Relation of the stacking fault energy to segregation at stacking faults and to the occurrence of phase boundaries in f.c.c. binary alloys

A calculation is made on the segregation of solute atoms at stacking faults in f.c.c. crystals under equilibrium conditions. The expression given by Dorm relating the concentrations of solute atoms in the faulted and unfaulted regions is rederived. A new expression is given for the equilibrium width of the stacking fault in terms of the solute concentration, which can be interpreted in terms of the balance of forces on each partial dislocation. On the assumption that the stacking fault energy is exactly the difference in the free energy of the f.c.c. and h.c.p. structures of the same composition and temperature, it is shown that the f.c.c. and h.c.p. phase boundary concentrations of the alloy may be related to the concentration dependence of the stacking fault energy. A numerical example for a hypothetical alloy illustrates the various aspects of the theory.