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

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 are also ever changing. The theory of plate tectonics, for example, was not part of any conversations when the senior author of this book was born. The structure of Earth's interior, including its crustal membrane, was not well understood then. In not so unfamiliar way, one may question the relevance of this topic to the high‐temperature response of materials of this book, if no consideration is given to the fact now established that various plates move because they are floating on their melt and the driving forces are initiated within this melt. Lower parts of the plates are close to the melting point and thus at extremely high temperatures. We have discussed details of this topic in Chapter 11. Thermomechanical interactions of plates produced mountain ranges, and the interaction of the mountains with the atmosphere affects the movement of air masses, thus influencing the weather patterns and the cryosphere of the earth.

1.2.1 Trinity of Earth's Structure

      Trinity of Earth's Structure

      CRUST

      MANTLE

      CORE

      Earth is believed to be a sphere with three concentric zones: the crust, the mantle, and the core. The solid shell of the earth, including the crust and the uppermost relatively rigid layer of the mantle, is known as the lithosphere, whereas the weaker zone beneath this layer is known as the asthenosphere. The division between the two layers is rather qualitative, based on seismic response, and improved understandings can be made on the basis of rheological response of geologic materials as a function of temperature.

      The mantle is subdivided into an upper boundary marked by the Mohorovicic discontinuity, a lower boundary marked by Gutenberg discontinuity, and the core (also further subdivided into the inner core and the outer core). Discussions on these subjects are outside the scope of this book. However, we will consider the lithosphere and asthenosphere as one “solid” layer at various temperatures below the melting point of all rocks. The topic is rather complex because of the inhomogeneity in the structure and the melting point depends on pressure and mineral contents.

1.2.2 Trinity of Earth's Cryospheric Regions

      The crustal surfaces of Earth have seen numerous cycles of glaciation and thawing. These cyclic events were interactively influenced by the “Trinity of Earth's Cryosphere.” Three sections of the cryosphere act as the cooling vent for the earth.

      Trinity of Earth's Cryosphere

      NORTH POLAR ZONE

      HIMALAYA (Alps, Andes, Rockies, etc.)

      SOUTH POLAR ZONE

      The Arctic region containing the North