Formation and growth of vacancy clusters in quenched dilute Al-Si alloys studied by positron annihilation

Abstract

Positron annihilation Doppler-broadening spectra were measured and lineshape parameters were estimated during isochronal aging of dilute alloys of Al-1.0 at.%Si and Al-0.1 at.%Si quenched from 773 and 823K to 183K. Two peaks in the lineshape parameters were observed at about 213 and 243K in both alloys. It is suggested that the increase and subsequent decrease of the parameters around 213K are due to the growth of vacancy clusters and the collapse of their open volumes, respectively. Positron annihilation experiments showed that the collapse of vacancy clusters in quenched pure Al seems to occur in a temperature region below about 213K. The size distribution of Si precipitates was determined by electron microscopy in various aging treatments of Al-1.0 at.%Si alloys quenched from 823K. Taking into account the results from electron microscopy, it is deduced that the change in lineshape parameter around 243K seems to be related to the nucleation of Si precipitates.