Oberflächenschutzschichten für metallische Hochtemperatur-Werkstoffe. Teil 1: Entwicklungstendenzen,Prüfung und Herstellung von Schutzschichten

Protection of High Temperature Metallic Materials against Oxidation . Part 1: Development Trends, Testing and Preparation of Protective Layers. A comparison is given of the chemical compositions of industrial Co- and Ni- high-temperature alloys used in the past years. Due to their relatively low Cr and Al contents the more recent super-alloys require additional oxidation protection. Theoretically, the refractory metals would allow much higher working temperatures than the super-alloys, but alloys based on such metals need a much more perfect oxidation protection. The fabrication testing and methods for such protective layers are briefly reviewed. The second part of this paper will describe some specific protective layers for super-alloys and refractory metals.     

Gradient-type sensors for heat flux measurements high temperatures

New gradient-type heat flux sensors (GHFSs) are created, the thermal stability of which is increased due to the use of refractory metals and alloys forming a heterogeneous layered macrostructure. The new GHFSs allow the heat flux measurements to be performed at temperatures up to 1300 K.     

Noise-pulse method of measuring the heat capacity of thin wire specimens

Heat capacity is determined by calculating the number of zeros in random temperature fluctuations induced in the specimen by a source of white noise and the number of zeros in the white noise of a reference transducer. The method is also used to measure absolute temperature and the temperature derivative in heart transfer. It was developed to study the heat capacity of thin wire specimens of refractory metals and alloys at high temperatures. Translated from Izmeritel'naya Tekhnika, No. 10, pp. 60–62, October, 1998.     

难熔高熵合金制备及性能研究进展EI北大核心CSCD

本文简述了难熔高熵合金的含义与特点,归纳了各类难熔高熵合金(块体、薄膜、涂层)的制备方法,重点阐述了难熔高熵合金的综合性能。建议通过构建专门的难熔高熵合金数据库优化成分设计,并着重于不同制备方法的工艺性研究。针对目前难熔高熵合金存在室温脆性大、密度大、成本高等不足,提出可根据所需难熔高熵合金的性能而选择不同的制备方法,以便未来工业化应用。     

多主元难熔高熵合金的研究进展

近年来,高熵合金(HEAs)因其新颖的设计理念和优异的综合力学性能成为了新材料领域的研究热点之一。作为HEAs一个重要分支的难熔高熵合金（RHEAs）由于将高熔点难熔元素作为主要合金成分而具有优异的高温抗软化性能。难熔高熵合金在高温下具有良好的相稳定性,有望成为新型高温结构材料。相比于传统的高温合金,难熔高熵合金的成分范围更广,密度区间更大,抗氧化性也更好。在过去的十余年中,难熔高熵合金的研究已经取得了很大进展。许多合金和合金体系都已经进行了广泛的测试和表征,包括力学性能和氧化行为等方面,有关固溶强化、变形机制和氧化行为的新模型也正在出现并不断完善。计算机构建模型和模拟计算也逐渐应用于难熔高熵合金的研究,促进了难熔高熵合金的开发和发展。主要介绍了难熔高熵合金的成分设计,对比分析了其制备工艺和相组成,并讨论了其室温和高温时的力学性能及高温抗氧化性。最后总结了难熔高熵合金研究目前存在的问题和瓶颈,并对未来研究方向提出了建议。     

High temperature resistivity determination of high-melting point metals and alloys

A method of determination of temperature-dependent electrical resistivity for metals and alloys of high-melting points at high temperatures is presented. It is based on the computer simulation of wire heating by direct current and basic electrical measurements. The authors present new results for 25Re75W and 47.5Re52.5Mo alloys along with the results of test measurements for tungsten. Electrical resistivity for tungsten obtained with presented method is in good agreement with well known data for the metal.     

难熔高熵合金的研究进展及应用

高熵合金不同于传统工程合金,是由多种元素以等摩尔或近等摩尔的比例混合,形成的以简单固溶体结构为基体的系列成分复杂合金。其中含高熔点元素的难熔高熵合金具有较高的高温强度和优异的高温抗氧化性能及耐蚀性能等突出特点,其潜在的高温应用价值引起了广泛关注。详细阐述了难熔高熵合金的研究现状及应用,根据晶体结构类型将难熔合金体系进行了分类,并对各类体系中的微观组织特征进行了概述;进而归纳总结了难熔高熵合金的各种性能,包括高强度、耐磨性、高温抗氧化性、耐蚀性能等;最后对难熔高熵合金的发展及应用前景进行了展望。     

Erosion of steels and alloys in their collision with a flux of abrasive particles

We describe a procedure and present results of an investigation of gas-abrasive erosion of metals and alloys at temperatures of 293-80 K.     

Considerations on the selection of alloys for use in pressure cells at low temperatures

Issues surrounding the choice of metals subject to large tensile stresses in a cryogenic environment are discussed. Problems with the use of materials under such conditions often arise from their loss of ductility and toughness. The focus of the present article is therefore mainly on these properties; their dependency on temperature, crystal structure, grain size and purity level; and their application to the quantitative prediction of fracture behaviour. It also discusses the selection of alloys (on the basis of their toughness, ductility and freedom from flaws) that can be used safely under ambient and cryogenic conditions. A recent proposal to employ a particular titanium alloy for the above purpose is considered, in light of the tendency of metals with a bcc crystal structure (including β-phase titanium alloys) to become brittle at low temperatures. Other materials, which are known to be (or are likely to be) satisfactory under such conditions are examined. For example, certain platinum alloys may possess very high cryogenic tensile strengths and ductilities, and low magnetic susceptibilities. Nanocrystalline metals have the potential to provide these characteristics, as well as low specific heats and high thermal conductivities. Such properties could make nanocrystalline materials very useful in (for example) work at microkelvin temperatures, or more generally where very accurate cryogenic temperature control is needed. Sources of cryogenic mechanical property information on a wide range of materials are also presented.     

高纯难熔金属及其合金单晶的发展

介绍了难熔金属及其合金单晶的制备技术,对电子束悬浮区域熔炼技术和等离子弧熔炼技术进行了比较。电子束悬浮区域熔炼法温度梯度易于控制、材料不受坩埚材料污染,但熔体表面张力对活性杂质和温度梯度敏感性高,所能制备的高纯难熔金属及其单晶材料尺寸规格受到很大限制,且材料内部位错密度较高。等离子弧熔炼法加热源能量密度高,原料规格形式多样,可制备单晶棒材、板材、管材及其他特定形状的单晶铸件,可最大程度地去除杂质元素（尤其是C元素）,但设备系统复杂,单晶材料位错密度大。讨论了单晶材料发展现状,通过固溶强化可进一步提高材料的高温性能及其稳定性。单晶管材的制备也是一个发展方向。此外还对单晶材料制备技术和发展前景提出了一些建议。     

High-Temperature Physicochemical Mechanics of Materials

We propose to regard investigations in the field of high-temperature strength of structural metals and alloys interacting with corrosive media as high-temperature physicochemical mechanics of materials (HTPCMM). The most important feature of HTPCMM is the principle of correlation between deformation processes and physicochemical phenomena, which allows one to describe the features and regularities of changes in the properties of materials under service conditions most completely and correctly. We emphasize the most important role of diffusion as a controlling factor in a metal–medium system at high temperatures. Results of analytical investigations aimed at the development and investigation of physicomathematical models of elastic and elastoviscous multicomponent solid solutions with inherent degradation processes (accumulation of damage) are presented. The fact that, as a rule, these models are constructed within the framework of continuum mechanics on the basis of principles of nonequilibrium mechanics is noted Experimental data obtained, in particular, on refractory metals and titanium interacting actively with components of a vacuum or an inert atmosphere testify to the intensification of saturation of metals by interstitial impurities under conditions of long-term loading and to significant changes in the character of their creep, namely, under the influence of oxygen diffusion the creep rate decreases as stresses increase.     

Manufacturing of Mo-25 Re and Mo-50 Re Alloys by Means of Powder Sintering at Medium Temperatures

Powder mixtures of molybdenum and rhenium were sintered at various temperatures to fabricate Mo-25% Re and Mo-50% Re alloys. For this purpose, stoichiometric mixtures of the component powder metals with particle sizes of 1 to 5 μm average, were first homogenized in a planetary mill and then compacted at 300, 400, 500 and 600 MPa, to the shape of tablets and small billets for rolling to thin foils. The obtained pieces were subjected to sintering at various temperatures in the range of 1400 to 1700°C, under argon-hydrogen protecting atmosphere. The solid solution of rhenium in the molybdenum matrix was followed by means of x-ray diffraction. It was found that at all sintering temperatures rhenium was totally dissolved in theMo-25% Re alloy, while small amounts of an intermediate phase were detected in the case of the Mo-50% Re alloy, in good agreement with the corresponding phase diagram. The densification of sintered parts increased with temperature and compaction pressure up to values less than 90%. Owing to the fine particle size of the powders, the sintering temperature can be lower than those normally utilized in the refractory metals industry.     

Some Characteristics of Automotive Exhaust Valve Alloys

The PH stainless steels and the nickel-base superalloys, can be evaluated for exhaust valve applications by considering their metallurgical, environmental and high temperature strength properties. The PH stainless steels are characterized by their good sulfidation and high temperature strength. Good PbO corrosion resistance is achieved with the low silicon, nickel containing alloys. Stable alloys show the greatest high temperature strength which can be improved further by a solution and age treatment. Aging below the optimum temperature of 760°C results in grain boundary sensitization and low impact properties while higher temperatures produce more of the discontinuous phase. The addition of refractory elements can be detrimental to the oxidation resistance of these alloys. The highest elevated temperature strength and best PbO and oxidation resistance is achieved with the nickel-base superalloys. These alloys are completely stable and highest strength at elevated temperatures is achieved with a solution and age treatment. These alloys show lower sulfidation corrosion resistance relative to the PH stainless steels, however this can be improved with higher chromium contents.     

Oxidation of chromia forming molybdenum-tungsten based alloys

The oxidation behaviour of tungsten and molybdenum based, chromia-forming alloys prepared by powder sintering activated with group VIII metals has been investigated. The influences of the alloy composition, nature of the sintering agent and oxidation temperature have been studied. A good oxidation resistance is observed with palladium as sintering agent. This metal is rejected at the grain boundaries and allows a fast diffusion of chromium to the metal-oxide interface. Contrary to palladium, nickel leads to a catastrophic oxidation of the sample. The formation of a two-phase interface enriched in nickel leads to a non-protective oxide layer constituted with Cr₂O₃ and NiWO₄.Catastrophic oxidation is observed when the refractory metals are oxidised into volatile oxides, i.e. in the case of the alloys with a high molybdenum content. Contrary to molybdenum, a high tungsten level leads to high oxidation resistance, even at temperature as high as 1300°C. In this latter case, alloys are two-phase: this result has led to the investigation of the ternary section of the Cr-Mo-W system at 1300°C.     

Specific Features of the Temperature Dependence of the Mechanical Properties of Some Transition Metals and Their Alloys

On the basis of the results of theoretical investigations confirmed by the experimental data, it is established that, at certain temperatures and strain rates, the interstitial atoms in transition metals and alloys based on these metals participate in the dynamic interaction with dislocations, which facilitates the process of hardening of these metals. We compute the temperatures of dynamic interaction for hydrogen, nitrogen, oxygen, and carbon in two modifications of iron and metals from the VA group. It is shown that these effects can be significant after holding in environments. Dynamic strain aging is also typical of alloys based on the investigated metals.     

High temperature corrosion of alloys containing rare earth or refractory elements: a review of current knowledge and possible future developments

The beneficial effects of minor (≤1%) additions of rare earth/reactive elements on the oxidation of HT alloys have been known for many years. Much interest has recently centred on the influence of certain refractory elements present at levels of several weight percent in a number of such alloys. The reasons why minor additions of these reactive elements, especially the rare earths, have beneficial effects have been the subject of systematic studies over the past 10–15 years. Ideas have been influenced by the understanding of the mechanisms of the development of protective chromia or alumina scales; in particular, the role such additions play in assisting the more rapid nucleation and growth of oxides on HT alloys, reducing the period of transient oxidation and the earlier attainment of steady-state scaling conditions. Much attention has also been paid to the mechanisms whereby such additions appear to promote improved scale adherence.

The various theories of high temperature corrosion of such alloys and supporting experimental evidence are reviewed in this paper. The reported information concerning the beneficial role or otherwise of the refractory metals, particularly with respect to hot corrosion in gas turbines, is appraised. Attention is drawn to the potentially beneficial effects of additions of certain of these Group III-VI elements in the development of novel alloys or coatings to resist corrosion in the complex environments of low oxygen potential, which occur in plants to produce synthetic natural gas from oil or coal. Areas for further research are also identified.     

高熵合金摩擦磨损性能的研究进展EI北大核心CSCD

近年来,高熵合金成为金属材料领域的研究热点。高熵合金处于相图中心区域,具有广阔的合金成分空间和组织结构形成可能;成分和制备工艺的协同调控,能够获得更丰富的组织结构;非常规的化学结构有望突破传统抗磨、润滑合金的性能极限。本文讨论了耐磨高熵合金的分类,分析了化学活泼金属、软金属、难熔金属的添加对高熵合金抗磨、润滑性能的影响规律;总结了非金属元素和陶瓷相的添加对高熵合金基复合材料摩擦磨损性能的影响;综述了热处理和表面工程技术对高熵合金表面组织结构和摩擦磨损行为的作用;讨论了苛刻工况下抗磨润滑高熵合金的设计方法。对未来高熵合金在摩擦磨损领域的研究和应用进行了展望,高熵合金在解决传统合金的瓶颈问题上具有巨大潜力,如在极端工况下实现稳定润滑抗磨、保证特定功能作用下实现抗磨。     

Development of chromia forming Mo-W-Cr alloys: synthesis and characterization

The sintering process and microstructural characteristics of tungsten-based and molybdenum-based alloys containing chromium and group VIII metals as sintering agents have been investigated. The influences of the alloy composition, the nature of the sintering agent and the synthesis process on the microstructure and microhardness of these materials have been studied. Homogeneous alloys can be obtained with palladium or nickel as the sintering agent. The mechanisms are totally different with these two metals. In the case of nickel, diffusion of the refractory metals through a nickel layer is responsible for the densification of the alloys, whereas with palladium, a CrPd liquid phase at the grain boundaries leads to homogeneous sintering. Formation of the CrPd phase is directly dependent on the chromium content wich influences the solubility of palladium in the MoW matrix. Consequently, a low chromium content leads to a high CrPd content, and to an increase in the grain size. On the contrary, with a nickel sintering agent, ahigh Ni content leads to an increase in thickness of the interdiffusion layer and thus a decrease in the grain size. Sresses generated by uniaxial sintering and mechanical alloying are not released during the annealing sequence and contribute to increase the microhardness of the alloys. Microhardness is also a strong function of the tungsten content.