SHEAR FAULTS AND DISLOCATION CORE STRUCTURES IN B2 COAL

Interatomic potentials of the embedded atom and embedded defect type w ere derived for the Co-Al system by empirical fitting to the propertie s of the B2 CoAl phase. The embedded atom potentials reproduced most o f the properties needed, except that, in using this method, the elasti c constants cannot be fitted exactly because CoAl has a negative Cauch y pressure. In order to overcome this limitation and fit the elastic c onstants correctly, angular forces were added using the embedded defec t technique. The effects of angular forces to the embedded atom potent ials were seen in the elastic constants, particularly C-44. Planar fau lt energies changed up to 30% in the {110} and {112} gamma surfaces an d the vacancy formation energies were also very sensitive to the non-c entral forces. Dislocation core structures and Peierls stress values w ere computed for the [100] and [111] dislocations without angular forc es. As a general result, the dislocations with a planar core moved for critical stress values below 250 MPa in contrast with the nonplanar c ores for which the critical stress values were above 1500 MPa. The eas iest dislocations to move were the 1/2[111] edge superpartials, and th e overall preferred slip plane was {110}. These results were compared with experimental observations in CoAl and previously simulated disloc ations in NiAl.