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Time, Temperature, and Environmental Effects on Properties
Published in David W. Richerson, William E. Lee, Modern Ceramic Engineering, 2018
David W. Richerson, William E. Lee
Creep in polycrystalline ceramics is usually controlled by the rate of diffusion or by the rate of grain boundary sliding. Diffusion involves the motion of ions, atoms, or vacancies through the crystal structure (bulk diffusion) or along the grain boundary (grain boundary diffusion). Grain boundary sliding often involves porosity or a different chemical composition at the grain boundary. Grain boundary sliding is an important (and undesirable) contribution to fracture in many ceramic materials densified by hot pressing or sintering. Additives are required to achieve densification. These additives concentrate at the grain boundaries together with impurities initially present in the material. If a glass is formed, it may soften at a temperature well below the temperature at which the matrix material would normally creep, allowing slip along grain boundaries. Grain boundary sliding is normally accompanied by the formation of pores at the grain boundaries, especially by cavitation at triple junctions (junctions at which three grains meet). The combination of reduced load-bearing capability due to softening of the grain boundary glass phase and formation of new flaws usually results in fracture before appreciable plastic deformation can occur.
Diffusion
Published in Gregory N. Haidemenopoulos, Physical Metallurgy, 2018
When the interstitial or substitutional diffusion takes place in the lattice, it is called lattice diffusion or volume diffusion or bulk diffusion. However diffusion can be assisted by structural imperfections. Grain boundary diffusion is the diffusion of atoms through grain boundaries, which exhibit a more “open structure” than the bulk of the crystal, as depicted in Figure 5.1c. In this case the rate of diffusion is higher than bulk diffusion. In fact diffusion is faster in metals with a finer grain size. This affects the deformation at high temperatures, i.e., creep, where diffusion plays an important role. As will be discussed in Chapter 9, alloys exhibiting high creep resistance, have either a coarse grain size or they are manufactured as single crystals. Dislocations assist diffusion, since the space below the extra half plane serves as a channel for the rapid diffusion of atoms, as depicted in Figure 5.1d. This type of diffusion is called dislocation pipe or core diffusion. Both grain boundary and dislocation diffusion proceed at higher rates than bulk diffusion. Grain boundaries and dislocations operate, therefore, as high-diffusivity paths and influence the kinetics of diffusional phase transformations.
Creep and Fatigue of Metals
Published in Yichun Zhou, Li Yang, Yongli Huang, Micro- and MacroMechanical Properties of Materials, 2013
Yichun Zhou, Li Yang, Yongli Huang
At low temperature and low stress, grain boundary diffusion plays a major role. Coble-type creep can be described by the following equation [18]: () ε˙sc≈50σb4DgbkTd3,
Microstructural evolution and grain-growth kinetics of Al0.2CoCrFeNi high-entropy alloy
Published in Philosophical Magazine Letters, 2021
Srinivas Dudala, Chenna Krishna S, Rajesh Korla
It is well established that the room-temperature mechanical properties of any structural material strongly depends on the average grain size, the size distribution, and the nature of the grain boundaries [1]. Thermomechanical processing can enhance both strength and ductility by refining the grains via recrystallization processes. The grain size then increases via grain growth during subsequent annealing at high temperatures. During grain growth, grain-boundary migration occurs by a local shuffling of atoms across the grain boundary, which is assisted by grain-boundary diffusion. As grain-boundary migration and grain-boundary diffusion involve similar mechanisms, similar activation energies[2–6] will be involved.
Effects of T5 heat treatment on extrusion welds in AZ80 hollow profile
Published in Materials and Manufacturing Processes, 2018
Gaoyong Lin, Yuyong Wei, Ke Zhou, Hongyang Wang, Weiyuan Song
Considering Figs. 45678, from 170°C to 250°C, the amount of continuous precipitates increases, while that of discontinuous precipitates decrease gradually, which indicates that discontinuous precipitates (mainly referring to lamellar and elliptical precipitates) are favored at low temperature, whereas at comparatively high temperature, continuous precipitates occur in whole samples uniformly, which is in agreement with finding of Duly et al. [18]. Based on previous studies [19,20], the process of discontinuous and continuous precipitates is controlled by two different diffusion mechanisms. At lower temperature, grain boundary diffusion is dominant diffusion mechanism, which favors discontinuous precipitates. With grain boundary diffusion mechanism, the discontinuous precipitates always nucleate at the grain boundaries and grow up with the migration of grain boundary, which from original position into adjacent grain. As mentioned above, the lamellar precipitate is the predominant form of precipitates at 170°C, the discontinuous precipitate (mainly referring to lamellar structure) reaction depends on interface energy and grain boundary orientation [21,22]. Presumably because the grain boundary along extrusion welds is not conducive for the initiation and the growth of lamellar precipitate, resulting in large width of PFZs at 170°C. Whereas at 200°C, same grain orientation along extrusion weld may be active. Moreover, the volume diffusion is activated, which favors continuous precipitates, and the continuous and discontinuous precipitates occur competitively. Because of active grain orientation and less volume fraction of lamellea precipitates, the width of PFZs is reduced compared with 170°C aged samples.