Creep deformation of Co-Re-Ta-C alloys with varying C content-investigated in-situ by simultaneous synchrotron radiation diffraction

The creep deformation of precipitation hardened Co-Re-Ta-C alloys is investigated during in-situ synchrotron diffraction experiment at 1373 K. At room temperature, the alloys have a structure consisting of ϵ-Co (hcp) and metastably retained γ-Co (fcc) and are strengthened by precipitates of the mono-carbide of Ta, which are finely dispersed in the alloy matrix. The alloy exhibits an allotropic ϵ→γ-Co phase transformation when heating to >1173K. A lower C content in the alloy generally promotes this transformation. It is shown that this transformation is strongly influenced by application of compressive load. The transformation ϵ→γ-Co at high temperature under load leads to microstructure refinement and subsequently to dissolution of hardening precipitates. This results in a considerable acceleration of the creep rate. Further, the equilibrium ratio of γ/ϵ-Co phase is significantly altered under compressive load. This behavior is attributed to a volume relaxation as the ϵ- and γ-Co phase have different unit cell volumes.