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

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СКАЧАТЬ most crystalline materials on Earth and metals and metallic alloys increase with increase in pressure. This is due to the increase in density on solidification unlike water changing its phase to ice. Naturally, applications of general rheological flow to glaciology must give consideration to pressure effects in the case of ice. However, such effects have not been completely sorted out within the glaciology field so it would be inappropriate to claim that the proposed ideas would somehow offer a unifying model applicable to all aspects of glaciology.

      High‐temperature mechanical properties of materials are characterized by their creep life. Creep and fracture properties are governed by structural changes, as well as damages in the form of cavities and cracks that occur during deformation. Most tests are performed in air. It is convenient to perform tests in air, simply because providing an inert environment during tests is very difficult. However, oxidation starting from the exposed surfaces of specimens in pure metals and alloys at elevated temperatures is unavoidable. Materials are degraded or damaged by the formation of voids preferably along the grain boundaries (being at higher energy states). For example, Wilshire and Battenbough (2007) performed tensile creep and rupture tests on fine‐grained (≈40 μm) polycrystalline pure copper under truly constant stress (using the machine described in Evans and Wilshire 1985), as well as constant load at 686–823 K (about 0.5–0.6 T _m), and demonstrated an increasing number of voids with increasing distance from the surface. They reported that fracture invariably occurs by cavitation, with the following observations noted: (i) Isolated grain‐boundary cavities are evident late in the primary stage during creep in both the n ∼ = 1 and n ∼ = 4.5 regimes, (ii) cavities form preferentially near specimen surfaces, with the numbers of cavities and cracks decreasing from the surface to the center due primarily to the formation of oxide particles created by oxygen ingress along grain boundaries during creep exposure (Parker and Wilshire 1980), and (iii) the incidence of cavities increases with increasing creep strain, eventually forming single‐grain‐facet cracks that are linked to producing large multigrain‐facet cracks at various locations along the gauge length of fractured specimens.

      In many respects, the above descriptions of creep cavitation and failure processes in metals (except for oxide formation) are remarkably similar to the observations of cracking activities during creep in ice (Gold 1972a, b). However, most importantly, the transparency of ice allows the use of visual and optical methods of observing the formation of cracks, as well as the use of acoustic emission methods for quantifying the kinetics of cracking activities during creep at experimental temperatures.

1.4 Rationalization of Temperature: Low and High

      The Celsius scale of temperature is almost universally accepted as the standard scale for temperature. It is based on the thermal state of pure water, which is the source for life and most abundant material on Earth's surface. In this scale, and for conditions under normal atmospheric pressure at sea level, the solidification temperature of pure water is considered as 0 °C and the boiling point is assumed to be 100 °C. Consequently, temperatures of the solid state of water (that is ice) are assigned with “negative” signs. The problem with this scale is that any temperature less than 0 °C is described as negative (−sign); a better scale is kelvin, as there is no negative temperature. However, is it a rational temperature for materials scientists to use? We will go to this question later, but let us first discuss human issues with the Celsius scale.

      Since ice is cold for the human body, “negative” temperatures are naturally considered as “cold.” Water temperature of +50 °C may be considered as “warm,” but any human body temperature of 40 °C is considered as rather “high” and feverish because the body temperature of about +37 °C is considered as “normal” for a human. These definitions for temperatures were convenient and extended to materials in general. Materials at temperatures greater than say 70 °C may be considered as “high temperature” for human safety. In this way, the title of this book may be misleading. For materials science and engineering, the definition of high temperature has to be based on the state of the solid material under scrutiny.

      The solid state is also subject to question. We will, of course, generally avoid solids, like polymers, in order to simplify the scope of this book. Ordinary glass is a solid with amorphous structure, but it could, phenomenologically speaking, deform like crystalline materials as temperature rises. Some rationalization can be made with glass because its structure may not show a long‐range order, but may have crystal‐like nanoscale structure. The primary issue then is how to characterize the thermal states of crystalline solids in general and relate them to all natural, ice and rocks, and fabricated materials, metals, and alloys.

      As for metallic alloys, in many sections of this book, we will concentrate on engineering properties of titanium‐base and nickel‐base superalloys due to the availability of relevant experimental results obtained directly by the authors of this book. These alloys are more complex than any metallic alloys being used today. Many metallurgists consider them as the most fascinating of all metallic alloys and they are mostly used for the hottest parts of gas turbine engines. Actually, their use encompasses the highest homologous temperature of any common alloy system (Ross and Sims 1987).

1.5 Deglaciation and Earth's Response

Ever since the formation of Earth, global‐scale changes, known as tectonic movements, are shaping the surface features exposed to the atmosphere. Global‐scale cooling and warming and the associated climate change are a cyclic phenomenon. The evaporation of water from the oceans and condensation in the form of snow on land during the cooling cycles lower the level of sea surfaces. A reverse phenomenon occurs during the warming part of the cycles. During warming, the changes in the geometry of the ocean floor and the volume of water in the oceans are linked to a number of interdependent and complex geophysical processes (Nakada and Lambeck 1987).

      Sea‐Level Change

      GLACIAL MELTING

      EARTH'S RESPONSE

      TECTONIC PROCESSES

      Depending on the location on Earth's surface, there are three main processes that govern the rise and fall of sea level relative to the crust. These are (i) the melting histories of ice sheets, (ii) the mechanical response of Earth's plates to the redistributed surface load of ice and meltwater, and (iii) tectonic processes causing uplift or subsidence of shorelines.

      Uses of Sea‐Level Changes

      GLACIOLOGISTS

      TECTONOPHYSICISTS

      GEOPHYSICISTS

      Nakada and Lambeck (1987) also pointed out three different disciplines with different approaches to examine and use the changes in sea level that occurred during the Holocene period: “Glaciologists have used the sea‐level information to constrain the volume of past ice sheets; tectonophysicists use the relative sea‐level changes to examine the processes that deform the crust and lithosphere; geophysicists use the sea‐level signatures to estimate the rheological properties of the mantle and the mechanical properties of the lithosphere.”

1.6 High‐Temperature Deformation: Time Dependency

      1.6.1 Issues with Terminology: Elastic, Plastic, and Viscous Deformation

      The deformation in a crystalline material consists of two major parts – an elastic part associated with the recoverable distortions of its atomic lattice structure, and an inelastic part. The term “inelastic” is associated with the part of the deformation that is “really a deviation” from the time‐independent (isothermal) elastic response. In classical СКАЧАТЬ