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

The phase transformation in Zn-4 Al-3 Cu alloy employing various solution-treatment temperatures (230 degrees C to 325 degrees C) was studied by means of microhardness, scanning electron microscopy (SEM), electron probe microanalysis (EPMA), transmission electron microscopy (TEM), and X-ray diffraction (XRD). The starting microstructure of the as-cast Zn-4Al-3Cu alloy consists of an alpha phase (aluminum-rich, fee structure) in the eta matrix (zinc-rich, h.c.p. structure) prior to solution-treatment. A platelike epsilon phase with 3-mu m length and 0.5-mu m thickness was found in the eta phase matrix after solution-treating the as-cast material at 240 degrees C for 1 hour. The epsilon phase was then dissolved gradually back into the eta matrix above that temperature. A four-phase transformation, alpha + epsilon --> T' + eta, was observed from the temperature 250 degrees C to 310 degrees C, wherein the T' phase formed at the interface of epsilon platelet and eta phase matrix. This T' phase was further identified as a rhombohedral structure. As the solution-treatment temperature was increased to above 310 degrees C, the epsilon phase was completely dissolved back into the eta matrix and numerous beta phase particles were distributed uniformly in the eta matrix. The beta phase subsequently decomposed at room temperature to a fine alpha phase embedded in the eta matrix. For the materials solution-treated above 250 degrees C, the microhardness of the eta matrix increased in 40 minutes during natural aging, which was associated with the formation of fine epsilon phase of 0.15 mu m diameter. The orientation relationship between this fine epsilon phase and eta phase was determined as [(1) over bar 011](eta) // [(1) over bar 011](epsilon), (01 (1) over bar 2)(eta) // (01 (1) over bar 2)(epsilon).