Mechanisms of Creep and Recovery in Nimonic 80A

Abstract

The effect of a dispersion of particles on high-temperature creep has been examined by comparing the creep and recovery properties of Nimonic 80A with those of a single-phase nickel–20% chromium alloy at 750° C. Over most of the creep curve, the strain at any instant can be described accurately as ɛ = ɛ₀ + ɛ _T (l − e^−mt ) + έ _s t, where ɛ₀ is the instantaneous strain on loading, ɛ _t the total transient strain, έ _s the steady-creep rate, and m a constant relating to the rate of exhaustion of transient creep. Measurements of the rate of recovery during transient and steady-state creep suggest that this equation applies when the creep rate is proportional to the rate of recovery. Deviations from this equation occur during the initial 10–15% of the transient stage, and are attributable to a rapid increase in the coefficient of work-hardening. Although the activation energy for creep and the stress-dependence of the creep rate of the Nimonic alloy are higher than the values obtained for the single-phase alloy, it is suggested that the mechanism of creep is essentially the same in both materials. The creep-resistance of Nimonic 80A is then primarily a result of retardation of recovery by the dispersion of Ni₃ (Al,Ti) particles.