Thermal recovery processes in deformed aluminum

Abstract

The effects of recovery on both microstructure and subsequent stress vs strain behavior were studied in pure aluminum single crystals prestrained at room temperature in 〈111〉 compression; they are presumed to be representative of polycrystal behavior, avoiding changes in grain structure. Transmission electron microscopy showed that two distinct dislocation processes occur: 1. (1) a rapid tightening of tangles and loose cell walls, eventually into subgrain boundaries, without a noticeable change in the scale of the structure, and 2. (2) subsequent coarsening of the subgrain structure. These two processes of microstructural change were correlated respectively with two distinct types (I and II) of stress vs strain behavior during postanneal straining. After ‘Type I recovery’, the relation between the strain-hardening rate, θ = dσ/ dϵ (σ: true stress, ϵ: true strain), and the true stress is identical to that of an unrecovered specimen, after a transient of a few percent of post-anneal straining. Type I recovery encompasses all of ‘metarecovery’ and part of ‘orthorecovery’, and bears resemblance to ‘dynamic recovery’ processes during stage III strain-hardening. Type II recovery, which succeeds Type I recovery, shows long-lasting effects on the mechanical behavior which are characterized in particular by unusually low θ values compared to unrecovered specimens deformed to the same stress. It may correspond to some transition stage between recovery and recrystallization.