In Chapter 7, we described the macroscopic aspects of the fracture behavior of materials. As with other characteristics, the microstructure of a material has a great influence on its fracture behavior. In what follows, we present a brief description of the microstructural aspects of crack nucleation and propagation, as well as the effect of the environment on the fracture behavior of different materials. Figure 8.1 shows, schematically, some important fracture modes in a variety of materials. These different modes will be analyzed in some detail in this chapter. Metals fail by two broad classes of mechanisms: ductile and brittle failure.
Ductile failure occurs by (a) the nucleation, growth, and coalescence of voids, (b) continuous reduction in the metal's cross-sectional area until it is equal to zero, or (c) shearing along a plane of maximum shear. Ductile failure by void nucleation and growth usually starts at second-phase particles. If these particles are spread throughout the interiors of the grains, the fracture will be transgranular (or transcrystalline). If these voids are located preferentially at grain boundaries, fracture will occur in an intergranular (or intercrystalline) mode. The appearance of a ductile fracture, at high magnification (500× or higher) is of a surface with indentations, as if marked by an ice-cream scooper. This surface morphology is appropriately called dimpled. Rupture by total necking is very rare, because most metals contain second-phase particles that act as initiation sites for voids.