Creep of tin dispersion hardened 20%Cr-25%Ni stainless steel below the transition stress

Abstract

Consideration is given to the mechanisms which determine the creep behaviour of TiN-dispersion strengthened 20Cr 25Ni steels at stresses above the threshold level of large-scale deformation but below that at which Orowan bypass of particles is widespread. This latter stress is termed the transition stress and is higher than the Orowan stress for initial bypass. It is that stress at which continuing slip bypass is possible even when dislocation tangles are associated with the TiN particles. Within this experimental stress range only those interparticle spacings (termed gates) for which the Orowan condition is satisfied will permit slip bypass (i.e. will be open). Elsewhere, particle by-pass must occur by thermally activated climb. Quantitative development of the model is made by demonstrating that the distribution function of interparticle spacings is closely related to the measured distribution of particle sizes. Using this result, calculations are made of the fraction of gates which are open at any given applied stress within the range considered. Due allowance is made for source shortening due to the accumulation of Orowan loops around the particles with increasing strain. A model which describes the stand off of dislocations from the looped particles is applied. Additionally, it is demonstrated that the time to climb past a particle is not affected significantly by its apparent increase in size due to loop accumulation and that this bypass time is expected to vary inversely with the difference between the applied stress and the threshold value. It is also shown that the increase in mean stress due to loop formation is unlikely to be significant in these alloys. On this basis, it is shown that a high stress dependence of steady-state creep is expected due largely to the rapid change in the number of open gates with increasing stress. Predicted values of the stress sensitivity are close to those observed. Finally, it is shown that a high rate of decrease of creep rate occurs in the early stages of testing, e.g. a factor of 10 4 over a strain interval of order 10 elastic deflections. This is accounted for qualitatively by postulating initial rapid changes in the apparent interparticle spacing (and, hence, number of open gates) as the cruciform morphology of the TiN particles becomes “filled in” by the accumulation of Orowan loops.