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

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СКАЧАТЬ of glass are unthinkable. Today, superalloys are closely linked to the aerospace industry. In aircraft and land‐based gas turbine engines, superalloys are used extensively. 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 air travel by jet aircraft is a common mode of transportation today, how many travelers can imagine that the reliability of modern jet engines based on superalloy technology is connected to the kitchens of the world. The development of nickel‐base superalloys, actually, started almost 100 years ago due to the demands, among others, of the kitchens of the world. It started 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.

      During the 1940–1960 period, the most widely used precipitation hardened nickel‐base alloys, belonging to the cubic system, were developed in the metallurgical laboratories for the hottest components of jet and rocket engines. In Chapter 2, chemical compositions of a few nickel‐base superalloys are given to provide a quick glance at the world of gas turbine engine materials. During the decade of 1960–1970, directionally solidified (DS) nickel‐base superalloys were developed for fabricating turbine blades used in modern gas turbine engines (Duhl 1987). Directionally solidified polycrystalline superalloys (belonging to the cubic system) are highly anisotropic, strong, and creep resistant. The long axis of turbine blades is parallel to the long axis of the columnar grains with large cross‐sectional areas. The <001> axis is parallel to the long axis of the grains. The Hall–Petch relation for strength giving preference to fine‐grained materials at low homologous temperatures is not applicable at elevated temperatures; large‐grained materials are used at high temperatures. Grain boundaries are the sources of weakness at high temperatures. Single crystals of nickel‐base alloys are now used for manufacturing turbine blades in today’s advanced jet engines (Duhl 1989). Creep, fracture, and creep modeling of nickel‐base single crystals are presented in detail in Chapter 5.

It is highly possible that most metallurgists, materials scientists, and engineers may not be aware of the reality that “Mother Earth” produces three distinct types of DS columnar‐grained polycrystalline ice every winter in freshwater lakes and vast oceanic surfaces of the earth. Natural ice, an oxide of hydrogen (H₂O), belongs to the hexagonal system (see Chapter 3). Although, granular snow–ice is commonly observed in thin surface layers of river, lake, and sea ice covers, as well as in glaciers and ice caps (Paterson 1994; Schulson and Duval 2009), engineering properties of this type of ice are not of significance because vast reservoirs of floating ice in lakes, rivers, and oceans are mostly columnar‐grained, DS type. Two of these DS ice types have <0001> or <c> axis, of the columnar crystals either (i) parallel to the long axis of the grains, (ii) randomly oriented in the plane normal to length of the grains (mostly for freshwater lake and river ice). These two types of freshwater DS ice (types i and ii) are classified as S1 and S2, respectively (Michel 1978). Michel (1978) was the first person who discussed the physics of the creep and fracture of ice, their interconnectivity, and the relationship between its structure and mechanical properties for a wide range of engineering applications. A third type of DS ice (type iii), classified now as S3, has <0001> or <c> axis clustered in the plane normal to the long axis of the crystals (Weeks and Gow 1978, 1980). Floating sea ice mostly belongs to the S3 type (Shokr and Sinha 2015). Thus, the physics of fabricating DS types of superalloys can be linked directly to the knowledge gained from the physics of columnar‐grained ice that naturally grows in lakes and rivers, as well as in salty water of the oceans (Dorsey 1940; Assur 1958; Gold 1965; Pounder 1965; Shokr and Sinha 2015). However, this nature‐based lesson is not discussed in metallurgical books or publications (Duhl 1987; Ross and Sims 1987) dealing with the fabrication of DS‐type superalloys.

An extremely important, yet startling, fact that leads some toward the philosophical issue of how humans (including even scientists and engineers) interpret temperature is the concept of ice as a high‐temperature material. Although the “cryosphere” of the earth is derived from “cryogenics” (Greek: kruos frost, −genes born) and is understood universally as snow‐ and ice‐covered areas, it should be realized that natural ice exists at temperatures extremely close to its melting point and is, therefore, the hottest material on the surface of the earth. Even an extremely cold body of ice at −40 °C (233 K) is equivalent to a very high temperature of 0.85 T _m (T _m = 273 K). This is still a dream operating temperature for gas turbine engine designers. Today, nickel‐base superalloys, in the form of DS material and single crystals, are used exclusively for making blades and vanes for the hottest turbine sections of modern jet engines. The operating temperatures are restricted to a maximum temperature of about 1500 K, which is only about 0.8 T _m of the base material, i.e. nickel (see Figure 1.2).

Figure 1.2 Melting points of some pure metals and ice.

      The perception of ice as a cryogenic material, i.e. a material at very low temperatures, comes not from the thermodynamic point of view, but from the very fact that the temperatures associated with ice are uncomfortably cold for humans. The notion that ice is cold is enhanced by the very fact that it exists at “negative” temperatures in Celsius. This negativity is deeply rooted even in scientific minds of material scientists even though they are familiar with temperatures in kelvin and thermodynamic “homologous” scale.

It is not surprising to incline toward the popular cold‐ice notion and treat snow/ice/permafrost‐related engineering as a synonym for “cold regions engineering.” There are several research and development laboratories in the world, dealing with issues related to glaciers, snow, snow avalanches, ice, and permafrost, and therefore these are named accordingly. For example, in Japan, there is the Institute of Low Temperature Science founded in 1941 in Sapporo. In the United States of America (USA), the US Army's Cold Regions Research and Engineering Laboratory (CRREL) was established in 1961 in Hanover, NH, during the peak of the Cold War. Incidentally, the CRREL used to be known with a temperature‐neutral identity, i.e. the Snow, Ice and Permafrost Research Establishment that was established in 1949. And as recently as 1999, three different laboratories of the Chinese Academy of Sciences were merged into one entity: Cold and Arid Regions Environmental and Engineering Research Institute. In many respects, the Arctic and Antarctic Research Institute of Russia, the Scott Polar Research Institute of the United Kingdom, and the Snow and Avalanche Study Establishment of India are appropriately named.

      Ice has been continued to be treated as the public enemy and “ice‐rich” oceans in the Arctic and the Antarctic, two of three Earth's cryosphere zones, are often described as “ice‐infested.” This is a misconception: ice is not a cryogenic material. On Earth's surface, as pointed out earlier, ice exists at extremely high homologous temperatures. It is crystalline and transparent. These facts make ice the best analog for studies related to creep, fracture and hence most engineering properties of crystalline materials at high temperatures.

1.2 Trinities of Earth's Structure and Cryosphere

      Geophysics and geology focus on the scientific study of the earth. These fields are in advanced stages of a huge subject dealing with various aspects of the earth. The environment we live in is ever changing as the surface of the earth and its interior are constantly shifting. Geological and geophysical studies are an integral part of physics, chemistry, and mathematics and use the scientific methods developed in general. As in most fields, it is natural that our hypotheses and beliefs СКАЧАТЬ