Figure 2 shows one of the most classical cleavage, or transgranular cracks, that I have ever found in a broken part. The red arrows highlight parallel sets of steps that form as the crack races through the part at the speed of sound. Cleavage features are a result of the fact that metals are crystalline, and some planes within a crystal break apart (cleave) more readily than others. The steps on the fracture face are a result of the physical orientation of the crack due to loading geometry not being exactly aligned with the specific preferred cleavage plane. The “mismatch” between these two planes is taken up by these small steps.
The white circle shows a fan or “river pattern,” referring to a river tributary system. When you see one of these river patterns, you can determine the crack direction for that tiny area of the crack. It can be very misleading to find one or two of these little patches and assume that you can determine the macro-scale crack direction. But assuming that you want to know the local (for that single grain) crack direction, in the above case it would be from the top to the bottom, as the tiny individual cracks at the grain edge are merging into one single crack as it grows. The big hexagonal shaped grain slightly left of center seems to have started cracking from multiple grain edges all at once. Cleavage fractures, in general but not always, happen fast.
Figure 3 shows a classical intergranular, often called “rock candy,” crack appearance. Hydrogen embrittlement often weakens the grain boundaries more than the interior of the grains. Other processing problems (such as very slow cooling during a quench operation on a very large part, tempering at an inappropriate temperature, or carburizing, nitriding or carbonitriding in an atmosphere that is too rich in carbon or nitrogen) can all facilitate intergranular cracks. Stainless steels, aluminums and other alloys have different processing problems or environmental exposure types that can promote intergranular cracking.
Often, but not always, intergranular cracks are a sign that something undesirable somewhere happened. The component of Figure 3 was caused by hydrogen attack, as far as could be demonstrated, which often means that one can find a known or suspected source of hydrogen and no evidence that there was a heat-treating or other processing problem. Intergranular cracks from hydrogen embrittlement that happens during electroplating usually have very sharp and “clean” features. The bits of debris on this part are a result of degredation during a longer period of exposure to the elements after the part broke and was left outside for a few months in the winter. Intergranular cracks also are very common in steel that has had a quench-cracking event. If the part has been quenched in oil or water, there will often be much more rounded edges to the exposed grain boundaries, and there may be traces of oil or rust on the crack surface. Compare to Figure 5 from the Oct. 2, 2008, post.