Trinity of Cracks
TRANSGRANULAR
INTRAGRANULAR
INTERGRANULAR
In order to define a “high temperature,” let us consider the general characteristics of cracks seen in polycrystalline materials during deformation and after failure in a wide range of temperatures. Experimentally observed microstructural characteristics of load‐induced cracks in a wide variety of polycrystalline materials can be divided into three basic types – transgranular (across grains), intragranular (within grains), and intergranular (along grain boundaries). It is particularly insightful to consider the characteristics of cracks within a material by considering the operating temperature with respect to the materials’ specific melting point, T m, in kelvin. Transgranular cracks are dominant at low temperatures (<0.3 T m). Intergranular, grain‐facet‐sized cracks readily form at temperatures higher than 0.4 T m.
Thus, the simplest thermodynamically based and fracture characteristic inspired definition of high temperature for a polycrystalline material is the regime of intergranular cracks at operational temperature higher than a third of the melting point: higher than 0.3–0.4 T m. In addition to this general fracture‐appearance‐based definition of high temperature, throughout the text, we will add another, and perhaps the most important, aspect of the term “high temperature,” based on the time‐dependent recoverable components of deformation characteristics: delayed elasticity.
It is appropriate to mention here that turbine blades used in aeroengines are exposed to temperatures as high as 0.8 T m and the design engineer's goal is to push the operating temperatures even higher. At present, single crystals of nickel‐base superalloys, without any grain boundaries (to avoid/minimize intergranular cracks), could take the challenge.
The above generalization of the “trinity of cracks” based on crack types, however, does not apply to amorphous materials with microheterogeneous (mechanically and chemically) structure, like natural silicates and complex inorganic glasses used in engineering applications. Unless there are inclusions trapped within the mass, glass objects fail primarily due to the propagation of surface cracks, which are either inherent following fabrications or nucleated during service life.
Glass is one of the oldest materials known to mankind. Glassy objects can be found in nature within the mass of a large variety of silicates. It is rather difficult to say what came first as consumable building materials for mass use: glass or ceramics (including fired bricks)? Actually, it is the glassy phase that gives the strength to sun‐dried clay blocks after prolonged firing. The knowledge of making relatively crack‐free fired (sintered) earthenware for storage of food and for cooking probably heralded the dawn of manufacturing fired bricks for mass consumption. All over the world, there are examples of early use of sun‐dried bricks in constructing houses, but fired bricks probably led to the building of cities. Unique among them were the planned and designed Harappan‐style settlements and cities along the Indus River in the Indian subcontinent. Residential quarters, identical to modern row houses, were constructed in these cities with prismatic fired bricks having rectangular cross‐sections. The dimensions of bricks used in Harappan cities were also standardized (Keay 2004). So were the procedures for making them using clay and wooden frames and then by drying them in the sun, stacking them in the form of pyramids, and firing them with wood, charcoal, or coal dust. Even today, one can see this ancient technology at work all over India. One does not have to imagine the different stages of the manufacturing process for making fired bricks in ancient times. Figure 1.1 illustrates how the Harappan technology for mass production of fired bricks from clay, including the quality of bricks and even manual labor involved in handling the bricks at construction sites of multistory buildings, has remained unchanged for millennia.
The PCM of fire also led the human species to invent techniques for extraction of metals. High‐temperature technology was also extended to the development of manufacturing tools for hunting and gathering from using tin and copper to making articles of bronze and then of iron and steel. With this knowledge came the development of high‐temperature metallic alloys and it can also be linked to the “kitchen.” During the early 1900s, cobalt‐based and cobalt–chromium–tungsten systems were developed to make cutlery in addition to machine tools and wear‐resistant hard‐facing applications (Morral 1968; Beltran 1987). Incidentally, the knowledge gained from the metallurgical processes of making tungsten filaments for incandescent light bulbs and simple alloys for making coils inside elements for electric stoves and ovens led to the beginning of the development of complex high‐temperature cobalt‐based, nickel‐based, and iron‐rich alloys. The simple nickel–chromium alloy, discussed in the following text, heralded a new era of the use of superalloys.
Making earthenware marked the beginning of the world of engineering physics of ceramics. These days, we rarely use earthenware for cooking, but high‐temperature ceramics are common in most of today's stoves, ranges, and cookware used in kitchens. Kitchens without bowls, jars, and bottles 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