Ab initio study of phase transformations in transition-metal disilicides

We suggest and investigate three possible displacive structural transformation paths between the ideal C11b, C40 and C54 structures in MoSi2, VSi2 and TiSi2. They involve shifts of the (110) atomic planes in the C11b structure, (0001) planes in the C40 structure and (001) planes in the C54 structure. These planes are shifted in the directions experimentally observed as slip directions during plastic deformation. An analysis of ab initio calculated total energies in MoSi2, VSi2 and TiSi2 along these transformation paths is presented. Since the transition-metal and Si atoms come close together during proposed transformations, and, consequently, the energy barriers between individual structures are high, the estimated transition temperatures are comparable with the melting temperatures of the disilicides studied. This confirms their high thermal stability and indicates that if a phase transformation between C11b, C40 and C54 structures of the disilicides takes place, then its prevailing mechanism should be diffusional rather than martensitic like. During the transformations studied, atoms come as close together as, for example, in configurations of interstitials. Hence, the present ab initio results can also be employed in fitting adjustable parameters of semi-empirical interatomic potentials for the transition-metal disilicides, in particular of the repulsion at short separations of atoms.

Graphical abstractWe suggest and investigate three possible displacive structural transformation paths between the ideal C11b, C40 and C54 structures in MoSi2, VSi2 and TiSi2. They involve the shifts of the (110) atomic planes in the C11b structure as well as of the (0001) planes in the C40 structure and of the (001) planes in the C54 structure. The planes are shifted in the directions experimentally observed during plastic deformation. An analysis of ab initio calculated total energies in MoSi2, VSi2 and TiSi2 along these transformation paths is presented. Since the transition-metal and Si atoms come close together during proposed transformations, and, consequently, the energy barriers between individual structures are high, the estimated transition temperatures are comparable with the melting temperatures of the disilicides studied. This confirms their high temperature stability and indicates that if a phase transformation between C11b, C40 and C54 structures of the disilicides takes place, then its prevailing mechanism should be diffusional rather than martensitic like. During the transformations studied, atoms come as close together as, for example, in configurations with interstitials. Hence, the present ab initio results can also help in fitting adjustable parameters of semi-empirical interatomic potentials for the transition-metal disilicides, in particular of the repulsion at short separations of atoms.Highlights► There exist higher-energy C40 structure of MoSi2 and TiSi2. We try to find mechanism of alte ring their structure. ► The C11b, C40 and C54 structures of MoSi2, VSi2 and TiSi2 transition-metal disilicides can be regarded as different stacking of (nearly) hexagonal atomic layers. ► These layers are the (110) bct layers in the C11b structure, the (0001) layers in the C40 structure and the (001) layers in the C54 structure. ► Changing these stacking by slipping of these layers in directions observed during plastic deformation of the MoSi2, VSi2 and TiSi2 disilicides can alter their structures. ► Atomic diffusion cooperate in the phase transformation.