On the fine-grained-lamellar microstructure transformation in Zn-20.2% Al-1.8% Cu alloy

Abstract

It is known that fine-grained and lamellar microstructures of Zn-22% A1 eutectoid alloy can be obtained by quenching or slow cooling, respectively, of their f c c phase a/, from above the eutectoid temperature (551 K). Also, a lamellar microstructure arises from the isothermal heating of finegrained samples at temperatures below 551 K [1]. On the other hand, certain transformations can be produced by plastic deformation of Zn-22% AI: a fine-grained microstructure arises from hightemperature deformation of lamellar microstructures (although it is not essential to have temperatures above the eutectoid temperature) [2]. Finally, a fine-grained microstructure appears on necking zones of plastically deformed Zn-A1 lamellar eutectoid alloy (in the range 300-473 K) [3]. The main object of this letter is to report some experimental results related to microstructural transformations occurring in a Zn-20.2% A1-1.8% Cu alloy during certain thermomechanical treatments of lamellar and fine-grained samples. In our experiments we found that the samples of Zn-20.2% A1-1.8% Cu alloy follow the same general microstructure transformations scheme shown by the eutectoid Zn-22% A1 alloy. For instance, quenching or slow cooling of the 0/-phase from a temperature above the eutectoid point gave, respectively, fine-grained or lamellar microstructures. Also, isothermal heating of fine-grained samples gave lamellar microstructures, but for low temperatures (T = 373 K) the rate of transformation was very low. The plastic deformation of samples with lamellar structure produced a finegrained microstructure on necking zones. Therefore, the addition of Cu near the eutectoid composition of Zn-22% A1 alloy does not give rise to any major change in the general transformation scheme of this type of alloy. Another general result is that transformations from lamellar to fine-grained microstructures (or vice versa) are possible only by the simultaneous existence on the samples of the cr(fcc structure, Al-rich) and fi(hcp structure, Zn-rich) phases. In other words, if in a given part of the sample the fl-phase is chemically removed from the alloy, then no microstructural transformation was detected (as is shown below). Scanning electron microscopy (SEM) images from