Effect of stress reductions on the stress exponent and subgrain size in an Al-Zn alloy

Abstract

Stress change and uninterrupted tests were performed on an Al-5 wt pct Zn alloy over the high temperature range (0.7 to 0.99T _m, whereT _mis the melting point) and at a normalized stress range extending from 10⁻⁵ to 2 × 10⁻⁴. Two techniques, etch-pit (EP) and transmission electron microscopy (TEM), were used to examine dislocation substructure developed during creep. The results of stress reduction tests, when compared with those of uninterrupted tests, lead to two findings: (a) there is no difference between the stress exponents determined from stress reduction tests and uninterrupted tests, and (b) after a stress reduction the subgrain size, measured by both EP and TEM, coarsens, reaching the steady-state value determined in an uninterrupted test at the reduced stress. TEM micrographs show that subgrain coarsening during the transient period after a stress reduction involves substructural activities such as subgrain boundary dissolution. Also, TEM examination of the interiors of subgrains occasionally reveals the presence of a cellular substructure which resembles very small subgrains. The characteristics of this cellular substructure are discussed with reference to recent conflicting evidence concerning the ability of the subgrain size to coarsen after a stress reduction.