NUCLEATION OF THE DIAMOND PHASE IN ALUMINUM-SOLID SOLUTIONS

Precipitation was studied from fcc solid solutions with silicon, germanium, copper and magnesium. Of all these elements only silicon and germanium form diamond cubic (dc) precipitates in fcc Al. Nucleation of the dc structure is enhanced if both types of atom are dissolved in the fcc lattice. This is interpreted as due to atomic size effects in the pre-nucleation stage. There are two modes of interference of fourth elements with nucleation of the dc phase in Al + Si, Ge. The formation of the dc phase is hardly affected if the atoms (for example, copper) are rejected from the (Si, Ge)-rich clusters. If additional types of atom are attracted by silicon and/or germanium, dc nuclei are replaced by intermetallic compounds (for example Mg2Si).