Correlation between Microstructures and Oxidation Resistance in Zr-Nb-Ti Alloys

Oxidation behavior of Zr-10Nb-10Ti and Zr-10Nb-20Ti (compositions are in atomic percent) alloys has been investigated in air between 300 °C and 700 °C. Higher Ti content in the alloy enhances the oxidation resistance. The calculated isotherms by PANDAT[1,2] show that 20Ti enters a three-phase (αZr-hcp, βNb-bcc, and βZr-bcc) region at 500 °C, while 10Ti alloy continues to be a two-phase (αZr and βNb) alloy until 550 °C and then enters the three-phase (αZr, βNb, and βZr) region. Both alloys have a single-phase βZr solid solution at 700 °C, which is detrimental for the oxidation resistance. The βNb phase greatly contributes to the oxidation resistance in these two alloys. The common oxidation products have been identified as TiO₂, ZrO₂, and Nb₂O₅. Both alloys suffer from pest oxidation at temperatures between 500 °C and 550 °C, respectively (20Ti and 10Ti), up to 700 °C. X-ray diffraction (XRD) indicates strong peaks for monoclinic structure of ZrO₂ at temperatures above 600 °C.     

High Temperature Oxidation Behavior of Conventional and Nb-Modified MAR-M246 Ni-Based Superalloy

The present investigations focused on the thermal oxidation of two variants of MAR-M246 alloy having the same contents of Ta and Nb in at. pct, considering the effects of total replacement of Ta by Nb. The alloys were produced by investment casting using high purity elements in induction furnace under vacuum atmosphere. The alloys were oxidized pseudo-isothermally at 800 °C, 900 °C and 1000 °C up to 1000 hours under lab air. Protective oxidation products growing on the surface of the oxidized samples were mainly Al₂O₃, Cr₂O₃. Other less protective oxide such as spinels (NiCr₂O₄ and CoCr₂O₄) and TiO₂ were also detected as oxidation products. The conventional alloy exhibited slight internal oxidation at 800 °C and an enhanced resistance at 900 °C and 1000 °C. The Nb-modified alloy presented an exacerbated internal oxidation and nitridation at 900 °C and 1000 °C and an enhanced resistance at 800 °C. At 1000 °C, Nb-modified alloy was particularly affected by excessive spalling as the main damage mechanisms. From a kinetic point of view, both alloys exhibit the same behavior at 800 °C and 900 °C, with k_p values typical of alumina forming alloys (2 × 10⁻¹⁴ to 3.6 × 10⁻¹³ g² cm⁻⁴ s⁻¹). However, Ta modified alloys exhibited superior oxidation resistance at 1000 °C when compared to the Nb modified alloy due to better adherence of the protective oxide scale.

     

Oxidation behavior of multiphase Nb-Mo-Si-B intermetallics

The mixed substitution of Nb and Mo in the ternary systems Mo-Si-B and Nb-Si-B was studied with the goal of balancing oxidation resistance with mechanical behavior. The microstructure and oxidation behavior of six compositions in the Nb-Mo-Si-B system were characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive spectroscopy, electron probe microanalysis (EPMA), and thermogravimetric analysis. Proper selection of the total metal content and the Nb/Mo ratio results in the co-existence of a T1 phase, as (Nb, Mo)₅Si₃B_x, and a solid solution (Nb,Mo) metal phase. At 800 °C, all compositions exhibited catastrophic oxidation, while changing to a quasi-steady-state mass gain at 1200 °C. The high rate constants at 1200 °C indicate that the scales formed were not passivating. A complex scale consisting of four layers formed that was about 350-to 450-μm thick after oxidation for 50 hours at 1200 °C. Borosilicate glass did form within the scale, but the significant prevalence of Nb₂O₅ within the glass, and the resulting inability of the glass to seal pores formed by the evaporation of MoO₃, contributed to the overall poor oxidation resistance compared to the ternary Mo-Si-B system. The Nb and Mo content of the alloy must be further studied and optimized before these alloys may be considered for further development for hightemperature applications. This article is based on a presentation made in the symposium entitled “Beyond Nickel-Base Superalloys,” which took place March 14–18, 2004, at the TMS Spring meeting in Charlotte, NC, under the auspices of the SMD-Corrosion and Environmental Effects Committee, the SMD-High Temperature Alloys Committee, the SMD-Mechanical Behavior of Materials Committee, and the SMD-Refractory Metals Committee.     

A Study on the Oxidation Behavior of Nb Alloy (Nb-1 pct Zr-0.1 pct C) and Silicide-Coated Nb Alloys

In the current work, silicide coatings were produced on the Nb alloy (Nb-1 pct Zr-0.1 pct C) using the halide activated pack cementation (HAPC) technique. Coating parameters (temperature and time) were optimized to produce a two-layer (Nb₅Si₃ and NbSi₂) coating on the Nb alloy. Subsequently, the oxidation behavior of the Nb alloy (Nb-1 pct Zr-0.1 pct C) and silicide-coated Nb alloy was studied using thermogravimetric analysis (TGA) and isothermal weight gain oxidation experiments. Phase identification and morphological examinations were carried out using X-ray diffraction (XRD) and scanning electron microscopy (SEM) techniques. TGA showed that the Nb alloy started undergoing accelerated oxidation at and above 773 K (500 °C). Isothermal weight gain experiments carried out on the Nb alloy under air environment at 873 K (600 °C) up to a time period of 16 hours exhibited a linear growth rate law of oxidation. In the case of silicide-based coatings, TGA showed that oxidation resistance of silicide coatings was retained up to 1473 K (1200 °C). Isothermal weight gain experiments on the silicide coatings carried out at 1273 K (1000 °C) in air showed that initially up to 8 hours, the weight of the sample increased, and beyond 8 hours the weight of the sample remained constant. The oxide phases formed on the bare samples and on the coated samples during oxidation were found to be Nb₂O₅ and a mixture of SiO₂ and Nb₂O₅ phases, respectively. SEM showed the formation of nonprotective oxide layer on the bare Nb alloy and a protective (adherent, nonporous) oxide layer on silicide-coated samples. The formation of protective SiO₂ layer on the silicide-coated samples greatly improved the oxidation resistance at higher temperatures.     

Nonequilibrium synthesis of nbai3 and Nb-Ai-V

In this study, the isothermal oxidation behavior of laser-clad NbAl₃ has been investigated in the temperature range between 800 °C and 1400 °C in air. The effect on oxidation of vanadium microalloying, used to increase the ductility of the otherwise brittle NbAl₃ and discussed in Part I, [1] has also been considered. Bulk and surface oxide chemistry has been investigated using X-ray diffraction and X-ray photoelectron spectroscopy (XPS), respectively. Oxidation kinetics have been determined from weight gain data. The XPS and X-ray diffraction data show that NbAl₃ does not exclusively form a protective A1₂O₃ layer when oxidized in air. The oxidation products at 800 °C are a mixture of Nb₂O₅ and A1₂O₃, while at 1200 °C, a mixture of NbAlO₄, Nb₂O₅, and Al₂O₃ is formed. At 1400 °C, a mixture of NbAlO₄, A1₂O₃, NbO₂, NbO_2.432, and Nb₂O₅ forms. Upon addition of vanadium, the oxidation rate is found to dramatically increase and may be related to the formation of (Nb, V)₂O₅ and VO₂, which grows in favor of protective A1₂O₃. Consequently, although vanadium may be a good additive in terms of its potential for improving ductility in NbAl₃, it is not in terms of its deleterious effects on oxidation.     

Refractory Oxide Coatings on Titanium for Nitric Acid Applications

Tantalum and Niobium have good corrosion resistance in nitric acid as well as in molten chloride salt medium encountered in spent fuel nuclear reprocessing plants. Commercially, pure Ti (Cp-Ti) exhibits good corrosion resistance in nitric acid medium; however, in vapor condensates of nitric acid, significant corrosion was observed. In the present study, a thermochemical diffusion method was pursued to coat Ta₂O₅, Nb₂O₅, and Ta₂O₅ + Nb₂O₅ on Ti to improve the corrosion resistance and enhance the life of critical components in reprocessing plants. The coated samples were characterized by XRD, SEM, EDX, profilometry, micro-scratch test, and ASTM A262 Practice-C test in 65 pct boiling nitric acid. The SEM micrograph of the coated samples showed that uniform dense coating containing Ta₂O₅ and/or Nb₂O₅ was formed. XRD patterns indicated the formation of TiO₂, Ta₂O₅/Nb₂O₅, and mixed oxide/solid solution phase on coated Ti samples. ASTM A262 Practice-C test revealed reproducible outstanding corrosion resistance of Ta₂O₅-coated sample in comparison to Nb₂O₅- and Ta₂O₅ + Nb₂O₅-coated sample. The hardness of the Ta₂O₅-coated Cp-Ti sample was found to be twice that of uncoated Cp-Ti. The SEM and XRD results confirmed the presence of protective oxide layer (Ta₂O₅, rutile TiO₂, and mixed phase) on coated sample which improved the corrosion resistance remarkably in boiling liquid phase of nitric acid compared to uncoated Cp-Ti and Ti-5Ta-1.8Nb alloy. Three phase corrosion test conducted on Ta₂O₅-coated samples in boiling 11.5 M nitric acid showed poor corrosion resistance in vapor and condensate phases of nitric acid due to poor adhesion of the coating. The adhesive strength of the coated samples needs to be optimized in order to improve the corrosion resistance in vapor and condensate phases of nitric acid.     

The effect of nb morphology on the cyclic oxidation resistance of MoSi2/20 vol pct Nb composites

The effect of Nb morphology on the 1200 °C cyclic oxidation resistance of MoSi₂/20 vol Pct Nb composites was investigated. Niobium was incorporated into MoSi₂ as particles, random short fibers, and continuous aligned fibers. After oxidation, it was found that all the composites had lost weight and essentially disintegrated. This was attributed to spalling of both the Nb₂O₅ scale and the MoSi₂ matrix. The spalling of the matrix was a result of cracks originating in the oxidized Nb and propagating through the MoSi₂ matrix. These cracks arose from two sources: (1) the volume expansion associated with the transformation of Nb to Nb₂O₃ and (2) the difference in thermal expansion between Nb₂O₅ and MoSi₂. However, it was found that the addition of smaller diameter Nb reinforcements tended to retard the disintegration of the composites. This was attributed to the effect of reinforcement size on CTE mismatch cracking. D.E. Alman, Formerly Graduate Student, Materials Engineering Department, Rensselaer Polytechnic Institute     

The balance of mechanical and environmental properties of a multielement niobium-niobium silicide-basedIn Situ composite

This article describes room-temperature and high-temperature mechanical properties, as well as oxidation behavior, of a niobium-niobium silicide basedin situ composite directionally solidified from a Nb-Ti-Hf-Cr-Al-Si alloy. Room-temperature fracture toughness, high-temperature tensile strength (up to 1200 °C), and tensile creep rupture (1100 °C) data are described. The composite shows an excellent balance of high- and low-temperature mechanical properties with promising environmental resistance at temperatures above 1000 °C. The composite microstructures and phase chemistries are also described. Samples were prepared using directional solidification in order to generate an aligned composite of a Nb-based solid solution with Nb₃Si- and Nb₅Si₃-type silicides. The high-temperature mechanical properties and oxidation behavior are also compared with the most recent Ni-based superalloys. This composite represents an excellent basis for the development of advanced Nb-based intermetallic matrix composites that offer improved properties over Ni-based superalloys at temperatures in excess of 1000 °C.     

Chlorination treatment to improve the oxidation resistance of Nb-Mo-Si-B alloys

Recent studies have shown that the quaternary Nb-Mo-Si-B system is not oxidation resistant. The difference in oxidation resistance between Mo-Si-B and Nb-Mo-Si-B may be interpreted in terms of the volatility of the metal oxide that forms. MoO₃ evaporates from the surface scale at about 650 °C, leaving a porous borosilicate glassy scale. Nb₂O₅ persists as a rapidly growing condensed phase that overwhelms the ability of the borosilicate glass to form a protective layer. In the present work, a novel chlorination process was employed to selectively remove Nb₂O₅ from the scale of the quaternary alloy as volatile NbCl₅. A Nb-Mo-Si-B alloy was studied with a nominal composition of 63(Nb,Mo)-30Si-7B (at. pct) with Nb/Mo = 1:1. The alloy consisted of a three-phase microstructure of (Nb,Mo)₅Si₃B _x (T1)-(Nb,Mo)₅(Si,B)₃ (T2)-(Nb,Mo)₅Si₃B _x (D8₈). The oxidation behavior of these alloys in air was studied both before and after chlorination. Results showed that Nb₂O₅ can be selectively removed from the scale to leave a borosilicate-rich scale, which then forms a dense scale after heat treatment at 1100 °C in argon. The oxidation rate of the chlorinated alloy was about one-third that of the unchlorinated alloy under identical conditions. Alloy oxidation during heating to the test temperature was studied, and a plausible mechanism for the formation of porosity in the oxide scale has been offered. This article is based on a presentation made in the symposium entitled “Beyond Nickel-Base Superalloys,” which took place March 14–18, 2004, at the TMS Spring meeting in Charlotte, NC, under the auspices of the SMD-Corrosion and Environmental Effects Committee, the SMD-High Temperature Alloys Committee, the SMD-Mechanical Behavior of Materials Committee, and the SMD-Refractory Metals Committee.     

Effect of dispersed oxides of rare earths and other reactive elements on the high temperature oxidation resistance of Fe-20Cr alloy

The isothermal oxidation resistance in air at 1273, 1323, and 1373 K of Fe-20Cr alloys with 1 wt pct dispersoid of Y₂0₃, La₂O₃, A1₂O₃, TiO₂, SiO₂, Cr₂O₃ and without dispersoid prepared by a conventional sintering and rolling procedures was examined. It was found that SiO₂ dispersoid reduced, while A1₂O₃ dispersoid slightly increased the oxidation resistance. The dispersoids of TiO₂ and Cr₂O₃ showed no beneficial effect on the oxidation resistance except for the oxidation after 10 h at 1373 K. The oxidation behavior after 10 h at 1373 K was rather complex including accelerated oxidation. The beneficial effect of La₂O₃ and Y₂O₃ dispersoids was excellent at all temperatures. The oxidation rates during the early stage of oxidation for the alloys with dispersoid were apparently dependent on the type of the dispersoid. There was no evidence that dispersoid accumulated at the scale-alloy interface. Comparison of results obtained for the oxidation of the alloys prepared by a powder metallurgical procedure with results for the alloys by arc-melting procedure indicated that the grain size of the alloy is an important factor for reduction of oxidation rate but does not seem to be critical, because the grain size of the alloys with dispersoid was not dependent on the type of the dispersoid. Ion Microanalyses of the Cr₂O₃ scale formed after 1 h oxidation at 1373 K showed an interesting feature in that all the dispersed elements were incorporated in the scale and the iron content of the scale was lower on the alloys which exhibited better oxidation resistance.     

Microstructural Evolution and Mechanical Behaviors of an Nb-16Si-22Ti-2Al-2Hf Alloy with 2 and 17 at. pct Cr Additions at Room and/or High Temperatures

X-ray diffraction analysis, scanning electron microscopy, and transmission electron microscopy were used to investigate the microstructures and orientation relationships (ORs) of Nb-16Si-22Ti-2Al-2Hf-(2,17)Cr alloys (hereafter referred to as 2Cr and 17Cr alloys, respectively). The mechanical properties of the two alloys at room and/or high temperatures were compared. The 2Cr alloy comprised Nb_SS and (α + β)-Nb₅Si₃ phases, while the 17Cr alloy consisted of Nb_SS, (α + β)-Nb₅Si₃ and Laves Cr₂Nb phases with a C15 structure. The β-Nb₅Si₃ and Laves Cr₂Nb phases exhibited variable ORs with respect to the Nb_SS phase. The Laves Cr₂Nb phase was found to play a negative role on the fracture toughness at room temperature and on the compressive strength at temperatures from 1523 K to 1623 K (1250 °C to 1350 °C). The fracture toughness and the compressive yield strength at 1623 K (1350 °C) both decreased from 14.4 to 10.3 MPa m^1/2 and from 300 to 85 MPa, respectively, when the nominal Cr content increased from 2 to 17 at. pct. Finally, the fracture modes of these typical Nb_SS/Nb₅Si₃ and Nb_SS/Nb₅Si₃/Cr₂Nb microstructures under bending and compression conditions at room and high temperatures were investigated and discussed.     

High-temperature oxidation of Ti3Al-based titanium aluminides in oxygen

The oxidation behavior of Ti-25Al, Ti-24Al-15Nb, and Ti-25Al-11Nb (at. pct) titanium aluminides was studied in dry oxygen at atmospheric pressure in the temperature range 1000 to 1300 K for 4 to 6 hours by thermogravimetry. The oxidation products were characterized by X-ray diffraction (XRD) and scanning electron microscopy with energy dispersive X-ray (EDX) analysis. Although some departures from the parabolic rate law were found, the analysis of data revealed that the parabolic rate law was a more reliable basis for interpretation of results as compared to the empirical power law. Parabolic rate constants (k _p ) for Ti-24Al-15Nb and Ti-25Al-11Nb were almost the same at the same temperature. However, k _p values for Ti-25Al were 2 to 8 times larger than niobium-containing alloys except at 1000 K, where k _p values for all three alloys were approximately the same. The effective activation energy (Q _eff) was 289 kJ/mole for Ti-25Al in the range of 1000 to 1300 K, while in the case of Ti-24Al-15Nb and Ti-25Al-11Nb,Q _eff values were 329 and 330 kJ/mole, respectively, in the range of 1100 to 1300 K. In general, the scales on Ti-25Al were porous and exhibited significant spallation above 1100 K. The scales were thinner, compact, and adherent for niobium-containing alloys. The scales formed on these alloys were predominantly composed of TiO₂, while Al₂O₃ was also present as a minor constituent. The superior oxidation resistance of the niobium-containing alloys has been attributed to the doping effect of niobium in TiO₂.     

Phase transformations during the interaction of Nb2O5 and FeNb2O6 with aluminum

The phase transformations that occur during the interaction of niobium pentoxide and iron niobate with aluminum are studied by differential scanning calorimetry, X-ray diffraction analysis and electronprobe microanalysis. The sequence of the formation of intermediate phases based on an NbO₂ solid solution is revealed. It is shown that the reduction of niobium from Nb₂O₅ is hindered by the formation of hard-to-reduce oxides Al₂O₃ · 25Nb₂O₅, Al₂O₃ · 9Nb₂O₅ and AlNbO₄. The interaction of FeNb₂O₆ with aluminum is accompanied by the formation of [(Fe,Nb)O₂]_s.s and NbO₂ solid solutions.     

Phase Evolution in and Creep Properties of Nb-Rich Nb-Si-Cr Eutectics

In this work, the Nb-rich ternary eutectic in the Nb-Si-Cr system has been experimentally determined to be Nb-10.9Si-28.4Cr (in at. pct). The eutectic is composed of three main phases: Nb solid solution (Nb_ss), β-Cr₂Nb, and Nb₉(Si,Cr)₅. The ternary eutectic microstructure remains stable for several hundred hours at a temperature up to 1473 K (1200 °C). At 1573 K (1300 °C) and above, the silicide phase Nb₉(Si,Cr)₅ decomposes into α-Nb₅Si₃, Nb_ss, and β-Cr₂Nb. Under creep conditions at 1473 K (1200 °C), the alloy deforms by dislocation creep while the major creep resistance is provided by the silicide matrix. If the silicide phase is fragmented and, thus, its matrix character is destroyed by prior heat treatment [e.g., at 1773 K (1500 °C) for 100 hours], creep is mainly controlled by the Laves phase β-Cr₂Nb, resulting in increased minimum strain rates. Compared to state of the art Ni-based superalloys, the creep resistance of this three-phase eutectic alloy is significantly higher.     

Development and elemental powder metallurgy of a Y-containing two-phase TiAl alloy

The Y modification of a two-phase (γ+α ₂) TiAl-(Mn,Mo,C) alloy was studied with an aim to improve, mainly, the oxidation resistance and the mechanical properties in a high-temperature air environment. The experimental alloy was prepared by the elemental powder metallurgy (EPM) method. The addition of up to 0.6 at. pct Y resulted in a significant improvement in tensile properties and compressive yield strength and an anomalous yielding phenomenon withal. Two structural characteristics were identified: first, microstructural refinement in terms of the grain size as well as the interlamellar spacing, and second, precipitation of fine oxides that might scavenge harmful oxygen. Deformation was found to be mainly provided by 1/2<110] ordinary dislocations and a much lesser amount of <011] superdislocations as compared to what has been reported in other (γ+α ₂) TiAl alloys. The oxidation resistance of the experimental alloy was evaluated by air-exposure tests at 800 °C, from which the oxidation kinetics and the morphological and phase characteristics of the oxide scales were analyzed. With a Y addition, the constituents of the oxide scale changed from those of the Y-free alloy. In the case of the Y-free alloy, the oxide scale which formed upon extended air exposure (350 hours) at 800 °C consisted of a mixture of TiO₂ and α-Al₂O₃. In the case of the alloy modified with Y (0.6 at. pct), however, the oxide scale formed in an identical environment was considerably different: it consisted of a complex mixture of TiO₂, α-Al₂O₃, Y₂O₃, and Al₅Y₃O₁₂. The formation of the multiphase (Y,Al)O-rich oxide scale impedes the oxygen transport and the thermal-expansion stress in the Al₂O₃ layer. It is also suggested that a Y addition reduces the oxygen solubility and concentration of oxygen vacancices in the TiO₂ layer. This article is based on a presentation made in the symposium entitled “Fundamentals of Structural Intermetallics,” presented at the 2002 TMS Annual Meeting, February 21–27, 2002, in Seattle, Washington, under the auspices of the ASM and TMS Joint Committee on Mechanical Behavior of Materials.     

Effect of nitrogen on the oxidation behavior of Ti3Al-based intermetallic alloys

The effect of nitrogen on the oxidation behavior of Ti-25Al, T-24Al-15Nb, and Ti-25Al-11Nb (at. pct) has been examined in this study. The gases employed were nitrogen and oxygen-nitrogen and oxygen-argon mixtures in various proportions at a total pressure of 1 atmosphere. The experiments were carried out in the temperature range of 1100 to 1300 K by thermogravimetry. The suitability of employing the parabolic rate law as the basis of interpretation of weight gainvs time data has been discussed. Oxidation resistance of Nb-containing alloys was superior to that of binary Ti-25Al, irrespective of the gas composition employed. The nitridation rates of alloys, with or without Nb, were more than an order of magnitude lower than those for oxidation. The scales were characterized by X-ray diffraction (XRD) and scanning electron microscopy with energy dispersive X-ray (EDX) analysis. The scales formed in all the conditions mostly consisted of TiO₂ and Al₂O₃. However, TiN was observed in scales of Nb-containing alloys in all nitrogen bearing gases and seemed to primarily account for improved oxidation resistance of the preceding in comparison to alloys without Nb. Nitrogen pretreatment was provided for some samples before oxidation for further elucidation of the role of nitrogen.     

Effect of Ti Addition and Microstructural Evolution on Toughening and Strengthening Behavior of as Cast or Annealed Nb–Si–Mo Based Hypoeutectic and Hypereutectic Alloys

Room temperature fracture toughness along with compressive deformation behavior at both room and high temperatures (900 °C, 1000 °C and 1100 °C) has been evaluated for ternary or quaternary hypoeutectic (Nb–12Si–5Mo and Nb–12Si–5Mo–20Ti) and hypereutectic (Nb–19Si–5Mo and Nb–19Si–5Mo–20Ti) Nb-silicide based intermetallic alloys to examine the effects of composition, microstructure, and annealing (100 hours at 1500 °C). On Ti-addition and annealing, the fracture toughness has increased by up to ~ 75 and ~ 63 pct, respectively with ~ 14 MPa√m being recorded for the annealed Nb–12Si–5Mo–20Ti alloy. Toughening is ascribed to formation of non-lamellar eutectic with coarse Nb_ss, which contributes to crack path tortuosity by bridging, arrest, branching and deflection of cracks. The room temperature compressive strengths are found as ~ 2200 to 2400 MPa for as-cast alloys, and ~ 1700 to 2000 MPa after annealing with the strength reduction being higher for the hypoeutectic compositions due to larger Nb_ss content. Further, the compressive ductility has varied from 5.7 to 6.5 pct. The fracture surfaces obtained from room temperature compression tests have revealed evidence of brittle failure with cleavage facets and river patterns in Nb_ss along with its decohesion at non-lamellar eutectic. The compressive yield stress decreases with increase in test temperature, with the hypoeutectic alloys exhibiting higher strength retention indicating the predominant role of solid solution strengthening of Nb_ss. The flow curves obtained from high temperature compression tests show initial work hardening, followed by a steady state regime indicating dynamic recovery involving the formation of low angle grain boundaries in the Nb_ss, as confirmed by electron backscattered diffraction of the annealed Nb–12Si–5Mo alloy compression tested at 1100 °C.

     

Isothermal oxidation behaviour of beta gamma powder metallurgy TiAl–4Nb–3Mn alloys

Abstract

A new TiAl–4Nb–3Mn beta gamma alloy was synthesised by a powder metallurgy process. HIPed and vacuum heat treated specimens were isothermally oxidised at 800 and 900°C in air up to 500 h. The TiAl–4Nb–3Mn alloys oxidised parabolically up to 500 h at both 800 and 900°C. The oxides consisted of outer TiO₂ layer, intermediate Al₂O₃ layer and inner TiO₂ rich mixed layer and the oxidation mechanisms of the alloy were identical at both temperatures. During oxidation, the degradation of α₂ lamellae in the vicinity of the interface forms a diffusion zone (lamellar depleted zone) leading to the formation of Nb and Mn rich white layers just below the interface by outward diffusion of Nb and Mn which are released from breakdown of α₂ lamellae. As exposure time increases, Nb begins to diffuse earlier than Mn and diffuses more actively at higher temperature. The activation energy for oxidation of TiAl–4Nb–3Mn alloy was lower than that of Ti–48Al alloy and was higher than those of Ti–48Al–2Nb–2Cr and Ti–48Al–2Nb–2Cr–W alloys.

On a synthétisé un nouvel alliage TiAl–4Nb–3Mn de type bêta gamma par un procédé de métallurgie des poudres. On a oxydé en isotherme à 800 et à 900°C à l’air, jusqu’à une durée de 500 heures, les spécimens pressés par HIP et traités thermiquement sous vide. Les alliages de TiAl-4Nb-3Mn s’oxydaient paraboliquement jusqu’à 500 heures tant à 800°C qu’à 900°C. Les oxydes consistaient en une couche externe de TiO₂, en une couche intermédiaire d’Al₂O₃ et en une couche interne mixe riche en TiO₂ et les mécanismes d’oxydations de l’alliage étaient identiques aux deux températures. Lors de l’oxydation, la dégradation de lamelles d’α₂ dans le voisinage de l’interface forme une zone de diffusion (zone lamellaire appauvrie) menant à la formation de couches blanches riches en Nb et en Mn juste au-dessous de l’interface par diffusion vers l’extérieur du Nb et du Mn, qui sont relâchés par la dégradation des lamelles d’α₂. À mesure que la durée de l’exposition augmente, le Nb commence à se diffuser plus tôt que le Mn et se diffuse plus activement à une température plus élevée. L’énergie d’activation pour l’oxydation de l’alliage de TiAl–4Nb–3Mn était plus basse que celle de l’alliage de Ti–48Al et était plus élevée que celle des alliages de Ti–48Al–2Nb–2Cr et de Ti–48Al–2Nb–2Cr–W.     

Overview of Strategies for High-Temperature Creep and Oxidation Resistance of Alumina-Forming Austenitic Stainless Steels

A family of creep-resistant, alumina-forming austenitic (AFA) stainless steel alloys is under development for structural use in fossil energy conversion and combustion system applications. The AFA alloys developed to date exhibit comparable creep-rupture lives to state-of-the-art advanced austenitic alloys, and superior oxidation resistance in the ~923 K to 1173 K (650 °C to 900 °C) temperature range due to the formation of a protective Al₂O₃ scale rather than the Cr₂O₃ scales that form on conventional stainless steel alloys. This article overviews the alloy design approaches used to obtain high-temperature creep strength in AFA alloys via considerations of phase equilibrium from thermodynamic calculations as well as microstructure characterization. Strengthening precipitates under evaluation include MC-type carbides or intermetallic phases such as NiAl-B2, Fe₂(Mo,Nb)-Laves, Ni₃Al-L1₂, etc. in the austenitic single-phase matrix. Creep, tensile, and oxidation properties of the AFA alloys are discussed relative to compositional and microstructural factors.     

The stability of Nb/Nb5Si3 microlaminates at high temperatures

The microstructural, phase, and chemical stability of Nb/Nb₅Si₃ microlaminates was investigated at temperatures ranging from 1200 °C to 1600 °C. Freestanding Nb/Nb₅Si₃ microlaminates were prepared by sputter deposition and their stability was investigated by annealing either in vacuum or in an Ar atmosphere. The microlaminates were generally structurally stable, with no evidence of layer pinchoff, even after annealing at 1600 °C. However, a small volume fraction (<2 pct) of voids formed in the silicide layers at 1500 °C and 1600 °C, which are attributed either to the Kirkendall diffusion of Si or to the growth of silicide grains. In terms of phase stability, there was no discernible dissolution of the Nb₅Si₃ layers and no silicide precipitates in the Nb layers following anneals at 1400 °C. Annealing at higher temperatures, though, resulted in the formation of non-equilibrium Nb₃Si on the Nb/Nb₅Si₃ interfaces. This phase is thought to precipitate from the supersaturated Nb-Si solid solution on cooling, and is stabilized by the development of tensile stresses in the Nb layers. The most pervasive observed high-temperature breakdown mechanism was chemical in nature, namely, the loss of Si via sublimation to the environment. The Si loss was partially suppressed either by annealing in a Si-rich atmosphere or by annealing in Ar.