Creep life predictions in nickel-based superalloys

Creep life prediction techniques such as the Larson-Miller parameter, life trend diagrams, the Monkman-Grant relationship and the Dobes-Milicka relationship are critically reviewed. Problems associated with the scatter in creep life data and the inability of these relationships to accommodate for service-induced material degeneration are discussed. These effects are rationalized in terms of the erratic tertiary creep behaviour of complex engineering alloys. A new creep design relationship based on the sum of the primary creep life tp plus the secondary creep life ts, normalized by the sum of the primary creep strain ϵp plus the secondary creep strain ϵs, varying as a function of secondary creep rate ϵ has been proposed. The relationship is described by tp+tsϵp+ϵsϵ̇M=K where M and K are constants. The equation is accurate over a wide range of stresses (350–700 MPa) and temperatures (760–890°C) in IN-738LC. The method systematically reveals the creep degeneration effects with increasing service life. The relationship appears to be independent of any changes in the predominant deformation mechanism.