Air Oxidation of Ni–Ti Alloys at 650–850°C

The oxidation of pure Ni and three Ni–Ti alloys containing 5, 10, and 15 wt.% Ti over the temperature range 650–850°C in air was studied to examine the effect of titanium on the oxidation resistance of pure nickel. Ni–5Ti is a single-phase solid solution, while the other two alloys consisted of nickel solid solution (-Ni) and TiNi₃. The oxidation of Ni–Ti alloys at 650°C follows an approximately parabolic rate law and produces a decrease in the oxidation rate of pure Ni by forming an almost pure TiO₂ scale. At higher temperatures, Ni–Ti alloys also follow an approximately parabolic oxidation, and their oxidation rates are close to or faster than those of pure Ni. Duplex scales containing NiO, NiTiO₃ and TiO₂ formed. Some internal oxides of titanium formed, especially at 850°C. In addition, the two-phase structure of Ni–10Ti and Ni–15Ti was transformed into a single-phase structure beneath the scales.     

Sulfidation Behavior of Inconel 738 Superalloy at 500–900°C

The high-temperature sulfidation behavior of the cast nickel-base superalloy Inconel 738 (IN-738) was studied over the temperature range 500–900°C in pure sulfur vapor over the range 10²–10⁴ Pa. The sulfidation kinetics followed the parabolic rate law in all cases. The sulfidation rates increased with increasing temperature and sulfur pressure. The scales formed were bilayered and temperature-dependent. At T700°C, the outer scale consisted of mostly NiS (with dissolved Co) and minor (CoS₂ and NiCo₂S₄, while the inner layer was a heterophasic mixture of NiS, NiCo₂S₄, and minor amounts of Al₂S₃ and chromium sulfide (Cr₂S₃/Cr₃S₄). At T750°C, the outer scale consisted of mostly Ni₃S₂ (with dissolved Co) and minor amounts of Co₃S₄ and Cr₂S₃/Cr₃S₄, while the inner layer was a complex, heterophasic mixture of Ni₃S₂, Cr₂S₃/Cr₃S₄, CoCr₂S₄, and minor Al₂S₃. Platinum markers were found to be located at the interface between the inner and outer scales, suggesting that the outer scale grew by the outward transport of cations and the inner scale grew by the inward transport of sulfur. The formation of Al₂S₃ and Cr₂S₃/Cr₃S₄ partly blocked the transport of cations through the inner scale and consequently reduced the sulfidation rates as compared to pure nickel.     

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.     

Sulfidation–Oxidation Behavior of FeCrAl and TiCrAl and the Third-Element Effect

Short-term sulfidation–oxidation exposures were conducted under high pS₂ and low pO₂ conditions for TiCrAl and FeCrAl alloys at 600 and 800 °C. Low mass gains and submicron Al-and Ti-rich oxide scales were formed on TiCrAl at 600 °C, while high mass gains and FeS-based scale formation were observed for FeCrAl. Based on the good behavior of TiCrAl, third-element effect additions of Cr are not inherently detrimental under sulfidation–oxidation conditions. Rather, differences in the mechanistic action of the third-element addition of Cr between FeCrAl and TiCrAl alloys and its relevance to low oxygen potential sulfidation–oxidation environments were the key factors in determining whether or not a protective alumina scale was established.     

Oxidation of chromium at 890°–1200°C

Cr was oxidized in 1 atm of oxygen at 980, 1090 and 1200°C for periods up to 100 hr. Surface preparation has a large effect on scaling; electropolished Cr oxidizes rapidly. Non-uniform oxide layers form exhibiting nodule growth, blistering, wrinkling and multilayered ballooning. These and other observations indicate that compressive stresses develop during film thickening. This suggests that anion as well as cation diffusion takes part in the growth process and that new oxide forms within the oxide layer. The resulting continuous plastic deformation is considered in interpreting the oxidation kinetics. Best values of the rate constants are derived from measurement of layer thickness at selected areas on the metallographic cross-section.. Moisture did not affect the rate. Cr is at least as oxidation resistant as Fe-25 Cr alloy.     

High-Temperature Oxidation Kinetics and Behavior of Ti–6Al–4V Alloy

Owing to the high-temperature reactivity of titanium, the oxidation and alloying of titanium during hot working processes is an important variable. The oxidation behavior of Ti–6Al–4V alloy in air was investigated at various temperatures between 850 and 1100 °C for different times. The oxidation kinetics were determined by isothermal oxidation weight gain experiments. The results showed that the oxidation kinetics approximately obeyed a parabolic law. The activation energy of oxidation was estimated to be 199 and 281 kJ mol^?1 when temperature was above and below the beta transformation temperature (T _β), respectively. A model to predict oxidation extent was established based on experimental observations. The oxide scales mainly consisted of TiO₂ with a small amount of Al₂O₃ and TiVO₄. The alpha case was defined as solid solution formed because of oxygen diffusion into the substrate. The difference in the morphology and the formation mechanism of the alpha case at different temperature ranges was mainly owing to the participation of the grain boundary and grain orientation of the nucleation site.     

Oxidation resistance of SHS Fe–Al–Si alloys at 800 °C in air

Alloys formed by Fe–Al intermediary phases have lower density than common metallic high-temperature alloys and good high-temperature oxidation resistance. Previously it was proved that silicon additions to these alloys enable to produce them efficiently by reactive sintering and improves the wear resistance. In this work, the oxidation resistance of the novel Fe–Al–Si alloys containing 10–30 wt. % of aluminium and 5–30 wt. % of silicon produced by the reactive sintering technology was tested. Cyclic and isothermal oxidation tests were carried out at 800 °C in air. Tested alloys exhibit excellent oxidation resistance, which increases with silicon content up to 20 wt. %. The reasons are discussed in terms of the phase composition of the oxide layer and the changes in chemical composition under the oxide layer during oxidation.     

High temperature oxidation of Ti–46Al–6Nb–0.5W–0.5Cr–0.3Si–0.1C alloy

A newly developed Ti–46Al–6Nb-0.5W-0.5Cr-0.3Si-0.1C alloy was oxidized isothermally and cyclically in air, and its high-temperature oxidation behavior was investigated. When the alloy was isothermally oxidized at 700 °C for 2000 h, the weight gain was only 0.15 mg/cm². The parabolic rate constant, k_p (mg²/cm⁴·h), measured from isothermal oxidation tests was 0.002 at 900 °C and 0.009 at 1000 °C. Such excellent isothermal oxidation resistance resulted from the formation of the dense, continuous Al₂O₃ layer between the outer TiO₂ layer and the inner (TiO₂-rich, Al₂O₃-deficient) layer. The alloy also displayed good cyclic oxidation resistance at 900 °C. Some noticeable scale spallation began to occur after 68 h at 1000 °C during the cyclic oxidation test.     

Oxidation of Fe–Si,Fe–Al and Fe–Si–Al alloys in CO2–H2O gas at 800 °C

Binary Fe–(1, 2, 3)Si and Fe–(2, 4, 6)Al, and ternary Fe–(2, 3)Si–(4, 6)Al alloys (all in wt%) were oxidised in Ar–20% CO₂, with and without H₂O, at 800 °C. All binary alloys except Fe–6Al, in all gases, formed a thin outer layer of Fe₃O₄, an intermediate Fe₃O₄ + FeO layer, an inner FeO + Fe₂SiO₄ (or FeAl₂O₄) layer and internally precipitated SiO₂ (or FeAl₂O₄). Ternary alloys and Fe–6Al developed a protective Al₂O₃ layer beneath Fe₂O₃ in Ar–20% CO₂. Water vapour affected ternary alloy oxidation only slightly, but Fe–6Al oxidized internally in high H₂O-content gas, and its scale was non-protective.     

Oxidation behavior of Nb–24Ti–18Si–2Al–2Hf–4Cr and Nb–24Ti–18Si–2Al–2Hf–8Cr hypereutectic alloys at 1250°C

Nb-24Ti-18Si-2Al-2Hf-4Cr and Nb-24Ti-18Si-2Al-2Hf-8Cr alloys were prepared by arc melting in a water-cooled crucible under argon atmosphere.Microstructural characteristics and oxidation resistance of the alloys at 1250 ℃ were investigated.The results show that,when the Cr content is 4 at%,the microstructures consist of(Nb,Ti)_(ss) and Nb_5Si_3;as Cr content increases to8 at%,C14 Laves phase Cr_2Nb is formed.The isothermal oxidation tests show that the oxidation kinetics of the two alloys follow similar features.The weight gains of the two alloys after oxidation at 1250℃ for 100 h are 235.61 and198.50 mg·cm~(-2),respectively.During oxidation,SiO_2,TiO_2,Nb_2O_5 and CrNbO_4 are formed at first.Then,Ti_2Nb_(10)O_(29) is formed after oxidation for 20 min and begins to change into TiNb_2O_7 as the oxidation proceeds.SiO_2 is formed as solid state at first but later evolves into glassy state to improve the cohesion of the scale.After oxidation for 100 h,oxidation products consist of SiO_2,TiNb_2O_7,Nb_2O_5 and CrNbO_4.     

Oxidation Behavior of a Ti3Al–Nb Alloy with Surface Thin Oxide Films

Thin films of aluminum, cerium, and yttrium oxides were applied onto the surfaces of Ti₃Al–11Nb samples using an electrodeposition technique. The oxidation behaviors of the Ti₃Al–Nb alloy, with and without these surface-applied films, were studied in air at 800–1000°C. The results showed that the oxidation rate of the alloy can be reduced by Ce oxide and Y oxide films, and the greatest improvement comes from oxidation of the Y oxide-coated specimens at 800°C. With increasing oxidation temperature, the difference between the Co-oxide and Y-oxide films becomes smaller. The results also indicated that the Ce-oxide and Y-oxide films can significantly improve the oxide scale-spallation resistance. On the other hand, Al-oxide films result in detrimental effects on the oxidation and scale-spallation resistance of the Ti₃Al–Nb alloy. Based on the experimental results, the effects of the different surface films on the oxidation mechanism are discussed.     

Cyclic Oxidation Behavior of the Ti–6Al–4V Alloy

The cyclic oxidation behavior of the Ti–6Al–4V alloy has been studied under heating and cooling conditions within a temperature range from 550 to 850 °C in air for up to 12 cycles. The mass changes, phase, surface morphologies, cross-sectional morphologies and element distribution of the oxide scales after cyclic oxidation were investigated using electronic microbalance, X-ray diffractometry, scanning electron microscopy and energy dispersive spectroscopy. The results show that the rate of oxidation was close to zero at 550 °C, obeyed parabolic and linear law at 650 and 850 °C, respectively, while at 750 °C, parabolic—linear law dominated. The double oxide scales formed on surface of the Ti–6Al–4V alloy consisted of an inner layer of TiO₂ and an outer layer of Al₂O₃, and the thickness of oxide scales increased with an increasing oxidation temperature. At 750 and 850 °C, the cyclic oxidation resistance deteriorated owing to the formation of voids, cracks and the spallation of the oxide scales.     

Oxidation behavior of MoSi2 and Mo(Si,Al)2 coated Mo–0.5Ti–0.1Zr–0.02C alloy

MoSi₂ and Mo(Si, Al)₂ coatings were prepared on Mo–0.5Ti–0.1Zr–0.02C alloy using pack cementation process. Oxidation studies revealed that Mo(Si, Al)₂ coating had a much superior oxidation resistance in the temperature range from 400 to 900 °C, where pest disintegration of MoSi₂ occurs due to internal oxidation. The growth kinetics of Al₂O₃ layer formed on Mo(Si, Al)₂ coating was much slower than that of SiO₂ layer formed on MoSi₂ coatings during oxidation.     

The Oxidation Behavior of Several Ti-Al Alloys at 900°C in Air

Ti-23Al, Ti-50Al and Ti-50Al-2Nb (at.%) wereoxidized in air at 900°C for times up to 1130 hr.The resulting oxide scale structures were analyzed ingreat detail by metallographic and microprobe investigations and theAl₂O₃ structures in the complexoxide scales were correlated with the course of thethermogravimetric curves. It appears that in order toachieve long-term protective behavior of the scales, it is necessary to stimulate theformation of a thin Al₂O₃ barrierat the scale-metal interface and not at a position inthe outer part of the scale. The Nb effect seems to bemostly due to this stimulation of anAl₂O₃ layer at theinterface.     

Lamellar orientation control of Ti–47Al–0.5W–0.5Si by directional solidification using β seeding technique

In this paper, we report a β seeding technique for the lamellar orientation controlling in TiAl alloy, which is a novelty and effective method for aligning the lamellar orientation of Ti–47Al–0.5W–0.5Si with primary α phase. The shorter composition transition zone and simpler process procedure can improve the deficiency of α seeding technique. The proper temperature gradient and normal growth rate are necessary for aligning the lamellar orientation in TiAl alloy with primary α phase using β seeding technique.     

Effect of Strontium on the Oxidation Behavior of Liquid Al–7Si Alloys

The oxidation behavior of small amounts of Al–7Si alloys containing strontium and magnesium has been investigated using thermal gravimetrical analysis (TGA). The results of TGA experiments showed that Sr-additions increase the oxidation rate of the Al–7Si alloys melt significantly. A noticeable content of SrO-containing mixed oxides was found, using scanning electron microscopy (SEM), on the oxidized surfaces after even short oxidation periods. The rapid rate of Sr-loss during the oxidation periods also showed that Sr was highly reactive in the small melts for oxidation. The oxidation of large Al–7Si melts was also studied by analyzing the surface layer at different periods. The surface oxide layers on the large melts consisted of Al₂O₃ and Al₂O₃ · MgO but were found to contain no detectable SrO. Also, in contrast with the small melts, the relatively slow rate of Sr-loss during the oxidation period showed that strontium has good stability in a large melt, revealing that in such cases the tendency of Sr for oxidation is not noticeable.     

Short-time Oxidation Behavior of Low-carbon, Low-silicon Steel in Air at 850–1,180 °C––I: Oxidation Kinetics

In this study, the oxidation kinetics of low-carbon, low-silicon steel in flowing air at 850–1,180 °C within 30 or 60 s were examined. The parabolic kinetics were established from the very early stage at 850° and 1,000 °C, whereas the oxidation kinetics at 1,100–1,180 °C appeared to obey a linear law initially and a more-parabolic one at a later stage. When the oxidation kinetics followed the linear law, “rough”-scale with an undulating, saw-teeth like microstructure developed, whereas when the parabolic law was followed, smooth scale developed. It appeared that a critical scale thickness existed, at which the scale-growth mechanism changed from linear to parabolic. This thickness was less than 7 μm at 850 °C, about 10 μm at 1,000 °C, about 50 μm at 1,100 °C and in the range of 60–80 μm at 1,180 °C under the conditions examined. Blister formation at 900 °C prevented clear observation of the linear-to-parabolic transition.     

The Influence of the Alloy Microstructure on the Oxidation Behavior of Ti–46Al–1Cr–0.2Si Alloy

The influence of microstructure of the two-phase alloyTi–46Al–1Cr–0.2Si on the oxidation behavior in air between600 and 900°C was studied. The oxidation rate, type of scale, and scalespallation resistance were strongly affected by the type of microstructure,i.e., lamellar in as-cast material and duplex after extrusion at1300°C. The oxidation rate was affected by the size and distribution ofthe ₂-Ti₃Al phase, being faster for the extrudedmaterial with coarse ₂-Ti₃Al. The type of oxide scaledetermines the spalling resistance. Cast material developed a uniform scalethat spalled off after short exposure times at 800 and 900°C when a criticalthickness was reached. The extruded material presented a heterogeneous scalewith predominant thick regions formed on -TiAl-₂-Ti₃Algrains and thin scale regions formed on -TiAl grains. Thistype of scale could permit an easier relaxation in the matrix of stressesgenerated by both thermal-expansion mismatch between scale and alloy andoxide growth, resulting in a higher spallation resistance.     

Microstructural Study of Fe-Cr-Al/Al Composite Coatings During Oxidation and Sulfidation at 900℃

The microstructure of a composite coating system,which was composed of an inner layer of Fe-Cr-Al and an outer layer of aluminum,was studied after it was respectively oxidized and sulfurdized at elevated temperatures. Apart from the Al2O3 scale formed on the surface,the microstructure of the composite coatings exposed at 900℃ in air for 4h was a three-layer structure. The first layer consisted of a solid solution of Cr and Fe in α aluminum and an intermetallic compound FeAl3,while the second layer was a single phase of the aluminide and the third layer still remained the same appearance as the original Fe-Cr-Al coating. The microstructural observation of the specimen tested at 850-900℃ at low oxygen pressure and high sulfur pressure for 576h revealed that the surface coatings of the specimen had transformed into a duplex structure containing an outer layer and a thicker aluminide layer beneath. X-ray diffraction results showed that the out layer was composed of Al2S3 and Al2O3 and that AlCrFe2 was the main phase composition of the aluminide layer,with a few of Al2S3 and Al2O3 accompanied.