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.     

Sulfidation properties of Fe-Al alloys at 1173 K in H2S-H2 atmospheres

The sulfidation kinetics and morphological development of reaction products are reported for Fe-9 and 18 at.% Al alloys exposed at 1173 K to H₂S-H₂ atmospheres at sulfur pressures in the range 10^–1–10³ Pa. The Fe-9 Al alloy sulfidized parabolically at Pa giving rise to a duplex scale composed of an outer Al-doped FeS layer and an inner FeS + FeAl₂S₄ lamellar layer and to an internal sulfidation zone containing Al₂S₃ precipitates. The Fe-18 Al alloy which was sulfidized at .     

Interfacial segregation during oxidation in O2 at 1173 K of CeO2-coated Fe-20Cr alloys

The interfacial segregation of sulphur and carbon during the oxidation in 1 torr O₂ at 1173 K of Fe-20Cr alloy, which was either free of Ce, alloyed with 0.078 wt.% Ce, or sputter-coated with a 4 nm-thick CeO₂ layer, was studied using polyatomic SIMS. Oxidation periods were up to 19 hr. During oxidation, sulphur and carbon accumulated at the alloy-oxide interface region of both uncoated and coated alloys. The amount of segregated sulphur did not vary appreciably with time, whereas carbon increased with time. The total amount of segregants was similar for both uncoated and coated alloys, although the scales formed on the sputter-coated alloy maintained adhesion and were about 10 times thinner than those on the uncoated alloys.     

The effect of yttrium addition on the oxidation resistance of EPM TiAl-based intermetallics

The effect of yttrium on the oxidation resistance of elemental powder metallurgy (EPM) processed TiAl-based alloys was studied. Y-addition improved the oxidation resistance of the experimental alloys exposed at 800 °C for 350 h. The improved oxidation resistance was attributed to a fine-grained (Y, Al)O-type oxides and the segregation of Y in the grain boundaries and at the interfaces of Al₂O₃ in the Y-rich layer.     

Microstructural refinement and improvement of mechanical properties and oxidation resistance in EPM TiAl-based intermetallics with yttrium addition

New Y-modified TiAl-based alloys, with nominal chemical compositions of Ti–46.6Al–1.4Mn–2Mo–0.3C–xY (x=0.1, 0.33 and 0.6), were developed with the elemental powder metallurgy (EPM) method. The alloys with Y addition had higher ultimate tensile strength, elongation and oxidation resistance than the Y-free alloy. Combined solid-solution strengthening of Mn, Mo, C and Y, and the microstructural refining effect of C and Y, mainly contributed to the strength at room temperature as well as at high temperature up to 800°C, in addition to strengthening due to Y₂O₃ and carbide precipitation. The room-temperature ductility was also improved by the microstructural refinement and the removal of interstitial oxygen in the α₂ and γ phases. Plastic deformation of γ plates during tensile testing occurred through the activation of ordinary dislocations of perfect Burger's vectors. In an oxidation test in air, the Y-containing alloys showed a significantly low weight gain compared with the Y-free alloy.     

Sulfidation mechanism of an Fe-26.6 at.% Cr alloy at temperatures 973–1173 K in H2S-H2 atmospheres at sulfur pressures 104–10−6 Pa

An investigation has been carried out to establish the sulfidation mechanism of an Fe-26.6 at. % Cr alloy at 973, 1073, and 1173 K in H₂S-H₂ atmospheres at sulfur pressures 10⁴ .     

Sulfidation properties of an Fe-26.6 at.% Cr alloy at temperatures of 973–1173 K in H2S-H2 atmospheres at sulfur Pressures 104–10−6 Pa

An investigation is reported on the sulfidation properties of an Fe-26.6 at. % Cr alloy at 973, 1073, and 1173 K in H₂S-H₂ atmospheres at sulfur pressures 10⁴ 10^–6 Pa. The sulfidation kinetics when plotted according to a parabolic relationship usually exhibited an early slow transient period before onset of parabolic kinetics. Scales contained up to three layers. A triplex (CrFe)S_x/(CrFe)₃S₄/-(FeCr)S_x scale was formed at high sulfur pressures (range I), a single-phase (FeCr)S_x or a duplex (CrFe)S_x/(FeCr)S scale at intermediate sulfur pressures (range II), and a single-phase (CrFe)S_x scale at low sulfur pressures (range III). These pressure ranges at 973 K were: range I = 10^–2Pa, 10^–2 > (range II) 10^–5 Pa, and range III .     

Oxidation of alumina formers at 1173 K: effect of yttrium ion implantation and yttrium alloying addition

The oxidation behaviour of three alumina forming FeCrAl alloys has been investigated during isothermal exposures in air at 1173 K. Two of them were Kanthal A1, differing by the presence or not of implanted yttrium. The third one, Kanthal AF contains alloying additions of yttrium. Kinetic results indicate that only yttrium implantation significantly reduces the growth rate of the oxide scale during the early oxidation stage. For longer oxidation times, the reactive element markedly influences the oxidation rate and the composition of the oxide scale, whatever its introduction mode in the alloy. In situ X-ray diffraction shows that yttrium suppresses the formation of transition alumina and promotes the growth of α-Al₂O₃, thereby leading to the earlier formation of a protective oxide scale.     

TiAlCr涂层对TiAl金属间化合物抗熔融硫酸盐热腐蚀的影响

研究了溅射ＴｉＡｌＣｒ 涂层对ＴｉＡｌ 金属间化合物抗硫酸盐热腐蚀性能的影响。结果表明,８５０ ℃时,９０ ％ Ｎａ２ＳＯ４ ＋ １０ ％ Ｋ２ＳＯ４ 中Ｔｉ５０Ａｌ 和Ｔｉ２４Ａｌ１４Ｎｂ３Ｖ 都遭受严重热腐蚀。ＴｉＡｌＣｒ 涂层能大大改善ＴｉＡｌ 金属间化合物的抗热腐蚀性能。带Ｔｉ５０Ａｌ１０Ｃｒ 涂层的Ｔｉ５０Ａｌ 表面形成一层完整致密的Ａｌ２Ｏ３ 膜, 涂层与基材之间无明显互扩散。带Ｔｉ５０Ａｌ１０Ｃｒ 涂层的Ｔｉ２４Ａｌ１４Ｎｂ３Ｖ 表面虽然也形成Ａｌ２Ｏ３ 膜, 但在其上面长出一些ＴｉＯ２ 瘤。这可能是涂层与基材之间的互扩散导致涂层中Ｔｉ 含量增加,Ａｌ 含量减少而生成ＴｉＯ２ 的结果。将涂层成分调整为Ｔｉ５０Ａｌ２０Ｃｒ 后, 带Ｔｉ５０Ａｌ２０Ｃｒ 涂层的Ｔｉ２４Ａｌ１４Ｎｂ３Ｖ 表面生成了连续的Ａｌ２Ｏ３ 膜, 其抗热腐蚀性能良好。     

Effects of Cr and Nb on the high temperature oxidation of TiAl

From isothermal and cyclic oxidation tests on thermomechanically treated Ti-51%Al, Ti-47%Al-4%Cr, and Ti-48%Al-2%Cr-2%Nb alloys at 800, 900, 1000°C in air, it was found that Ti-48%Al-2%Cr-2%Nb and Ti-47%Al-4%Cr had the best and the worst oxidation resistance, respectively. The oxide scales consisted primarily of TiO₂ and Al₂O₃, with and without a small amount of dissolved Cr and Nb ions, depending on the alloy composition. These ions were slightly enriched inside the inner oxide layer, and strongly enriched around the scale-matrix interface. The outer TiO₂-rich layer was formed by the outward diffusion of Ti ions, while the inner (TiO₂+Al₂O₃) mixed layer was formed by the inward transport of oxygen. The outward movement of Al ions formed the intermediated Al₂O₃-rich layer, above the prepared alloys.     

不同微观组织TiAl金属间化合物高温动态力学行为与断裂特性

研究Gamma等轴NG组织和近片层NL组织的Ti-46.5Al-2Nb-2Cr合金在不同温度和应变率下的力学行为和断裂模式,揭示温度和应变率对不同微观组织的力学行为和断裂特性的影响。结果表明,两种微观组织的TiAl合金均与温度和应变率相关,且具有类似的温度和应变相关性：动载下的强度明显高于静载下的强度,但在高应变率范围内（320-1350s-1）无明显的应变率相关性;韧脆转变温度（BDTT）随应变率的增加而升高;等轴组织的TiAl合金的真应力—真应变曲线中可以看到明显的屈服现象,而近片层组织的屈服现象不明显。在韧脆转变温度以下,随着温度的上升,两种组织的TiAl的断裂方式均由穿晶解理断裂转变为穿晶断裂与沿晶断裂的混和模式,最后转变为沿晶断裂方式。     

The effect of aluminum ion implantation on the oxidation of an Fe-6 at.% Al alloy at 1173 K

Samples of an Fe-6 at.% Al alloy were implanted with doses of 5×10¹⁶–3×10¹⁷ Al⁺ ions/cm² at 100 kV, and subsequently oxidized at 1173 K in O₂ gas at 27 kPa. By comparison with the behavior of unimplanted specimens, no effect on oxidation was observed if the dose of implant was less than 1×10¹⁷ Al⁺ ions/cm². At doses of 1×10¹⁷ Al⁺ ions/cm² or higher, a change in the oxide scaling morphology was effected from a duplex -Fe₂O₃--Al₂O₃ scale to an -Al₂O₃-enriched film with a dispersion of -Fe₂O₃ nodules. Oxygen uptake by the high-dose samples was due principally to nodule formation and weight gains after 45 hr were lower by 50–70% as compared with the unimplanted material. Nucleation and growth of the nodules occurred only within the first 5 hr of reaction, but their presence acted as an upper limit on the benefits provided by implantation of aluminum. Nodule formation was shown to be characteristic of the oxidation properties of the alloy material, not an extraneous feature introduced by the implantation process itself. Surface preparation prior to implantation had little effect on oxidation: high-dose electropolished samples behaved similarly to mechanically polished samples when implanted to comparable doses.     

Sulfidation properties of an Fe-23.4Cr-18.6Al alloy at temperatures 1073 and 1173 K in H2S-H2 atmospheres of sulfur at pressures 104-10−5 Pa

An investigation is reported on the sulfidation properties of an Fe-23.4Cr-18.6Al(at.%) alloy at 1073 and 1173 K in H₂S-H₂ atmospheres, 10⁴ > P_{S 2} 10⁵Pa. The sulfidation kinetics exhibited an early transient period before onset of parabolic kinetics. Values of the parabolic sulfidation rate constants increased by as much as 10⁵ from their smallest values at low sulfur pressures, P_{S 2} 10^–4 Pa, to maximum values at sulfur pressures P_{S 2} 10² Pa. Multilayered scales were formed, the number and types of layers dependent on sulfur pressure. A fully developed scale at sulfur pressures P_{S 2} > 10^–3 Pa.     

Sulfidation properties of austenitic Fe-Mn and Fe-Mn-Al alloys under low sulfur vapor pressure (8 Pa) in the temperature range 873–1173 K

The sulfidation properties of austenitic Fe-Mn and Fe-Mn-Al alloys containing small amounts of carbon have been characterized with respect to the sulfidation kinetics, scale morphological development, structures, and composition of the sulfide phases. The alloys contained 21–40 wt. % Mn and 2.5–8 wt.% Al. The sulfide phase was monosulfide of manganese and iron containing the other metallic elements in solid solution. Two regimes of sulfidation categorized by slow and fast reaction rates were exhibited by all alloys when sulfidized in sulfur vapor at = 8 Pa and over the temperature range 873–1173 K. In the slow regime, a compact duplex -Mn(Fe)S/Fe(Mn)S scale evolved by a classical parabolic law associated with metal diffusion in scale. A porous microcrystalline mixed scale of the above sulfides evolved in the regime of rapid sulfidation by quasilinear kinetics associated with sulfur ingress through the porous scale.     

Effect of Hf addition (0.24 w/o) on the oxidation behavior of TiAl at high temperatures

The isothermal oxidation behavior of TiAl coupons containing 0.24 wt.% Hf has been studied in the temperature range 1100 to 1400 K in a flow of purified oxygen at atmospheric pressure. The addition of Hf is very effective in decreasing the overall oxidation rate at temperatures up to 1300 K, although the oxidation rate was slightly increased during the initial period of about 10 ks, after which a very low oxidation rate was maintained. However, at 1350 K the effect becomes small, and at 1400 K it is inverted. The excellent oxidation resistance obtained is attributable to the formation of two alumina-rich layers in the scale; one is beneath the outer rutile layer and the other at the scale/substrate interface. In addition, no internal oxides were formed in the substrate up to 1300 K.     

Selective oxidation of Fe-30Cr at low temperatures: 743–823 K

Fe-30 Cr specimens were oxidized for 6×10²–3.6×10⁴ s at 743–823 K in pure oxygen at a pressure of 1.33×10⁴ Pa. Depth profiling of oxidized surfaces was performed with the simultaneous use of Auger electron spectroscopy (AES) and inert gas-ion (Ar⁺ or Xe⁺) sputter-etching technique. Chromium was selectively oxidized, and a chromium-depletion zone was formed in the underlying alloy. The values (10^–16–10^–15 cm² s^–1) of the interdiffusion coefficient, D, of the underlying alloy evaluated from the depth-composition profiles were compared in magnitude with the values extrapolated from lattice diffusion data of the corresponding alloy obtained at high temperature. The apparent activation energy obtained from an Arrhenius plot of the evaluated diffusion coefficient, however, was one-third of the value for the corresponding alloy at high temperatures. Discussion was made on the possible mechanism of selective oxidation at low temperatures.     

Tensile Properties and Structure of Several Superalloys after Long- Term Exposure to LiF and Vacuum at 1173 K

The use of the solid- to- liquid phase transformation of LiF to store thermal energy is under consideration for a space- based solar dynamic system. Although advantageous in terms of its energy density, the melting point of this salt (1121K) is beyond the commonly accepted upper- use temperature of 1100 K for chromium- bearing superalloys in vacuum. However, one commercially available nickel- base superalloy (Hastelloy B- 2) is chromium free; unfortunately, because of its high molybdenum content, this alloy can form phases that cause high- temperature embrittlement. To test the suitability of Hastelloy B- 2, it has been exposed to molten LiF, its vapor and vacuum at 1173 K for 2500, 5000, and 10 h. For control, the chromium- containing cobalt- base Haynes alloy 188 and nickel- base Haynes alloy 230 were also exposed to LiF and vacuum at 1173 K for 5000 h. Neither LiF nor vacuum exposures had any significant effect on Hastelloy B- 2 in terms of microstructural surface damage or weight change. Measurement of the post exposure tensile properties of Hastelloy B- 2, nevertheless, revealed low tensile ductility at 1050 K. Such embrittlement and low strength at elevated temperatures appear to preclude the use of Hastelloy B- 2 as a containment material for LiF. Little evidence of significant attack by LiF was seen in either of the chromium- containing superalloys; however, considerable weight loss and near- surface microstructural damage occurred in both alloys exposed to vacuum. Although measurement of the post exposure room-temperature tensile properties of Haynes alloys 188 and 230 revealed no significant loss of strength or ductility, the severe degree of microstructural damage found in unshielded alloys exposed to vacuum indicates that chromium-bearing superalloys might also be unsuitable for prolonged containment of LiF in space above 1100 K. Keywords     

TiAl基合金离子渗碳与高温抗氧化性研究

研究了辉光离子渗碳处理对TiAl基合金的渗层组织、表面硬度和抗高温氧化性的影响。实验显示,渗碳处理可在TiAl基合金的表面形成由碳化物和过渡层组成的复合相结构,经不同渗碳处理的试样,其表面硬度和抗高温氧化性均得到明显提高。     

Comparison of Nb- and Ta-effectiveness for improvement of the cyclic oxidation resistance of TiAl-based intermetallics

High-temperature cyclic oxidation resistance of Ti–45.2Al–7.2Nb and Ti–44.8Al–6.6Ta (in at.%) alloys is studied. The alloys were oxidized at 800–1000 °C for up to 380 h and oxidation kinetics was expressed as a function of weight gain versus time. Both as-cast and oxidized alloys were examined by optical and scanning electron microscopy, energy- and wave-dispersion spectrometry, X-ray diffraction and X-ray fluorescence spectrometry. In contrast to previously reported studies, tantalum is found to retard oxidation more effectively than niobium, despite its lower content in the investigated alloy. It also shows a significantly better adherence of oxide scales to the substrate, i.e. a lower tendency to scale spallation, particularly at 1000 °C. Chemical and phase analysis of oxidized alloys reveals considerable differences in element and phase distribution in oxidation-affected zones: 1. Niobium is observed to increase alumina amount in scales more effectively than tantalum. 2. Concentration of tantalum in scales is lower than of niobium. 3. Tantalum causes the occurrence of nitrides both in scales and in metallic substrate, while niobium supports formation of nitrides only in metallic substrate beneath scales. Differences between the investigated alloys are discussed in relation to oxidation mechanism, stability of Ta- and Nb-containing compounds.     

Sulfidation of manganese in sulfur vapor at low pressure and temperatures between 873 and 1273 K

An investigation was carried out on the sulfidation kinetics of manganese plates by thermogravimetry in pure sulfur vapor at low pressures, 4.5×10^–5< p_S2 (atm)<7.2×10^–4, at temperatures between 973 and 1173 K. The reaction kinetics were parabolic and the sulfidation rate constant at 1073 K was proportional to where n = 6.4±0.8. The activation energy for the sulfidation reaction at atm was 25,000±3000 cal·mol^–1. The -MnS developed a preferred orientation texture, and grain growth was observed even though the metal exhibited no preferred orientation.