysis of the solids and an inspection of graphed relationships, indicated the existence of the compounds Ca(AsO₂)₂ and CaAsO₂OH 1/2H₂O in the system Ca(II)-As(III)-H₂O; and Ca(H₂AsO₄)₂, CaHAsO₄ H₂O, Ca₃(AsO₄)₂ 4H₂0 and Ca₂AsO₄OH 2H₂O in the system Ca(II)-As(V)-H2O. The solubility and stability regions for these compounds were assessed as a function of pH, and solubility constants and conditional free energies of formation for each compound calculated from the solubility data obtained.

A brief historical review of literature in which calcium arsenites and calcium arsenates have been reported is included in this paper.     

Influence of the Co/Ni Ratio and Dendritic Segregations on the High-Temperature Oxidation Resistance of Multinary Co-Rich Superalloys at 850 °C and 1050 °C

Metallurgical and Materials Transactions A - Excellent inherent oxidation resistance is a prerequisite for the use of superalloys in many high-temperature applications. To achieve this goal,...     

Structure and Wear Resistance of Coatings on Titanium Alloy and Steels Obtained by Electrospark Alloying with AlN ― ZrB2 Material

We have studied the process of electrospark alloying of titanium alloy VT6 and steels with a composite material based on AlN ZrB₂ with ZrSi₂ additive. We have established that Al₂SiO₅ and ZrSiO₄ forming in the coating directly during its formation play the role of a solid lubricant. Under optimal conditions for electrospark alloying of the titanium alloy, the coefficient of friction is 0.13, the wear is 6 m/km for a sliding velocity of 14 m/sec and a load of 2.56 MPa. Ceramics based on AlN ZrB₂ can be recommended for use as wear-resistant and corrosion-resistant coatings.     

Interdiffusion in Ni-rich,Ni-Cr-Al alloys at 1100 and 1200 °C: Part II. Diffusion coefficients and predicted concentration profiles

Ternary interdiffusion coefficients were measured in the Ni solid solution γ (fcc) phase of the Ni-Cr-Al system at 1100 and 1200 °C. Extensive use was made of both γ/γ and γ/γ + β (β-NiAl structure) diffusion couples. Two analysis techniques were employed to calculate the interdiffusion coefficients. When the Matano planes for Al and Cr were not coincident, numerous integral calculations were made to determine an average diffusion coefficient and to assess the effect of the noncoincidence of the Matano planes. The results of the diffusivity measurements showed that is approximately four times greater than , while and are of the same magnitude. For all concentrations, is two to three times greater than . Both and increase with increasing Al concentration, whereas and show little concentration dependence on Cr alone. A ternary, finite-difference interdiffusion model was employed to predict concentration profiles for the γ/γ couples utilizing the concentration dependence of the measured diffusivities. Good agreement was observed between the predicted and measured concentration profiles for both 1100 and 1200 °C.     

Mechanical Properties and Heat Resistance of Ta–Zr–Si–B–C–N Coatings Obtained upon the Magnetron Sputtering of the TaZrSiB Target in Ar,N2, and C2H4 Atmosphere

Russian Journal of Non-Ferrous Metals - Ta–Zr–Si–B–C–N coatings are obtained by magnetron sputtering in argon, nitrogen, and ethylene atmosphere. The structure of the...     

Thermodynamics and Structure of CaO-Al2O3-3 Mass Pct B2O3 Slag at 1773 K (1500 °C)

Metallurgical and Materials Transactions B - Activity values of Al2O3 in CaO-Al2O3-B2O3 systems at 1773 K (1500 °C) were determined experimentally using a...     

Reactions of Fe-Cr and Ni-Cr alloys in CO/CO2 gases at 850 and 950 °C

A series of Fe-Cr and Ni-Cr solid solution alloys was reacted at 850 and 950 °C in CO/CO₂ gas mixtures in which FeO and NiO were unstable. The compctitive tendencies toward the carburization and oxidation of the chromium solute, as compared to a graphical thermodynamic "metastability" criterion, were tested experimentally. Relatively good agreement was found between predictions and experiments for the occurrence of Cr carburization beneath Cr₂O₃ internal oxides or external scales. The chromium contents required for the transition from internal oxidation of Cr to the formation of Cr₂O₃ external scales in CO/CO₂ gas mixtures were established for Fe-Cr and Ni-Cr alloys. The Cr₂O₃ external scales formed on Fe-Cr alloys were found to be relatively impervious to carbon penetration for short (12-hour) experiments. No carburization was observed in the Ni-Cr alloys, but the only alloys that were predicted to carburize were the ones that formed external scales. Formerly Graduate Student, The Ohio State University     

Influence of CeO_2 on the Microstructure and Wear Resistance of M_(80)S_(20) Thermal Spray and Thermal Spray Welding Coatings

The influence of rare earth oxide CeO_2 on microstructure and wear resistance of M_(80)S_(20) thermal sprayand thermal spray welding coatings is studied using optical microscope,scanning electron microscope,X-rayenergy dispersion spectroscope,X-ray diffractometer and wear testing machine.The results show that the addi-tion of 8% CeO_2 can improve the microstructure,microhardness and wear resistance of coatings significantly.     

Effects of grain size and precipitate size on the fatigue crack growth behavior of alloy 718 at 427 °C

The fatigue crack growth rates of four Alloy 718 microstructures comprising a two-by-two matrix of grain size and γ" precipitate size were determined in air at 427 °C and 0.33 Hz. For a stress ratio of 0.05, slower Region II rates were obtained for coarse-grained microstructures, independent of γ" size, and microstructures with large γ", independent of grain size. In the near-threshold regime, the coarse-grained microstructures again showed slower growth rates (higher ΔK_th), whereas the effect of γ" size was mixed. Deformation modes were studied using scanning and transmission electron microscopy. Fatigue deformation resulted in the formation of slip bands which were longer for coarse-grained microstructures and typically spaced farther apart and more planar for large γ micro-structures. The concepts of dislocation reversibility and slip band strain localization were used to explain the microstructural effects. Fatigue morphologies and cyclic constitutive behavior were con-sistent with the observed deformation modes. For a stress ratio of 0.75, the effects of grain size and γ" size were essentially identical to those observed for a stress ratio of 0.05. This indicated that roughness-induced closure had a minimal influence on the differences that were observed in fatigue crack growth behavior.     

The Al2O3-ZrO2-Yb2O3 phase diagram. I. Isothermal sections at 1250 and 1650°C

Isothermal sections at 1250 and 1650 °C of the Al₂O₃-ZrO₂-Yb₂O₃ phase diagram are plotted for the first time. Phase equilibria are established at these temperatures. No ternary compounds and appreciable solid solution regions based on components or binary compounds are found in the ternary system. A partially pseudobinary section Yb₃A₅-F triangulating the Al₂O₃-ZrO₂-Yb₂O system is established.     

Evolution and coarsening of Al2O3 dispersoids at 500 °C to 600 °C in a mechanically alloyed Al-Ti-O based material

The formation and coarsening of Al₂O₃ dispersoids have been investigated at 500 °C, 550 °C, and 600 °C in a mechanically alloyed (MA) extrusion of composition Al-0.35wt pct Li-1wt pct Mg-0.25wt pct C-10vol pct TiO₂ for times up to 1500 hours. In the as-extruded condition, the dispersed phases included Al₃Ti, Al₄C₃, MgO, cubic TiO (C-TiO), monoclinic TiO (M-TiO), TiO₂, and a small amount of Al₂O₃. However, numerous Al₂O₃ dispersoids (various polymorphs: η, γ, α, and δ) with “block-shaped” morphology were formed after heat treatment due to reduction of C-TiO, M-TiO, and TiO₂. Transmission electron microscopy (TEM) and X-ray diffraction (XRD) showed conclusively the transformation of these phases to additional Al₂O₃ and Al₃Ti. High resolution TEM showed that the α-Al₂O₃ dispersoids exhibited some lattice matching with the α-Al matrix. Coalescence of the block-shaped Al₂O₃ dispersoids occurred after heat treatment, and Al₄C₃ also became attached to them. The length and width of the block-shaped Al₂O₃ dispersoids increased by a factor of ∼1.55 between 340 and 1500 hours at 600 °C.