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 (ε_s) can be related to stress (σ) by the equation

ε_s = A *(σ−σ₀)^p exp (−Q_c */RT).

The exponent p ≃ 4, Q_c *≃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 ε_s/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 (σ_0.05) or yield (σ_y) stress of the alloy at the appropriate creep temperature, resulting in the expression

ε_s = B(σ−σ₀/σ_0.05)^3.5

with B independent of material, crystal lattice, microstructure, and temperature.