Effect of Carbon on the Long-Term Oxidation Behavior of Fe3Al Iron Aluminides

Electroslag, remelted-iron aluminides having the compositions: (1) Fe–16Al–0.05C, (2) Fe–16Al–0.14C, (3) Fe–16Al–0.5C, and (4) Fe–16Al–1.0C were investigated to understand the effect of carbon on their oxidation behavior in the temperature range 700–1000°C. The oxidation behavior of these aluminides was compared with that of 310 SS, a reference alloy used in the study. Regardless of carbon content, the iron aluminides exhibit marginally higher oxidation tendency than that of 310 SS at 700°C. However, between 800 and 1000°C, they exhibit better oxidation resistance than 310 SS. Although the oxidation resistance of aluminides at 1000°C is better than that of 310 SS, they suffer severe spallation during long-term exposure and C exacerbates this effect. Examination of the early stages of oxidation of the alloys at 800 and 900°C shows that they do not gain a corresponding weight as they do for a temperature rise from 700 to 800°C. A further rise to 1000°C leads to a marginal inversion in the oxidation tendency of the alloys. Based on the literature, this inversion is attributed to the possible dissolution and/or change in compo- sition of Fe₃AlC_0.69 carbide phase with temperature.     

The oxidation of Co−Nb alloys under low oxygen pressures at 600–800°C

The oxidation of two Co–Nb alloys containing 15 and 30 wt.% Nb has been studied at 600–800° C in H₂–CO₂ mixtures providing an oxygen pressure of 10^–24 atm at 600°C and 10^–20 atm at 700 and 800°C, below the dissociation pressure of cobalt oxide. At 600 and 700°C both alloys showed only a region of internal oxidation composed, of a mixture of alpha cobalt and of niobium oxides (NbO₂ and Nb₂O₅) and at 700°C also the double oxide CoNb₂O₆, which formed from the Nb-rich Co₃Nb phase. No Nb-depleted layer formed in the alloy at the interface with the region of internal oxidation at these temperatures. Upon oxidation at 800°C a transition between internal and external oxidation of niobium was observed, especially for Co–30Nb. This corrosion mode is associated with the development of a single-phase, Nb-depleted region at the surface of the alloy. The corrosion mechanism of these alloys is examined with special reference to the effect of the low solubility of niobium in cobalt and to the relation between the microstructures of the alloys and of the scales.     

Oxidation Behavior of ODS Fe–Cr Alloys

Four experimental oxide dispersion strengthened (ODS)Fe-(13–14 at. %)Cr ferritic alloys were exposed for up to 10,000 hr at 700–1100 °C in air and in air with 10vol.% water vapor. Their performance has been compared to other commercial ODS and stainless steel alloys. At 700–800°C, the reaction rates in air were very low for all of the ODS Fe–Cr alloys compared to stainless steels. At 900°C, a Y₂O₃ dispersion showed a distinct benefit in improving oxidation resistance compared to an Al₂O₃ dispersion or no addition in the stainless steels. However, for the Fe-13 %Cr alloy, breakaway oxidation occurred after 7,000 hr at 900°C in air. Exposures in 10 % water vapor at 800 and 900°C and in air at 1000 and 1100°C showed increased attack for this class of alloys. Because of the relatively low Cr reservoirs in these alloys, their maximum operating temperature in air will be below 900°C.     

Oxidation of an Fe-12% Ni alloy in oxygen at 700–1000°C

The oxidation of an Fe-12% Ni alloy has been studied at 700–1000°C using thermogravimetric, metallographic, and electron probe microanalysis techniques. At all temperatures parabolic kinetics were observed and the activation energy for the process was 48±4 kcal mole^–1. At 700°C Fe₃O₄ and Fe₂O₃ were present in the external scale and scaling was accompanied by a progressive Ni enrichment of the underlying alloy. When the Ni-enriched zone contained 50–60% Ni, this metal entered the spinel phase, eventually leading to the formation of Ni_xFe_3–xO₄ where x had a value of 0.24 close to the alloy and <0.01 close to the Fe₂O₃ phase boundary. At higher temperatures (900–1000°C) Ni entered the spinel phase very early in the oxidation process. There was a buildup in Ni concentration in the Ni_xFe_3–xO₄ phase to x values of 0.4 and at 900°C only this corresponded to a transition to a lower parabolic rate of oxidation. The internal oxide phase was identified as Ni_0.7Fe_2.3O₄. The mechanism of oxidation of the alloy is discussed in the light of present knowledge concerning the Fe-Ni-O system.     

Improvement of Oxidation Behavior of a Ti–48Al–2Cr Alloy by a Nitridation Treatment

The effect of a nitridation treatment for 10 hr at 800°C on the oxidation resistance of a Ti–48Al–2Cr (at.%) alloy in air at 800°C was evaluated. Results prove that nitridation decreases by about 40% the total mass gain of nonnitrided material, although the oxidation mechanism is the same for both materials. The oxidation can be divided into two stages. The formation of a nonprotective mixed alumina–rutile scale during the transient stage results in a high oxidation rate. A further decrease in the oxidation rate arises from the establishment of an external alumina-rich layer during the steady stage. The main difference between the scale developed on both materials is the continuous nature of the nitride layer present in the nitrided material during the entire exposure. The thin continuous nitride layer formed during the nitridation treatment acts beneficially as a diffusion barrier, preventing oxygen dissolution in the ₂-Ti₃Al phase during the transient stage. Furthermore, the oxygen gradient through the oxide scale is kept low, because no oxygen is removed at the scale–alloy interface.     

The oxidation resistance of a sputtered,microcrystalline TiAl-intermetallic-compound film

The oxidation behavior of a cast TiAl intermetallic compound and its sputtered microcrystalline film was investigated at 700–900°C in static air. At 700°C, both the cast alloy and its sputtered microcrystalline film exhibited excellent oxidation resistance. No scale spallation was observed. However, at 800–900°C, the oxidation kinetics for the cast TiAl alloy followed approximately a linear rate law, which indicates that it has poor oxidation resistance over this temperature range. The poor oxidation resistance of TiAl was due to the formation of an Al₂O₃+TiO₂ scale which spalled extensively during cooling. Nevertheless, the sputtered, TiAl-microcrystalline film exhibited very good oxidation resistance. The oxidation kinetics followed approximately the parabolic rate law at all temperatures. Although the composition of the scales was the same as that of scales formed on the cast alloy, the scales formed on the sputtered microcrystalline-TiAl film are adherent strongly to the substrate. No scale spallation was found at 700–850°C, while a small amount of spallation was observed only at 900°C. This indicates that microcrystallization can improve the oxidation resistance of the TiAl alloy.     

Effect of heat treatment on the properties of deformed alloy 36N

Iron-nickel alloy 36N (Invar) is widely used in industry as a material having an anomalously low and almost constant thermal coefficient of linear expansion (TCLE) in the temperature range of 20 – 100°C. This value of the coefficient is attained after heat treatment of the deformed semifinished product by the regime of quenching from 830°C in water, tempering at 315°C for I h, and aging at 95°C for 48 h. The minimum value of the TCLE is provided by the quenching operation, whereas the tempering and aging prevent growth of the TCLE during long-term operation of Invar. The use of such heat treatment for rods and wire of alloy 36N guarantees a TCLE of at most 1.5 × 10^–6 °C^–1. It is known that the value of the TCLE and the level of the mechanical properties of Invar can be changed by changing the temperature and deformation regime of its treatment. The aim of the present work is to determine an optimum regime of heat treatment of the alloy after drawing that would ensure, without a finishing treatment, a TCLE not exceeding 1.0 × 10^–6 °C^–1 in the temperature range 20 – 100°C.Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 4, pp. 31 – 32, April, 1996.     

Effect of chromium on the oxidation resistance of TiAl intermetallics

The effect of 10 at.%Cr on the oxidation resistance of TiAl intermetallic compound at 800–1100°C in air was investigated. The results indicated that 10 at.%Cr equally substituting for Ti and Al in TiAl alloy had duplex effects on the isothermal kinetics of DAL At lower temperatures (800–900°C), Cr increased the oxidation rates as a result of the doping effect of Cr in the scale and at higher temperatures (1000–1100°C), especially at 1100°C, Cr significantly reduced the oxidation rates as a result of the formation of a continuous Al₂O₃ film on the surface. 10 at.%Cr only substituting for Ti in TiAl alloy remarkably reduced the oxidation rates at all temperatures by about two orders of magnitude. Moreover, 10 at%Cr significantly improved the cyclic-oxidation rsistance of TiAl alloy.     

New phases in Fe−Nd alloys obtained by quenching from the liquid state

Conclusions In alloys of the Fe–Nd system obtained by quenching from the liquid state, we observe the following phases: for (28–58)% Fe–P₁ (T_C240°C); for <28% Fe–P₂ [T_C=(220±5)°C]. In the alloy based on Nd containing 5% Co and 23% Fe, we observe the P₃ phase with T_C=160°C (tentatively amorphous).The presence of the P₁ phase in Nd-(37–58)% Fe alloys is responsible for the increase in the coercive force_IH_C up to 0.4 MA/m. During annealing, the amount of P₁ phase is reduced, a stable Nd₂Fe₁₇ phase appears, which is accompanied by a sharp decrease in the coercive force. In the alloy based on Nd containing 5% Co and 23% Fe, after annealing the P₄ phase is formed (T_C=340°C, H_a4.8 MA/m), which causes the appearance of the coercive force_IH_C=1.52 MA/m.Moscow Institute of Steel and Alloys. Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 9, pp. 13–16, September, 1992.     

The Effect of Annealing on the Electrochemical Behavior of Amorphous Alloys Fe73.5–Si13.5–B9–Nb3–Cu1in Acidic Perchlorate Media

The effect of annealing in a temperature interval of 300 to 600°C on the properties of an amorphous alloy Fe_73.5–Si_13.5–B₉–Nb₃–Cu₁is studied by potentiodynamic, x-ray diffraction, and x-ray photoelectron techniques, as well as by scanning probe microscopy. The highest corrosion resistance is characteristic of a nanocrystalline alloy completely crystallized at 600°C. The main factor enhancing the corrosion resistance of the alloy is shown to be not the alloy's crystallization degree but the high segregation activity of such components as silicon and boron.     

The sulfidation of pure Co, Y, and of a Co-15 wt.% Y alloy in H2-H2S mixtures at 600–800°C

The corrosion of pure Co and Y and of a Co-15 wt.% Y alloy in H₂-H₂S mixtures providing a sulfur pressure of 10^–8 atm. at 600–800°C and also of 10^–7 atm. at 800°C was was studied to examine the effect of yttrium on the sulfidation resistance of pure cobalt. The alloy was nearly single phase, containing mostly the intermetallic compound Co₁₇Y₂ plus a small amount of cobalt solid-solution. For all conditions except for 800°C under 10^–8 atm. S₂, the alloy formed multilayered scales consisting of an outer region of pure cobalt sulfide, an intermediate region of a mixture of cobalt sulfide with yttrium oxysulfide and finally an innermost layer of a mixture of yttrium oxysu fide with cobalt metal. At 800°C under 10^–8 atm. S₂, below the dissociation pressure of cobalt sulfide, the alloy formed only a single layer composed of a mixture of metallic cobalt with yttrium oxysulfide. Pure yttrium produced only the oxysulfide Y₂O₂S, as a result of the large stability of this compound and of the presence of some impurities in the gas mixtures used. The corrosion kinetics were generally rather complex, but except at 800°C under 10^–8 atm. S₂, the addition of yttrium reduced the sulfidation rate of cobalt, even though the formation of a continuous protective external layer of a pure yttrium compound was never achieved. Finally, when the gas-phase sulfur pressure was above the dissociation of cobalt sulfide the corrosion rate of yttrium was significantly lower than that of Co-15 Y. The internal sulfidation of Y in Co-15 Y was not associated with depletion of Y in the alloy. This difusionless kind of internal attack is typical of binary A-B alloys presenting a very small solubility of the most-reactive component B in the base metal A, which restricts severely the flux of B from the alloy toward the alloy-scale interface.     

Estimation of the Effect of Molybdenum on Chemical and Electrochemical Stability of Iron-Based Alloys

The E–pH diagram of Mo–H₂O system is refined. A cross-section of the Fe–Mo–O phase diagram at 25°C is constructed. The E–pH diagrams for the systems Fe–Mo–H₂O and alloy X17H13M2–H₂O are calculated. Thermodynamic aspect of the effect of molybdenum on chemical and electrochemical stability of iron alloys is discussed.     

Effects of yttrium and zirconium additions on the high-temperature sulfidation behavior of an Fe−10Mo−20Al−8Mn alloy

The effects of zirconium and yttrium additions on the sulfidation behavior of an Fe–10Mo–20Al–8Mn(a/o, atom percent) alloy were examined in flowing H₂/H₂S gas of 4Pa sulfur partial pressure at 900°C. Good scale protection was obtained during the initial reaction stage of the base alloy. However, after 7–8 hr, the formation of internal (Mn,Fe) Al₂S₄ platelets triggered breakdown of the protective scale. The reaction products of the zirconium-containing alloy were nonprotective. Yttrium addition resulted in an Y(Fe_1–xAl_x)₁₂ network along the alloy ferrite grain boundaries. Preferential sulfidation of this phase led to almost complete manganese depletion from the engulfed ferrite, and consequently avoided the manganese-promoted scale breakdown.After an even slower initial stage, this alloy sulfidized at a parabolic rate two orders of magnitude slower than that of pure iron. The protection during the initial and following stages was believed to be provided by an Al₂O₃-containing layer and an Al_0.55Mo₂S₄+Fe_xMo₆S_8–z layer, respectively. The formation of Al₂O₃ is thought to be due to oxygen impurities in the H₂S gas, which cannot be removed by conventional means.     

Oxidation of Ni-base alloys in atmospheres with widely varying oxygen partial pressures

The oxidation behavior of the Ni-base alloys IN 617, IN 713 LC, Ni20Cr, and Ni20Cr+Si has been investigated in the temperature range from 850°C to 1000°C in air and at low-oxygen partial pressure p(O₂) (10^–19 to 10^–16 bar). With the exception of alloy IN 713 LC, the materials show no influence of p(O₂) on the oxidation mechanisms and the kinetics. This result can be explained by the formation of a dense Cr₂O₃ layer, the growth rate of which is controlled by the Cr ion interstitial concentration in Cr₂O₃ at the phase boundary oxide/alloy and the mobility of Cr ions in Cr₂O₃. For the alloy IN 713 LC which develops a dense Al₂O₃ layer in air, a modified transition mechanism at low p(O₂) leads to the formation of Cr₂O₃ at the surface and a strong internal oxidation of Al.     

Influence of heat treatment on the microstructure and mechanical properties of DZ951 alloy

DZ951 directionally solidified nickel-base superalloy is mainly strengthened by y phase.Regularly aligned cuboidal and bimodal γ precipitates were attained by two heat treatments.The effect of microstructure on the mechanical properties of DZ951 alloy has been investigated.The results indicate that MC carbide changes to little blocks during aging treatment at 1050℃ (HT1).MC carbide partly degrades into M23c6 and there is a layer of γ around the carbide during aging treatment at 115℃ (HT2),which is beneficial to the elongation of DZ951 alloy.Small γ volume fraction and the uneven deformation structure are contributed to low mechanical propexties of the as-cast alloy.HT1 alloy has a better stress rupture life at 1100℃50 MPa and yield stress at 20℃,800℃ and 1100℃,which is attributed to regularly aligned cuboidal γ phase and even deformation structure.HT2 alloy has a good combination of strength and ductility.This arises fi'om the bimodal γ precitates and the degeneration of MC carbide.     

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.     

Heat treatment of cast aluminum alloys obtained by application of regulated pressure during crystallization

conclusion For castings of AK8 alloy obtained by application of regulated pressure during crystallization, we recommend the following heat treatment conditions: homogenization at 460–470°C for 8 h, quenching from 505–510°C, aging at 150–170°C for 3.5–4h. After treatment according to this procedure, a hardness of 72–74 HRB is achieved for the alloy.Vladimir Polytechnical Institute. Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 10, pp. 36–39, October 1992.     

TEM studies of cross sections of oxidized Fe-25Cr-6Al-La alloy

The oxidation behavior of the alloy Fe–25%Cr–6%Al-RE (rich in lanthanum) was investigated by means of TEM analysis. The results show that after 2 hr oxidation of the alloy, in pure oxygen at 1200° C, La precipitated in the oxide scale at the top of -Al₂O₃ grains and at the grain-boundary regions in the form of tiny particles of hexagonal La₂O₃. These tiny particles prevented aluminum from diffusing toward the surface and suppressed lateral growth of the oxide scale. The rare-earth constituents accelerated the internal oxidation of the alloy during thermal cycling between 1200° C and room temperature. They appeared as particles of aluminum oxide and lanthanum oxide. Particles of cubic La₂O₃ precipitated in the alloy matrix near the oxide scale-metal interface in a direction parallel to grain boundaries.     

Simultaneous chromizing — Aluminizing coating of low-alloy steels by a halide-activated,pack-cementation process

The simultaneous chromizing — aluminizing of low-alloy steels has achieved Kanthal-like surface compositions of 16–21Cr and 5–8 wt.% Al by the use of cementation packs with a Cr-Al masteralloy and an NH₄Cl activator salt. An initial preferential deposition of Al into the alloy induces the phase transformation from austenite to ferrite at the 1150°C process temperature. The low solubility of carbon in ferrite results in the rejection of solute C into the austenitic core, thereby preventing the formation of an external Cr-carbide layer, which would otherwise block aluminizing and chromizing. The deposition and rapid diffusion of Cr and Al into the external bcc ferrite layer follows. Parabolic, cyclic-oxidation kinetics for alumina growth on the coated steels in air were observed over a wide range of relatively low temperatures (637–923°C).     

Structure and short-term creep resistance of alloy TsM2A after nitriding

Conclusions Internal intriding of the alloy TsM2A at 1500–1700°C causes particles of the strengthening phase ZrN, 0.0425–0.0640 m in size, to appear in the structure of the alloy. Such treatment makes it possible to increase short-term creep resistance at 1500°C by 2–2.5 orders of magnitude, and to increase the microhardness of the alloy to H 280–340.Moscow Institute of Steel and Alloys. Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 2, pp. 29–31, February, 1983.