Analysis of tertiary creep in an NiAl hardened austenitic steel

Abstract

The present study aims to identify the various factors responsible for the early onset and extended tertiary creep observed in a precipitation hardened Fe–Ni–Cr–Al alloy by analysing the tertiary creep at 600°C. Although the forms of the individual creep curves qualitatively appear to be compatible with the concept of particle coarsening control, quantitative analysis of the results in this present study are totally inconsistent with the predictions of a particle-coarsening model for tertiary creep. Creep rate acceleration with creep strain accumulation during the tertiary creep regime of the Fe–Ni–Cr–Al alloy indicates that strain softening is the origin of the onset of tertiary creep in this alloy. Further, acceleration of tertiary creep rate depends on the applied stress and is also related to the rupture ductility such that the rate of increase of tertiary creep rate becomes progressively more rapid as the applied stress is reduced or when rupture ductility is lower. The higher incidence of grain boundary cavitation combined with limited rupture ductility, and an exponential variation of creep rate with strain over most part of tertiary creep regime suggests that cavitation damage (internal loss of section) is primarily responsible for tertiary creep in the Fe–Ni–Cr–Al alloy when in the peakaged condition with lower rupture ductility. Wider but limited grain boundary cavities, which are localized very close to the fracture face together with a higher rupture ductility, indicate that the contribution from cavitation damage to tertiary creep of the alloy in the over-aged condition is significant only late in the tertiary creep regime. While linear softening makes a significant contribution early in the tertiary regime. Linear softening is subsequently replaced by exponential softening (external loss of section) during later part of deformation and can exist over longer periods.