Tensile stress induced microstructural change and phase transformations in extruded Zn-Al alloy

Abstract

During the last decade, much attention has been paid to the evaluation of the tensile and creep behaviours of Zn-A1 based alloys of various chemical compositions [1-4]. In order to understand the microstructure dependence of tensile and creep behaviours, an extensive research programme was recently carried out on thermal and thermo-mechanically treated eutectoid Zn-A1 alloys. This work reports some results of microstructure change and phase transformations in cast-extruded eutectoid Zn-A1 based alloy Zn76A122Cu2 (wt%) subjected to tensile stress. As-cast eutectoid Zn-A1 alloy ingots were extruded into rods of 20 mm diameter at 250 °C. The extruded rod was machined into standard specimens of 10 mm diameter with 50 mm gauge length for tensile testing. The tensile test was carried out at 100 4-2 °C on an Instron machine at a crosshead speed of 7.0 × 10-3mm/s. The tensile specimen ruptured when the strain reached 24.66% with an instantaneous tensile stress of 49.9 MPa. The ultimate tensile strength (UTS) and the 0.2% proof stress of the extruded specimen were 148.9 MPa and 127.5 MPa, respectively. Figure 1 shows the stressstrain curve. Various parts of the specimen after tensile test: bulk, neck zone and rupture part, were examined on an X-ray diffractometer with Ni filtered CuKa radiation and a scanning electron microscope to identify phase transformations and microstructural changes. The microstructure of the as-extruded eutectoid Zn-A1 alloy consisted of Al-rich fcc aphase, Zn-rich hcp q~ phase and decomposed fis phase regions, as reported in previous work [5]. The fi£ phase, a Zn-rich supersaturated phase of fcc structure, decomposed during extrusion at 250 °C, according to the cellular reaction, fi~ --+ a~ + e + q [6-8]. During tensile testing at 100 °C, the bulk part of the specimen underwent thermal treatment, i.e. ageing at 100°C, while the neck zone and the rupture part of the specimen suffered thermalmechanical treatment, i.e. ageing at 100°C plus plastic deformation under a high local strain. As shown in Figs 2 and 3, the microstructural changes and phase transformations were quite different in the different parts of the specimen. In the bulk part of the specimen both the a-phase and decomposed fi~phase decomposed further to form fine (0.1/xm lamellar thickness) and coarse (0.3/xm lamellar thickness) lamellar structures, respectively, as shown in Fig. 2a. The metastable q~ phase decomposed to