Grain boundary segregation of an Al-Zn-Mg ternary alloy

Auger electron spectroscopy (AES) has been used to measure the grain boundary concentration profiles of alloy additions in an A1-5.5 pct Zn-2.5 pct Mg ternary in as-quenched, under-, peak-, and over-aged conditions. The AES depth profiles show marked segregation of Mg and Zn to the grain boundary, in contrast to that reported previously on similar A1 alloys. It is found that this apparent contradiction can be resolved by exploiting the plasmonloss features of the AES spectra to help elucidate the grain boundary segregation. With the AES/plasmon-loss measurements, one can determine not only the concentration of Mg and Zn at the grain boundary, but also the metallurgical environments surrounding the alloy additions. It is shown that, for over-aged specimens of the Al alloy, only a fraction of the total Mg at the grain boundary is incorporated in MgZn₂ precipitates, the remainder being segregated to within a few atomic layers of the boundary.