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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has been made in mapping the regime of dislocation creep on the basis of experimental observations on the stress–temperature–grain size dependence of the secondary or actually minimum creep rate (Ashby 1972; Frost and Ashby 1982). While this was convenient for a working‐model kind of description, minimum creep rates, as pointed out by Evans and Wilshire (1985) and shown in Chapter 8, represent evolved characteristics, not fundamental properties of polycrystalline materials. The deformation maps also describe the conditions for diffusional creep, which is extremely important for the processing industry. Burton (1977) has given a brief, but comprehensive, introduction to diffusion creep of polycrystalline materials.

      The concept of “strength” implies a specific material property. Methods have been developed to quantify tensile, compressive, bending, shear, etc. strength and standard procedures have been developed for conducting appropriate tests (see Chapter 4). At ordinary temperatures, strength is taken for granted to be independent of the rate of loading. However, the rate effects are recognized, especially for materials with pores and microcracks, and standard tests are recommended to perform in a specific range of loading rate. Stress–strain (σε) diagrams are used to characterize the yield and ultimate strengths. These diagrams have no room for time. Traditionally, the initial linear part of σε curves implies elastic regime, and deviation from the linearity is considered as plastic flow. At elevated temperatures, the linear σε response does not necessarily mean pure elastic response. Material strength, defined by yield, is thus a low‐temperature concept. This low‐temperature concept does not apply at the elevated temperatures relevant to many engineering applications. Nonetheless, the concept prevailed and was extended to elevated temperatures. It has retarded growth in the understanding of failure processes involving inelastic deformation or creep and hence microstructure–property relationship in engineering components subjected to high temperatures. 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 design problems. At high temperatures, strength, creep, and relaxation properties are intertwined and require a holistic approach to understand the complexities of engineering properties at high temperatures.

      There are a number of excellent reviews and innumerable books and monograms on strength of materials, including response at high temperatures on physics and mechanics of creep and strength of metals and alloys. The situation has improved significantly over the last century in extending this knowledge to other materials, such as rocks and ceramics. Investigators in the field of mechanics of nonmetallic materials looked toward the knowledge gained and concepts developed in metallic materials. Theories on diffusion and dislocation creep in metals, concentrated on explaining the steady‐state flow, dominated the minds of innumerable experimentalists irrespective of materials. The main reason, of course, includes the technical difficulties in performing experiments at high temperatures together with in situ measurements and in‐depth microstructural analysis at experimental temperatures. While presenting a damage mechanics treatment of creep failure in rock salt, Chan et al. (1997) remarked that it is not difficult to give a rational explanation for the deficiency, but the fact admits of no doubt of experimental difficulties and limitations, but also challenging traditional approaches. The transient creep, although recognized, is given only a passing importance and practically no importance is given to the delayed elastic phenomenon even though delayed elastic strain can be measured and analyzed. The contribution of transient creep, particularly the recoverable part, is assumed to be negligible without any serious challenge to this belief. Fortunately, the situation has improved considerably over the last 40 years in engineering physics of ice, a material that naturally exists at extremely high homologous temperatures. In situ measurements can be made and microstructural investigations can be made at experimental temperatures. Engineering physics of ice emphasized the importance of micromechanics of transient creep and its role in developing grain‐boundary microvoids, kinetics of void formation, void‐enhanced creep, etc. The transparency of pure polycrystalline ice and relatively large grain sizes has also been proved to be most important, in fact unique, in making in situ observations on grain‐facet‐sized void formation.

      As described in Chapter 5, SRRT methodology was based on experimental observations on glass, an amorphous material. It was extended to ice that is crystalline. Eventually, SRRTs were performed on a titanium‐base alloy, Ti‐6246, a wrought nickel‐base alloy, Waspaloy forgings, nickel‐base precipitation hardened superalloys, IN‐738LC, and single‐crystal superalloys. Minimum creep rates were also examined using SRRTs. Both viscous strain rate and the minimum creep rate were shown to obey the same power‐law dependence on stress with equivalent stress exponents. SRRTs also allowed investigations of the stress sensitivity of the delayed elastic strain. Similar to the viscous flow, varying from a linear to a highly nonlinear response, delayed elasticity was also shown to vary from linear to nonlinear, though to a lesser extent. In Chapter 5, we have also discussed SRRTs performed on a [001]‐oriented nickel‐base single‐crystal, CMSX‐10, in the temperature range 1073–1273 K for conditions where rafting of γ′ did not play a major role. Long‐term