Segregation in Al3(Sc,Zr) precipitates in Al-Sc-Zr alloys

An unusual precipitate structure in ternary Al(Sc,Zr) alloys has been revealed by analytical and high resolution transmission electron microscopy. Coherent precipitates with L12 structure and highly anisotropic shapes were found to consist of a core containing Al and Sc surrounded by a Zr-rich shell. The shell thickness ranged from about 4 to 7 nm in different particles. An abrupt change in composition between core and shell was indicated by both, energy-dispersive X-ray microanalysis and high resolution phase contrast imaging. It is likely that this phenomenon is responsible for the increased resistance to coarsening of Al3(Sc,Zr) precipitates in these ternary alloys.