Strengthening Mechanisms in γ′ Precipitating Alloys

Abstract

The mechanical properties of several austenitic steels containing γ′ precipitate have been studied. A large increase in proof stress occurs during ageing as a result of particle growth at almost constant volume fraction of precipitate. Up to the peak proof stress, dislocations are paired owing to the high antiphase-domain boundary energy and this, in conjunction with Friedel's theory relating effective particle spacing to particle strength, satisfactorily accounts for the increasing proof stress. The value of the antiphase-domain boundary energy is higher at higher Ti/Al ratios. Misfits between 0 and 0·4% have no influence on yield strength. Large particles are by-passed by Orowan looping irrespective of misfit. During the looping process a transition from paired to single dislocations occurs that reduces the fall in stress with increasing particle size. Stable Orowan loops give rise to a back-stress that is detectable at low strains. This causes work-hardening by the Fisher, Hart, and Pry mechanism. Alloys with high Ti/Al ratios show more marked work-hardening associated with more debris in the slip planes. This is apparently a result of the by-passing of particles by cross-slip, which gives rise to complex slip interactions. However, except at very large particle sizes slip continues to be planar.