Studies of segregation and the initial stages of precipitation at grain boundaries in an aluminium 7 wt. % magnesium alloy with an energy analysing electron microscope

Abstract

A new high resolution technique has been developed for the study of segregation and as­sociated phenomena; a microanalysis is, in effect, performed on a highly magnified image of a specimen when examined by transmission electron microscopy. The method depends on the facts that electrons lose energy on transmission through the specimen, that the energy losses are characteristic of the material of the specimen, and that these energy loss electrons contribute to a normal image in the electron microscope. The spatial resolution of the technique is estimated to be about 10 nm. In the special case presented here, the measurements are semi-quantitative and concentration changes of about 1 wt. % are detectable. The technique is applied to the study of precipitation and associated segregation of mag­nesium at high angle grain boundaries in an aluminium 7 wt. % magnesium alloy after it has been quenched from different solution treatment temperatures. The results of the investiga­tion are consistent with precipitate nucleation at the boundaries during the quench at a quench rate of 3 x 10 -4 °C s -1 . Non-equilibrium segregation of magnesium to the bound­aries is also indicated, and a model based on the converse of the Kirkendall effect is proposed to explain this.