Mechanical properties of Ti-Cr-Nb alloys and prospects for high-temperature applications

Four Ti-Cr-Nb alloys containing the C15 Laves phase have been given a series of mechanical tests, including compression, bending, notch toughness, elastic moduli measurement, microhardness, and a simple impact toughness test. Strength remains high between 900 °C and 1000 °C; moduli and specific moduli exceed those of titanium; 1000 °C ductility is substantial; but roomtemperature notch toughness is low. Since room-temperature impact toughness is substantial in unannealed samples, the effects of heat treatment will need to be studied. Oxidation rates have been measured on two of the alloys.     

Effect of cerium content on the deformation behavior of rapidly solidified Al-Fe-Ce alloys

The deformation behavior of a rapidly solidified Al-8.9Fe-6.9Ce (wt pct) alloy was studied in the temperature range of 250 °C to 350 °C and stress range of 20 to 175 MPa. The stress exponents and activation energies suggest that the alloy exhibits a pronounced diffusional creep regime with a transition to power law creep behavior at stresses beyond 60 MPa. Comparing these data with those obtained earlier for an Al-8.8Fe-3.7Ce alloy, it was found that in the diffusional creep regime, the Ce content had no effect on the creep rate. However, in the power law creep regime, a strong dependence on the precipitate spacing, as predicted by the structureinvariant creep law,^[5] was observed. The higher volume fraction of precipitates in the Al-8.9Fe6.9Ce alloy causes a decrease in the power law creep rates by a factor of 5. Formerly Graduate Student. Formerly Assistant Professor, University of Illinois at Urbana-Champaign.     

Microstructure and high-temperature tensile deformation of tiai(si) alloys made from elemental powders

Two ternary TiAl-based alloys with chemical compositions of Ti-46.4 at. pct Al-1.4 at. pct Si (Si poor) and Ti-45 at. pct Al-2.7 at. pct Si (Si rich), which were prepared by reaction powder processing, have been investigated. Both alloys consist of the intermetallic compounds y-TiAl, α₂-Ti₃Al, and ξ-Ti₅(Si, Al)₃. The microstructure can be described as a duplex structure(i.e., lamellar γ/α₂ regions distributed in γ matrix) containing ξ precipitates. The higher Si content leads to a larger amount of ξ precipitates and a finer y grain size in the Si-rich alloy. The tensile properties of both alloys depend on test temperature. At room temperature and 700 °C, the tensile properties of the Si-poor alloy are better than those of the Si-rich alloy. At 900 °C, the opposite is true. Examinations of tensile deformed specimens reveal ξ-Ti₅(Si, Al)₃ particle debonding and particle cracking at lower test temperatures. At 900 °C, nucleation of voids and microcracks along lamellar grain boundaries and evidence for recovery and dynamic recrystallization were observed. Due to these processes, the alloys can tolerate ξ-Ti₅(Si, Al)₃ particles at high temperature, where the positive effect of grain refinement on both strength and ductility can be utilized.     

Composite powders for controlled abradability applications

Abstract

The use of composite powders for abradable seals is increasing and is a unique solution to the problem of maintaining pressure differentials in the flow of hot gases in high-performance turbine engines. This paper reviews the powder production methods and the properties of various commercial composite powders. The methods used to form these powders into abradable structures are outlined and the properties of these structures are discussed. Specific engine applications are detailed and the use of advanced composite powders in future turbine engines is outlined.

Résumé

Il y a accroissement de l'utilisation de composites frittés pour des joints d'étanchéité usables par abrasion; ceux-ci sont une solution unique au problème du maintien de différences de pression des gaz chauds dans les diverses sections des moteurs à turbine à haute performance. Les méthodes de production de poudre et les propriétés des divers composites frittés commerciaux sont pas sées en revue dans cet article. Les grandes lignes des méthodes utilisées pour transformer les poudres en structures usables par abrasion sont présentées et les propriétés de ces structures sont discutées. On présente en détail des applications à des moteurs particuliers et on esquisse l'utilisation de composites frittés plus avancés dans les moteurs à turbine de l'avenir.     

Physicochemical approaches to designing NiAl-based alloys for high-temperature operation

The structure and properties of new-type materials based on light refractory nickel monoaluminide NiAl as a structural material are analytically reviewed. The choice of various alloying systems and structural-phase states of NiAl-based structural materials, including structural materials, is analyzed, and the choice of the processes of production of the materials is grounded, as applied to their composition.     

Electrospark powders of shape memory alloys

Powders of Ti-Ni and Ni-Mn-Ga alloys are obtained by the electrospark method. It is established that the largest particles are of irregular shape and the smaller ones are spherical. Some spherical particles are hollow. No carbon contamination is found within the Ti-Ni and Ni-Mn-Ga powders obtained in cryogen liquids. The Ti-Ni powders of all compositions are mixtures of austenite and martensite phases. Preliminary heat treatment of spherical-particle Ni-Mn-Ga alloys is needed to activate their martensitic transformation. The martensitic transformation proceeds over quite a wide temperature range, which is indicative of high dispersion, and the martensitic temperatures increase, which is indicative of a minor difference between the chemical compositions of powders and massive samples. __________ Translated from Poroshkovaya Metallurgiya, Vol. 46, No. 5–6 (455), pp. 3–15, 2007.     

钒合金抗高温氧化腐蚀研究进展

以V?(4?5)Cr?(4?5)Ti合金为代表的钒合金具有高温性能优异、抗辐照肿胀性能好、中子辐照活化性低等诸多优点,被视为先进核聚变反应堆最有潜力的候选包层结构材料之一。然而,钒合金在较高温度下的氧化腐蚀及吸氧脆化问题仍是目前制约其实际应用和长寿命服役的重要因素。因此,提升钒合金的抗高温氧化腐蚀性能,对于提高其服役温度、延长其服役寿命以及拓宽其应用领域均具有重要意义。本文综述了国内外有关提升钒合金抗高温氧化腐蚀性能的三种主要方案,即添加抗氧化性元素、应用扩散型涂层和包覆型涂层,并对这些方案的主要特点、应用实例以及存在的问题进行了分析和讨论。上述三种方案中,包覆型涂层由于可以将钒合金基体和服役环境完全隔离,因而具备更大的应用潜力。根据钒合金的应用特点,对先进包覆型抗氧化腐蚀涂层的发展趋势和技术需求进行了展望,以期为钒合金抗高温氧化腐蚀研究工作的深入开展提供借鉴。      

Cyclic creep-rupture behavior of three high-temperature alloys

Some important characteristics of the cyclic creep-rupture curves have been studied for the titanium alloy 6Al-2Sn-4Zr-2Mo at 900° and 1100°F (755 and 865 K), the cobalt-base alloy L-605 at 1180°F <910 K), and for two hardness levels of 316 stainless steel at 1300°F (980 K). Such curves have been used successfully in a previous investigation for the evaluation and prediction of strain-cycling life at elevated temperatures within the creep range. The cyclic creep-rupture curve relates tensile stress and tensile time-to-rupture for strain-limited cyclic loading and has been found to be independent of the total strain range and the level of compressive stress employed in the cyclic creep-rupture tests. The cyclic creep-rupture curve was always found to be above and to the right of the conventional (constant load) monotonic creep-rupture curve by factors ranging from 2 to 10 in time-to-rupture. This factor tends to be greatest when the creep ductility is large. Cyclic creep acceleration was observed in every cyclic creep-rupture test conducted. The phenomenon was most pronounced at the highest stress levels and when the tensile and compressive stresses were completely reversed. In general, creep rates were found to be lower in compression than in tension for equal true stresses. The differences, however, were strongly material dependent.     

Mechanical properties of high-temperature alloys of AlRu

A number of alloys based on the binary CsCl (B2) compound AIRu are tough and also are ductile in compression at room temperature. Alloys that are substoichiometric in Ru tend toward intergranular brittleness, are enriched in Al at the grain boundaries, and are improved by the addition of 0.5 at. pct B. Al₄₇Ru₅₃ + 0.5 at. pct B underwent 33.6 pct true strain in compression at 23°C and work hardened to 3.1 GPa true stress. Dislocations of slip vectors 〈100〉, 〈110〉, and 〈111〉 have been identified by transmission electron microscopy (TEM) study of plastically deformed samples, so that ample slip systems appear to be present for compatible deformation of polycrystalline samples.     

Manufacture of powders of nickel-base brazing alloys

Conclusions A process has been developed for the manufacture of a Ni-Cr-Si-Fe-B brazing alloy in powder form, designated as PKh12N75S8R, and also brazing alloy powder No. 6MA, which is a mechanical mixture of 85% of PKh12N75S8R powder and 15% of molybdenum. Using alloy powder No. 6MA (instead of PKh12N75S8R) for brazing high-alloy steels raises the secondary melting (unbrazing) temperature of the brazed joint.Translated from Poroshkovaya Metallurgiya, No. 5 (113), pp. 82–88, May, 1972.     

Modeling high-temperature stress-strain behavior of cast aluminum alloys

A modified two-state-variable unified constitutive model is presented to model the high-temperature stress-strain behavior of a 319 cast aluminum alloy with a T7 heat treatment. A systematic method is outlined, with which one can determine the material parameters used in the experimentally based model. The microstructural processes affecting the material behavior were identified using transmission electron microscopy and were consequently correlated to the model parameters. The stress-strain behavior was found to be dominated by the decomposition of the metastable θ′ precipitates within the dendrites and the subsequent coarsening of the θ phase, which was manifested through remarkable softening with cycling and time. The model was found to accurately simulate experimental stress-strain behavior such as strain-rate sensitivity, cyclic softening, aging effects, transient material behavior, and stress relaxation, in addition to capturing the main deformation mechanisms and microstructural changes as a function of temperature and inelastic strain rate.