Substructure and fatigue fracture in aluminum

Abstract

Fatigue induced substructure has been studied in polycrystalline aluminum (99.99 per cent) cycled at constant strain amplitude. X-ray and transmission electron microscope techniques were used. At low strains (3 × 10 −4) large concentrations of dislocation loops are often formed near subgrain boundaries, and the average subgrain volume is much larger than that found at higher strain amplitudes. The limiting subgrain volume was determined as a function of strain amplitude. Above ∼2 × 10 −3, this volume is constant at approximately 9 cubic microns. A consistent model is proposed to account for the variability of substructure with strain amplitude and crystal orientation. It is concluded that the strain concentration near a fatigue crack is always sufficient to insure that propagation of the crack will occur in material which is strongly subdivided. The mechanism of fracture in thin foils containing many fatigue-induced subgrains was investigated using a tensile stage in the electron microscope. Although the fracture mode in thin films is quite different from that which must occur in bulk material under alternating stress, the behavior of sharp notches in the vicinity of subgrain boundaries provides a clue to possible fatigue fracture mechanisms in aluminum.