Relation between precipitate-free zone width and grain boundary type in 7075-T7 Al alloy

The effect of grain boundary (GB) type on precipitate-free zone (PFZ) width in friction stir-processed 7075-T7 Al alloy is investigated by transmission electron microscopy (TEM) and stereology. The average half width of PFZs at random GBs is 70.4 +/- 0.7 nm. For low-angle GBs, an apparent transition of PFZ half width is observed at a misorientation of 11 degrees. For coincidence site lattice (Sigma) GBs, only Sigma 1, Sigma 3 and Sigma 5 have smaller PFZ width than that of random GBs. Crystal-frame stereology is used to recover the GB plane distribution. It is found that the GB plane distribution is relatively isotropic for most Sigma GBs. Low/high index plane combinations are observed for most Sigma GBs; furthermore, most Sigma GBs have both tilt and twist components. The combined results of TEM and stereology suggest that smaller PFZ width is associated only with low Sigma GBs since the formation and growth of PFZs at GBs depends closely on the core structure, in addition to the geometric structure of GBs. (c) 2006 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.