ORIGIN OF GRAIN BOUNDARY PRECIPITATE FREE ZONE

The origin of the grain boundary precipitate free zone is discussed in relation to Al-Zn-Mg and Al-Cu alloys using the occurrence of homogeneously distributed precipitate as a semi-quantitative indication of the excess vacancy concentration and its variation in the quenched alloys. It is shown that all boundaries except those with a misorientation 2̌° act as ideal sinks for vacancies. In order to explain the occurrence of the precipitate free zone and its behaviour under different heat treatment conditions it is necessary to consider: (a) the presence of a vacancy concentration profile in the quenched alloy resulting from loss of vacancies to the grain boundary and, on occasions, to intragranular sinks such as dislocation loops; (b) the presence of a solute concentration profile in cases where grain boundary precipitation has occurred as a result of a slow quench or a quench interruption treatment. © 1969.