Читать книгу Engineering Physics of High-Temperature Materials - Nirmal K. Sinha - Страница 33

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 (σ–ε) diagrams are used to characterize the yield and ultimate strengths. These diagrams have no room for time. Traditionally, the initial linear part of σ–ε curves implies elastic regime, and deviation from the linearity is considered as plastic flow. At elevated temperatures, the linear σ–ε 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.

There was a need to overhaul the basic experimental techniques. Classical use of the dead‐load creep test procedures, ideal for long‐term tests, was complemented with the introduction of closed‐loop‐controlled constant‐stress “strain relaxation and recovery tests” (SRRTs) for short‐term tests (Chapter 4). Dead‐load systems cannot provide the necessary data for the initial and early part of creep curves, necessary for the interpretation of engineering (say 0.2% offset) yield strengths or low‐cycle fatigue or dwell‐fatigue properties. SRRTs can generate a significant amount of data on reversible elastic and delayed elastic strain and the irreversible (permanent) viscous strain components in a short time using only one specimen, if necessary, to avoid specimen‐to‐specimen microstructural variations and thus keep the structure fixed. Three basic components of strain (elastic, delayed elastic, and viscous) during primary creep over a wide range of stresses and temperatures can be examined. SRRT is more powerful than any mechanical tests popular today for investigating “constant stress” and “constant microstructure” creep properties at high temperatures.

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 creep recovery, over several months, allowed examination of reversible delayed elastic strain. For a total strain of 15%, the delayed elastic strain was found to be about 0.15 or 25% of the elastic strain. A possible relationship has been proposed between the delayed elastic strain measured at 15% strain and area fraction of γ′ facet‐sized cracks for the onset of ultimate fracture. Nonetheless, delayed elastic effect in the single‐crystal material could not be detected during the primary creep period of small strain of engineering importance.

The lack of a reliable, simple – with a limited number of material constants or parameters – microstructurally sensitive, three‐dimensional constitutive model of polycrystalline materials at high homologous temperatures is the stumbling block against the use of sophisticated computer modeling and numerical simulations available today. A successful constitutive equation should have a physical basis and be able to describe the features that can be readily seen in the real field conditions. Such a model for inelastic deformation must be able to predict the usual empirically derived relations and be adequate for extrapolation to account for the effects of microstructural altercations or loading conditions. Moreover, such a model must be capable of predicting delayed elastic phenomena that can be quantified on the removal of the external driving forces. This means, predictability of the rebound of the structure on partial or full unloading should be built in the model. To date, most constitutive equations are highly empirical in nature and are useful for interpretation in the range investigated, but inadequate for extrapolation.

The chronology of the development of the three‐term rheological model, called EDEV model, is described in Chapter 5. This model relies on the incorporation of grain‐boundary shearing processes to relate with the delayed elastic phenomenon. A review of the phenomenological aspects of the failure processes at elevated temperatures is presented in Chapter 6. The crucial roles played by the phenomenon of delayed elasticity are examined in depth. Delayed elasticity can be linked to the predictability of the conditions necessary for the onset of grain‐facet‐sized crack formation and the kinetics of microcracking (Chapter 7). This leads to the development of the crack‐enhanced EDEV model that can handle, for example, the strain‐rate sensitivity of strength of polycrystalline solids at high temperatures (Chapter 8). EDEV is a nonlinear constitutive equation for high‐temperature applications. It consists of elastic, delayed elastic, and viscous components corresponding to three micromechanisms: lattice deformation, intergranular shearing/sliding, and intragranular dislocation motion. The model incorporates the predictability of the onset of cracking activity and damage accumulation due to the mechanism(s) of high‐temperature grain‐boundary embrittlement.

Grain facet long cracks develop when a critical grain‐boundary sliding or shearing (gbs) displacement or an equivalent delayed elastic strain is reached. Further damage is given in terms of the excess gbs, displacement over its critical value. As cracks form, they enhance the deformation matrix affecting the overall creep rate, leading to a minimum rate and then tertiary creep. Formulations have been developed in Chapter 8, using this model, for predicting the deformation and cracking activity for conditions of constant strain‐rate strength tests. The theory was tested with published experimental data on the strain‐rate sensitivity of the compressive strength of transversely isotropic, columnar‐grained, pure polycrystalline ice with a load applied in the plane of isotropy. Calculations using material constants were obtained from constant‐stress creep and recovery (SRRT) experiments totally independent of the strength tests. One‐to‐one correspondence of theory and experiments was noted for the dependence of strength, failure strain, and failure time on strain rate. The theory has the capabilities for predicting the popular empirically obtained relations between these quantities, such as the MG relationship. The EDEV theory also points out the limitations of the empirical relations, for example the inapplicability of the empirical power‐law relation between t _f and σ _f or between ε _f and strain rate. It is shown that strain‐rate sensitivity of volumetric dilatation can also be reasonably predicted.

The EDEV model has been extended to stress relaxation tests (SRTs) at elevated temperatures in Chapter 9 and applied successfully to nickel‐base superalloys, like In‐738LC, titanium‐base alloys, like Ti‐6246, and polycrystalline ice (especially the grain‐size effects on stress relaxation). Theoretical predictions on total strain as well as the components of elastic, delayed elastic, and viscous strain at any time during creep and SRTs can be examined and compared with experimental observations. EDEV model is based on observations that the viscous strain rate (for dislocation creep) is constant throughout the primary creep under constant stress (for conditions of no microcracking activities) and the shape of the primary creep is governed by delayed elastic deformation that is recoverable on unloading. The acid tests of the developed model are provided by the comparisons between the predicted permanent strain and the recovered elastic and delayed elastic strain components with the corresponding experimental observations of both SRRTs and SRTs. Since delayed elastic response in single‐crystal materials may be neglected, the EDEV model reduces to a simple nonlinear elasto–viscous relation, as presented in Chapter 5, for most of the deformation of engineering design purposes.

But why does the viscous strain rate, associated with dislocation creep, appear to be constant during normal primary or transient creep? This SRRT‐based experimental observation in single‐phase crystalline materials and complex crystalline materials, including at least one rock type, ice, and complex alloys, goes against the dislocation theories developed over many decades. Theoreticians have to modify some of the classical assumptions and start working on the development of some new ideas.