Mechanistic control regimes for intergranular cavity growth in 2.25Cr-1 Mo steel under various stresses and stress states

Abstract

Quantitative models for creep-cavity growth are assessed in relation to results obtained in 2.25Cr-1Mo steel at 565°C. Measurements of cavity size and spacing previously performed in interrupted tests under uniaxial tension are analysed further in the light of results obtained under a state of triaxial tension obtained in axially loaded circumferentially notched bar tests. Such tests enable us to distinguish between the principal stresses σ1 or hydrostatic stress (σH and the effective stress σ controlling shear deformation. While at high stresses (>200 MN m−2) small cavities can grow by diffusion-controlled growth, for stress levels of major interest results are interpreted on the basis that the grain boundaries behave as imperfect vacancy sources. In the early stages it is proposed that an inhibited diffusional mechanism prevails such that growth is limited by the availability of dislocations climbing in the grain boundaries. With further growth a plasticity-controlled or continuum-growth process becomes dominant and this is considered to occur at an enhanced rate owing to the presence of weak precipitate-free zones at the grain boundaries. During the late stages cavity growth becomes geometrically constrained by creep in the surrounding material. This situation is favoured under conditions of high triaxiality (σH/σ). On the basis of these interpretations the rupture life and ductility are shown to decrease with increasing triaxiality.