Microcracks act as precursors for crack propagation in brittle fracture. The process of cleavage fracture involves three steps: (a) plastic deformation to create

dislocation pileups, (b) crack initiation, and (c) crack propagation. The initiation of microcracks can be affected by the presence of second-phase particles. Figure 5.16a shows one example of microcrack formation in iron after tensile deformation at —140 ° C. The appearance of clevage fracture surface in a low carbon steel is shown in Figure 5.16(b). Cleavage cracks can also be initiated at mechanical twins.

The ductile fracture starts with the initiation of voids, most commonly at second — phase particles. The particle geometry, size, and bonding play an important role. Dimpled rupture surface (ductile fracture) contains cup-like depressions (dimples) that may be equiaxial, parabolic, or elliptical, depending on the precise stress state. Microvoids are generally nucleated at second-phase particles, and the voids grow and eventually the ligaments between the microvoids fracture. The different stages of this process are shown in Figure 5.17(a) and an SEM fractograph with dimples shown in Figure 5.17(b).

Ductility (as expressed in true strain to fracture) of a material may depend on the volume fraction of second-phase particles present, as shown in Figure 5.18 with examples in steel.

5.1.4