Effects of tensile stress on microstructural change of eutectoid Zn-Al alloy - Discussion

In a recent contribution,[1] Zhu and Orozco presented a phase transformation of the ternary alloy Zn-20.2 wt pct Al-1.8 wt pct Cu, studied under tensile stress by using X-ray diffraction and scanning electron microscopy techniques. The authors report the existence of three phases in the alloy at room temperature after furnace cooling,α,ε, and a newη′Tinstead of the zinc-rich solid solutionη, as appears in the phase diagrams. The reported parameters for this hcp metastable phase are[1,2]a = 0.2663 andc = 0.4873 nm; these values are close to the parameters of pure zinc,[3] witha = 0.2664 nm andc = 0.4946 nm. The difference betweenη′Tand zinc in thea parameter is around 0.03 pct, and it is 1.47 pet for thec parameter. When zinc is saturated with aluminum in the Zn-AI alloys, thea parameter shrinks[3] to 0.2660 nm. It is possible to see that the value ofa of theη′Tphase lies in-between the values of pure zinc and zinc-aluminum solid solution. The solubility of Al and Cu in Zn[4] at 100 °C is 0.3 wt pct Cu and 0.06 wt pct Al. The covalent radius of Cu (0.117 nm) is smaller than the covalent radius of Al (0.118 nm) and Zn (0.125 nm), so the introduction of Cu in the zinc structure can result in a reduction of thec parameter. These values suggest that the metastable phaseη′T could be the hcp zincrich solid solution with low aluminum and copper contents. The article of Zhu and Goodwin,[5] cited by Zhu and Orozco in their Reference 14, is related not to the eutectoid alloy, as they argue, but to an alloy with 27 wt pct Al, and no reports about the transformation ofε intoT′ were found. The presence of the metastable e phase (CuZn4, sometimes called CuZn5) at room temperature and its transformation to the stable phaseT′ (rhombohedral intermetallic phase, Al4Cu3Zn) have been observed by other authors.[6,7] © 1996, The Minerals, Metals & Material Society.