On a proposed theory for the disappearance of serrated flow in f.c.c. Ni alloys

Abstract

The disappearance of serrated flow was investigated for Inconel 718 and 600. The activation energy for the disappearance was found to be 2.7 eV for both alloys. It is proposed that the mechanism leading to the disappearance of serrated flow is a reaction occurring between carbide-forming substitutional atoms and carbon atmospheres which are forming on the arrested dislocations. This leads to the subsequent depletion of atmospheres and formation of finely dispersed carbides along dislocation lines and to the disappearance of serrated flow. The rate controlling step for the process is the rate of diffusion of the substitutional carbide-forming species to the arrested dislocation lines. The disappearance occurs when a critical balance is reached between the rate of atmosphere formation and the rate at which carbon is drained from arrested dislocations by the carbide-forming species. This behavior manifests itself by the disappearance of serrations off the outer end of the flow curve at progressively lower strain as the temperature is increased or the strain rate decreased. In contrast, the disappearance of serrated flow in age-hardened Waspaloy took place by a progressively larger delay to the onset of serrations as the temperature is increased or the strain rate decreased. It was proposed that the mechanism leading to the disappearance in age-hardened Waspaloy is due to the draining of carbon down the dislocation line to the Ni 3(Al,Ti) percipitate while the line is arrested at the precipitate. The rate controlling step was proposed to be the reaction rate at the carbide-forming sink ( ΔH = 1.3 to 1.6 eV). Thus, in each case, it is proposed that the underlying mechanism for the disappearance of serrations at higher temperatures and lower strain rates is a precipitation reaction. The continuous and reproducible pattern of an increase in stress decrement ( σ D ) with either decrease in strain rate or increase in temperature was also investigated. With either of the above conditions the atmosphere size increases, thus more than offsetting the decrease in binding energy which occurs simultaneously with increasing temperature. This increase in σ D continues progressively right up to the point at which the serrations disappear.