Cold Work and Recovery in Creep at Ostensibly Constant Structure

Abstract

The Bailey-Orowan equation έ_s = r/h (έ_s is the steady-state creep rate, r the rate of recovery, and h the strain-hardening coefficient) has been applied to creep tests in which the stress was reduced in stages. The object was to observe the influence of stress on a structure of fairly constant dislocation density, and examination showed that this density did not alter during a typical sequence of stress reductions. Nickel (99.7%) was tested in this way at 650, 670, 695, 720, and 800°C and aluminium (99.98%) at 200, 225, and 250°C. The recovery rate was determined from the delay times after a stress reduction, and the strain-hardening coefficients were obtained from tensile tests made after completion of the creep tests. Good agreement with the Bailey-Orowan equation was always found; this therefore applies even though the stress is varied during a creep test. The stress-sensitivities of creep and recovery rate were high: έ_s ∝ (stress)¹⁰ for nickel and (stress)^11.5 for aluminium; r ∝ (stress)⁷ for nickel and (stress)^8.5 for aluminium. Since a delay or incubation period follows stress reduction, some recovery takes place even though it was found not to affect average dislocation density. It is suggested that this recovery consists of a further weakening of the small proportion of dislocation links that are already the weakest, as they must control the flow stress. The results demonstrate the potential creep strength obtainable from dense dislocation networks and help to relate the slip and recovery concepts about creep.