High Temperature Aerospace Research Articles

In situ synthesis of titanium boride whisker reinforced titanium by mechanical alloying : T. Takahashi (Hyogo Prefectural Inst. of Industrial Research, Kobe, Japan). J. JapanInst. Metals, vol 59, no 3, 1995, 244–250. (In Japanese.)

Energy filtering TEM (EFTEM) studies of a fibre-reinforced Ti-6A1-4V alloy designed for use in high temperature aerospace applications have been performed using a Philips CM20 TEM equipped with a Gatan imaging filter. The fibre employs a developmental duplex coating based upon a compliant layer of gadolinium metal and a gadolinium boride reaction layer both deposited by physical vapour deposition. EFTEM elemental maps, employing a background subtraction procedure, as well as the more simple jump-ratio images were recorded using the Si L23−, Gd N45−, B K-, C K-, Ti L23−, O K-, Gd M45-edges and a comparison between the two methods is presented. Important information on inhomogeneities in the deposited layers, reactions occurring at the coating-fibre and coating-matrix interfaces and the nature of the diffusion processes was derived from these results. In order to support the interpretation of the EFTEM element distributions, electron energy-loss spectra (EELS) have been recorded on regions of interest and subsequently quantified to yield the composition of the phases. This investigation highlights the benefit of combining EELS and EFTEM in the characterization of complex microstructures.

Energetics of point defects in γ-TiAl

{gamma}TiAl has been receiving a great deal of attention in recent times owing to its industrial importance. This structural intermetallic is a candidate material for high temperature aerospace applications. Therefore, a study of point defect properties is useful in elucidating its physical metallurgy. In this brief communication, the authors discuss the vacancy and antisite defect properties of {gamma}-TiAl.

An evaluation of fiber-reinforced titanium matrix composites for advanced high-temperature aerospace applications

The current capabilities of continuous silicon-carbide fiber-reinforced titanium matrix composites (TMCs) are reviewed with respect to application needs and compared to the capabilities of conventional high-temperature monolithic alloys and aluminides. In particular, the properties of a first-generation titanium aluminide composite, SCS-6/Ti-24Al-11Nb, and a second-generation metastable beta alloy composite, SCS-6/TIMETAL 21S, are compared with the nickel-base superalloy IN100, the high-temperature titanium alloy Ti-1100, and a relatively new titanium aluminide alloy. Emphasis is given to life-limiting cyclic and monotonic properties and to the influence of time-dependent deformation and environmental effects on these properties. The composite materials offer a wide range of performance capabilities, depending on laminate architecture. In many instances, unidirectional composites exhibit outstanding properties, although the same materials loaded transverse to the fiber direction typically exhibit very poor properties, primarily due to the weak fiber/matrix interface. Depending on the specific mechanical property under consideration, composite cross-ply laminates often show no improvement over the capability of conventional monolithic materials. Thus, it is essential that these composite materials be tailored to achieve a balance of properties suitable to the specific application needs if these materials are to be attractive candidates to replace more conventional materials.

On the increase in young's modulus of Ti24Al11Nb alloy as a result of hold time during low cycle fatigue at 650 °C

Titanium aluminide Ti[sub 3]Al based Ti-24Al-11Nb alloy has been the subject of considerable interest for high temperature aerospace applications in advanced systems. Although a number of studies on the mechanical properties are reported, only a limited amount is known on fatigue behavior, particularly regarding hold-time effects on the high temperature low cycle fatigue (LCF). Here the authors report the observations made on the variation of Young's modulus during the investigation carried out to evaluate hold-time effects on the LCF of Ti-24Al-11Nb at 650 C.

A Review of the Status and Developmental Issues for Continuously-Reinforced Ti-Aluminide Composites for Structural Applications

AbstractA significant materials-based activity to develop Ti-aluminide metal matrix composites for high temperature aerospace structural applications is now underway. A review of the approaches, progress, and status of the development of continuously-reinforced Ti-aluminide metal matrix composites with matrices which contain a significant volume fraction of the ordered orthorhombic Ti2AlNb phase will be presented. The principal application considered is a gas turbine compressor rotor ring, and this influences the development approaches and properties goals. Specific development activity that will be presented includes modification and control of the matrix composition and microstructure, fiber coating treatments to control interdiffusion between the fiber and the matrix, and to improve the ability of the interface to support a mechanical load, and efforts to improve the properties of SiC monofilaments used as reinforcements. Critical issues that define the requirements for additional studies will be presented.

Combustion synthesis of single phase Ti5Si3

Ti[sub 5]Si[sub 3] is an important compound of the Ti-Si binary system with potential usage in high temperature aerospace applications. Silicides are important because of their superb oxidation resistance. Other characteristics for structural service temperatures of up to 1,600 C include low density, good strength at high temperature, good creep resistance, and microstructural stability over the temperature range between 25 C and the maximum service temperature. Because they are high temperature materials, the processing of the refractory silicides may not be easy. Typical processes for producing the silicides include chemical/ physical vapor deposition, rapid thermal processing for thin films and traditional casting or powder metallurgical processing. The drawbacks of these processes include high capital investments and an extended amount of time at high temperatures. As detailed in several reviews, Combustion Synthesis (CS) processing, provides several benefits, including the generation of substantial heat in situ, use of a simple reactor, ease of scaling up, tailoring of the composition, and reduction of impurities.

Effect of Cold Work and Aging Upon the Properties of a Ni-Mo-Cr Fastener Alloy

HAYNES® alloy 242 is a promising new Ni-Mo-Cr alloy for high-strength, high-temperature aerospace fastener applications. Age-hardenable by virtue of a long-range-order strengthening mechanism, 242™ alloy is capable of developing very high strength, particularly in the cold-worked condition. A key difference from other fastener materials is its attendant good ductility. The relationship between the amount of cold work, aging temperature and time, and the properties of this alloy has not been previously explored in detail. In the present work, the nature of the interaction between cold work and aging will be examined, and the properties of the material relevant to aerospace fastener applications will be described.

Deformation at interfaces of Ti-6Al-4V/SiC fiber composites

Titanium-aluminum alloy metal matrix composites (MMC) and Ti-Al intermetallic matrix composites (IMC), reinforced with continuous SCS6 SiC fibers are leading candidates for high temperature aerospace applications such as the National Aerospace Plane (NASP). The nature of deformation at fiber / matrix interfaces is characterized in this ongoing research. One major concern is the mismatch in coefficient of thermal expansion (CTE) between the Ti-based matrix and the SiC fiber. This can lead to thermal stresses upon cooling down from the temperature incurred during hot isostatic pressing (HIP), which are sufficient to cause yielding in the matrix, and/or lead to fatigue from the thermal cycling that will be incurred during application, A second concern is the load transfer, from fiber to matrix, that is required if/when fiber fracture occurs. In both cases the stresses in the matrix are most severe at the interlace.

Computational technology for high-temperature aerospace structures

The status and some recent developments of computational technology for high-temperature aerospace structures are summarized. Discussion focuses on a number of aspects including: goals of computational technology for high-temperature structures; computational material modeling; life prediction methodology; computational modeling of high-temperature composites; error estimation and adaptive improvement strategies; strategies for solution of fluid flow/thermal/structural problems; and probabilistic methods and stochastic modeling approaches, integrated analysis and design. Recent trends in high-performance computing environment are described and the research areas which have high potential for meeting future technological needs are identified.

Thermal properties of MoSi2 and SiC whisker-reinforced MoSi2

The heat capacity, thermal conductivity and coefficient of thermal expansion of MoSi2 and 18 vol % SiC whisker-reinforced MoSi2 were investigated as a function of temperature. The materials were prepared by hot isostatic pressing between 1650 and 1700 °C, the hold time at temperature being 4 h. The heat capacity of MoSi2 showed an increase from about 0.44 Wsg−11K−1 at room temperature to 0.53 at 700 °C. Whisker reinforcement increased heat capacity by about 10%. Thermal conductivity exhibited a decreasing trend from 0.63 Wcm−1 K−1 at room temperature to 0.28 Wem−1 K−1 at 1400°C. Whiskers reduced conductivity by about 10%. The thermal expansion coefficient increased from 7.42 °C−1 between room temperature and 200 °C to 9.13 °C−1 between room temperature and 1200 °C. There was a 10% decrease resulting from the whiskers. The measured data are compared with literature values. The trends in the data and their potential implications for high-temperature aerospace applications of MoSi2 are discussed.

Tribological performance of MoS 2 compacts containing MoO 3, Sb 2O 3 OR MoO 3 and Sb 2O 3

Selected oxides and sulfides have been previously investigated as bulk additives to enhance the tribological performance of MoS 2 in air from ambient temperature to about 315 °C. Sb 2O 3, in particular, has been identified as a superior additive for high temperature aerospace applications. In experiments designed to investigate a hypothesis that the superior tribological performance of MoS 2-Sb 2O 3 formulations results from formation of an Sb 2O 3-MoO 3 eutectic, compacts of MoS 2 with MoO 3, Sb 2O 3 or MoO 3 and Sb 2O 3, were prepared and tribologically evaluated at 316 °C. Formulations containing Sb 2O 3 or MoO 3 resulted in higher friction but much less wear than MoS 2 compacts, with the wear volume of MoS 2-MoO 3 compacts at about half that of compacts containing Sb 2O 3. Data from compacts containing mixtures of MoO 3 and Sb 2O 3 indicate no synergism between the additives, as would be expected if an eutectic of MoO 3-Sb 3O 3 yields any significant benefits in MoS 2-Sb 2O 3 formulations as hypothesized previously. In fact, X-ray diffraction and differential thermal analysis data indicated no evidence of eutectic formation. Alternatively, it is hypothesized that these soft oxides, owing to their low shear strength, permit MoS 2 crystallites to retain a preferred orientation achieved during asperity contact, improving the overall tribological performance. The protective role of MoO 3 in MoS 2 compacts is anomalous in that MoO 3, a product of the oxidation to be avoided in high temperature applications of MoS 2, is tribologically beneficial in compacts with MoS 2 in such applications.

Recent advances in aerospace refractory metal alloys

The development of refractory metal alloys for aerospace applications is reviewed. Refractory metals are prime candidates for many high temperature aerospace components because of their high meltin...

Recent advances in aerospace refractory metal alloys

The development of refractory metal alloys for aerospace applications is reviewed. Refractory metals are prime candidates for many high temperature aerospace components because of their high melting points and inherent creep resistance. The use of refractory metals is often limited, however, by poor room temperature properties, inadequate oxidation resistance at elevated temperatures, or difficulties associated with joining or welding. Recent advances in the understanding of the role of oxygen on these room temperature brittle behaviour problems, especially in molybdenum alloys, are described. The oxidation behaviour of refractory alloys at elevated temperatures in complex environments has also been re-evaluated recently and a clear understanding of the appropriate environments requiring protective coatings has emerged. Current research is emphasising the development of creep resistant niobium and tantalum alloys that are inherently oxidation resistant. The long term creep resistance of refractory alloys has been the subject of considerable research for space nuclear applications and progress in this area is discussed. The effectiveness of carbides in molybdenum and tungsten alloys designed for high temperature creep resistance has been reanalysed. Brittle behaviour after joining or welding of refractory metals has limited their applications. The origins of brittle behaviour in welded alloys are described. Examples are also given of novel solid state joining developments in tungsten and molybdenum alloys below and above their recrystallisation temperatures. Finally, novel developments are underway in particulate refractory metals, specifically encompassing rapid solidification processing. Although in its infancy, the application of rapid solidification to refractory metals is an exciting and fruitful area for further research.

High Temperature Aerospace Materials Prepared by Powder Metallurgy

Metal alloys used in the aerospace industry for high-performance com­ ponents are normally prepared initially as quality castings. In the cast form, these materials are subject to heterogeneities in structure as a result of factors such as chemical macrosegregation on the scale of the dimension of the casting, micro segregation on the scale of the dendrite arm spacing (fractions of a millimeter), microporosity due to gas and solidification shrinkage, and second-phase inclusions with a scale of several micrometers. Since castings can be produced with intricate detail including internal passages, they find special use as articles cast to final shape, e.g. air-cooled, gas turbine vanes and blades. In general, however, high-quality castings, which are cooled as rapidly as is practical to minimize microstructural heterogeneities, are subjected to subsequent hot working and thermal treatment. In the worst case, heterogeneities may render the casting unworkable; even in the best cases, a wrought material may inherit undesirable features from the initial casting, despite the homogenizing effects of the thermal and mechanical processes. An example of such difficulty is the homogenization of the microsegre­ gation that results from solute enrichment in interdendritic regions during solidification (1). Reducing the micro segregation of a solute species to 5% of its initial value requires a time approximately equal to the square of the dendrite spacing divided by a solute diffusion coefficient. For a segregated

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