Tem Observations Of Precipitation Of Coherent L1(2) And D0(22) Ordered Phases In Ni-V-Ge Alloys

Phase separations of Al (gamma) supersaturated solid solution into gamma, cubic-L1(2) (gamma') and tetragonal-D0(22) (gamma '') phases were investigated in Ni-V-Ge alloys by means of transmission electron microscopy (TEM). When Ni-15.8at%V-9.0at%Ge alloy is aged at 1073 K, at the early stage of ageing gamma '' phases are observed in the gamma matrix as triangle- or diamond-shaped precipitates. With further ageing, colonies of lamellar structure consisting of two variants of gamma '' phase are dispersedly formed in the gamma matrix and then gamma '' plates grow along the (110) direction. When Ni-14.5at%V-8.8.at%Ge alloy is isothermally annealed at 1023 K, first spherical gamma' particles precipitate homogeneously in the gamma matrix, followed by the formation of lamellar structure of gamma '' phases. In the course of further ageing, the lamellar structure develops, and only gamma' particles around the lamellar structure grow and others gradually fade out.