Modifications on solidification thermal parameters, microstructure and hardness induced by Cu additions to a hypereutectic Zn[sbnd]8Al alloy

Abstract

Binary ZnAl alloys are interesting alternatives when tensile strength and hardness performances are required. However, such alloys exhibit limitations on lubrication, ductility at room temperature and mechanical strength at high temperatures. It is known that small alloying additions of Cu to ZnAl alloys can modify the microstructure and properties such as creep and wear resistances, hardness, fluidity, and lubrication. In this sense, this work aims to study the effects of Cu additions on the solidification thermal parameters such as cooling and growth rates (ṪL/VL-liquidus isotherm and ṪE/VE-eutectic front), microstructure (dendritic and eutectic arrangements), and hardness of a hypereutectic Zn-8wt.%Al alloy. Techniques such as X-Ray Diffraction (XRD), X-Ray Fluorescence (XRF), Optical Microscopy (OM) and Scanning Electron Microscopy (SEM) were used to characterize the as cast samples. The results show that the resulting microstructures consist of a eutectic matrix [(Zn) + (Al + Zn)] with lamellar and fibrous morphologies and τ’-Al4Cu3Zn intermetallic particles disseminated within the (Zn) phase; pro-eutectic Al-rich (Al') dendrites containing alternating lamellae of Al and Zn phases, which resulted from the eutectoid transformation [(Al')↔(Al) + (Zn)]. The Cu additions have coarsened the fibrous and lamellar eutectic of the binary Zn-8wt%Al alloy, with exception of the Zn-8 wt% Al-1.3 wt% Cu alloy, which showed a refinement of the fibrous eutectic for VE > 0.30 mm/s. Increase in hardness has been observed for both Cu-modified alloys due to the solid-solution strengthening mechanism and presence of the τ’-Al4Cu3Zn intermetallics. An expressive increase of about 73.4% has been observed due to the addition of 1.3% Cu, which is also related to lower eutectic fibrous spacings.