LOCAL MELTING IN AL-MG-ZN-ALLOYS

The internal melting of several Al-Mg-Zn-alloys has been studied by rapid upquenching in a salt bath of specimens slowly cooled at a rate of 2-degrees-C/h down to 375-degrees-C. The melting reaction was studied metallographically in the light- and electron-scanning microscope, and local concentrations were measured in the microprobe. Local melting of both the equilibrium phases T and eta was observed to occur. There were, however, essential differences between the melting kinetics for the two phases. While the T-phase particles melted spontaneously at temperatures at or above the invariant temperature, 489-degrees-C, and after some period of time at lower temperatures, the eta-phase particles either melted spontaneously at or above the invariant temperature, T = 475-degrees-C, or dissolved into the matrix at temperatures below 475-degrees-C. This difference in behavior can be accounted for if the alpha(Al)-eta section is not a quasi-binary section. The industrial implications of the internal melting in these alloys are discussed and compared to the same reaction in the Al-Mg-Si alloys. A model is developed in the Appendix to quantify the different behaviors of these two classes of alloys.