The Growth of Intergranular Cracks at High Temperatures

Abstract

Intergranular cracks at high temperatures are classified into two groups, i. e., the wedge type and the cavity type. The former is sharp and long, and is considered to grow by grain boundary sliding. The latter is round in shape, and is thought to grow by diffusion. The present work shows that, by taking into account both grain boundary and surface diffusion, a crack of the cavity type can grow laterally flat along a grain boundary which is normal to tensile stress. The effect of strain energy which is negligible when the spherical growth is assumed must come into the estimation in the present treatment.
The lateral growth is feasible especially when the wedge type cracking by grain boundary sliding is prohibited by coarse grain boundary precipitates and hence a high stress is applied. For usual structures without such coarse grain boundary precipitates, when cavities grow to a certain size which depends on the applied stress, they begin to become flat, linking each other more rapidly than in the case in which the spherical growth is assumed to continue.
Effects of the gas content and the resistance to creep on the cavity growth are discussed.