The variation with torsional strain of the Barrett anomalous after effect

Abstract

The variation with torsional strain of the magnitude of the transient in the Barrett abnormal after effect has been studied. The results provide evidence that the oxide film yields at a strain of about 1°/cm in 1 mm dia. aluminium specimens, i.e. at a surface strain of about 8.7 × 10 −4, but that at much larger twists there are again stresses which can be released by acid attack on the oxide layer. The after-effect which is produced by the acid in the small twist region is in the opposite direction to the Barrett effect which occurs in the large twist region. The transient takes but a short time in the small twist region, while in the large twist region it takes much longer. Transverse, longitudinal and 45° lines were visible on eleetropolishod and twisted specimens, but only 45° lines were visible on anodized and twisted specimens. Electron microscopic examination showed that the 45° lines consisted of wide cracks in the oxide film whereas the transverse and longitudinal lines produced detachment of the oxide film or narrow cracks in the oxide film. It appears that the transverse and longitudinal primary glide systems are responsible for releaseable stresses in the region below 1°/cm and the noncrystallographic 45° cracks are associated with at least part of the releaseable stresses in the Barrett effect region. The effect of the anodizing time on the structure and properties of the film was studied. The results are discussed in terms of dislocation theory and in relation to the literature on surface effects in the plastic deformation of metals.