Phase transformation of Zn-4Al-3Cu alloy during heat treatment

摘要

The phase transformation in Zn-4 Al-3 Cualloy employing various solution-treatmenttemperatures (230℃ to 325℃) was studied by means ofmicrohardness, scanning electron microscopy (SEM),electron probe microanalysis (EPMA), transmissionelectron microscopy (TEM), and X-ray diffraction (XRD).The starting microstructure of the as-cast Zn-4Al-3Cualloy consists of an α phase (aluminum-rich, fccstructure) in the ηmatrix (zinc-rich, h.c.p.structure) prior to solution-treatment. A platelike εphase with 3-μm length and 0.5-μm thickness was foundin the η phase matrix after solution-treating theas-cast material at 240℃ for 1 hour. The ε phase wasthen dissolved gradually back into the η matrix abovethat temperature. A four-phase transformation, α+ε→T'+η, was observed from the temperature 250℃ to310℃, wherein the T' phase formed at the interface ofε platelet and η phase matrix. This T' phase wasfurther identified as a rhombohedral structure. As thesolution-treatment temperature was increased to above310℃, the ε phase was completely dissolved back intothe η matrix and numerous β phase particles weredistributed uniformly in the η matrix. The β phasesubsequently decomposed at room temperature to a fineα phase embedded in the η matrix. For thematerials solution-treated above 250℃, themicrohardness of the η matrix increased in 40 minutesduring natural aging, which was associated with theformation of fine ε phase of 0.15-μm diameter. Theorientation relationship between this fine ε phaseand η phase was determined as [1011]η//[1011]ε,(0112)η//(0112)ε.