Grain boundary ductile fracture in precipitation hardened aluminum alloys

Abstract

Published microstructural studies of grain boundary (gb) fracture in precipitation hardened aluminum alloys are reviewed with respect to the three main ideas that have been developed to explain the gb fracture surfaces. The ideas are 1. (1) microvoid growth at large gb precipitates, 2. (2) strain localization in the soft, and sometimes solute-free, gb precipitate free zones (pfz) and 3. (3) the influence of matrix precipitate shear giving rise to inhomogeneous “planar” slip that may apply large stress concentrations to the gb at the end of slip bands. Although the last two processes have a supporting role in many cases, the published evidence strongly suggests that the first process is of overwhelming importance. This conclusion has been tested by reversion experiments in model Al-Li alloys in which microstructures with increasing area fractions, A f , of large stable δ precipitates (Al-Li) were produced, but with equivalent matrix structures and yield strengths. The materials show marked falls of toughness and of fracture strain as A f was increased. Studies of surface slip markings in the Al-Li alloys suggested that slip was initiated at the large gb δ precipitates. Only very limited evidence for a role of planar slip in the fracture of the Al-Li alloys was found in contrast to observations on high purity Al-Zn-Mg-Cu alloys where planar slip seemed to show more importance. Brief studies on a nickel based alloy, MAR-M200, suggested that even in the absence of a pfz, strong room temperature embrittlement by gb precipitates was produced. The results of this study suggest that the marked problems of gb fracture in Al-Li alloys are associated with large gb δ precipitates. Jensrud and Ryum [ Mater. Sci. Engng 64, 229 (1984)] have shown how gb precipitate growth is facilitated in this system as the gb δ phase is very much less soluble than the strengthening δ′ (Al 3Li) phase.