IRREVERSIBILITY OF STRAIN DURING LOW-CYCLE FATIGUE EXPERIMENTS OF A PRECIPITATION-HARDENED ALLOY

In this precipitation-hardened alloy, containing delta' (Al3Li) cohere nt particles, under conditions of peak-aged heat treatment and high ap plied plastic strain amplitude, the slip mode is mainly inhomogeneous and planar. It is shown that dwell time periods at ambient temperature superimposed on the basic cycling test induces, on reloading, a signi ficant transient hardening. The mean cumulative irreversible shear str ain within the bands may be evaluated from a simple geometrical model. This model is based on the variation in the mean sheared section of d elta' particles and takes into account the transient hardening measure ments. From these results and the quantity of slip bands estimated fro m transmission electron microscopy observations it has been found that the shearing is relatively irreversible during the first cycles and b ecomes more and more reversible once a stable deformation substructure is formed. The volume fraction of softest bands, where the particles are entirely sheared, and their mean cumulative shear strain are deriv ed from the preceding results. The present study suggests that these i ntense slip bands are sufficient to accommodate the applied plastic st rain. Accordingly, it seems that the drastic cyclic softening observed after a number of cycles equal to 0.5N(f) (N-f is the life duration) may probably be attributed to the initiation and propagation of microc racks.