Planar faults in the L12, lattice Stability and structure

Abstract

The stability of various planar faults on the {111} and {100} planes of A₃B alloys having the L1₂ structure is investigated using two methods. First, the crystallography of the faulted lattice is examined for indications of which types of faults are expected to be stable on the basis of symmetry. It is shown that superlattice intrinsic stacking fault, SISF, on the (111) plane with a fault vector, f, of 1/3[121] must always be stable. These same considerations show that the antiphase boundary, APB, can be stable with a fault vector of 1/2[110] +f′ where f′ lies along [112]. Similarly the complex stacking fault, CSF, con be stable with a fault vector of 1/6[211] + f′, whore f′ lies along [211]. In both cases f′ may be zero. Thus the prediction of the hard sphere model for the SISF is correct, but it is not necessarily correct for the APB and CSF. Secondly, the γ surfaces (i.e. the dependence of fault energy on displacement) for the {111} and {100} planes are calculated using a series of central force potentials which all represent a mechanically stable L1₂ lattice but correspond to different ordering energies and thus lead to different APB energies on {111} planes. In this way the stability of various faults as a function of the ordering energy is investigated. It is found that the SISF is always stable, and the fault vector is determined from the hard sphere model; however, the APB and CSF can be unstable (the CSF always becomes unstable before the APB), and the fault vectors may differ from those based on the hard sphere model. The deviations are in the directions predicted from the symmetry considerations. The implications of these results for dislocation dissociations and dislocation cross-slip are then discussed.