Stacking-fault energy and yield stress asymmetry in molybdenum disilicide

Stacking-fault energies in MoSi2 due to shear along 〈331〉 have been calculated by ab-initio and modified embedded-atom method (MEAM) calculations. The results are used to investigate the configurations of ½〈331〉 dislocations and their mobility. Shear of ⅙〈331〉 in the {103} plane of MoSi2 produces an antiphase boundary (APB) whose geometry. called APB(1), is different from that produced by ⅙〈331〉 in the opposite direction, APB(2). MEAM calculations show that APB(1) is stable while both types of calculation show that APB(2) is unstable. Both ab-initio and MEAM calculations show that there is a stable fault close to APB(2) with a displacement of about ⅛〈331〉 in the same direction. The calculations also show that there is a stable fault in the {110} plane with a displacement of ¼〈111〉. The identical fault is produced by a shear of ¼〈331〉. There is good agreement between the fault energies calculated by the two methods and also with the experimental value (200-370 mJ m−2). The agreement between the calculated fault energies in the {013} plane is not so good. One factor is that the relaxation procedures are different; the MEAM method has more flexibility as well as a larger number of atoms, possibly explaining why it gives lower stable fault energies. The {103} planes have an unusual five-layer ABCDE stacking sequence with successive planes offset by ⅕〈301〉. Shear of 1/10〈351〉 in the correct direction gives a low-energy fault with Mo atoms surrounded by the correct number (ten) of Si nearest neighbours. This vector is close to the ⅛〈331〉 shear that produces a stable fault and may explain its low calculated energy. Various dissociated configurations of ½〈331〉 dislocations are considered on the basis of ⅙〈331〉, ⅛〈331〉, ¼〈331〉 and 1/10〈351〉 partials. All can have asymmetrical arrangements which will respond differently to the direction of the applied stress, explaining why {103}〈331〉 slip is much easier for crystals compressed along [100] than along [001].