Der Einfluß des Gefügezustandes auf die Oxydationseigenschaften von Zirkonium-Vanadium-Legierungen

Influence of structure on the oxidation behaviour of zirconium vanadium alloys The oxidation of the alloys ZrV 1 and Zr V 3 has been studied on a comparative basis to zirconium after various mechanical and thermal pretreatements; the aggressive medium was steam of 300°C.It has been shown that recrystallizing annealing after cold deformation has but little influence on the oxidation resistance; a considerable diminution of the oxidation susceptibility has been achieved, however, by grain refining after a cold deformation or annealing of the martensitic structure. This type of treatment has a positive effect in particular on the structure of the protective layer. In view of the fact that the oxidation susceptibility increases with the concentration of the alloying element it may be suggested that the intermetallic compound ZrV₂ has a negative effect. Consequently, the oxidation resistance is improved by quenching from high temperatures which prevents the formation of segregations.     

Potentialities of Mössbauer Spectroscopy for Studying Zirconium Alloys and Their Oxide Films

A brief review and an analysis of results of a study of iron- and tin-bearing zirconium alloys and their oxide films by the method of Mössbauer spectroscopy (MS) are presented. The potentialities of MS for studying the phase composition of zirconium alloys are described and the changes in the states of iron and tin atoms are presented as a function of additional alloying and thermomechanical treatment. The conditions of formation of Zr₂Fe and Zr₃Fe intermetallic compounds and chromium- and niobium-bearing compounds are considered. It is shown that some intermetallic compounds transform into other compounds at room temperature. Metallic iron and tin are shown to be present in oxide films of zirconium alloys, and their concentration is shown to affect the corrosion resistance of zirconium alloys.     

Der Einfluß der intermetallischen Phase ZrV2 auf die Oxidations- und Korrosionseigenschaften von Zirkonium-Vanadium- Legierungen

The influence of the intermetallic ZrV₂ phase on the oxidation and corrosion behaviour of zirkonium vanadium alloys The good oxidation resistance of zirkonium and some of its alloys is due to the formation of an oxide layer characterized by good adhesion. Since, however, cracks are formed in such a layer upon prolonged oxidation the corrosion rate can be rather high in the later stages. An improvement of the properties of the oxide layer may be obtained by providing for the incorporation into the scale of atoms able to prevent oxygen diffusion. In the case of vanadium, however, no protective effect is obtained and the corrosion rate even increases as the vanadium content is increased. This phenomenon is due to the poor solubility of vanadium in the zirconium so that the incorporation of vanadium in the scale is inhibited during the initial stages of corrosion. The corrosion mechanism is the same in saturated vapour and NaCl-solution: there are ZrV₂-Particles which are not attacked and are embedded in a largely destroyed zirkonium matrix.     

Alloying effects on properties of Al2O3 and TiO2 in connection with oxidation resistance of TiAl

In the present work, a first-principles method is used to calculate the oxidation energies of Al₂O₃ and TiO₂ as well as the formation energy of oxygen vacancy in TiO₂ containing various alloying elements, in order to shed some light on the alloying effects on the oxidation resistance of γ-TiAl. Our calculations demonstrate that almost all alloying elements increase the oxidation energies of Al₂O₃ and TiO₂. The alloying elements with number of d electrons from 2 to 5 in the forth and fifth rows of the periodic table (e.g., Zr, Nb, Mo, Hf, Ta, W) increase significantly the oxidation energy difference between Al₂O₃ and TiO₂, i.e., reduce the relative stability of Al₂O₃ to TiO₂. On the other hand, these alloying elements increase the formation energy of oxygen vacancy in TiO₂. The effects of other alloying elements are less significant or opposite. Observing the experimental mass gains of TiAl alloys and unalloyed TiAl due to oxidation, we find that the elements reducing the relative stability of Al₂O₃ to TiO₂ and increasing the formation energy of oxygen vacancy enhance the oxidation resistance of TiAl whereas others do not. Such correlations are rationalized by analyzing the alloying effects on the internal oxidation of Al in the γ-TiAl matrix and the diffusion of oxygen in TiO₂ surface scale.     

Determination of Oxygen Diffusion Along Nickel/Zirconia Phase Boundaries by Internal Oxidation

Internal oxidation tests with nickel alloys that contained up to 8 at.% zirconium were carried out. All alloys were two-phase consisting of γ-Ni and the intermetallic phase Ni₅Zr. Their behavior under low oxygen partial pressures in the range of 800–1,000?°C could not be described by the Wagnerian analysis. Oxygen diffusivity along the interface nickel/monoclinic zirconia plays an important role for the rate of internal oxidation. The early stages of internal oxidation show the in situ mode where diffusion of the less noble element zirconium cannot diffuse in the matrix and is oxidized instantly. Later in the process the mode shifts from in situ towards the diffusive mode as zirconium has the possibility to diffuse. This change could also be observed as the size of the oxide particles varied with ongoing oxidation. A method for the determination of the oxygen diffusivity in nickel/monoclinic zirconia phase boundaries is presented.     

Untersuchungen über das Korrosionsverhalten von Zirkoniumlegierungen II. Verhalten bei Wasserdampfkorrosion

Investigation into the corrosion behaviour of zirconium alloys. 11. Behaviour under steam corrosion conditions An investigation into the corrosion behaviour of about 50 Zr alloys in high pressure high temperature steam (240 atm, 400°C) and hot air (4OO°C) has revealed a clear correlation between the behaviour in the two media. Alloys characterized by lower scaling susceptibility are also less susceptible to corrosion in steam, while, on the other hand, thicker scale layers tend to reduce hydrogen embrittlement. From the alloying additions studied Sn, Fe, Cr and Ni have a clearly favourable influence and W may have a similar effect. Ti has an outspoken negative influence on the behaviour in steam (spalling off of the scale) and Si, too, is deleterious because the SiO₂ being formed is soluble in steam, so that the scale layers retain their penetrability. Mo cannot be recommended either, while Nb is favourable in particular because of its grain refining effect. The favourable effect of Cu additions has been confirmed. Alloys with Fe, Cr and Ni and single types with Mo, Nb and Cu are superior to pure Zr with respect to steam resistance and are in part superior to Zircaloy 2 with respect to their mechanical strength.     

Effect of Alloy Composition and Exposure Conditions on the Selective Oxidation Behavior of Ferritic Fe–Cr and Fe–Cr–X Alloys

Selective oxidation behavior of ferritic martensitic Fe–Cr base alloys, exposed in various atmospheres containing combinations of O₂, CO₂, and H₂O, were studied at various temperatures relevant to oxy-fuel combustion. This paper begins with a discussion of the required Cr content to form a continuous external chromia scale on a simple binary Fe–Cr alloy exposed in oxygen or air based on experiments and calculations using the classic Wagner model. Then, the effects of the exposure environment and Cr content on the selective oxidation of Fe–Cr alloys are evaluated. Finally, the effects produced by alloying additions of Si, commonly present in various groups of commercially available ferritic steels, are described. The discussion compares the oxide scale formation on simple binary and ternary Fe–Cr base model alloys with that on several commercially available ferritic steels.     

Identification and analysis of intermetallic phases in overaged Zr-containing and Cr-containing Al-Zn-Mg-Cu alloys

This paper investigates the effect of alloying elements on the characteristics of intermetallic phases in Zr-containing and Cr-containing 7xxx Al-Zn-Mg-Cu alloys at overaged conditions. Four Al-Zn-Mg-Cu alloy plates with different alloying element contents were studied by optical microscopy based image analysis, differential scanning calorimetry, scanning electron microscopy combined with energy disperse X-ray spectroscopy and transmission electron microscopy. The grain structures, recrystallisation, intermetallic phases and precipitates in the selected alloys have been analyzed and the presence of coarse intermetallic phases has been interpreted using established phase diagrams. The different effects of Zr or Cr addition to the alloys have been compared. The experimental results showed that the recrystallised area fraction of Zr-containing alloys is less than that of Cr-containing alloys, being attributable to Zr reducing recrystallisation more effectively than Cr. The detected particles are mainly S phase, Al₇Cu₂Fe, as well as dispersoids of Al₃Zr for Zr-containing alloys and Cr-rich E phase for Cr-containing alloys. These coarse particles, especially the S phase which cannot be dissolved during solution treatment, are detrimental to the fracture toughness of the alloys.     

High‐temperature oxidation behaviour of model Co?Re?Cr alloys at low oxygen partial pressure

The oxidation behaviour of four model Co? Re? Cr alloys and a commercial Co‐based alloy was investigated at 1000 °C and a low‐oxygen partial pressure of p(O₂) = 10^?16 bar, in order to prove the feasibility of a pre‐oxidation treatment. Under suitable conditions of the pre‐oxidation treatment, the oxidation of the highly reactive alloying element Cr is possible. All the studied alloys form a continuous and dense Cr₂O₃ scale on the metal surface. The transport of chromium to the surface occurs mainly from the Cr‐rich σ‐phase, which becomes completely dissolved in the surrounding matrix after long exposure times. As a result of the Cr₂O₃ scale, growth depletion of Cr occurs in the near surface region, leading to internal oxidation.     

Melting and solidification of TiNi alloys by cold crucible levitation method and evaluation of their characteristics

Abstract

The addition of Re, Fe and Cr into Ti–50 mol.-%Ni has been carried out to improve the oxidation and mechanical properties. The mono phase consisting of TiNi with the B2 type structure was identified in micro-alloyed materials proposed on the basis of the d-electrons concept. Experimentally, TiNi alloys were melted and solidified by the cold crucible levitation melting (CCLM) method. The TiNi–(Cr, Fe, Re) alloys with high purity and without contamination from a crucible were prepared, and the homogeneous microstructure was achieved by the diffusion mixing effect of CCLM even in the as-cast alloys which contained Re and Cr with higher melting temperatures and different specific gravities. The transformation from austenite to martensite phases occurred in all alloys below or above room temperature. Some alloys had the ability of shape memory even at room temperature. Ternary alloys showed a higher flow stress level compared with the binary TiNi alloy. On the other hand, the oxidation at 1273 K was promoted by the formation of titanium oxides (TiO₂) on the alloy surfaces. The oxidation resistance was improved by the formation of the continuous Cr₂O₃ film in TiNi–Cr alloys. The alloying effects by ternary elements (Re, Fe, Cr) in the intermetallic TiNi as well as metallic materials were explained well using two parameters used in the d-electrons concept.     

The oxidation of Co-20% Cr base alloys containing Nb or Ta

Alloys based on Co-20% Cr containing approximately 4, 7 or 10wt% Nb or Ta were oxidized in oxygen and air at 900, 1000 and 100°C for times up to 350 h. In general, the addition of Nb accelerated the oxidation rate, although this effect was small at the lowest temperature. Futhermore, little evidence could be found for the development of a protective Cr₂O₂ layer. In contrast, the addition of Ta proved beneficial at all temperatures, promoting the development of protective oxide scales. A critical difference appeared to be the ability of the Ta-containing alloys to form a compound oxide, CrTaO₄, whereas no similar phase could be detected in the scales on the Nb-containing alloys. The Ta-rich oxides formed a layer adjacent to the metal, while a Cr-rich layer was formed outside it. It is possible that the Ta reduced the oxygen activity at the surface of the alloy, preventing the formation of cobalt-containing oxides which might otherwise disrupt the protective scale. Both elements have restricted solubility in Co-20% Cr, forming intermetallic compounds which oxidize internally. In the case of the Nb-containing alloys, a process occurs, during oxidation, which produces a change in the intermetallic deep in to the alloy, though there is no similar change in the Ta-containing alloys. This process has not yet been defined.     

Ion Implantation as a tool to study the oxidation behaviour of TiAl-based intermetallics

TiAl-based Intermetallic alloys are being considered as structural materials for high-temperature applications due to their low density and substantial mechanical strength at high temperatures. The effect of various elements added by ion implantation on the oxidation behaviour of near-γ Ti-48Al-2Cr and Ti-48Al-2Cr-2Nb (compositions are given in at-%) in air at 800°C has been studied. Ions implanted (10¹⁷ Ions/Cm²) were Al, Ti, Cr, Mo, Y, Mn, Pt, Nb, and Si in case of Ti-48Al-2Cr and Nb in case of Ti-48Al-2Cr-2Nb. Comparison was made with the oxidation behaviour of Ti-48Al2Cr-2Nb and Ti-47Al-2Cr-0.2Si alloys in which quaternary elements were added by alloying. It is Concluded that ion implantation can serve as a research tool to study, in the frame of screening tests, the influence of various elements on the corrosion behaviour of materials.     

The influence of alloying elements on the chlorine-induced high temperature corrosion of Fe-Cr alloys in oxidizing atmospheres

The effect of the alloying elements Al, Cr, Mn, Mo, Si and Ti on the corrosion behaviour of ferritic Fe-15Cr model alloys was studied in a N₂/He-5 vol.% O₂ gas mixture with and without additions of 500–1500 vppm HCl at 600°C. The main corrosion mechanism is “active oxidation”, characterized by the formation of volatile metal chlorides at the metal/oxide interface. Volatilization and subsequent conversion of the chlorides into oxides results in the formation of porous and poorly adherent oxide scales. Large mass gains were observed for Fe-15Cr, Fe-35Cr and Fe-15Cr with additions of 5 wt.% Ti, 10 wt.% Mn or 10 wt.% Mo. The specific morphology of the corrosion products depends strongly on the alloying elements. For the Fe-Cr alloys, a model for the formation of the scales, which are characterized by alternating dense and porous layers, is presented. The addition of 5 wt.% Si or Al to Fe-15Cr leads to much better corrosion resistance by the formation of protective Cr₂O₃/Al₂O₃-layers, however in the case of Al addition the behaviour depends strongly on the experimental conditions, as surface treatment and flow velocity. In Fe-15Cr-10Mo preferential removal of the more reactive metals Fe and Cr was observed resulting in a Mo-enriched porous metal zone underneath the metal-oxide interface. The effect of carbon on the corrosion behaviour was examined by addition of 0.3–0.8 wt.% C to the model alloys. Cr-rich M₂₃C₆-carbides were attacked preferentially while Mo-rich M₆C-carbides are very stable relative to the matrix and the attack occurs in regions surrounding the carbides.     

The oxidation of Fe-Cr alloys containing an oxide dispersion or reactive metal additions

The oxidation of an iron-16% chromium alloy containing a dispersion of yttria particles and of iron-16 to 18% chromium alloys containing small additions of yttrium or zirconium has been studied at 1100 and 1200°C in 100 Torr oxygen. The yttria-containing alloy exhibited the excellent oxidation resistance usually associated with oxide-dispersion-containing alloys, having a thin, adherent, virtually iron-free scale which resisted the breakaway rapid oxidation behavior commonly found in Fe-Cr alloys in this composition range. Of the alloying additions intended to form a fine oxide dispersion during oxidation, only zirconium affected the oxidation behavior in a beneficial way, the scale on the yttrium-containing alloy being possibly less protective than that on the equivalent binary alloy.Supported by Battelle, Columbus Laboratories, Columbus, Ohio.     

Volume and size effects of intermetallic compounds on the high-temperature oxidation behavior of Mo-Si-B alloys

In this study, we investigated the high-temperature oxidation behavior of Mo-Si-B alloys with different volume fractions or sizes of intermetallic compound phases. Mo-Si-B alloys with uniformly dispersed intermetallic compound phases (Mo₅SIB₂ and Mo₃Si) in Mo solid solution matrix phase were fabricated using a novel powder metallurgical route, as introduced in our previous study. An isothermal oxidation test was conducted at 1300 °C for up to 10 h. The high-temperature oxidation resistance of Mo-Si-B alloys improved by increasing the volume fraction of intermetallic compound phases; this was a result of the increased amount of protective oxidized layers, which protect the Mo_ss phase from oxidation by covering the surface. In addition, Mo-Si-B alloy with smaller intermetallic compound phases pulverized by high-energy ball milling had better high-temperature oxidation resistance compared to Mo-Si-B alloy with as-synthesized intermetallic compound phases.     

High Temperature Performance of an FeAl Laser Coated 9Cr1Mo Steel

The isothermal high temperature corrosion behavior of an FeAl coating, coated on 9Cr1Mo steel through laser surface alloying, was studied in atmospheres of pure oxygen and O₂ + 1 %SO₂. The specimens were tested at 500, 600 and 700 °C for 4–100 h. The mass change of the specimens versus time of exposure was used to study the kinetics of oxidation. The coating degradation through interdiffusion of alloying elements between the surface layer and substrate was investigated by long-term oxidation tests in air. OM, SEM, FESEM, EDS and EPMA analyses were used to study the oxidation behavior of the intermetallic coating. The results showed excellent oxidation/sulfidation resistance of the coated material due to a negligible growth rate of the oxide scale. However, the coating was degraded because of the interdiffusion of Al and Fe atoms between the coating and substrate after prolonged exposure to elevated temperatures.     

Fundamental Factors Determining the Mode of Scaling of Heat-Resistant Alloys

The compact scale structures developed on heat-resistant alloys are determined to a considerable degree by interplay of the following factors:–

• (a) The free energies of formation of the various possible doped simple and complex oxides, sulphides, etc.
• (b) The growth rates of these compounds.
• (c) The bulk alloy composition.
• (d) The alloy interdiffusion coefficient, taking into account transport both through the alloy grains and through grain boundaries.
• (e) The solubility and diffusivity of oxygen, sulphur, etc., in the substrate.
• (f) The influence of the alloy substructure.

These structures may be modified by the consequences of loss of adhesion, spalling, breakaway, oxide interdiffusion, etc. The principles involved in transient and steady-state oxidation are illustrated briefly in terms of the comparative scaling behaviour of Fe-Cr. Ni-Cr and Co-Cr alloys, and of Fe-Cr-Al, Ni-Cr-A1 and Co-Cr-A1 and Fe-Cr-Si alloys, in oxygen. The aim of this discussion is to present a general background, in terms of which the behaviour of further addition or tramp elements may be assessed. Attention is then turned briefly to the theories that have been postulated for explaining the effectiveness of rare earth and similar elements in improving scale adhesion and/or reducing scale growth rate. A probable role of rare earths in diminishing hot corrosion is outlined. Possible contributions to the scaling pattern of other additives, such as manganese and carbon, are described.     

Microstructure and properties of Ti–Co–Si ternary intermetallic alloys

Ti–Co–Si ternary intermetallic alloys with Ti₅Si₃ as the main reinforcing phase and intermetallic TiCo as the toughening matrix were fabricated by the laser-melting deposition (LMD) process. Microstructure of the intermetallic alloys was characterized by OM, SEM, XRD and EDS. High-temperature oxidation resistance of the alloys was evaluated by isothermal oxidation at 1173 K and metallic dry-sliding wear property was evaluated at room temperature. The effect of reinforcing phase Ti₅Si₃ content on hardness, oxidation and wear resistance of the alloys was investigated. Results indicate that microstructure of the alloys transforms from hypoeutectic to hypereutectic, while hardness and oxidation resistance increases with the increasing Ti₅Si₃ content. The alloys have good oxidation resistance at 1173 K and the oxidation kinetic curves are approximately parabolic. Wear resistance of the alloys is insensitive to the microstructure and is up to 15–19 times higher than the hardened tool steel 1.0%C–1.5%Cr under dry-sliding wear test conditions. The excellent wear resistance of alloys is attributed to the effective reinforcement of Ti₅Si₃ and the excellent toughness of the intermetallic TiCo.     

Welding of unique and advanced ductile intermetallic alloys for high-temperature applications

Intermetallic alloys represent a unique class of materials with atomic arrangements that are different from those of conventional disordered alloys. Among them are alloys based on Ni₃Al, Fe₃Al, and TiAl. Intermetallic alloys have unique properties, such as high melting point, low density, high-temperature strength, and high-temperature corrosion and oxidation resistance. Their only disadvantage is the lack of ductility at room temperature and at elevated temperatures. However, they can be ductilised by micro- and macroalloying. Application of intermetallic alloys for structural use at elevated temperature depends on their ability to be welded using conventional welding procedures. This paper focuses on the development of these alloys, their behaviour when subjected to weld thermal cycles, and their weldability. Most intermetallic alloys are susceptible to cracking during or after welding, but some can be modified to have good weldability. The paper discusses welding and weldability of Ni₃Al-, Fe₃Al-, and TiAl-based intermetallic alloys. In addition, the weldability of other long-range ordered alloys, of the type (Fe, Ni)₃V and (Fe, Co)₃V, are briefly discussed.     

Untersuchungen über das Korrosionsverhalten von Zirkoniumlegierungen I. Verhalten gegenüber wäßrigen Medien

Investigations into the corrosion behaviour of zirconium alloys I. Behaviour in the presence of aqueous media Extensive corrosion research covering 5O Zr alloys have shown that the good corrosion resistance of pure Zr cannot be improved to any greater extent by alloying. Pur Zr as well as most of its alloys have a high corrosion resistance; even in boiling mineral acids (20% H₂SO₄, HNO₃, and HCl) the corrosion rate is below 0.5 g/m²·d and no attack can be recognized in neutral salt solutions. The metal also resists attack by boiling 2O% KOH. Strong attack, however, it caused bv Fe-III-chloride. The metals do not resist attack by HF, while neutral KF attacks but little, however produces pitting. The effect of alloying additions is due to the structure produces by them: pure grain boundaries or very fine heterogeneous structures show good corrosion resistance. Ti, Hf, Sn and small amounts of Nb have little influence, Mo, W, larger amounts Nb and small pt and Re additions give rise to Local cells, while Fe, Cr and Ni have practically no influence.