Engineering Physics of High-Temperature Materials. Nirmal K. Sinha. Читать онлайн. Newlib. NEWLIB.NET

Автор: Nirmal K. Sinha
Издательство: John Wiley & Sons Limited
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Жанр произведения: Техническая литература
Год издания: 0
isbn: 9781119420460
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      Mechanical metallurgy is important because the structural response to mechanical forces can cause a huge impact on macroscopic properties. Processes, such as rolling and extruding, can impart strong texture within a polycrystalline material. This preferred orientation can lead to anisotropic properties on the macroscale. Understanding the effects of forming processes on final component properties is increasingly being recognized as a tool to obtain properties of interest, as well as avoiding unwanted impacts.

      When dealing with alloys at high temperatures, an additional point to consider is the definition of the melting point. For pure elements, T m can be determined precisely, but complications arise for engineering materials with complex compositions for which the melting point cannot be defined well. For metallic alloys, liquidous point – the temperature at which an alloy is completely melted – is convenient to use.

      

      At low homologous temperatures, “elastic” response dominates the deformation in most crystalline solids. Very little inelastic strain occurs during load application time until the stress reaches a limiting level called yield strength or after a reasonably long time of sustained load. If a polycrystalline specimen is uniformly loaded to a uniaxial tension or compression, the specimen deforms elastically to a limiting strain, known as elastic limit. Simplest assumption is that all the grains also suffer the same strain as that of the specimen as a whole. Most solids, with very low porosities, exhibit linear elastic behavior. Unless specifically mentioned, linear elastic response is assumed to be the norm for all solids of engineering importance. Customarily, the porosity is assumed to be very low. However, the compaction of materials with high porosity subjected to load results in nonlinear response. Porous materials, such as firebricks and ceramic foams (used as insulations and protective covers), exhibit stress‐wise nonlinear elastic response, but we will not address such materials in this book.

      With increase in temperature, two other mechanisms start to contribute to the deformation: “delayed elastic” and “viscous.” The delayed elastic response may also be called “anelastic,” but we will refrain from using it as per the suggestion of British Standard Institution (1975). Mechanical response of materials at high temperatures can be described as “Elasto – Delayed‐Elastic – Viscous” or simply response (see Chapter 5). Irrespective of loading conditions, monotonic or cyclic, total strain can be described generally as:

      Trinity of High‐Temperature Deformation Mechanisms

      EDEV‐1

      EDEV‐2

      EDEV‐3

       EDEV‐1: Materials exhibiting “linear elastic, linear delayed elastic, and linear viscous” behavior. Ordinary soda‐lime–silica glass used as structural materials in buildings and cars belongs to this class.

       EDEV‐2: Materials exhibiting “linear elastic, linear delayed elastic, but nonlinear viscous” response. As an example, natural water ice (H2O) on Earth's surface belongs to this class.

       EDEV‐3: Materials exhibiting “linear elastic, nonlinear delayed elastic, and nonlinear viscous” response. Most metals and alloys belong to this class of materials.

      It should be noted that, in this book, we will use the term “viscoelasticity” in a general sense to refer to time‐dependent mechanical response, irrespective of linear or nonlinear dependence of flow on stress.

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