The friction-stress concept during high-temperature creep

Abstract

Under high-temperature creep conditions the dependence of the steady-state creep rate (ε_s) on applied stress (σ) and temperature (T) can be described by an equation

ε_s = Ao ⁿ exp (−Q_c/RT)………(1)

where A and n are constants and Q_c the activation energy for creep. For most pure metals n is ∼4.0 and Q_c = Q_SD (activation energy for self-diffusion). Creep theories can readily account for these stress- and temperature-dependences. However, with two-phase alloys n and Q_c are generally substantially higher than those for pure metals and single-phase alloys. For example, with T.D. nickel n ≃ 40 and Q _c = 795 kJ/lmol compared with n = 5.0 and Q _c = 276 for pure nickel. Many precipitation-hardening alloys show the same trends. No creep theories can account for these high n and Q _c values. In particular, recovery-creep theories suggest maximum values of n ≃ 4.0 and Q _c = Q _SD.