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

Чтение книги онлайн.

СКАЧАТЬ about this aspect and define engineering properties of materials accordingly. Hysteresis loop or stress–strain diagrams for constant stress (load) creep tests are unheard or not commonly discussed, perhaps because of uncertainty of the loading and unloading phases of the common creep tests using dead loads.

      Computer‐controlled SRRTs provide unique opportunities for examining the effective elastic modulus during both loading and unloading times, and examine the differences (if any) associated with creep damages. An example is shown in Figure 1.7a for the 200 s SRRT of Figures 1.4 and 1.5 and this illustrates the possibilities of determining E during loading and unloading sequences. Such stress–strain diagrams for creep tests are unheard of to most materials scientists.

      Comparing the E values of 180.0 GPa during the rise time to apply the full load and that of 177.6 GPa obtained for fall time (unloading) demonstrates that these values are complimentary and close to the dynamic values of previously undeformed and undamaged Young's modulus E of Waspaloy at comparable temperatures determined from seismic methods.

Figure 1.7b shows a stress–strain diagram for the longer (2341 s) duration SRRT of Figure 1.5. In this case, the E value of 178.2 GPa during the loading sequence is very close to those for short‐term loading and unloading periods in Figure 1.7a, but noticeably lower E value of 168.0 GPa obtained during the unloading period. This lower E value after longer‐term creep is indicative of a measure of the structurally “damaged” state of the material undergoing minimum creep rate and subsequent tertiary creep stage.

      Figure 1.7 also reveals a particularly important aspect of constant‐stress creep tests by noting the experimentally determined amounts of the three components of strain: elastic, delayed elastic, and viscous. Most important is the fact that the delayed elastic contribution to the total strain is measurable and not negligible. In the case of the 200 s test, ε _d is comparable to ε _v, whereas it is significantly lower in the case of the longer‐term 2341 s test. There was, therefore, a significant contribution of delayed elastic (anelastic) strain to the total “inelastic strain” at the time of the minimum creep rate. Realistic rheological models for high‐temperature engineering applications cannot ignore the contributions of delayed elastic strain, known to be associated with grain‐boundary shearing processes (Sinha 1979).

      Rheological models must be able to quantify the trinity or three‐component aspects of creep and relate to failure processes; this is covered in Chapters 5–8. It is shown in Chapter 5 that the delayed elastic effect could vary from a linear to a highly nonlinear response, but during primary creep the ratio, n _v/s, of stress exponents, n _v, for viscous flow (dislocation creep) and, s, for delayed elasticity could be very similar for crystalline materials in general. Since delayed elasticity has been linked strongly to grain‐boundary shearing processes, metallurgical and process engineering for superalloys may be directed toward (grain‐boundary engineering) increasing this n _v/s ratio to decrease the propensity for generating intergranular voids and cracks.

Figure 1.8 presents five sets of results for different specimens on the stress dependence of viscous strain rate during the primary creep obtained from short‐term SRRTs, and the corresponding minimum creep rate obtained from the long‐term tests. The same (or numerically very close) power law with a stress exponent of 11.0 applies for both n _v for viscous strain rate during primary creep and n _min for the minimum creep rate. These results and more are discussed in detail in Chapter 6 for a number of different materials in conjunction with development of rheological equations for the primary or transient stage of creep.

      

1.8.4 Romanticism for a Constant‐Structure Creep Test

      Creep theories for polycrystals are essentially based on idealized microstructures. This is essential to keep the number of material parameters as low as possible. To check the theoretical predictions, ideally experiments should be performed on specimens with exactly the same microstructure and chemical characteristics. In reality, it is impossible to have polycrystalline samples with exactly the same microstructure, i.e. the same number of grains, sizes, distribution, orientations, etc. CL creep tests are mostly carried out. Tests aimed at exploring the stress exponent (n _min or n _m) characterizing the dependence of steady state or minimum creep rate (mcr) on stress and temperature are therefore performed on different specimens with similar physical qualities. However, mcr may occur at a relatively large strain with evolved microstructure depending on the initial structures, stress, temperature, and test environment (air, gas, or vacuum). All the tests are different even though performed under similar conditions. Is it possible to use one specimen (i.e. constant microstructure) to perform a number of very short‐term creep tests and determine n _v for viscous flow (dislocation creep)? The answer will be a quick “no,” “impossible.” This quick answer is based on classical approaches that have been taken for many decades. This is where a change in paradigm is required; Figures 1.4–1.7 are illustrations of this possibility using SRRT – a stylized name for a simple creep and recovery test originally developed for ice (Sinha 1978a), later coined by Sinha (2001), and used extensively for advanced aerospace alloys.

Figure 1.8 Stress dependence of average viscous strain rate during primary creep and the corresponding minimum creep rate from short‐term and long‐term SRRTs on five different specimens of Waspaloy at 1005 K.

      Source: N. K. Sinha.

The numerical equivalency of n _v and n _min, for stress levels of engineering importance, has far‐reaching consequences. Since failure and fractures in metals and alloys at high temperatures are traditionally linked to n _min, does it mean that SRRTs requiring “a single specimen” can be used for the characterization of failures? Sounds incredible! One of the primary goals for writing this book is to convince metallurgists, ceramicists, and rock mechanics researchers to use SRRTs and explore certain important aspects of materials (delayed elasticity and crack nucleation and multiplication during primary creep stages) neglected so far in the general field of materials science and engineering.

One approach of utilizing the methodology of “hindsight” is to perform a series of creep and recovery tests (SRRTs) on one specimen. For each test, the full load is applied as quickly as possible and, at the end of the creep time, completely unloaded from the specimen very quickly. The strain is continuously monitored during the recovery. This approach allows one to explore the “trinity of creep” in a quantitative manner at different stages СКАЧАТЬ