Engineering Physics of High-Temperature Materials

Preface

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...