Superalloys fiber reinforcement

Tungsten metal and tungsten alloy wires have attracted interest for fiber reinforcement of high-temperature materials, in particular of superalloys. However, until now, they have remained exotic materials with only a few specific applications or for model studies of mechanical behavior and are not produced on a large scale. This is at least partially due to the difficulties encountered during composite fabrication and the resulting high cost of the material. 

More recently, DiCarlo (1997) has compared ceramic composites to the best superalloy using the strength-to-density ratio versus the Larson-Miller parameter, q. In this analysis (Figure 3-25), the Nextel 720 fiber-reinforced composite is inferior to the metal in short-term fracture but comparable to the superalloy in long-term rupture. On this basis, the Hi-Nicalon fiber is superior to the superalloy except at short times and high stress levels, when fiber failure is dominated by slow... 

FIGURE 3-25 Comparison of optimum strength map of AI2O3 fiber-reinforced CMC with strength behavior of an advanced superalloy. Source DiCarlo, 1997. 

This means that until more oxidation tolerant matrix-interface-fiber systems are developed, service conditions will be rather limited for non-oxide fiber-reinforced CMCs. CMCs do have a large weight advantage over superalloys, however, as well as low observability to radar. 

The superalloys, as well as alloys of aluminum, magnesium, titanium, and copper, are used as matrix materials. The reinforcement may be in the form of particulates, both continuous and discontinuous fibers, and whiskers concentrations normally range between 10 and 60 vol%. Continuous-fiber materials include carbon, silicon carbide, boron, aluminum oxide, and the refractory metals. However, discontinuous reinforcements consist primarily of silicon carbide whiskers, chopped fibers of aluminum oxide and carbon, or particulates of silicon carbide and aluminum oxide. In a sense, the cermets (Section 16.2) fall within this MMC scheme. Table 16.9 presents the properties of several common metal-matrix, continuous and aligned fiber-reinforced composites. 

FIGURE 6.19. Micrographs of a tungsten-wire reinforced superalloy sectioned perpendicular to the wire direction (left) strength and ductilities of typical tungsten-wire q reinforced superalloys with 40 vol% fibers (right) [6.50]. By courtesy of R. Warren, Lulea University of Technology, Sweden. ... 

MMCs are usually reinforced by either monofilaments, discontinuous fibers, whiskers, particulates, or wires. With the exception of wires, which are metals, reinforcements are generally made of advanced ceramics such as boron, carbon, alumina and silicon carbide. The metal wires used are made of tungsten, beryllium, titanium, and molybdenum. Currently, the most important wire reinforcements are tungsten wire in superalloys and superconducting materials incorporating niobium-titanium and niobium-tin in a copper matrix. The most important MMC systems are presented in Table 18.5. 

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