High-temperature oxidation of Fe-2 1 4 Cr-1Mo in oxygen

The oxidation of a 2 ₁ ⁴ Cr-1Mo steel in dry flowing oxygen has been studied in the temperature range 550–700°C for periods of up to 100 hr. A detailed low-resolution microstructural investigation revealed a layered oxide consisting of a very fine-grained and finely pored innermost layer of doped spinel, a central columnar-grained relatively coarsely pored layer of magnetite, and an outer fine-grained hematite layer with fine pores and covered with whiskers of -Fe₂O₃. This structure is compared with previous results on Fe and model Fe-Cr alloys, as are the kinetics of the oxidation reaction.     

Kinetics and mechanisms of the oxidation of cobalt at 600–800°C

Two-phase layered scales comprising CoO and Co ₃O₄ formed on cobalt during oxidation at 600°, 700°, and 800°C and at oxygen partial pressures in the range 0.001–1 atm. The kinetics, which were obtained by thermogravimetric analysis, obeyed a parabolic rate law after an initial, non-parabolic stage of oxidation. The monoxide consisted of relatively large grains (10 ) and the spinel comprised small grains (3 ) for all conditions of oxidation. Grain boundary diffusion of cations played a significant role in the growth of the spinel layer. Thermogravimetric data and the steady-state ratio of the oxide layer thicknesses were employed to calculate the rates of thickening of the individual oxide layers and the rate of oxidation of CoO to Co₃O₄.     

Oxidation properties of Fe-5Cr-4Al (wt.%) alloys in oxygen at temperatures 1000°C–1320°C

Oxidation kinetics of a parent Fe-5Cr-4Al alloy subjected to two types of anneals were investigated at temperatures ranging from 1000°C to 1320°C. The alloy annealed at 850°C exhibited a rapid transient oxidation stage associated with growth of nodules containing iron oxides and internal precipitation of -Al₂O₃ in the alloy beneath these nodules. The nodules nucleated and grew from sites located in the regions of the alloy grain boundaries during the period of rapid alloy grain growth. Nodular growth virtually ceased when a continuous -Al₂O₃ film formed at the nodule-alloy interface. The alloy subjected to anneal at 1000°C and at the reaction temperature to stabilize the alloy grain size tended upon oxidation to form a protective -Al₂O₃, layer by parabolic kinetics at temperatures to 1250°C. If this alloy was oxidized in stages at 1000°C, a protective -Al₂O₃ scale was formed up to 1320°C. The temperature coefficient of the parabolic oxidation kinetics was consistent with diffusion processes at boundaries of the -Al₂O₃ grains playing an essential role during growth of this protective oxide layer.     

Oxidation Behavior and Breakdown of an Aluminide Coating on DZ40M Alloy at High Temperatures

DZ40M alloy is a new Co-base superalloy, which is suitable for the blade material of gas turbines. In this paper, isothermal oxidation of an aluminide coating on this alloy was examined at 900–1100°C in air. It was observed that the weight gain at lower temperatures (900 and 1000°C) was greater than that at the higher temperature (1050°C), which was due to the formation of both -Al₂O₃ and -Al₂O₃ at 900 and 1000°C but only -Al₂O₃ at 1050 and 1100°C.     

Oxidation Behavior of Ti--50Al Intermetallics with Thin TiAl3 Film at 1000°C

Isothermal oxidation tests at 1000°C in air indicate that the Ti--50Al alloy with about 8 m TiAl₃ layer on the surface can resist the oxidation for 10 hr. From the FESEM and EPMA/EDS results, the rapid oxidation behavior is attributed to the formation of oxide nodules through the protective Al₂O₃ and TiAl₂ layers on the outer surface. Upon increasing the oxidation time at 1000°C, the size and the number of oxide nodules increase. After 3 hr of oxidation at 1000°C, a laminated layer is formed in between the oxide nodule and substrate, which consists of two nearly parallel phases. The EDS results suggest that these two phases are Ti--Al--O compounds. After 20 hr oxidation, the oxidation nodules and laminated layers disappear and a complex oxide scale is formed which is similar to the bare Ti--50Al oxidized at 1000°C.     

Phase equilibria in the subsolidus region of the NiO-α-Al2O3 system between 1000 and 1920°C

Nickel spinel, Ni_1–xAl_2(1+x/3)O₄, is the only intermediate compound in the quasibinary NiO--Al₂O₃ system at temperatures between 1000 and 1920°C. The spinel equilibrated with NiO occurs at its stoichiometric composition, NiAl₂O₄, independent of temperature. An alumina rich spinel, 0.17 x 0.62, equilibrated with -Al₂O₃ increases in alumina content with increasing temperature. Aluminum oxide solubility in NiO increases from 1 mole % at 1000°Cto 3 mol % at 1800°C. Nickel oxide solubility in -Al₂O₃ was found to increase from 2 mole % at 1000°C to 3 mole % at 1920°C.     

High-temperature oxidation behavior of pure Ni3Al

The high-temperature oxidation behaviour of pure Ni₃Al alloys in air was studied above 1000°C. In isothermal oxidation tests between 1000 and 1200°C, Ni₃Al showed parabolic oxidation behavior and displayed excellent oxidation resistance. In cyclic oxidation tests between 1000 and 1300°C, Ni₃Al exhibited excellent oxidation resistance between 1000 and 1200°C, but drastic spalling of oxide scales was observed at 1300°C. When Ni₃Al was oxidized at 1000°C, Al₂O₃ was present as -Al₂O₃ in a whisker form. But, at 1100°C the gradual transformation of initially formed metastable -Al₂O₃ to stable -Al₂O₃ was observed after oxidation for about 20 hr. After oxidation at 1200°C for long times, the formation of a thick columnar-grain layer of -Al₂O₃ was observed beneath a thin and fine-grain outer layer of -Al₃O₃. The oxidation mechanism of pure Ni₃Al is described.     

Effect of Cr,Co, and Ti additions on the high-temperature oxidation behavior of Ni3Al

The oxidation behavior of Ni₃Al+2.90 wt.% Cr, Ni₃Al+3.35 wt% Co, and Ni₃Al+2.99 wt.% Ti alloys was studied in 1 atm of air at 1000, 1100, and 1200°C. Isothermal tests revealed parabolic kinetics for all three alloys at all temperatures. Cyclic oxidation for 28 two-hour cycles produced little spallation at 1000°C, but caused partial spallation at 1100°C. Especially, at 1200°C severe spallation in all three alloys was observed. Although additions of Cr, Co, or Ti to Ni₃Al alloys slightly increased the isothermal-oxidation resistance, the additions tended to decrease the cyclic-oxidation resistance. The major difference in the oxidation of the three alloys compared with the oxidation of pure Ni₃Al alloys was the existence of small -Al₂O₃ particles in the middle of the -Al₂O₃ scale and the formation of irregularly shaped Kirkendall voids at the alloy-scale interface.     

Development,growth, and adhesion of Al2O3 on platinum-aluminum alloys

The development, growth, and adhesion of -Al₂O₃ scales on platinum-aluminum alloys containing between 0.5 and 6 wt.% aluminum have been studied at temperatures in the interval between 1000 and 1450° C. The morphologies and microstructures of the -Al₂O₃ scales were found to be influenced by the temperature, oxygen pressure, and the microstructures of the alloys. The oxidation rates of the alloys appeared to be controlled by transport of oxygen along grain boundaries in the -Al₂O₃ scales. The -Al₂O₃ scales adhered to the platinum-aluminum substrates even after extensive periods of cyclic oxidation. The good adhesion of the -Al₂O₃ may result from mechanical keying of the oxide to the alloys due to the development of irregular oxide-alloy interfaces.This work was supported by the U.S. Army Research Office, Durham, under Contract Number DAHCO 4 73 C 0021.     

The effect of particle size of alumina dispersions on the oxidation resistance of Ni-Cr alloys

The oxidation behavior of Ni-Cr alloys with various chromium concentrations and particle sizes of a dispersion of 10 vol.% Al₂O₃ was observed in 1 atm of oxygen at 1000°C. This study was intended to determine the critical chromium concentration to form a protective Cr₂O₃ oxide layer for different Al₂O₃ particle sizes. The oxidation rate of Ni-Cr alloys containing 10 vol.% Al₂O₃ followed a parabolic rate law and a Cr₂O₃ protective layer continuously formed when the oxidation rate decreased rapidly. Times to form a continuous and protective Cr₂O₃ layer during the initial oxidation shortened as the size of the dispersion decreased. The critical chromium concentration to form a protective Cr₂O₃ layer in the oxide scale was 69 wt.% and was related strongly to the particle size of the Al₂O₃ dispersion.     

High-temperature oxidation of rhodium

The oxidation kinetics of Rh were measured in air at 1 atm. in the temperature range 600–1000°C. The oxidation weight gain proceeds logarithmically at the lower temperatures (600°C, 650°C) followed by a transition to power law behavior at the higher temperatures (800°C). The logarithmic growth kinetics result from thickening of a hexagonal Rh₂O₃ scale. The transition from logarithmic to power law growth kinetics occurs in the range 700–800°C, and reflects thickening of hexagonal and orthorhombic Rh₂O₃ scales. Above 800°C, the growth kinetics result from thickening of a predominately orthorhombic Rh₂O₃ scale. At 1000°C the oxide becomes volatile, leaving the metal surface exposed.     

Oxidation of the intermetallics MoSi2 and TiSi2—A comparison

The oxidation behavior of two MoSi₂ variants, one Mo-rich and one Si-rich, and TiSi₂ was investigated between 1000 and 1400°C in air, oxygen and an 80/20-Ar/O₂ mixture. A protective SiO₂ scale develops on MoSi₂ in all atmospheres in the temperature range investigated. The SiO₂ modification changes around 1300°C from tridymite to cristobalite. This change in SiO₂ modification seems to cause an enhanced formation of SiO₂ and evaporation of MoO₃. The SiO₂ grows at the MoSi₂-scale interface. In air a two-layer scale grows on TiSi₂ between about 1000 and 1200°C with an inner inwards growing fine-grain mixture of SiO₂ + TiO₂ and an outer outward-growing TiO₂ partial layer. TiN formation in the transient oxidation is responsible for the formation of the inner mixed partial layer because in N -free atmospheres a scale of a SiO₂ matrix with some Ti oxide precipitates inside is formed. A one-layer scale structure similar as that in N-free atmosphere is found on TiSi₂ in air at T > 1200°C. In oxygen the TiO₂ precipitates grow as needles mostly oriented perpendicular to the surface. Due to the faster oxygen transport in TiO₂ compared with SiO₂, these TiO₂ needles act as oxygen pipes, causing an enhanced oxidation of TiSi₂ in front of these needles. The SiO₂ scale dissolves about 1–2% TiO₂. This doping causes a mixed oxygenand Si transport with the consequence that the SiO₂ scale on TiSi₂ grows partly by oxygen transport inwards and Si transport outwards. The SiO₂ modification is cristobalite over the entire temperature range investigated.     

Oxidation Mechanism of Ni—20Cr Foils and Its Relation to the Oxide-Scale Microstructure

The oxidation behavior of Ni—20Cr foils of 100- and 200-m thickness wasstudied in air between 500 and 900°C. Simultaneously, the morphology,microstructure, and composition of the oxide layers were determined byscanning and transmission electron microscopies. Depending on thetemperature, the oxide layer differed significantly. The scale formedat all temperatures was complex, with an outer NiO layer having columnargrains, and an inner layer of equiaxedNiCr₂O₄+NiO+Cr₂O₃ grains. At low temperatures (500 and 600°C),the chromium content was insufficient to form a continuousCr₂O₃ layer, while such a continuous layer formed at theinner interface at oxidation temperatures of 700 to 900°C. At 600°C,internal oxidation of chromium occurred in the substrate. The oxidationmechanism is described taking into account these morphologies and theoxidation kinetics. The observation of no significant differences betweenthe oxidation behavior of thin strips and thick materials is related to thelimited exposure times of the study.     

On the transient oxidation of a Ni-15Cr-6Al alloy

Stages in the development of a protective -Al₂O₃ scale on a Ni-15Cr-6Al (wt.%) alloy have been examined. It is shown that prior to the formation of a continuous -Al₂O₃ layer, a transient stage of oxidation occurs that consists of a rapid uptake of oxygen with conversion of a thin surface layer of alloy to predominantly spinel and the subsequent development of a discrete layer of Cr₂O₃. It is also shown that during the transient period of oxidation metastable phases of aluminum oxide are formed which transform to -Al₂O₃ upon incorporation into the external oxide scale.     

The high-temperature oxidation behavior of vanadium-aluminum alloys

The oxidation behavior in air of pure vanadium, V-30Al, V-30Al-10Cr, and V-30Al-10Ti (weight percent) was investigated over the temperature range of 700–1000° C. The oxidation of pure vanadium was characterized by linear kinetics due to the formation of liquid V₂O₅ which dripped from the sample. The oxidation behavior of the alloys was characterized by linear and parabolic kinetics which combined to give an overall time dependence of 0.6–0.8. An empirical relationship of the form: W/A=Bt + Ct^1/2 + D was found to fit the data well, with the linear contribution suspected to be from V₂O₅ formation for V-30Al and V-30Al-10Cr, and a semi-liquid mixture of V₂O₅ and Al₂O₃ for V-30Al-10Ti. The parabolic term is presumed related to the formation of a solid mixture of V₂O₅ and Al₂O₃ for V-30Al and V-30Al-10Cr, and TiO₂ for V-30Al-10TiThe addition of aluminum was found to reduce the oxidation rate of vanadium, but not to the extent predicted by the theory of competing oxide phases proposed by Wang, Gleeson, and Douglass. This was attributed to the formation of a liquid-oxide phase in the initial stages of exposure from which the alloys could not recover. Ternary additions of chromium and titanium were found to decrease the oxidation rate further, with chromium being the most effective. The oxide scales of the alloys were found to be highly porous at 900° C and 1000° C, due to the high vapor pressure of V₂O₅ above 800° C.     

Tensile Deformation of Iron Oxides at 600–1250°C

Tensile tests of virtually pure FeO, -Fe₃O₄, and -Fe₂O₃ were performed at 600–1250°C at strain rates of 2.0×10^–3–6.7×10^–5 s^–1 under controlled gas atmospheres. Mechanical properties and deformation/fracture behavior were investigated. For -Fe₂O₃, brittle fracture resulted at 1150–1250°C and the fracture strain was below 4.0% at a strain rate of 2.0×10^–4 s^–1. -Fe₃O₄ deformed plastically above 800°C. Steady-state deformation was indicated at 1200°C; elongation of 110% was obtained. Plastic deformation observed at 800 to 1100°C was considered to result from dislocation glide. Using TEM, the Burgers vector of dislocations observed in deformed -Fe₃O₄ was determined to be <110>, its slip system was estimated to be {111}<110>. FeO deformed plastically above 700°C. Steady-state deformation became predominant above 1000°C. Elongation of 160% was obtained at 1200°C. Strain rates of FeO at 1000°C and 1200°C were proportional to the fourth power of the saturated stress, indicating that plastic deformation was affected by dislocation climb.     

Deformation of Iron Oxides upon Tensile Tests at 600–1250°C

Tensile tests of virtually pure FeO, -Fe₃O₄, and -Fe₂O₃ were performed at 600–1250°C at strain rates of 2.0×10^–3–6.7×10^–5 s^–1 under controlled gas atmospheres. Mechanical properties and deformation/fracture behavior were investigated. For -Fe₂O₃, brittle fracture resulted at 1150–1250°C, and the fracture strain was below 4.0% at a strain rate of 2.0×10^–4 s^–1. Oxide of -Fe₃O₄ deformed plastically above 800°C. Steady-state deformation was indicated at 1200°C; elongation of 110% was obtained. Plastic deformation observed at 800–1100°C was considered to result from dislocation glide. Using TEM, burgers vector of dislocation observed in deformed -Fe₃O₄ was determined to be 110, and its slip system was estimated to be {111}<110>. Oxide of FeO deformed plastically above 700°C. Steady-state deformation became predominant above 1000°C. Elongation of 160% was obtained at 1200°C. Strain rates of FeO at 1000 and 1200°C were proportional to the fourth power of the saturated stress, indicating that the plastic deformation was affected by dislocation climb.     

Effect of theθ-α-Al2O3 transformation on the oxidation behavior ofβ-NiAl + Zr

Isothermal oxidation of NiAl + Zr has been performed over the temperature range of 800–1200°C and studied by TGA, XRD, and SEM. A discontinuous decrease in growth rate of two orders of magnitude was observed at 1000° C due to the formation of -Al₂O₃ from -Al₂O₃. This transformation also resulted in a dramatic change in the surface morphology of the scales, as a whisker topography was changed into a weblike network of oxide ridges and radial transformation cracks. It is believed that the ridges are evidence for a shortcircuit outward aluminum diffusion growth mechanism that has been documented in a number of¹⁸O tracer studies.     

Oxidation Behavior of a Single-Crystal Ni-Base Superalloy in Air. I: At 800 and 900°C

The oxidation behavior of a Single-crystal Ni-base superalloy was studied using discontinuous thermogravimetric analysis (TGA) and prolonged exposure in air at 800 and 900°C. The mass gain of specimens at 900°C was found to be lower than that of specimens at 800°C because of the formation of a protective inner -Al₂O₃ layer at 900°C. A subparabolic time dependence (n=0.16 at 800°C and n=0.10 at 900°C) of the oxide growth rate was determined at both temperatures. At 800°C, the superalloy exhibited nonuniform oxidation—in some areas a thin scale with an outer NiO layer and an inner layer of an Al-rich oxide was found and, in other areas, complex oxides [CrTaO₄, NiCr₂O₄, (Ni,Co)Al₂O₄, etc.] below the NiO outer layer formed by growing into the superalloy. The scale formed at 900°C is more uniform than that formed at 800°C, consisting of several layers: an NiO outer layer, spinel-rich sublayer, a CrTaO₄-rich layer, and an -Al₂O₃ inner layer. The -Al₂O₃ inner layer provides good oxidation protection and the specimen mass gain is low for test up to 1925 hr.     

High-Temperature Oxidation Behavior of Ti2AlC in Air

The isothermal oxidation behavior of bulk Ti₂AlC in air has been investigated in temperature range 1000–1300°C for exposure time up to 20 hr by TGA, XRD, and SEM/EDS. The results demonstrated that Ti₂AlC had excellent oxidation resistance. The oxidation of Ti₂AlC obeyed a cubic law with cubic rate constants, k_c, increasing from 2.38×10^-12 to 2.13×10^-10 kg³/m⁶/sec as the temperature increased from 1000 to 1300°C. As revealed by X-ray diffraction (XRD) and SEM/EDS results, scales consisting of a continuous inner -Al₂O₃ layer and a discontinuous outer TiO₂ (rutile) layer formed on the Ti₂AlC substrate. A possible mechanism for the selective oxidation of Al to form protective alumina is proposed in comparison with the oxidation of Ti–Al alloys. In addition, the scales had good adhesion to the Ti₂AlC substrate during thermal cycling.