The development of a universal equation for secondary creep rates in pure metals and engineering alloys

It has previously been shown that, during creep deformation, secondary creep rates (εs) can be related to stress (σ) by the equationεs = A*(σ−σ0)p exp (−Qc*/RT).The exponent p ≃ 4, Qc*≃an activation energy for diffusion (usually self-diffusion), and σ0 is a friction stress. The parameter A* is material-dependent so that creep data for a variety of materials appear as a set of parallel lines on a log εs/log (σ−σ0) graph. In the present paper, the general applicability of the expression with p ≃ 3.5 is confirmed on a variety of nickel- and iron-base alloys encompassing a range of microstructures commonly found in engineering materials. Further, a single universal equation to describe the creep behaviour of the various materials has been developed. It involves normalization of the effective stress (σ−σ0) by the proof (σ0.05) or yield (σy) stress of the alloy at the appropriate creep temperature, resulting in the expressionεs = B(σ−σ0/σ0.05)3.5with B independent of material, crystal lattice, microstructure, and temperature.