Mechanical stresses developed in high temperature resistant alloys during isothermal and cyclic oxidation treatments: The influence of yttrium additions on oxide scale adherence

Deflection measurements on thin foils of refractory alloys protected on one side by SiO₂ were made. The protection by a crystallized silica layer is effective up to 1100°C. Deflection values allowed the stresses created in the oxide scale to be estimated. Under all oxidation conditions, compressive stresses appear in the oxide scale. Yttrium additions decrease these stresses by increasing the concentration of oxide vacancies.     

The influence of yttrium on oxide scale growth and adherence

Alloys and coatings for high-temperature service are designed to form selectively chromia scales, alumina scales, or, to a limited extent, silica scales upon exposure to the environment. For such oxide scales to be protective, they should be both slow growing and adherent. It turns out that the addition of yttrium to such alloys can often impart both characteristics to the oxide scale. However, the actual operating mechanisms continue to be a matter of controversy among researchers in the area of oxidation. In the present study, the growth and adherence of alumina and chromia scales on alloys containing yttrium, either as an oxide dispersion or as an intermetallic phase, have been investigated in conjunction with detailed oxide scale characterization using the techniques of scanning electron microscopy (SEM), transmission electron microscopy (TEM), and secondary ion mass spectrometry (SIMS). The results of the study are used for critical assessment of the proposed mechanisms, especially the more recent ones, and to suggest some new mechanisms for adherence.     

On the influence of metal lattice diffusion on oxidation of metals and alloys

The influence of metal lattice diffusion on oxidation kinetics is discussed for two simple cases: (i) a pure metal, where vacancies generated at the scalemetal interface diffuse to sinks within the metal; and (ii) a binary alloy of metals A and B, with A forming the more stable oxide. In the first case it is shown that vacancy effects are generally negligible. Analyses suggesting the contrary have failed to replace atom concentration gradients by the more appropriate chemical potential gradients. For the alloy, Wagner's condition for breakdown of A oxide is confirmed. It is shown that growth of A oxide cannot be controlled by diffusion of A in the metal, if B atoms can react at the scale-metal interface; scale-breakdown intervenes.     

Some factors influencing the adherence of oxides on metals

Some of the differences in adherence behaviour may be explained by differences in the respective physical properties of the films and the metals. The strength and plasticity of some of the oxides has been measured; the relative expansion coefficients are also important and it is thought to be the favourable nature of this which is responsible for the good adherence of NiO on nickel and more complex films on nickel-base alloys. Another important factor is internal stress. The sources of stresses during the oxidation of copper have been estimated using a unilateral oxidation technique. With sufficient knowledge of the mechanical properties it is possible to calculate the actual stress levels in the growing oxide film. A further factor influencing adherence is the accumulation of trace or residual elements on the metalloxide interface. The technique of Peters and Engell has been used to estimate the effect of various elements at a concentration of 0.4 wt% on the adherence of scales on iron. The results fall into four groups; those that form mixed oxide scales with wüstite either spalled or were weakly adherent; those on noble metal alloys were nearly comparable with pure iron, but copper and tin had a detrimental effect. Scales containing As, Sb and P were weakly adherent but left a thin adherent layer on the metal surface. Cobalt and niobium had no effect.     

铝合金铸件壁厚对机械性能的影响

研究了铸件壁厚对ＺＬ１０１合金，ＺＬ１０４合金，ＺＬ２０５合金，ＺＬ２０１合金及ＺＬ２０５Ａ合金机械性能的影响。研究表明：这些合金随铸件壁厚的增加机械性能下降。ＺＬ２０１，ＺＬ２０５Ａ合金的机械性能受壁厚的影响比ＺＬ１０１合金，ＺＬ１０４合金和ＺＬ１０５合金的机械性能受壁厚的影响大。     

Length scale and time scale effects on the plastic flow of fcc metals

We examine size scale and strain rate effects on single-crystal face-centered cubic (fcc) metals. To study yield and work hardening, we perform simple shear molecular dynamics simulations using the embedded atom method (EAM) on single-crystal nickel ranging from 100 atoms to 100 million atoms and at strain rates ranging from 10⁷ to 10¹² s⁻¹. We compare our atomistic simulation results with experimental data obtained from interfacial force microscopy (IFM), nano-indentation, micro-indentation and small-scale torsion. The data are found to scale with a geometric length scale parameter defined by the ratio of volume to surface area of the samples. The atomistic simulations reveal that dislocations nucleating at free surfaces are critical to causing micro-yield and macro-yield in pristine material. The increase of flow stress at increasing strain rates results from phonon drag, and a simple model is developed to demonstrate this effect. Another important aspect of this study reveals that plasticity as reflected by the global averaged stress–strain behavior is characterized by four different length scales: (1) below 10⁴ atoms, (2) between 10⁴ and 10⁶ atoms (2 μm), (3) between 2 μm and 300 μm, and (4) above 300 μm.     

The influence of scale/metal interface characteristics on the oxidation behaviour of iron at elevated temperatures

The oxidation kinetics of cold-rolled Remko iron coupons in purified oxygen at atmospheric pressure in the temperature range 973–1173K and the morphologies of both oxide scales and subtrate surfaces were studied using thermogravimetric, metallographic and scanning electron-microscope techniques.Parabolic oxidation rate constants were found to decrease after initial varying periods of constancy, an effect believed to be due to void formation at the wüstite/iron interfaces by the coalescence of vacancies. The observed subsequent recoveries of the values of the rate constants were considered to be due to an increase in oxide growth stresses to values large enough to cause appreciable plastic flow of oxide, thereby decreasing the number of interfacial voids.The extent of the deviations from the initial values of the rate constants decreased as the oxidation temperature was increased and this was considered to be due to the enhanced plasticity of the scale and increased rate of stress generation which favoured the elimination of the interfacial voids.In the temperature range 973–1073K, there were periods in which logarithmic growth relationships were observed, the mechanism being similar to that suggested by Evans, and involving a continuous increase in the number of interfacial voids.     

Oxide coatings modified with transition and rare-earth metals on aluminum and their activity in CO oxidation

Oxide catalysts, which, in addition to Al₂O₃ + SiO₂, contain from one (Ni) to four oxides or compounds of transition metals (Ni, Cu, Mn, and Co) and oxides or compounds of rare-earth elements (Ce, La), are produced on D16 aluminum alloy by plasma electrolytic oxidation combined with impregnation and subsequent annealing. The composites formed begin to catalyze the CO oxidation in a temperature range of from 100 to 300°C. The catalysts used can be arranged in the following series of decreasing catalytic activity: Ni-Cu-Mn-Co-Ce > Ni-Cu-Mn-Co-Ce-La ≈ Ni-Cu-Mn-Co > Ni-Cu-Mn > Ni-Cu > Ni. Oxygen compounds of Cu⁺, Cu²⁺, Mn⁴⁺, Co³⁺, Ce³⁺, and Ce⁴⁺, which seem to determine the catalytic properties of the oxide systems studied, are found on the surface and in the subsurface layer with a total thickness of ～6 nm of the most active Ni-Cu-Mn-Co-Ce catalyst.     

Analysis on failures of protective-oxide layers and cyclic oxidation

A condition of oxide-scale spalling is derived from the concept of strain energy proposed by Evans. By considering the probabilistic distribution of fracture strain energy across the metal oxide interface, the fraction of oxide scale spalled is described as a function of the weight of oxide before cooling and temperature change. The cyclic-oxidation behavior is modeled with this result. The prediction by the model is shown to be in good agreement with data on the oxidation behavior under thermal cycling.     

In-situ deposition of silica layers during the operation of a micro-gas turbine and their effect on the oxidation of superalloy blades

Silica-based oxide layers were deposited in-situ on turbine blades made from Inconel 713 during the operation of a 13 kg_f-class gas turbine, and their effect on the ex-situ oxidation behavior of the blades at 1050 °C was examined. The two turbines were driven by burning liquid petroleum gas (LPG), one turbine at a rotation speed of 35,000 rpm for 4 h (TB04), and the other at 42,000 rpm for 8 h (TB08). For deposition, tetraethylorthosilicate (TEOS) was sprayed into the fuel line immediately ahead of the combustion chamber. The TEOS-to-LPG ratio for TB04 and TB08 was maintained at 5.4 wt. % and 2.3 wt. %, respectively. Directly after operation, the turbine blades were coated with silica layers to a thickness of ∼10 μm, independent of the operating conditions. These oxide layers on the blades provided excellent protection against oxidation during both operation and the ex-situ isothermal oxidation test.     

Mechanical properties and substructure of metals

The effect of the angle of misorientation of substructural elements, structure reversibility during plastic deformation, and the formation of microcracks on the surface of and within a metal on its mechanical properties are examined. It is shown that it is possible to define a degree of degradation (damage) of a metal using the true stress-residual strain diagram.Institute of Machinery Science. Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 10, pp. 12–17, October, 1994.     

Size effect and friction in cutting of metals on the small scale

An experimental study of the “size effect” in machining – substantial increase in the specific cutting energy with decrease in chip size – is described. An instrumented ultramicrotomy technique employing “ideally” sharp glass and diamond knives is developed to characterize the forces and specific energies in slow-speed orthogonal cutting of pure copper, with depths of cut in the range of 30 nm to a few micrometers. The increased role of friction and intermolecular adhesion at small-scale tool-chip contacts is shown to be the primary factor underlying the size effect. Implications for using machining chip-formation to explore the mechanics of adhesive (plastic) sliding contacts are discussed.     

High temperature cyclic oxidation of aluminide layers on titanium

Aluminide layers containing TiAl₃ or TiAl on the surface were formed on titanium samples using the pack-cementation process. Cyclic oxidation (1 hr at the desired temperature and 20 min at room temperature) was carried out over the temperature range 850–1000°C. TiAl surfaces showed poor oxidation resistance compared to TiAl₃. The oxide morphology showed a peculiar protrusion formation on TiAl₃ surfaces at lower temperatures. A cross-section of the oxidized sample showed discrete Al₂O₃ and TiO₂ layers.     

Effect of yttrium on the oxide scale adherence of pre-oxidized silicon-containing heat-resistant alloy

This paper investigates the effect of the rare earth element yttrium on the rupture behaviour of the oxide scale on the silicon-containing heat-resistant alloy during cooling. After 10 h of oxidation, yttrium is found to facilitate the formation of internal oxides (silica) at the scale–matrix interface. Due to the twinning observed by scanning transmission electron microscopy (STEM) in silica, the critical strain value for the scale failure can be dramatically improved, and the formation of cracks at the scale–matrix interface is inhibited.     

Mechanical behavior of nanocrystalline metals and alloys

Nanocrystalline metals and alloys, with average and range of grain sizes typically smaller than 100 nm, have been the subject of considerable research in recent years. Such interest has been spurred by progress in the processing of materials and by advances in computational materials science. It has also been kindled by the recognition that these materials possess some appealing mechanical properties, such as high strength, increased resistance to tribological and environmentally-assisted damage, increasing strength and/or ductility with increasing strain rate, and potential for enhanced superplastic deformation at lower temperatures and faster strain rates. From a scientific standpoint, advances in nanomechanical probes capable of measuring forces and displacements at resolutions of fractions of a picoNewton and nanometer, respectively, and developments in structural characterization have provided unprecedented opportunities to probe the mechanisms underlying mechanical response. In this paper, we present an overview of the mechanical properties of nanocrystalline metals and alloys with the objective of assessing recent advances in the experimental and computational studies of deformation, damage evolution, fracture and fatigue, and highlighting opportunities for further research.     

Effect of mechanical stresses on heterogeneous oxidation of metals

The existence and influence of mechanical stresses, appearing at a metal-oxide interface, on the rate and regularities of metal oxidation is analyzed. The source of these stresses involves, in particular, the difference in thermal-expansion coefficients and densities of metal and oxide. From this point of view, the oxidation parameters of Al, Ti, Zr, and Mg are calculated. It is shown that the suggested approach well describes qualitatively, and in some cases quantitatively, the numerous experimental data concerning oxide-layer structures, complex multistep parabolic oxidation, transition from protective to nonprotective oxidation, etc. Results of the calculations indicate that for cylindrical vessels internal pressure may result in significant intensification of the oxidation process.     

镁合金微弧氧化陶瓷层显微缺陷与相组成及其耐蚀性

利用SEM，XRD及盐雾腐蚀等试验手段，研究了MB8镁合金微弧氧化陶瓷层生长过程中显微缺陷与相组成的变化规律及其对耐蚀性的影响。结果表明：微弧氧化初期，陶瓷层致密，几乎观察不到显微缺陷，随着处理时间的延长及陶瓷层的增厚，其外侧开始出现孔洞类缺陷，直至90％厚度范围布满相互交错的不规则孔洞；陶瓷层主要由MgO，MgSiO3，MgAl4O4和 非晶相组成，随着厚度的增加，陶瓷层中MgO的比例不断增加，而非晶相含量逐渐减少；短时间微弧氧化处理有利于制取艰非晶相为主的致密无缺陷的耐蚀陶瓷层。     

Characterization of the surface layers formed on titanium by plasma electrolytic oxidation

This paper is concerned with the surface modification of titanium by the PEO method (plasma electrolytic oxidation) in the solutions which contain Ca, P, Si and Na. The chemical composition of the thus formed surface layers was examined by XPS and EDS. The morphology of the surface was observed by SEM. The phase composition was determined by X-ray diffraction (XRD). The adhesive strength of the oxide layers was evaluated by the scratch-test. The corrosion resistance was determined in a simulated body fluid (SBF) at a temperature of 37 °C by electrochemical methods for various exposure times.The oxide layers obtained were porous and enriched with Ca, P, Si and Na and their properties depended on the electrolyte solution and the parameters of the oxidation process. The results of the electrochemical examinations show that the surface modification by PEO does not worsen the corrosion resistance of titanium after a 13 h exposure in SBF. The electrochemical impedance spectroscopy (EIS) results indicate that the surface layers have a complex structure and that their electric properties undergo changes during long-term exposures in SBF.