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

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      Glass is amorphous in structure, but most natural and manufactured metal‐based engineering materials are polycrystalline materials. However, a rheological model, called Elasto – Delayed‐Elastic – Viscous (EDEV) model developed originally for stabilized glasses at elevated temperatures (Sinha 1971), can be, and has been, used as the foundation for developing grain‐size‐dependent EDEV models for high‐temperature creep and fractures of pure polycrystalline metals and complex alloys, ceramics, and rocks. The delayed elastic effect and its recoverable aspects have been linked to grain‐boundary shearing processes. The connectivity to engineering materials, such as complex titanium‐base and nickel‐base superalloys used in gas turbine engines for power generation and in the aerospace industry, is based on the physics of the grain‐boundary areas. The history of this scientific and technical development is fascinating and has been described in Chapters 5–9. The description covers the whole range from phenomenological to microstructure‐based, grain‐structure‐sensitive EDEV modeling for deformation, cavitation, dilation, and strength. The predictive power of this EDEV model has been demonstrated by explaining the ductile‐to‐brittle transition and success of the classical and ever popular minimum creep‐rate‐based models (e.g. the Monkman–Grant (MG) relationship) for failure or fracture. EDEV model is also capable of quantitatively predicting the strain‐rate sensitivity of 0.02% yield and upper‐yield (ultimate) strength, as well as relaxation of stresses under constraint conditions. The possible extension of this EDEV model to get an understanding of the post‐glacial uplifting is covered in Chapter 10 while the hypothetical uses of the EDEV model for understanding certain issues with plate tectonics are presented in Chapter 11.

1.1.1 Defining High Temperature Based on Cracking Characteristics

      For any load‐bearing engineering component, the propagation of inherent flaws or cracks at the surfaces or in the bulk and/or freshly nucleated microcracks and their multiplication under load is the most challenging factor to design for. At high temperatures, materials also exhibit time‐dependent deformation or creep. Such rheological properties, including the kinetics of nucleation, growth, and propagation of microcracks, and their size and shapes with respect to the crystalline structure, are essentially the limiting factors that control the usage of these materials. Of course, the design of most engineering components is also limited by the amount of creep deformation. The question is – How can we define high temperature in a rational manner?

      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 СКАЧАТЬ