Direct Observation by Electron Microscopy of Fatigue-Induced-Substructure in Aluminium

Abstract

Transmission electron microscopy was used to study the structure developed in pure and very fined polycrystalline aluminium, several microns in grain size. Observations were made on the specimens fatigued at stress amplitudes, 8.5, 5.8, 4.8 and 3.8 kg/mm2. At the highest stress tested, a cell structure was formed even with such small grains and very dense parts of dislocation were observed near the grain boundary. The transition from dislocation tangle to cell-boundary was clearly seen. Dislocation loops which were likely to be formed by the condensation of vacancies were rarely observed. Main feature of the structure at the medium stress levels was the so-called dislocation patch which consists of the elongated loops arrayed in the direction <211>. Dislocation loops distributed in wide area were observed at the lowest stress. Surface observations were also done with a light microscope. Speculation was made on the propagation of the fatigue crack.