Microstructure and phase stability in MOSi2-TSi2 (T=Cr, V, Nb, Ta, Ti) pseudo-binary systems

The phase stability of molybdenum disilicide (MoSi2, C11(b) structure) rela tive to other phases, C40 and C54 phases, in the pseudo-binary systems of M OSi2 and other types of disilicides of transition metals including Cr, V, N b, Ta and Ti was investigated by establishing the MOSi2-TSi2 (T=Cr, V, Nb, Ta, Ti) pseudo-binary phase diagrams. It was found that V, Nb, Ta and Ti wh ich substitute for Mo in MOSi2 strongly stabilize the C40 phase while Cr on ly shows a weak C40 structure-stabilizing effect. The phase stability was a lso discussed on the basis of geometrical chan-e in the three phases when c omposition varies. Change in lattice parameter of each phase indicated that phase stability of C11(b), C40 and C54 structures greatly depend on the re lative stacking spacing of the equivalent hexagonal atomic plane for the th ree structures. Based on the results of phase diagram investigation, the pr esent work attempted to design a C11(b)/C40 lamellar microstructure. Existe nce of the equivalent hexagonal stacking atomic plane among C11(b), C40 and C54 phases makes it possible to design a coherent C11(b)/C40 two-phase mic rostructure. Assuming the equivalent atomic planes in C11(b) and C40 phases coincide with each other, high lattice coherency between C11(b) and C40 ph ases is available in MOSi2-CrSi2 System with a lattice misfit of less than 0.5% and in the other four systems with a misfit of 1.3 to 2.7%. A lamellar microstructure was observed in MoSi2-TaSi2 and MoSi2-NbSi2 systems but no lamellar microstructures were obtained in MoSi2-CrSi2, MoSi2-VSi2 and MoSi2 -TiSi2 systems.