Crystallisation of oxygen-stabilised amorphous phase in a Zr50Cu50 alloy

The crystallisation of the oxygen-stabilised amorphous phase in a Zr₅₀Cu₅₀ alloy has been investigated by means of neutron diffraction and electron microscopy. The crystallisation microstructure consists of ZrO₂, Zr₂Cu and Zr₇Cu₁₀. A two-stage crystallisation mechanism is suggested: (i) primary crystallisation of Zr₂Cu and (ii) formation of nanocrystals ZrO₂ and Zr₇Cu₁₀. In (i), it is proposed, Zr₂Cu crystallises from the oxygen-stabilised amorphous phase, leaving an oxygen- and copper-enriched matrix ; Zr₂Cu rapidly grows and eventually attains a grain size of about 100 nm. In (ii), it is suggested, the residual amorphous matrix crystallises into nanocrystals ZrO₂ and Zr₇Cu₁₀ due to the sluggish growth of ZrO₂ and to the already formed ZrO₂ which acts as a growth barrier to Zr₇Cu₁₀. In this case there is no particular orientation relationship between Zr₂Cu and Zr₇Cu₁₀.     

Anomalous high-temperature oxidation in the zirconium-copper system

The high-temperature oxidation of the Zr-3 mass% Cu alloy and Zr₂Cu in oxygen is characterized by selective oxidation of zirconium while the excess of copper is accumulated at the alloy-oxide interface forming the Zr₈Cu₅ phase. The oxidation of Zr₂Cu at elevated temperatures shows an anomalous decrease of the oxygen consumption rate in the temperature range 890-975 °C. The oxide layer consists of monoclinic ZrO₂ mainly, with preferentially oriented crystallites in depth region at 900 °C and tetragonal ZrO₂ on the surface below 600 °C, and small amounts of CuO and Cu₂O. The reaction kinetics obeys a parabolic rate law. The activation energy of 117.5 and 54.4 kJ/mol has been estimated for the oxidation of the Zr-3 mass% Cu alloy and Zr₂Cu, respectively.     

Impact of oxyfuel atmospheres H2O/CO2/O2 and H2O/CO2 on the oxidation of ferritic–martensitic and austenitic steels

In future power plant technologies, oxyfuel, steels are subjected to steam rich and carbon dioxide rich combustion gases. The effect of simulated combustion gases H₂O/CO₂/O₂ (30/69/1 mol%) and H₂O/CO₂ (30/70 mol%) on the corrosion behavior of low alloyed steels, 9–12% chromium steels and an austenitic steel were studied. It was discovered that the formation of protective chromium rich oxides is hampered due to the carburization of the base material and the formation of chromium rich carbides. The kinetics of corrosion and carburization are quantified. The effect of temperature and the effect of gas pressure are analyzed statistically.     

Recession behavior of Yb2Si2O7 phase under high speed steam jet at high temperatures

Recession behavior of Yb₂Si₂O₇ phase was examined under high speed steam jet environment between 1300 °C and 1500 °C. Yb₂SiO₅ phase was formed on the bulk surface by the decomposition of Yb₂Si₂O₇ phase and the elimination of silica component at elevated temperatures. The phase ratio of Yb₂SiO₅/Yb₂Si₂O₇ increased up to 1400 °C and then decreased above 1400 °C. The relative intensity of 2 2 0 peak for Yb₂Si₂O₇ phase increased with increasing the temperatures. Fine grains were generated on the bulk surface at 1300 °C. The phase decomposition caused on the grain interior. A porous structure was formed on the bulk surface during the test at 1400 °C. Surface cracks were generated for 1400 °C test sample. A smooth surface was generated on the surface of 1500 °C test sample. The triple points of the grains were bridged with a glassy phase.     

Electrochemical behavior and localized corrosion associated with Al7Cu2Fe particles in aluminum alloy 7075-T651

Initiation of localized corrosion upon high strength aluminum alloys is often associated with cathodic intermetallic particles within the alloy. Electrochemical measurements and metallurgical characterization have been made to clarify and quantify the physical properties of Al₇Cu₂Fe particles in AA7075-T651. Prior studies regarding either the stereology or electrochemical properties of Al₇Cu₂Fe are scarce. Quantitative microscopy revealed a significant population of Al₇Cu₂Fe in the alloy; comprising up to 65% of the constituent particle population and typically at a size of 1.7 ± 1.0 μm. It was determined that Al₇Cu₂Fe may serve as a local cathode in the evolution of localized corrosion of AA7075-T651 and is capable of sustaining oxygen reduction reactions at rates of several hundreds of μA/cm² over a range of potentials typical of the open circuit potential (OCP) of AA7075-T651 in NaCl solution of various concentrations and pH. The presence of Al₇Cu₂Fe leads to the development of pitting at the particle–matrix interface.     

The role of microstructure in the high temperature oxidation mechanism of Cr3C2-NiCr composite coatings

Composites of Cr₃C₂-NiCr provide superior oxidation resistance to WC-Co composites, which has seen them applied extensively to components subjected to combined high temperature erosion and oxidation. This work characterises the variation in oxidation mechanism of thermally sprayed Cr₃C₂-NiCr composites at 700 °C and 850 °C as a function of heat treatment. Carbide dissolution during spraying increased the Ni alloy Cr concentration, minimising the formation of Ni oxides during oxidation. Compressive growth stresses resulted in ballooning of the oxide over the carbide grains. Carbide nucleation with heat treatment reduced the Ni alloy Cr concentration. The oxidation mechanism of the composite coating changed from being Cr based to that observed for NiCr alloys.     

Corrosion behaviour of Y2O3-ZrO2 coatings on IN713LC in a LiCl-Li2O molten salt

Hot corrosion studies were performed on superalloy specimens. The IN713LC superalloy were sprayed with an aluminized NiCrAlY-bond coat and then with an yttria-zirconia top coat. The bare superalloy reveals an obvious weight loss due to spalling of the scales by the rapid scale growth and thermal stress. The top coatings showed a much better hot corrosion resistance in the presence of LiCl-3 wt.% Li₂O molten salt when compared with those of the bare superalloy and the aluminized bond coatings. These coatings have been found to be beneficial for increasing hot corrosion resistance of structural materials for lithium molten salts.     

Oxidation kinetics of high strength low alloy steels at elevated temperatures

High strength low alloy (HSLA) steels are candidate Rockbolt materials for use as underground roof supports at Yucca Mountain nuclear waste repository. Oxidation kinetics of International Rollforms Split Set Friction Rock Stabilizers (SS46), and Swellex Mn24 steels have been determined by temperature modulated thermogravimetry at temperatures ranging between 600 and 900 °C in pure oxygen atmosphere for 100 hr. The imposed sinusoidal temperature modulations (±5 °C for a period of 1 cycle per 200 s) on the isothermal temperature did not have any noticeable effect on the weight gain characteristics during oxidation. Weight gain data on the steels indicate two distinct regions with different oxidation profiles, where a definite change in rates of oxidation is observed: a first oxidation regime where the steels followed a rate law y = kt^0.40-0.63 (changing index of rate law depending upon steel and temperature) and a second stage oxidation regime that follows the parabolic law. The results of characterization of the oxide films using SEM/EDAX, X-ray diffraction and Synchrotron white beam X-ray microdiffraction are presented. The oxidation data of the steels presented here is expected to be useful for characterizing those steels for use in underground rock bolt system and as roof support for the DOE proposed Yucca Mountain Nuclear Waste Repository. To the best of our knowledge this is the first time thermogravimetric studies of this kind have been done on these steels.     

Effects of water vapour on isothermal oxidation of chromia-forming alloys in Ar/O2 and Ar/H2 atmospheres

Fe9Cr, Fe17Cr and Fe25Cr alloys were subjected to isothermal oxidation in Ar/O₂ and Ar/H₂ atmosphere at 700 °C as high temperature corrosion for 48 h. Oxidation weight change measurement showed increasing Cr content reduced the oxidation rate. The oxidized Cr alloys were analysed using SEM, TEM and XRD. The addition of water vapour accelerates the onset of breakaway oxidation kinetics for Fe9Cr. The presence of water vapour promotes internal oxidation of Cr within Fe9Cr. For Fe17Cr and Fe25Cr, the water vapour effect is not significant due to the large Cr reservoir due to continue growth of Cr₂O₃.     

Corrosion of Ti3AlC2 at 800–1100 °C in Ar–0.2% SO2 gas atmosphere

Ti₃AlC₂ was corroded between 800 and 1100 °C in an Ar–0.2% SO₂ gas atmosphere according to the equation: Ti₃AlC₂ + O₂ → rutile-TiO₂ + α-Al₂O₃ + (CO or CO₂). The scales that formed on the Ti₃AlC₂ were thin and rich in α-Al₂O₃, whose growth rate was exceedingly slow. The TiO₂ was present either as the outermost surface scale or a mixture inside the α-Al₂O₃-rich scale. In the Ti₃AlC₂, the activity and diffusivity of Ti were low, whereas those of Al were high. This was the main reason for the superior corrosion resistance of Ti₃AlC₂ over TiAl.     

Microstructure studies of Zr65Cu17.5Al7.5Ni10 and Zr65Cu15Al10Ni10 glass forming alloys: Phase morphologies and undercooled melt solidification

Zr₆₅Cu_17.5Al_7.5Ni₁₀ (at.%) and Zr₆₅Cu₁₅Al₁₀Ni₁₀ (at.%) glass forming alloy microstructures have been investigated by means of optical and electron microscopies. They are composed of a fine eutectic matrix with eutectic dendrites (EDs) that have peculiar morphologies. Al and Cu concentrations, in these alloys, favour primary dendrites and determine the ED morphologies and compositions. Their locations within the microstructures suggest a two-step solidification process of the two undercooled melts. The identified crystalline phases indicate the occurrence of solid state phase transformations in agreement with the structural defects observed in the grains. The crystalline phases can be classified into Zr-rich, Cu-rich, Ni-rich and Al-rich compounds resulting from competing diffusion between Cu, Ni, and Al in the melts.     

High-temperature SO2-gas corrosion of TiN–Ti5Si3 composites prepared by polymer pyrolysis

By pyrolyzing a mixture of Si-containing pre-ceramic polymers and TiH₂ powders in a N₂ atmosphere, a TiN–Ti₅Si₃ composite was synthesized. The composite was then corroded between 700 °C and 1000 °C for 20 h in an Ar–0.2% SO₂ atmosphere. TiN was mainly oxidized to rutile TiO₂. Ti₅Si₃ was oxidized to TiO₂ supersaturated with Si ions, and sulfidized to Ti₂S supersaturated with Si ions. At initial stage of corrosion, oxidation dominated sulfidation. As corrosion proceeded, sulfidation progressively occurred underneath the oxide scale based on the decreased oxygen potential and increased sulfur potential near the scale/matrix interface.     

Hot corrosion behaviour of Yb2Zr2O7 ceramic coated with V2O5 at temperatures of 600-800 °C in air

To better understand the hot corrosion behaviour of Yb₂Zr₂O₇ ceramic in molten V₂O₅, hot corrosion experiments were performed in a temperature range of 600-800 °C in air. Different reaction products of ZrV₂O₇, YbVO₄ and m-ZrO₂ were identified depending upon the hot corrosion conditions, for example, ZrV₂O₇ and YbVO₄ at 600 °C for 2 h and 8 h; ZrV₂O₇, m-ZrO₂ and YbVO₄ at 700 °C for 2 h; m-ZrO₂ and YbVO₄ either at 800 °C for 2 h or at 700-800 °C for 8 h. The hot corrosion reaction mechanisms were further discussed based on the thermal instability of ZrV₂O₇ at elevated temperatures.     

High temperature oxidation resistance of FeCrAl alloys covered with ceramic SiO2–Al2O3 coatings deposited by sol–gel method

One-, three- and five-layer SiO₂–Al₂O₃ coatings were deposited on a FeCrAl alloy basis by the sol–gel method. Sols in which the molar ratio of tetraethoxysilan to aluminium tri-sec-butoxide was 1:1 and 1:3 were used.As the samples were being soaked at T = 1200 °C for t = 700 h the mass of the samples increased. Thermal shock (T = 1200 °C, 10,000 cycles) causes greater degradation of the surface than soaking at a constant temperature. The XPS and EDS results show that the composition of the top layer of the coatings changes during high temperature oxidation and thermal shock. The outward movement of aluminium cations results in surface enrichment with aluminium, particularly for the single-layer coatings. The measured energies of bonds Si 2p and Al 2p in the multilayer coatings indicate that a structure of aluminosilicates with a composition between that of mullite and that of sillimanite forms during sintering.     

Analytical models for the internal oxidation of a binary alloy that forms oxide precipitates of high stability

Since the experimental work of Smith on internal oxidation many attempts have been made to quantify the observations. This paper outlines the mathematical development of the extended model starting from the pioneering work of Rhines et al.A proper definition of the mass balance of individual components at the reaction interface predicts the transition behaviour. A proper definition of the convective velocity due to precipitation of oxide of larger molar volume compare to matrix phase in the differential mass balance introduces a volumetric flow towards the exposed surface that explains the outward extrusion of the matrix reported by many researchers.     

Oxidation of a chromia-forming nickel base alloy at high temperature in mixed diluted CO/H2O atmospheres

Corrosion of a chromia-forming nickel base alloy, Haynes 230^®, has been investigated under impure helium containing a few Pa of CO and H₂O at 900 °C. It has been found that this alloy reacts simultaneously with CO and H₂O. Oxidation by CO has been revealed to occur mainly in the first hours. CO diffuses through the scale via short-circuit pathways and oxidizes Al, Cr and Si at the oxide/metal interface. Kinetics of CO oxidation has been investigated and several rate limiting steps are proposed. In the long term, H₂O is the major oxidant of chromia-forming nickel base alloys in impure helium.     

Characterization of microstructures and oxidation behaviour of Nb–20Si–20Cr–5Al alloy

Static oxidation in air was performed on Nb–20Cr–20Si–5Al alloy at high temperatures ranging from 700 to 1400 °C. Pesting occurred at 700 °C while internal oxidation took place at 1300 and 1400 °C where Al₂O₃ initiated at the interface between NbCr₂ and Nb₉Cr₃Si₂ phases. Phases present were Nb₅Si₃, NbCr₂, Nb solid solution and Nb₉Cr₃Si₂ depending on the temperature. The aluminium content on each of the phases was analyzed. Al content in Nb₉Si₂Cr₃ has been found to be as high as 5–6 atomic percent. SEM, EDS and XRD techniques were utilized in order to characterize the specimens.     

High-temperature carburization behaviour of HASTELLOY X in CH4 gas

Ni-base wrought alloy HASTELLOY X tube was exposed to Ar–CH₄ at 800 and 1000 °C in order to understand the carburization kinetics of the alloy used for fuel injection nozzles of micro-gas turbine combustors. Three different internal carbides, (Cr,Mo)₃C₂, (Cr,Mo)₇C₃ and (Cr,Mo)₂₃C₆ were observed in this order from the surface, and the partial damage to the outer surface of the specimen tube appeared similar to metal-dusting. The internal carburization kinetics on both the inner and outer parts of tube followed the parabolic rate law. The carbon permeability in HASTELLOY X was obtained, and was slightly smaller than that of Ni–20%Cr.