Читать книгу Engineering Physics of High-Temperature Materials - Nirmal K. Sinha - Страница 57

Solid materials are rarely made of pure single crystals. Different grains in polycrystalline material can be recognized by a mismatch in atomic alignment between regions in the solid. The grain boundaries are, in themselves, a source of great imperfection within a solid.

Small mismatches (less than 10–15° depending on the material) are called low‐angle grain boundaries or more commonly “subgrain boundaries.” Subgrain boundaries include “tilt” and “twist” boundaries and contain dislocations of various types. The energy of subgrain boundaries depends upon the degree of misorientation and types of dislocations.

High‐angle grain boundaries – or simply grain boundaries – separate regions with high mismatch in lattice arrangement. Grain boundaries are disordered regions associated with high energy. The disorder of grain boundaries is a result of not only lattice mismatch of adjacent grains, but also the segregation of various impurities and voids into the edge of the grains. Grain boundaries tend to have lower density and higher reactivity than the grains themselves. They can also act as conduits or pathways for various processes, such as corrosion, into the bulk.

There is a driving force to reduce grain boundaries energy by decreasing the grain boundary area. At elevated temperatures, some grains will tend to grow at the expense of others. Grain boundary mobility often depends on bulk diffusion processes and follows an Arrhenius‐type relationship. Grain boundary motion can also depend on the radius of curvature of the boundary, structure of the boundary (pores, ledges steps, etc.), crystallography of the grains involved, and the presence of impurity and solute atoms and second‐phase particles. For example, the dispersion of fine particles of a second phase reduces the energy of grain boundaries and increases their stability through a process known as Zener pinning.

Grain boundaries are an extremely important influence in a variety of material properties and processes, including yield and tensile strength, elongation, and formability. Often, GB effects are expressed as a variation of the property with grain size. In fact, the impact of grain size (and thus GB area), grain boundary chemistry, and grain boundary morphology are all important parameters. Many of these effects will be discussed throughout the text.