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

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      Source: (a) N.K. Sinha, unpublished; (b) Sinha (1988a) with permission from Springer Nature.

      There are a number of excellent books published in the past with a primary emphasis on metals and alloys. These publications have received wide‐ranging attention from metallurgists over the last 50 years or more. However, none of these well‐known publications have (to the authors’ knowledge) provided any information on grain‐size‐dependent nucleation and the kinetics of grain‐facet size microcracking activities and crack‐enhanced matrix creep, which starts during early stages of primary (transient) creep, leading to minimum creep rates and tertiary stages. Minimum creep rates are evolved properties and are in fact predictable. Minimum creep rate does not necessarily mean steady‐state creep due only to the dynamics of dislocation creep/climb mechanisms. The use of the usual experimentally evaluated characteristics of the minimum creep rate as a fundamental material property was recognized as being inappropriate by several investigators, but this is still largely ignored. None of the available books that focus on metallurgical processes take notice of the fact that strain‐rate‐sensitive 0.2% yield stress depends on characteristics of transient creep. This yield stress is actually predictable for real engineering materials (e.g., Ni‐ or Ti‐base superalloys used in gas turbine engines) on the basis of the EDEV model using material characteristics that can be obtained from independent SRRT tests (elaborated and substantiated in Section 5.16 of Chapter 5).

      The preceding text summarizes fundamental concepts that, although duly acknowledged in different ways by materials experts in different fields, are yet to be addressed comprehensively in a text that ties it all together. Moreover, the implications of applying the knowledge to different fields is vast: from predicting/designing to account for the creep of nickel‐base turbine blades in aerospace or power engineering, to guidelines for ice fishermen about how long a vehicle can remain parked on a floating ice sheet, or even to describe certain aspects of post‐glacial uplift and plate tectonics, including man‐made reservoir‐induced earthquakes, known as reservoir‐triggered seismicity (RTS).

      The traditional concept of “strength” implies a specific material property. But the strength of a material is a low‐homologous temperature concept, say, less than about 0.3T _m. This low‐temperature concept, based primarily on stress‐strain diagrams without any reference to time, does not apply at the elevated temperatures relevant to all high‐temperature engineering, for example, hot sections of gas turbine engines or nuclear and power‐generation applications. Strength at elevated temperatures is rate sensitive and is therefore not a specific property. Nonetheless, the concept of strength as a specific property (a low‐temperature concept) has retarded growth in the understanding of microstructure–property relationships and failure processes in engineering components in general. The application of this concept has misleading implications, drawing away from one basic fact: transient or primary creep stage, involving the initial periods of damage accumulation, plays a dominant and perhaps decisive role in many engineering problems. In Chapter 8, we will use the crack‐enhanced Elasto – Delayed‐Elastic – Viscous (EDEV) model for predicting the rate sensitivity of strength in a rational manner.

      One of the primary intentions of writing this text is to draw attention to the fact that polycrystalline ice can be used as an “ideal analogue” material to explain certain peculiarities of the elevated temperature response of engineered as well as natural materials. One such peculiarity is the observation that a polycrystalline material may exhibit both ductility and brittleness in a simultaneous manner. And this may happen at rather low levels of stress for engineering applications. But again, what do we mean by low or high stresses appropriate for high temperatures? Only by examining and analyzing the initiation of grain‐facet size cracks that can lead to tensile fracture can we offer a satisfactory mathematical and physical description for the stress as low or high. There is sufficient evidence to show that stresses higher than about 1 × 10⁻⁵ E at T > 0.4T _m (where E is the Young’s modulus) may be considered as high stress for polycrystalline materials at high homologous temperature.

      The onset of microcracking activities in pure, transparent ice can be monitored both visually and using AE technology. This dual process of evaluation is not possible for most opaque materials like metals, ceramics, and most rocks. Since it is not possible to visually identify the tiny grain‐facet size cracks inside most engineering materials, including bubbly ice, one‐to‐one correspondence between AE or microseismic activity (MA) signals and cracks could never be made. This is the dilemma for all metallic and ceramic materials. The predicament due to the opacity of specimens in most engineering materials allows AE/MA signals, even with 3D locator systems, only for monitoring the overall crack‐damage processes. We will discuss these issues in Chapter 4 (Section 4.10) and Chapters 7 and 8 for clarifying the advantages of using pure ice as an ideal analogue material for studies on engineering materials in general.

Although the very powerful Elasto – Delayed‐Elastic – Viscous (EDEV) model, described and applied for a wide range of engineering applications (Chapters 6–10), was developed from the rheological investigations on glass (Section 5.5 in Chapter 5), we are still unaware of any satisfactory answer as to why glass exhibits delayed elasticity identical to polycrystalline ceramics, metals, and complex alloys (see Section 5.6, Chapter 5). Why is the deformation‐induced birefringence (photoelastic effect) in glass independent of viscous strain and coupled “only” to pure elastic response of its complex structure?

      Sea ice in the Arctic Ocean plays one of the most important roles in modifying the climate of the world. Sea ice in the Antarctic region is marginal and seasonal, as described in our earlier book, Sea Ice: Physics and Remote Sensing (AGU/Wiley, 2015). No doubt, we must pay attention to the formation and decay of sea ice as a measure of climate change. Coincidentally, air‐ or space‐borne images of sea ice bear all the likeness of micrographs of metals, alloys, rocks, and ceramics, as pointed out in Chapter 11. Ice floes in the oceans can be characterized as grains in polycrystalline materials. On the other hand, an image of floating pack‐ice may also evoke likeness to Earth’s tectonic plates and sub‐plates. Relative movements of sea ice floes with respect to each other and rafting can be described as divergence, convergence, subduction, etc. Can we apply the lessons learned from the bearing‐capacity of floating ice, on the basis of the EDEV model, to large‐scale global phenomena such as post‐glacial uplifting (see Chapter 10), which is a very complex issue related to the convergence and subduction of plate tectonics or RTS (see Chapter 11)?

In Chapter 1, we introduce three major cooling vents for Earth as the “Trinity of Earth’s Cryospheric Regions.” Cryosphere is historically an accepted term, depicting cold (from the human point of view), ice‐rich areas, including the atmosphere. Thermomechanically speaking, ice can be considered as cold only if the temperature is significantly below 0.4T _m or 109K (−165°C). Earth’s cryosphere, therefore, consists of three relatively hot zones: primary (two polar areas) and secondary (the Alps, Andes, Himalayas, Rockies, etc.). The Trishul or Trinity of major cryospheric zones of the world is: the North and South poles, and СКАЧАТЬ