Lattice parameter variation and its effect on precipitation behaviour of ordered Ni2(Cr,Mo) phase in Ni-Cr-Mo alloys

Abstract Ni-Cr-Mo based superalloys often precipitate out particles of an ordered Ni2(Cr,Mo) phase in the γ-matrix, whose kinetics varies with composition of alloys. Precipitation of the Ni2(Cr,Mo) phase involves a change in the unit cell from cubic to orthorhombic structure. Lattice misfit strains associated with such cubic to non-cubic transformations significantly affect precipitation behaviour. In the present work, a series of Ni-Cr-Mo alloys have been investigated in order to study the effect Cr and Mo solutes on lattice parameters of disordered as well as ordered phases, and its manifestation on precipitation behaviour of the Ni2(Cr,Mo) phase. Lattice parameters have been determined on the basis of Rietveld refinement of x-ray and neutron diffraction data obtained from different alloys in solution treated as well as aged conditions. Present results have shown that lattice misfit strains in these alloys increases monotonously with the increase of Mo/Cr ratio, which significantly affect the precipitation behaviour of the ordered Ni2(Cr,Mo) phase.