Mode of combined fracture

It must not be assumed that brittle fracture always occurs solely by the cleavage or the intergranular fracture mode as described above. In most cases one mode predominates but is not necessarily the only mode. For example, in a predominantly intergranular fracture, there probably will be regions, large or small, in the fracture surface that contain cleavage fracture as well. The reverse also is true. In other words, the mode of fracture that occurs at a particular location depends on the local composition, stress, environment, imperfections, and crystalline orientation of the grains. There may also be regions of tough, fibrous, dimpled-rupture fracture, particularly away from the origin of the major fracture.
It should be noted that cast metals generally tend to be less ductile (or more brittle) than wrought metals of the same composition under the same conditions. The reason is that various types of casting imperfections-particularly shrinkage porosity, gas porosity, and certain types of inclusions-act as internal stress concentrations in castings. In wrought metals the hot working process closes the porosity and changes the shape of many inclusions to long, thin stringers, which are usually less harmful.