Читать книгу Engineering Physics of High-Temperature Materials - Nirmal K. Sinha - Страница 11

The development of knowledge in all branches of science and engineering has been so varied and rapid during the last century that it has become extremely difficult, if not impossible, for investigators to pay attention to different fields outside of their own expertise. As time progresses, each and every branch of scientific endeavor is getting subdivided and micro‐ divided, with specific jargon developing even within micro units, making it even more difficult to communicate with each other across specialties. The physics and engineering of high‐temperature materials is one such special area, and yet it touches many fields in many ways.

There is an ever‐growing number of human‐made materials like ceramics, metallic alloys, and superalloys used specifically in high‐temperature applications in areas such as the nuclear, chemical and aerospace industries. This may also include materials developed by design on the basis of nanotechnology and grain‐boundary engineering for very specific uses. Then, there are rocks of geophysical interest (such as with respect to tectonics and post‐glacial uplifting) existing at high temperatures within the depths of Earth and floating on magma, and ice (freshwater and saline sea ice) floating in its molten state in lakes and oceans. It would be impossible to cover all the complicated phenomena of different materials in a single book. However, the principal strengths of a book like the present one is the manner in which it covers many different materials all together. This could also be a weakness if descriptions are not clear enough to facilitate an understanding of the complicated physics and mechanics in widely differing materials. Some difficulties can be overcome by restricting topics relevant only to inorganic crystalline materials that would include the most abundant materials on Earth – ice and rocks, in addition to manu‐made (gender‐neutral term derived from Manushya in Sanskrit) and manufactured metallic‐based engineering materials used in various industries such as aerospace, power generation, and nuclear technology. Further obstacles can be removed by concentrating on materials at or used at high homologous temperatures greater than about one‐third of the melting point, T _m in Kelvin. In this manner, it is indeed possible to draw attention to a common string that unites most, if not all, apparently different polycrystalline materials and topics. Many time‐honored, empirically derived relations will be explained on the basis of a simple, microstructure‐sensitive, Elasto – Delayed‐Elastic – Viscous (EDEV) model.

High‐temperature materials science and engineering sounds like a specialized branch of applied science, but it can actually be considered as one of the most general areas of modern science and technology. This book is prepared with the intention of making it known that apparently dissimilar polycrystalline materials, such as metals, alloys, ice, rocks, and ceramics – and even glassy materials – behave in a very similar manner at high temperatures. This book, therefore, is aimed at a variety of experts, such as metallurgists to metal physicists, glaciologists to ice engineers, solid‐earth geophysics, earth scientists to volcanologists, and cryospheric and interdisciplinary climate scientists. The critical question addressed is, what is really meant by “high temperature,” and why? What is the microstructural‐based rationale for defining high temperatures?

Materials scientists (materialogists) universally agree that temperatures, T, above about one‐third of the melting point, T _m in degrees Kelvin, are high. For metals and alloys, it is unanimously recognized that T > 0.4T _m is unquestionably categorized as high‐temperature because intergranular cracks (called wedge or w‐type) along the grain‐boundaries (comparable to the size of grain facets) are predominantly observed at such temperatures, particularly in polycrystals. Grain‐boundary spherical or elliptical voids (called cavitation or r‐type) are also commonly noticed features in deformed or fractured materials. To this list of readily observable microstructural features, we consider a very special aspect of high‐temperature deformation and failure processes – that, to‐date, has not derived much attention from materialogists in general. It is the recoverable delayed elastic strain (des) in addition to elastic and viscous (matrix dislocation creep) deformation. For example, complex aerospace alloys exhibit a significant amount of delayed elastic effect not only during the primary or transient stages, but also during the tertiary creep regime. Progress made in ice mechanics, experimental as well as theoretical, have proved to be a fertile ground for explorations toward understanding the onset of interfacial failure processes in polycrystalline materials during the primary creep and eventual failures at high temperatures. The modern knowledge summarized in this book demonstrates that delayed elastic strain can be measured precisely at any stage of high‐temperature deformation through the careful design of experimental techniques (e.g., Chapter 4). This is illustrated in Figure P.1.

As mentioned earlier, a constitutive model, named as the Elasto – Delayed‐Elastic – Viscous (EDEV) model, was developed that recognizes delayed elasticity (that can be measured experimentally for quantitative verifications) as one of the most important aspects of high‐temperature engineering materialogy. As this text will show, it has been demonstrated that delayed elastic strain plays crucial roles in governing every aspect of primary (often called transient) creep curves and engineering stress‐strain diagrams and strain‐rate‐dependent strength (such as 0.2% offset yield and ultimate strength) properties. Finally, and very importantly, grain‐facet size cracks are initiated during primary creep, when des reaches a critical stage (Chapters 5, 6). The kinetics of microcracking and crack‐enhanced viscous (or dislocation) creep, essence of the EDEV model, leads to tertiary or accelerating stages in constant‐stress creep or constant strain‐rate deformation (Chapters 7, 8). The processes of grain‐boundary shearing (often referred to as sliding in the literature) induce recoverable delayed elastic strains. The grain‐boundary shearing mechanisms also govern the initial-strain (or initial-constrain) sensitivity of stress‐relaxation (SR) at high homologous temperatures, as presented in Chapter 9. The crack‐enhanced EDEV model, therefore, provides a physics‐based elucidation for the phenomenological observations on a huge number of engineering materials. And the methodology is very simple. Material characteristics for creep, and the kinetics of grain‐facet size cracking during creep, like those provided in Table 7.1 for ice, can be obtained for other materials by performing the appropriate strain relaxation and recovery test (SRRT) (Chapter 4), including the use of acoustic emission (AE) technology, and emphasizing, of course, evaluation of recoverable delayed elastic response.

Engineering design is most often based on “effective” elastic response, yield strength such as 0.1 or 0.2% offset yield stress, and/or design curves summarizing stress‐time‐temperature dependence of some specified strain. All these characteristics are strain‐rate sensitive and have been shown to be governed by primary or transient creep at high temperatures. It is shown in this book that primary creep is linked strongly to observable and precisely quantifiable delayed elastic phenomena, and that it is of utmost importance not only for characterizing the propagation of seismic waves in rocks (well recognized by geophysicists and volcanologists), but also for the prediction of strain‐rate‐sensitive 0.2% offset yield strengths, extremely important for design engineers. This book fills this gap in materials science in a significant manner.

Figure P.1 Delayed elastic strain (des) recovery. (a) constant‐stress creep of nickel‐base Waspaloy forgings at 1005K and 724 MPa; (b) constant strain‐rate strength test of directionally solidified (DS) ice at 263K (0.96T _m) and strain rate of 3 × 10⁻⁵ s⁻¹, as described in Chapters 4 and 6.

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 the Himalayan belt in the middle (Figure P.2).

Materialogists would perhaps give limited thought to the geophysically established fact that the secondary cryospheric zones of Earth – the Himalayas, Andes, Rockies, etc. – are products of high‐temperature phenomena active deep underneath Earth’s crust. Plate tectonics, very similar to sea ice dynamics, is briefly presented in Chapter 11. It is shown that the zone of reservoir‐induced earthquakes (or RTS), such as the Koyna–Warna area in India, may be predicted on the basis of the Elasto – Delayed-Elastic (EDE) aspect of the EDEV equation.

Figure P.2 “Trishul” (trident) of the two primary – North (N) and South (S) – polar regions, and the secondary regions represented by the Himalayas (H), with concentrations of snow and ice at extremely high homologous temperatures.

Source: Visionary sketch by N.K. Sinha.

History based on engineering physics looks to be the domain of professionals in metallurgy and materials science or materialogists. Where so much of the past, even the chronology, has to be teased from articulated intellectual objects emphasized in textbooks, scientific papers, and monographs, there surely must be need for a new perspective. However, much of the information required with state‐of‐art experimental observations was missing. The principal author, in particular, decided therefore to take a path that was a deviation from the normal.

Nirmal K. Sinha and Shoma Sinha