A review of the high temperature oxidation of iron- manganese- aluminium alloys

Research on the oxidation behaviour of alloys from the Fe-Mn-Al system has been devoted primarily to the austenitic alloys with particular attention being paid to their mechanical properties. Information regarding their oxidation behaviour and scale morpology is sparse. There is very little on the behaviour of the ferritic alloys and these show great promise as heat resisting alloys, particularly with small additions of chromium. The oxidation behaviour of iron, manganese and binary iron-manganese alloys is reviewed in this paper, to clarify the behaviour of the more complex ternary alloys.     

难熔高熵合金NbMoTaWTi/Zr的高温氧化行为

采用真空电弧熔炼法制备NbMoTaWTi和NbMoTaWZr难熔高熵合金,分析合金组织结构与元素分布,研究两种合金从室温到 1 500 ℃的动态氧化行为以及 1 200 ℃的恒温氧化行为.结果表明:NbMoTaWTi合金主要由单一 BCC固溶体相组成,而 NbMoTaWZr合金则由BCC固溶体和富Zr相两相组成.两种合金在 700 ℃以上温度均发生了剧烈的氧化反应.相比较而言,NbMoTaWTi合金在 1 300 ℃以下的抗氧化能力优于 NbMoTaWZr合金.两种合金在 1 200 ℃恒温氧化时都以氧向内扩散为主,氧化 3 h后均发生了严重氧化.Ti和 Zr的添加均未发生选择性氧化现象,虽然与其他难熔金属氧化物形成复合氧化物层,但其致密度不够,阻止氧化能力不足.     

On the behaviour of minor active elements during oxidation of selected Ni-base high-temperature alloys

We have examined the distribution of active minor elements in the oxide scales developed by selected Ni-base alloys with commercial grades. Emphasis is placed upon Mn, La and Si in a chromia-forming alloy and Y in an alumina-forming alloy. Initially, La and Y have been segregated at free surfaces and then become constituents of the oxides in contact with the substrates. A continuous layer of MnCr₂O₄ is formed above La- and Si-modified inner chromia layer. Silicon has been homogenously distributed throughout the grain structure, however, some La is present as LaCr₂O₃ particles and most of the remainder has been segregated at grain boundaries. The results indicate that the collective effect of Mn, Si and La is to extend protection by chromia to temperatures in excess of 1000 °C. Yttrium in the alumina- forming alloy is found to predominantly segregate at grain boundaries of nanostructured oxide with improved mechanical strength.     

微晶化及活性元素效应在合金抗高温氧化中的协同作用

总结了目前广为研究的微晶化和活性元素效应在合金高温氧化过程中的作用，并讨论了二者的协同效应。微晶化和活性元素效应均能够降低合金的高温氧化速率，提高氧化膜的黏附性。然而，微晶化中晶粒的长大趋势和促进内氧化发生的缺陷对其实际应用造成不利影响，少量活性元素的添加恰好弥补微晶化的这些缺失；活性元素添加须适量，这点限制了它作为保护性氧化膜形核中心的数量及降低氧化速率的幅度，且使得合金氧化增重较未添加活性元素试样略有增加，微晶化则不存在这样的问题，可以弥补应用活性元素效应的不足之处。本工作旨在加深对微晶化和活性元素协同效应对合金高温氧化性能影响的微观理解，为制备性能更为优异的抗高温氧化合金提供理论和实践基础。     

Effect of dendritic orientation on the high temperature oxidation kinetic of tantalum-containing nickel-base alloys

The aim of this work is the study of the possible effect of surface dendritic orientation on the high-temperature oxidation behavior of two chromia-forming Ni-base alloys with two tantalum contents (3%_wt and 6%_wt). Samples were prepared by special cutting orientation with regard to the external surface. Thermogravimetry tests were run at 1,000 °C, 1,100 °C and 1,200 °C and parabolic oxidation constants were determined taking into account the effect of the transient oxidation. When the sample surface is mainly parallel to the dendritic network, the oxidation parabolic constant can be higher than for a surface perpendicular to dendrites. It occurs only for temperatures that depend on the alloy. Inversely, the transient oxidation linear constant is generally higher for the perpendicular orientation than for the parallel one.     

Load and temperature interaction modeling of fatigue crack growth in a Ni-base superalloy

An approach was developed to predict the thermo-mechanical fatigue crack growth rates under typical gas turbine engine spectrum loading conditions. The material studied in the development of this model was a polycrystalline superalloy, Inconel 100. Load interaction effects were determined to have a major effect on the crack growth life. A yield zone load interaction life prediction model was modified to include temperature dependent properties. Multiple overload effects were included in the model to incorporate enhanced retardation compared to single overload retardation behavior. Temperature interaction effects were included and proved to be very important because of the wide temperature ranges to which turbine engine components are subjected. The effects of oxidation and temperature changes were accounted for in the model by accelerating crack growth in regions that had been previously affected by elevated temperatures. Experimental data of isolated, first order effects were used to calibrate and verify the model. Temperature dependent mechanical properties were determined and were essential in the model’s development. Parametric studies were performed using this model to assess the sensitivity of specific crack growth variables on life predictions.     

High temperature oxidation behavior of Ti-Al-Nb ternary alloys

The oxidation behavior of four Ti-Al-Nb ternary alloys with different microstructures were investigated at 1000°C using interrupted oxidation test in air. Alloys with single-phase -TiAl, two-phase -TiAl + ₂-Ti₃Al, multi-phase -TiAl + ₂-Ti₃Al + Nb₂Al and two-phase ₂-Ti₃Al + Nb₂Al were prepared. The oxidation resistance of the Ti-Al-Nb ternary alloy at high temperature was found to be better than that of the binary Ti-Al alloy. Among the four Ti-Al-Nb ternary alloys, the + ₂ two-phase alloy has the best oxidation resistance. The presence of Nb-enriched phase such as Nb₂Al and Nb₃Al decrease the oxidation resistance at elevated temperature presumably due to the formation of Nb₂O₅, which would accelerate the exfoliation of oxide.     

Recovery of high temperature deformed Ni-Al alloys

The flow-stress recovery of two high purity nickel aluminium alloys, containing zero and 5vol% (Ni₃Al), deformed at 735° C has been studied for recovery times ranging from 0.1 to 100 h. Results show that the presence of a finely dispersed second phase does inhibit the recovery rate following high-temperature deformation. For short recovery times, t<5 h, neither alloy obeys the predictions of the diffusion-controlled dislocation networkgrowth recovery model. For longer times, the recovery of the solid solution alloy does agree with the model while that of the precipitation-hardened alloy continues to deviate.The short-time recovery tests were used to obtain values for the recovery rate, r=d_i/dt and these values of r are used to predict values of the hardening rate, h=d_i/d, from the relation where is the steady-state creep rate. The values of h obtained in this manner are in reasonable agreement with expected values, i.e. h /100 where is the shear modulus. Electron microscopy observations on as-deformed and recovered samples show good agreement between the measured changes in flow stress and the observed variations in dislocation density.     

High temperature internal oxidation behaviour of dilute Ni-Al alloys

The internal oxidation of dilute Ni-Al alloys, either pure or containing small additions of tantalum or hafnium, was investigated at 1200° C in air. The advance of the precipitation zone followed a parabolic relationship at a slightly decreasing rate with increasing aluminium content. The presence of active elements had no appreciable effect on the growth rate. The precipitate shape, size and spacing depended upon the aluminium concentration and more significantly on the active element additions. The morphology varied from polyhedral crystallites and epitaxial platelets in the Ni-0.5 wt% Al alloy, to well defined cylindrical rods extending across the precipitation zone approximately normal to the reaction interface, in the alloys of higher aluminium content. An also continuous and similarly oriented plate-like morphology was observed in the active element-containing alloys. X-ray microanalysis indicated that the continuous precipitates consisted of NiAl₂O₄ and Al₂O₃. The former phase comprised approximately 65±5% of the outer region of the precipitation zone. Aluminium depletion in the alloy ahead of the precipitation front and a consequent enrichment in the form of oxide within the reaction zone inferred that growth was controlled by simultaneous outward aluminium and inward oxygen diffusion. No correlation was found between either the growth rate or oxygen permeability and the distribution of the precipitates. It was, therefore, concluded that the interfacial boundaries were ineffective in accelerating oxygen transport at this elevated temperature.     

The high temperature oxidation of CoCrAl alloys containing yttrium or hafnium additions

The effect of small amounts of yttrium (up to 1 wt.%) or hafnium (up to 1.5 wt.%) on the oxidation behaviour of CoCrAl alloys in the temperature range 1000–1200°C for times of up to 1000 h in air was studied. Both isothermal and cyclic tests were carried out; the cycle used consisted of 20 h at a high temperature after which the specimen was withdrawn from the furnace and was allowed to cool. The major part of the study was concerned with Co10 wt.% Cr11 wt.% Al alloys.In isothermal oxidation, increasing the yttrium decreased the overall weight gain, principally by decreasing the length of the initial transient period before the establishement of steady state kinetics. Hafnium also decreased the rate, with the lowest concentrations (0.05 and 0.1 wt.% Hf) having the greatest effect; further increases produced an increase in oxidation rate. In thermal cycling tests, increasing the yttrium again progressively improved the oxidation behaviour. Hafnium also produced an improvement, but in contrast with the isothermal behaviour a minimum of 0.3 wt.% Hf was required to produce a significant effect; further increases resulted in a small increase in oxidation.Metallographic examination suggested that the improved scale adhesion is due principally to a pegging mechanism; the active elements promote the growth of intrusions of Al₂O₃ into the metal. However, if the intrusions are too large, they can act as initiators of scale failure.Overall, hafnium additions appear to be preferable to yttrium additions.     

Behaviour of Al-Mg alloys at high temperature

The behaviour of a few Al-Mg alloys (up to 7.1 wt% Mg) has been studied on heating in air at 500°C for 8 h or more. The precipitation of a spinel phase, MgAl₂O₄, is found to take place on the surface and along the transverse section (depth) of the specimen. The alloys seem to be resistant to internal oxidation below 4.5% Mg and thereafter prone to it. Beryllium modification does not suppress the progress of internal oxidation taking place in the Al-Mg alloys. The results have been interpreted on the basis of anodized photomicrographs and X-ray diffraction data.     

Development of oxidation resistant high temperature intermetallics

Abstract

The phase NiAl with its wide homogeneity range, and especially when of high Al content, is a reliable alumina scale former and exhibits remarkable oxidation resistance at high temperatures. By contrast, the line phase NbAl₃ oxidises with formation of mixed layers of Al₂O₃ and fast growing oxides. However, the mechanical strength of NiAl is not sufficient for application at high temperatures, but the strength is enhanced in multiphase alloys in the systems Ni–Al–Cr and Nb–Ni–Al. Increasing the Cr content of two phase NiAl–Cr alloys reduces the oxidation resistance, but for three phase NbAl₃–NbNiAl–NiAl alloys the oxidation resistance is comparable to that of NiAl, since the Al content of the phases NiAl and NbNiAl is high in the phase equilibrium.

MST/1570     

Endless recrystallization of high-entropy alloys at high temperature

In traditional physical metallurgy, once recrystallization occurs, it will proceed to 100% along with time even at relatively low temperatures, resulting in the limited thermal stability of partially recrystallized alloys. Here, we proposed the strategy of achieving the endless recrystallization state at high temperature(～0.6Tm) in high entropy alloys for the first time. The partially recrystallized microstructures remained stable after annealing at 700 °C for 1440 h toward endless recrystalliza...     

Fe-Cr合金钢高温性能研究

本文试验研制了四种Fe-Cr合金材料,其在900 ℃时的热膨胀率在10～11×10-3范围内,其中FeCr-4合金的热膨胀率为10.27×10-3,与钇稳定氧化锆(YSZ)相比,热膨胀率逐渐接近;与传统合金材料相比,热膨胀率之差均有了明显下降,因而降低了界面热应力,有效改善层间的热失配现象.随着温度的升高,各合金试样的电阻率呈平缓增大的趋势.在850℃时,四种合金试样的电阻率在0.95～1.17×10-3 Ω·cm范围内,其中Fe-Cr-4试样的电阻率较低,为1.17×10-3 Ω·cm,满足用作SOFC连接体材料导电性能的要求.Fe-Cr -1、4两种合金试样的抗氧化性能较好,随着氧化时间的延长,试样表面形成稳定的3Cr2 O3 Fe2 O3氧化层,氧化增重逐渐趋于稳定.累计氧化455 h后,Fe-Cr-1、4两种试样对应的氧化增重在0.0002 g/g左右;而Fe-Cr-2、3两种试样仍在0.0004 g/g以上.     

Nickel based hardfacing alloys for high temperature applications

Abstract

Conventional nickel based hardfacing alloys deposited by arc welding usually have a nominally single phase microstructure and derive their hot strength primarily from solid solution strengthening. The present work is an attempt at designing improved alloys containing large volume fractions of ordered precipitates or intermetallic compounds. The alloy design has been carried out using a computer model capable of estimating microstructure and strength as a function of many variables. The results are tested experimentally against cast samples which simulate welding conditions during manual metal arc welding.

MST/1035     

Low temperature oxidation of copper alloys—AEM and AFM characterization

Oxidation kinetics and oxide structures of three polycrystalline copper grades at different temperatures were studied by analytical transmission electron microscopy (AEM) and atomic force microscopy (AFM). The copper samples were oxidized in air at 200 and 350 °C for 1–1,100 min. AEM and AFM studies indicated that alloying and increase of temperature, accelerated oxidation. At 200 °C local oxidation was observed in the unalloyed copper samples while a uniform oxide layer formed on the alloyed coppers. At 350 °C a uniform oxide layer formed on all copper samples. The oxide structure was nanocrystalline cubic Cu₂O after all oxidation treatments at 200 °C and after 5 min oxidation at 350 °C. After 25 and 100 min oxidation at 350 °C the crystal size of copper oxide had grown and the oxide structure was monoclinic CuO.     

NiAl高温形状记忆合金的组分分析

采用提拉法无污染磁悬浮冷坩埚技术生长出NiAl合金,具有马氏体结构,NiAl合金的组分对其结构和相变温度有着重要的影响,然而其组分分析受Ni,Al难以分离的困挠,针对这一现状,对终合置换法,沉淀滴定法与电子探针（EPMA）仪器分析法进行了比较,综合分析表明,络合置换化学分析法对NiAl基合金的组分分析是一种简便,准确的分析方法。     

Studying the role of the alloy-grain-boundary character during oxidation of Ni-base alloys by means of the electron back-scattered diffraction technique

Abstract

Most polycrystalline steels and nickel-base alloys exhibit a tendency to preferred oxidation attack along the alloy grain boundaries during exposure to corrosive atmospheres at high temperatures. This is due to the much faster intergranular transport of the reacting elements in combination with easy nucleation of precipitates. In general, Ni-base superalloys exhibit a superior oxidation resistance compared to low-alloy steels due to the presence of elements with very high oxygen affinity, e.g., Cr, Al and Ti, which are often responsible for pronounced intergranular oxidation. Oxidation tests on the Ni-base superalloy IN718 were carried out at temperatures between 850°C and 1000°C using thermogravimetry supported by analytical scanning electron microscopy in combination with EBSD (electron back-scattered diffraction). Evaluation of oxidation kinetics have revealed that special grain boundaries with a high fraction of coincident lattice sites (low Σ values) seem to exhibit a higher resistance to intergranular attack as compared to random high-angle grain boundaries. Hence, grain-boundary engineering might be a promising way to improve high-temperature oxidation resistance.