The close-packed lattices have a specific atom stacking sequence. For example, FCC crystal has ABCABC… stacking sequence, as shown in Figure 2.39a. Any disrup­tion in this stacking sequence causes local region to get out of perfect sequence, as shown in Figure 2.39b. Locally, this region is improper or faulty, and so this region is called a stacking fault. Stacking faults possess surface energy. The surface tension due to this tends to minimize the area of the faulted region. Stacking fault energies of some typical metals/alloys are as follows: Al: 0.2 J m-2, Cu: 0.04 J m-2, Ni: 0.03 J m-2, Cu-25Zn: 0.007 Jm-2, Fe-18Cr-8Ni: 0.007 Jm-2. This topic will be revisited in detail in Chapter 4.