Cyclic Oxidation of Nickel-Base Alloys and Heat-Resisting Steels

Abstract

The weight changes of alloys, based on nickel-chromium and nickel-iron-chromium oxidised in air for up to 1000 hours under cyclic temperature conditions have been determined and related to the microstructures of the alloys after exposure. Initial oxidation to form a protective scale occurred rather faster than predicted on the basis of isothermal oxidation data; subsequent spalling of the scale was reduced by high contents of nickel and chromium, and by minor additions of silicon, cerium and aluminium.     

Influence of Chromium Carbides on the High Temperature Oxidation Behavior and on Chromium Diffusion in Nickel-Base Alloys

Two simple cast nickel alloys Ni–30Cr–0.2C and Ni–30Cr–0.8C were oxidized at 1,000, 1,100 and 1,200°C. Their behaviors were characterized using thermogravimetry techniques and their parabolic and chromia volatilization constants were determined by analyzing the versus −m curves. The constants obtained were generally in good agreement with experimental mass-gain kinetics and chromium-balance aspects. A higher carbon content i.e., a higher carbides density leads to higher values of the transient linear-oxidation constant K _l , of the parabolic constant K _p , and obviously also of the volatilization constant K _v . Chromium diffusion coefficients through the zone affected by oxidation were calculated from the oxidation kinetics and chromium gradients. They increase when the carbide density increases.     

Intergranular Corrosion of Nickel Alloys (Review)

The role of impurities and alloying components in intergranular corrosion (IGC) of nickel alloys is considered in relation to their composition and redox properties of the medium. The mechanism of IGC of single-phase alloys and alloys containing second phases is discussed, and the methods of enhancing their resistance to the IGC are considered.     

Intergranular Energy of Iron and Some Iron Alloys

The energy of the γ-iron grain boundary was determined to be 850 ergs per cm² at 1105°C. The α/α and the α/γ boundaries possess somewhat less energy. The microstructures of several iron alloys are discussed in terms of the interfacial energy relationships.     

On the Mechanism of Intergranular Corrosion of Aluminum Alloys

It is found that intergranular corrosion can proceed on aluminum alloys in either of two forms. The first is characteristic of low-alloy or pure aluminum; it is called crystallographic and is controlled by recrystallization process. High-angle boundaries of growing recrystallized grains accumulate numerous vacancies which partly coagulate. This causes thermodynamical instability of the boundaries, and their electrode potential approaches the potential of active corrosion. The second form characteristic of medium- or high-alloy aluminum is called structure-decomposition and is controlled by peculiarities of the decomposition of supersaturated solid solutions.     

Modelling of High Temperature Oxidation of Alumina-Forming Single-Crystal Nickel-Base Superalloys

Microstructure evolution in single-crystal superalloys is modelled as the interplay between oxide growth and substrate response. This is of particular importance for specimens with reduced wall thicknesses, where the affected substrate constitutes a significant fraction of the material. A model based on thermodynamic and kinetic data only is presented in order to predict the growth kinetics of oxides and the resulting influence on microstructure evolution of the substrate. The present work focuses on alumina (Al₂O₃) growth as it is the most important oxide regarding long-term behaviour. Al₂O₃ growth is described using a dynamic growth parameter which is derived using thermodynamic and kinetic principles. The substrate response model calculates the distribution of the alloying elements as well as the evolution of the phase fractions as a function of depth and oxidation time. The model has been applied on the strong alumina-forming alloy René N5 and the weak alumina-forming alloy SCA425+. Since γ′ fraction is one of the most relevant factors for high temperature creep properties, the present work concentrates on the calculation of the time- and space-dependent γ′ precipitate fraction profile. The model predictions were verified with very good agreement with respect to Al₂O₃ growth, element distribution and γ′ fraction distribution.     

Oxidation of Heat-resistant Alloys

The oxidation performance of three novel, heat-resistant alloys, namely KHR35C HiSi, KHR45A LC, and UCX, was studied at relatively low temperatures (i.e. 650 and 750 °C) for 1,000 h. The study was focused mainly on exploring oxide-layer growth and characterizing the oxide phases in order to prevent or minimizing metal dusting in carbonizing environments. The specimens were examined by visual and metallographic examination, SEM/EDX, XRD, and weight-change measurements. The results have been compared with a previous, short-term study (100 h) in order to understand the influence of exposure time on oxidation. It is concluded that exposing the alloys to air at 650 and 750 °C led to the formation of oxides of chromium, nickel, iron, silicon, and iron-containing spinels. Moreover, increasing the exposure time, temperature or both resulted in further oxide growth, leading to more continuous, adherent, and thicker oxides. The alloys, however, did not form a completely protective scale, especially at 650 °C, even after 1,000 h of exposure.     

Internal Nitridation of Nickel-Base Alloys. Part II. Behavior of Quaternary Ni-Cr-Al-Ti Alloys and Computer-Based Description

Whereas in Part I of this study the process ofinternal nitridation was described for binary andternary alloys within the Ni-Cr-Al-Ti system, this partfocuses on quaternary Ni-Cr-Al-Ti alloys, which are similar to commercial Ni-base alloys used inhigh-temperature applications regarding their chemicalcompositions. These alloys can simultaneously form twodifferent nitride-precipitation zones consisting of TiN and AlN. In order to quantify thenitridation process, thermogravimetric measurements inan oxygen-free nitrogen atmosphere in the temperaturerange 800-1100°C were carried out and supplemented by extensive microstructural studies. Whilesingle-nitride internal nitridation can easily bedescribed by Wagner's theory of internal oxidation,modeling of the more complex internal-precipitationreactions that involves more than one nitride requires anumerical treatment of both the diffusion and thethermochemical processes in the alloy. For this purpose,a computer simulation was developed in which the commercial thermodynamic software ChemApp iscombined with a finite-difference diffusion calculation.It was shown that this calculation technique can beapplied successfully to quantitatively describe the internal-nitridation process of theNi-Cr-Al-Ti model alloys used in this study.     

Oxidation Characteristics of Magnesium Alloys

Magnesium oxide, MgO, has a number of features that make it useful in a variety of applications; however, it does not offer protection to magnesium alloys against thermal oxidation. Although the thin oxide films, formed at ambient and elevated temperatures, create a diffusion barrier, they are sensitive to environmental factors and often experience severe damage. The thick oxide morphologies, formed at high temperatures, do not block the outward ion diffusion or, at a certain growth stage, also the inward penetration of gaseous species, leading to nonprotective oxidation. While additions of common alloying elements generally increase the oxidation rate of Mg-based alloys, doping of MgO with reactive-element atoms essentially improves its protective behavior.     

Ni-Cr-Mo-Cu合金耐晶间腐蚀及点蚀性能研究

在Cr、Mo成分一定的情况下,通过改变铜的质量分数(1％～5％),制备新型Ni-Cr-Mo-Cu耐蚀合金.利用化学浸泡法、电化学法(极化曲线法、循环伏安法)对其耐晶间腐蚀以及耐点蚀能力进行分析研究.实验结果表明,w(cu)=1％～5％的Ni-Cr-Mo-Cu合金具有优良的耐晶间腐蚀以及点蚀性能,相对而言w(cu)=2％的镍基合金耐晶间腐蚀以及点蚀性能较差,因此说明合适的铜含量可以提高镍基合金的耐晶间腐蚀和点蚀性能;并且试验合金晶间腐蚀与点蚀的电化学行为和特征与其浸泡腐蚀的结果是吻合的.     

Internal Nitridation of Nickel-Base Alloys. Part I. Behavior of Binary and Ternary Alloys of the Ni-Cr-Al-Ti System

The internal-nitriding behavior of several modelalloys of the Ni-Cr-Al-Ti system in an oxygen-freenitrogen atmosphere at 800-1100°C was studied.Thermogravimetry as well as various metallographic techniques (SEM and TEM) were used. It wasshown that both the nitrogen solubility and the nitrogendiffusion coefficient are strongly affected by the Crcontent of the Ni alloy. Hence, in Ni-Cr-Ti alloys a higher chromium content leads to an increaseddepth of the internal precipitation of TiN. Nitridationof the alloying element Cr takes place only at highconcentrations of Cr. In general, the nitridation rate was found to obey Wagner's parabolic ratelaw of internal oxidation. Changes in the parabolic rateconstant with alloy composition can be understood bymeans of thermodynamic calculations in combination with microstructural observations.     

Oxidation Resistance of Cast Magnesium Alloys

The purpose of the present study was to examine the oxidation of Mg–Al alloys in the atmosphere of oxygen at various temperatures. The basic examination methods used were differential scanning calorimetry (DSC) and thermogravimetry (TG). The results show that the oxide layer can either protect the material against progressive oxidation or it can cause complete disintegration of material. The nature of the oxide layer formed depends on the external conditions, i.e. on the atmosphere, temperature, and type of alloy. A model of the Mg–Al alloy oxidation was calculated from the TG curves.     

Thermal Oxidation Behavior of TiNi Alloys

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纯镁的高温氧化特性研究

通过差热分析法和氧化动力学曲线测定法对纯镁的高温氧化特性作了详细研究,结果表明,纯镁在低温下氧化缓慢,更不会发生燃烧,只有当温度上升至873 K以上近熔点温度范围时,与氧的反应才会剧烈起来,因此建议阻燃镁合金的开发应将重点放在近熔点温度范围内表面保护膜的生成热力学、动力学及其组织结构上,这对阻燃镁合金的进一步发展具有指导意义.     

湿热条件下超塑性锌铝合金的晶间腐蚀研究

评估了超塑性锌铝合金在高温高湿环境下晶间腐蚀的程度.在铝含量为(1～22)mass%的范围内,耐蚀性呈不单调变化.退火有利于提高耐蚀性.退火温度达300℃时,耐蚀性提高最多.晶间腐蚀深度由富铝相的百分比和晶粒细化两大因素综合决定.富铝相加速腐蚀,而晶粒的细化作用则阻碍腐蚀.晶间腐蚀深度随腐蚀时间延长呈线性增长.