COHERENCY STRAIN EFFECTS ON TEM IMAGES OF COMPOSITE PRECIPITATES IN AL-LI-ZR ALLOYS

Anomalous superlattice dark-field images of composite precipitates con sisting of a dark core surrounded by a bright shell in Al-2.52Li-0.15Z r alloys, has been investigated by many beam dynamical image simulatio ns employing a scattering matrix approach. The coherency strain field in the shell of the precipitate, as well as in the matrix, has been ca lculated by use of relations derived, and these are incorporated into the image calculations to determine whether the unexpected images can be elucidated by the coherency strain field of the precipitates. It ha s been found that the coherency strain field may modify the observed c omposite precipitate images, but it definitely cannot be the origin of the observed anomaly in these images. It has been also shown by the i mage matching technique, that the mismatch between the beta' (Al3Zr) p recipitates and aluminium matrix in Al-Li-Zr alloys should have a valu e of about 0.3%.