Combined in situ and EBSD studies of discontinuous precipitation in Al-22 at.%Zn alloy

Abstract

For the first time, the in-situ observation combined with Electron back scatter diffraction (EBSD) measurements were applied to discontinuous precipitation (DP) in the Al-22 at.% Zn alloy. So far, all the studies were restricted to “frozen” images of grain boundaries (GBs) after certain time of ageing. This did not allow the precise estimation of the incubation period preceding the initiation and growth discontinuous precipitates, as well it was not possible to distinguish still growing cells from those for which the growth was terminated. New approach permitted to determine that a large fraction of GBs does not undergo DP process. This was especially true for low angle GBs with misorientations below 19°. Growth in one direction occurred for a relatively large fraction of GBs. However, no correlation was found for the relationship between misorientation angle and growth rate on one and both sides of the GB as well for incubation period versus growth rate. Only few GBs were characterised with special CSL misorientation and they showed the growth in one direction with an exception of Σ3 grain boundary (no growth). Moreover, there was no evidence that the GBs close to the special Σ misorientations exhibited a very low DP reaction rate.