What Types of Stacking Faults and Dislocation Dissociations Can Be Found in Transition-Metal Disilicides

Identical atomic planes of transition-metal disilicides can form different stacking when they are ordered in several combinations of four different positions A, B, C, D. The following arrangements can be formed: AB in C11(b) structure of e.g. MoSi2, ABC in C40 structure of e.g. VSi2 and ABDC in C54 structure of e.g. TiSi2 disilicides. The ABC atomic plane stacking along the < 111 > cubic directions is well known in the fcc lattice, where three basic types of stacking faults are known: intrinsic or extrinsic faults and elementary twin, however, other types of stacking faults can be detected in transition-metal disilicides due to the occurrence of the fourth position D. On the other hand, the faults well known in metallic systems as antiphase boundaries need not be metastable in disilicides. Based on the results of ab initio calculations, it can be predicted which types of planar defects are metastable corresponding to the local minima on the energy surface of generalized stacking faults or unstable when they are represented, for example, by saddle points. The character of dissociation of the dislocation cores is directly related to the existence of metastable stacking faults. Moreover, the space distribution of dislocation cores has a direct impact on dislocation mobility and, therefore, also on macroscopic mechanical properties of materials. The behaviour of extended crystal defects in disilicides that is caused by covalent interatomic bonding, is discussed starting from the geometrical analysis, and it is demonstrated that predictions of materials properties can be deduced.