Theoretical Analysis of Anomalies in High-Temperature Oxidation of Fe–Cr, Fe–Ni, and Fe–Ni–Cr Alloys

The following anomalies are theoretically analyzed: weakening of the protective ability of dense Cr₂O₃ film during its long-term thermal exposure (because of iron oxidation under the film); lowering of the heat resistance of Fe–Cr and Fe–Ni–Cr alloys during the oxidation (800°C) with an increase in the chromium content over 40 at. %; improving of the protective ability of the films formed at Fe–Ni alloys because of nickel oxidation under the dense FeO film; and the internal oxidation of the Fe 30Ni alloys under the FeO films with the internal formation of FeO oxides and spinel of NiFe₂O₄ type. It is shown that these anomalies can be explained, and the composition of the most heat-resistant alloys calculated, if one takes into account that associates with significantly stronger interatomic bonds than those in ideal solutions can form in solid solutions and cause unlimited solubility of the metallic components in each other.     

The Formation of Protective Alumina-Based Scales During High-Temperature Air Oxidation of γ -TiAl Alloys

The effect of Ag additions on the oxidation behavior of -TiAl hasbeen studied. The materials investigated containing 47–50 at.% Aland 0–5 at.% Ag were tested with respect to oxidation resistanceduring exposure in air at 800°C. The exposures up to around 1600 hrshowed that suitable Ag additions can promote formation of long-term,protective, alumina scales on -TiAl alloys. Extensive analysesof the oxidation products using optical metallography SEM, XRD, EPMA,and SIMS revealed that Ag stabilizes the Z-phase (Ti₅Al₃O₂) in thesubscale-depletion layer thereby preventing formation of ₂-Ti₃Alas well as Ti-rich nitrides, which are responsible for the destructionof alumina scales in common -TiAl alloys. The best results wereobtained for the alloy Ti–50Al–2Ag; even during exposures aslong as around 1600 hr, this alloy still appeared to form a stable aluminalayer. It was found that high Ag additions of 5% were detrimental afterlonger exposure times due to extensive Ag precipitationat the interface between the alloy and depletion layer, resulting inlocalized formation of rapidly growing, mixed-oxide scales.     

High-Temperature Oxidation Behavior and Oxide Scale Structure of Yttrium-Modified Ni–16Mo–7Cr–4Fe Superalloy at 1273 K

Oxidation of Metals - Oxidation of a Ni–16Mo–7Cr–4Fe superalloy containing various yttrium concentrations (0.00, 0.05, 0.12, 0.21 and 0.43 wt%) was undertaken in air at...     

Kinetic and Morphological Study of the Oxidation of an Fe–36% Ni Alloy in Carbon Dioxide at 750–1000°c

Abstract

The oxidation of ‘Nilo’ alloy 36, an Fe–36% Ni alloy, has been studied by thermogravimetric, metallographic and electron-probe microanalysis techniques. At 750°–950°, after a short period of ill-defined oxidation the parabolic law was obeyedthroughout the entire exposure period which vaned from ～150h at 750° to ～25h at 950°. At 1000° there was a paralinear kinetic transition after 2–3 h (wt. gain 8–9 mg/cm²). The activation energy for the oxidation reaction derived from the parabolic rate constants was 52 ± 9·3 kcal/mole.

During the parabolic stage at 1000° the scale was mainly magnetite with pockets of wustite along the scale/metal interface. There was also considerable intergranular oxidation and nickel enrichment of the alloy grains in the region of the alloy/scale interface. The intergranular oxide was wustite. This pattern of oxidation was followed at lower temperatures. Followmg the transition from parabolic to linear kinetics at 1000° the scale was found to consist of wustite containing ≤0·4% Ni. The manganese concentration in the scaleswas similar to that in the alloy. The results are discussed and mechanisms of oxidation are suggested.     

Visualization of Concentration Micro-Inhomogeneities in Fe–Ni Alloys

Physics of Metals and Metallography - The method of visually identifying microregions with increased nickel concentration in austenitic Fe–Ni alloys by the appearance of dark diffraction...     

High-Temperature Oxidation Behavior of ODS–Fe3Al

The high-temperature oxidation behavior of an oxide dispersion-strengthened (ODS) Fe₃Al alloy has been studied during isothermal and cyclic exposures in oxygen and air over the temperature range 1000 to 1300°C. Compared to commercially available ODS–FeCrAl alloys, it exhibited very similar short-term rates of oxidation at 1000 and 1100°C, but at higher temperatures the oxidation rate increased because of increased scale spallation. Over the entire temperature range, the oxide scale formed was -Al₂O₃, with the morphological features typical of reactive-element doping and was similar to those formed on the ODS–FeCrAl alloys. Although initially this scale appeared to be extremely adherent to the Fe₃Al substrate, an undulating metal–oxide interface formed with increasing time and temperature, which led to cracking of the scale in the vicinity of surface undulations accompanied by a loss of small fragments of the full-scale thickness. In some instances, the surface undulations appeared to have resulted from gross outward local extrusion of the alloy substrate. Similar features developd on the FeCrAl alloys, but they were typically much smaller after a given oxidation exposure. The ODS–Fe₃Al alloy has a significantly larger coefficient of thermal expansion (CTE) than typical FeCrAl alloys (approximately 1.5 times at 900°C) and this appears to be the major reason for the greater tendency for scale spallation. The stress generated by the CTE mismatch was apparently sufficient to lead to buckling and limited loss of scale at temperatures up to 1100°C, with an increasing amount of substrate deformation at 1200°C and above. This deformation led to increased scale spallation by producing an out-of-plane stress distribution, resulting in cracking or shearing of the oxide.     

Formation of Vacancy-Type Dislocation Loops in Hydrogen-Ion-Implanted Fe–Cr Alloy

Fe-10 at.%Cr alloy was implanted with hydrogen ions at room temperature, followed by annealing at high temperatures.The annealing process made the defects develop into large dislocation loops. The nature of the dislocation loops formed after annealing was studied by the evolution of loops under in situ electron irradiation in high-voltage electron microscope.It indicated that only interstitial-type loops were observed when annealed at 550 °C and below, but vacancy-type loops started to form at the temperature higher than 600 °C. According to the previous study of our group, the presence of chromium element made the formation temperature of vacancy-type loops higher than that in pure iron. The effect of alloying elements on the formation temperature of the vacancy-type loops was discussed.     

Influence of Alloying Element Addition on Cu–Al–Ni High-Temperature Shape Memory Alloy without Second Phase Formation

In this study,the effects of rare earth Gd and Fe elements on the microstructure,the mechanical properties and the shape memory effect of Cu–11.9Al–3.8Ni high-temperature shape memory alloy were investigated by optical microscopy,scanning electron microscopy,X-ray diffraction and compression test.The microstructure observation results showed that both Cu–11.9Al–3.8Ni–0.2Gd and Cu–11.9Al–3.8Ni–2.0Fe–0.2Gd alloys displayed the fine grain and singlephase b01martensite,and their grain size was about several hundred microns,one order of the magnitude smaller than that of Cu–11.9Al–3.8Ni alloy.The compression test results proved that the mechanical properties of Cu–11.9Al–3.8Ni alloy were dramatically improved by alloying element additions due to grain refinement and solid solution strengthening,and the compressive fracture strains of Cu–11.9Al–3.8Ni–0.2Gd and Cu–11.9Al–3.8Ni–2.0Fe–0.2Gd were 12.0%and 17.8%,respectively.When the pre-strain was 10%,the reversible strains of 5.4%and 5.9%were obtained for Cu–11.9Al–3.8Ni–0.2Gd and Cu–11.9Al–3.8Ni–2.0Fe–0.2Gd alloys after being heated to 500°C for 1 min,and the obvious two-way shape memory effect was also observed.     

High-Temperature Oxidation of Ni–20 wt.% Cu from 700 to 1100°C

Ni–20 wt.% Cu was oxidized in different oxygen pressures from 1×10^–5 to 1 atm at 700–1100°C. The oxidation consisted of an initial transient period in which a composite scale of NiO and Cu oxides formed, and a subsequent quasi steady-state regime during which parabolic growth of NiO determined the overall oxidation rate. Based on the oxide composition and the oxygen- pressure dependence of the parabolic rate constant, it is concluded that outward transport of Ni via vacancies determines the growth rate of the oxide during the steady-state period, either in the grain boundaries or in the lattice. The influence of Cu dissolved in NiO on the oxidation rate and the oxygen-pressure dependence of the parabolic rate constants is discussed.     

Structure-Selective Dissolution of Al–Fe–Ni Bronze in Chloride Electrolyte

The selectivity of the anodic dissolution of the iron-rich phases of . 9-4-4-1 cast bronze in 0.5 N NaCl + 0.01 N HCl was studied. X-ray diffraction analysis revealed that the Fe₃Al and ₁-phases present at the surface of the initial bronze become predominantly etched during the early period of anodic dissolution at E = –0.12 V (NHE). According to electrochemical and analytical data, the above phases dissolve at this potential most rapidly. The x-ray diffraction data agree well with the results obtained by atomic absorption analysis of the solution.     

Effects of Sol–Gel-Derived Silica Coatings on High-Temperature Oxidation of Ni

Effects of sol–gel-based coatings on high-temperature oxidation of Ni were studied in the temperature range 700–1200°C in air by thermogravimetry, SEM, and TEM. The oxidation kinetics are parabolic and lower than for uncoated Ni, which also shows subparabolic kinetics below 1000°C. The temperature dependence of the parabolic rate constant for oxidation of silica-coated Ni corresponds to a single Arrhenius relation. The NiO scale formed on silica-coated Ni consists of three regions; an outer region with columnar grains and an inner region of equiaxed grains, separated by a fine-grain, Si-rich layer. For uncoated specimens only, the outer columnar and inner equiaxed regions were encountered. The beneficial effect of the silica on the rate of oxidation has been interpreted to reflect that Si segregated along grain boundaries in the intermediate layer decreases the outward Ni flux along grain boundaries.     

High-Temperature Oxidation of Fe3Al and Fe3Al–Zr Intermetallics

The oxidation behavior of Fe₃Al and Fe₃Al–Zr intermetallic compounds was tested in synthetic air in the temperature range 900–1200 °C. The addition of Zr showed a significant effect on the high-temperature oxidation behavior. The total weight gain after 100 h oxidation of Fe₃Al at 1200 °C was around three times more than that for Fe₃Al–Zr materials. Zr-containing intermetallics exhibited abnormal kinetics between 900 and 1100 °C, due to the presence and transformation of transient alumina into stable α-Al₂O₃. Zr-doped Fe₃Al oxidation behavior under cyclic tests at 1100 °C was improved by delaying the breakaway oxidation to 80 cycles, in comparison to 5 cycles on the undoped Fe₃Al alloys. The oxidation improvements could be related to the segregation of Zr at alumina grain boundaries and to the presence of Zr oxide second-phase particles at the metal–oxide interface and in the external part of the alumina scale. The change of oxidation mechanisms, observed using oxygen–isotope experiments followed by secondary-ion mass spectrometry, was ascribed to Zr segregation at alumina grain boundaries.     

Effect of γ-(Fe,Ni) crystal-size stabilization in Fe–Ni–B amorphous ribbon

The effect of stabilizing crystal size in a melt-quenched amorphous Fe₅₀Ni₃₃B₁₇ ribbon is described upon crystallization in a temperature range of 360–400°С. The shape, size, volume fraction, and volume density have been investigated by transmission electron microscopy and X-ray diffraction methods. The formation of an amorphous layer of the Fe₅₀Ni₂₉B₂₁ compound was found by means of atomic-probe tomography at the boundary of the crystallite–amorphous phase. The stabilization of crystal sizes during annealing is due to the formation of a barrier amorphous layer that has a crystallization temperature that exceeds the crystallization temperature of the matrix amorphous alloy.     

Formation and Evolution of Microstructures in Fe–9%Cr Heat-Resistant Steel

Microstructures of casting samples of Fe–9%Cr steel and samples subjected to different heat treatments were investigated to determine their formation and evolution mechanism. The results show that there is no macroscopic segregation in the casting Fe–9%Cr steel. During cooling from solidification temperature to room temperature, d-ferrite→austenite transformation is obviously influenced by cooling rate, while subsequent transformation of austenite does not obviously depend on the cooling rate. In the casting samples, a great number of precipitates distribute inside martensitic laths while there are almost no precipitates inside d-ferrite. When the casting samples were reheated to and isothermally held at 800 °C, the original precipitates and the lath boundaries disappeared gradually. Meanwhile, new precipitates nucleate and grow at the prior lath boundaries.     

Influence of Fe on Microstructure and Mechanical Properties in Pdoped Ni–Cr–Fe Alloys

The influence of Fe on the microstructure and mechanical properties of P-doped Ni–Cr–Fe alloys has been investigated.Results showed that increasing Fe content refined the dendrite microstructure and enhanced the solubility of P in as-cast alloys. The change of microhardness in different dendrite regions was attributed to the segregation of P atoms in solid solution state, which had strengthening effects. Increasing Fe contents from 15.2 to 60.7 wt% reduced the yield strength and tensile strength but had little influence on the elongation of alloys. The stress rupture life of alloys after heat treatment decreased with the increment of Fe contents, and the failure fracture modes transferred from transgranular to intergranular fracture mode. The change of fracture modes was due to the weakness of grain boundaries caused by the increment of Fe.In addition, the precipitation of M_(23)C_6 was believed to be related to the segregation of P toward grain boundaries, which led to the fluctuation of carbon and chromium atoms near the grain boundaries in alloys with low Fe contents. Consequently, the increment of Fe decreased the strength of matrix and changed the existence of P atoms and the precipitates at grain boundaries.     

High-Temperature Oxidation Behaviour of SS 216L (Fe–16Cr–6Ni–6Mn–1.7Mo)

Oxidation behaviour of a 216L austenite stainless steel (Fe–16Cr–6Ni–6Mn–1.7Mo) was evaluated at temperature between 700 and 900?°C by thermogravimetric analysis and compared with that of SS 316L. Transmission electron microscopy in combination with energy dispersive X-ray analysis was used to study surface morphologies and chemical composition of the oxide scales formed. Replacement of Mn with Ni in SS 316L enhances its oxidation rate. SS 216L exhibits an anomalous temperature dependence of the oxidation behaviour. A kinetic inversion was observed at temperature 900?°C. Surface analysis reveals domination of Mn and iron mixed oxides in oxide scale.     

Investigation of Fe content in Cu–Al–Ni Shape Memory Alloys

Polycrystalline Cu–Al–Ni–Fe-based shape memory alloys with different chemical composition were produced in an arc-melting furnace under an argon atmosphere. Homogenized and aged specimens were prepared for multiple analyses. The temperatures of reversible martensitic transformations, namely A_s, A_f, M_s, M_f, A_max and ΔH enthalpy values were determined by a DSC device. The phase transition analysis from the room temperature to 850°C was undertaken by DTA. To characterize the lattice structure, an XRD analysis was conducted, the results of which were confirmed by microstructure images obtained from optical microscope observations.     

Glow discharge plasma brazing of Fe–Ni alloy in vacuum

Abstract

A novel vacuum brazing technique, termed glow discharge plasma brazing, is investigated. During the brazing process, the heating temperature of base metals is proportional to the square of the operating barometric pressure and the operating voltage, and the temperature distribution of the base metals is easily regulated by means of suitable measures. The ion beam from the glow discharge anode can efficiently sputterclean the surface of the base metals and the filler metal, which improves the wetting and spreading properties of the filler metal. Unlike the traditional vacuum brazing process, a high quality braze of Fe–Ni alloy is achieved at lower vacuum (a pressure of 5– 30 Pa or higher) by using the glow discharge brazing method. The brazing technique has a promising application in industry.     

Roles of Mn in the High-Temperature Air Oxidation of 9Cr Ferritic–Martensitic Steel After Severe Plastic Deformation

Oxidation resistance of 9Cr ferritic–martensitic steel before and after cold-swaging process in air at 923 K was compared, and the oxide scales were characterized in detail by secondary ion mass spectrometry (SIMS). Both the cold-swaged sample and post-annealed samples showed considerably greater oxidation resistance than the initial as-tempered sample. Mn played a key role in scale formation of the cold-swaged sample and post-annealed sample; although the Mn content was only 0.5 wt% in the matrix, formation of Mn oxide (MnCr₂O₄ and Mn₂O₃) was favored. Roles of Mn in the oxidation behaviors of the cold-swaged sample and post-annealed sample were discussed.