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 °C/h down to 375 °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 phasesT and η was observed to occur. There were, however, essential differences between the melting kinetics for the two phases. While theT-phase particles melted spontaneously at temperatures at or above the invariant temperature, 489 °C, and after some period of time at lower temperatures, the η-phase particles either melted spontaneously at or above the invariant temperature,T - 475 °C, or dissolved into the matrix at temperatures below 475 °C. This difference in behavior can be accounted for if the α(Al)-η 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.