INFLUENCE OF THE γ' FRACTION ON THE γ/γ' TOPOLOGICAL INVERSION DURING HIGH TEMPERATURE CREEP OF SINGLE CRYSTAL SUPERALLOYS

Evolution of the γ/γ’ microstructure during creep at 1050°C and 150 MPa of the AM1, MC2 and MC544 single crystal superalloys was followed and quantified by evaluating the varia tion of the specific connectivity number of the γ’ phase N A(γ’). Complementary assessments of the γ’ phase connectivity were performed on a modified MC544 alloy with a reduced amount of γ’ phase and on Rene N6 and CMSX-10M third generation superalloys. The transition from the secondary to t he tertiary creep stage is correlated to the topological inversion of the γ/γ’ microstructure. The γ’ phase initially surrounded by the γ phase matrix becomes progressively the connected phase as the creep deformation increases. This topological inversion o ccurs during creep of single crystal superalloys when the γ’ phase fraction is higher than about 50% and therefore depends on the alloy chemistry and on the temperature. The γ/γ ' topological inversion occurs sooner during creep as γ ' amount increases above 50%.