Engineering Physics of High-Temperature Materials. Nirmal K. Sinha

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СКАЧАТЬ CN = 6 forming a polyhedral coordination structure with six neighbors. The ZnS structure is also based on the FCC structure. It can be thought of as two interpenetrating FCC lattices with one displaced by 1/4 of the diagonal along a cubic cell such that each site is tetrahedrally coordinated (CN = 4), as shown in Figure 2.14b. If all the atoms in the ZnS structure are carbon atoms, this would form diamond.

Ceramic materials are generally particularly strong in compression and shear. Dislocation movement is much harder in ceramics than metals due to the dissimilar nature of the atoms in the crystal structure – especially when opposite charges exist as the strong repulsive forces of nearby ions act as strong barriers to motion. The number of possible slip directions is generally lower in ceramics and the force required to cause movement is greater. Ceramics also tend to have higher melting points and are, generally, more resistant to chemical attack than metals and alloys. On the other hand, because ceramics do not flow under stress, they tend to be very brittle and will likely fail by cracking mechanisms. Acoustic emissions have been used to study the development and propagation of cracks; this is discussed in more detail in Chapter 4.

      As crystalline ceramic materials are brittle in nature, they are generally not very processable. Ceramic components are generally made in the desired shape either by in situ reactions or powder processing. In powder processing, powders or pasts are formed into the desired shape, and then undergo sintering to form a solid body. Such processes include injection molding, dry pressing, slip casting, and tape casting (which is used to make thin sheets). Certain processing techniques, such as hot pressing of ceramic powders, can be used to minimize the initial defects in the material and reduce the onset of cracking (Carter and Paul 1991).

      Several other techniques have been used to reduce the brittle behavior of ceramics. For example, the addition of small amounts of certain metals strengthens the cohesion between ceramic crystal grains at which fractures normally develop. Ceramic‐matrix composite (CMC) materials have also been developed to substantially increase fracture toughness. Such matrix composites can take different forms, but a common form that has been found to reduce cracking failure is that of ceramic fibers embedded within a ceramic matrix. The reinforcing fibers increase the energy expended to propagate cracks, as well as bridging cracks without fracturing, thereby increasing the ultimate tensile strength of the overall material. The reinforcing fibers may be polycrystalline – like the matrix – or amorphous and are often composed of carbon, special silicon carbide, alumina, and mullite.

      A special class of CMCs, called ultra‐high‐temperature ceramic‐matrix composites, is specifically designed for high‐temperature use, such as in thermal protection systems and rocket nozzles in the aerospace industry. Carbon fiber reinforced carbon‐matrix composites (C/C), carbon fiber reinforced silicon carbide matrix composites (C/SiC), and silicon carbide fiber reinforced silicon carbide matrix composites (SiC/SiC) are some of the materials being developed. CMCs are particularly desirable for their high strength‐to‐weight ratios and high thermal shock resistance. However, they can suffer from erosion and can have anisotropic properties following the orientation of the fibers (Vinci et al. 2018).

2.8 Metals and Alloys

      The world of metals, metallic alloys, and emerging intermetallics is both vast and complex. It touches our daily lives practically in all aspects. Metallurgy has played such an important role in civilization that several historical epochs have been named after the metals/alloys that enabled the next stage of advancement – often with the exploration of higher and higher temperature applications in society – such as in the Copper Age, Bronze Age, and Iron Age (Ferguson 2008). The engineering of metals and alloys to attain specific properties is an ongoing exploration that requires a detailed understanding of the structure of materials.

      The search for high strength materials that would satisfy required creep and corrosion resistance under extremely severe conditions of elevated temperatures and pressures led to the development of “heat‐resistant alloys” or “high‐temperature alloys.” These alloys are known as “superalloys” to go along with the popular fictional television hero of late 1940s, Superman (Boesch 1989, p. 1). These alloys are either (i) solid‐solution strengthened or (ii) precipitation strengthened with or without additional strengthening by the incorporation of fine stable dispersoid (dispersion strengthened). The precipitation‐strengthened alloys are based on nickel, iron, or cobalt (Betteridge and Heslop 1974; Ross and Sims 1987; Stoloff 1987; Stephens 1989). Dispersion strengthening has been applied to many alloy systems, such as Al, Cu, Fe, Ni, Pb, Pt, and W alloys (Arzt 1991).

      Superalloys are used extensively in aircraft and land‐based gas turbine engines. About 60% by weight of most modern gas turbine engine structural components, such as blades, vanes, and integral wheels, are made of nickel‐base superalloys. Although superalloys are closely linked to the aerospace industry, the history of nickel‐base superalloys, actually, started almost 100 years ago from the development of 20 wt.% Cr in an 80 wt.% Ni solid‐solution alloy for electrical heating elements (Betteridge and Heslop 1974; Ross and Sims 1987; Stephens 1989). Chromium was added to nickel to improve its strength and oxidation resistance.

      Here, we present a basic overview of the structure of several important metals and alloys for high‐temperature applications. This is not intended to be comprehensive, but rather to serve as a reference to understand the models presented in subsequent parts of the text.

2.8.1 Iron‐base Alloys

Iron makes up approximately 8.1% by weight of Earth's crust (Lutgens and Tarbuck 2000) and is naturally found in the form of an ore, most often an iron oxide, such as magnetite (Fe₃O₄) or hematite (Fe₂O₃). The use of steel, an iron‐base alloy, dates far back to the Middle Ages, with the earliest known production of steel dating nearly 4000 years, from 1800 BCE (Akanuma 2005). Iron alloys are generally inexpensive compared to other materials and are highly versatile.

      Trinity of Iron Phases

      α‐Fe [BCC, Ferrite]

      γ‐Fe [FCC, Austenite]

      δ‐Fe [BCC]

      Pure iron exists in three phases. Below 1185 K, α‐Fe (known as ferrite) has a BCC structure, but it transitions to γ‐Fe (known as austenite) with an FCC structure above this temperature. This transition to a close‐packed structure with an increase in temperature is highly unusual and is the basis of many of the useful properties of steel. Above 1667 K, pure iron transitions back to BCC and is known as δ‐Fe at high temperatures. It is of interest to note that a β‐Fe phase, which was historically thought to exit, was found to be a form of α‐Fe above its Curie temperature (nonmagnetic).

The foundation for understanding the microstructure of steel is the iron‑carbon phase diagram (Figure 2.15). Iron alloyed with carbon is called carbon steel and has a carbon content that ranges from 0.002 to 2.14% by weight (or from 0.009 to 9.2 mol%). Alloys with a higher carbon content generally have phases of pure carbon graphite within the structure and are known as cast irons. Steels, on the other hand, have low carbon precipitates and benefit from the presence of iron carbides, namely Fe₃C. A key feature of the phase diagram is the transformation, upon cooling, of austenite to an intimate mixture of ferrite and carbide. Thin platelets of Fe₃C become immersed in an α‐Fe matrix in a two‐phase mixture called pearlite, and the interlamellar spacing can be controlled with transformation temperature.

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