Effect of phase transformations on hardness in zn-al-cu alloys

Sixteen Zn-Al-Cu alloy compositions were prepared by melting pure elements. The as-cast alloys were homogenized at 350 °C for 180 h. Both the as-cast and homogenized alloys were analyzed with X-ray diffractometer and EDX-scanning electron microscope. The Rockwell "B" hardness of both the as-cast and homogenized alloys was determined using the standard procedure. The X-ray diffraction patterns and scanning electron micrographs indicated the presence of several phases in the as-cast alloys. Some of them do not correspond to those shown in the equilibrium Zn-Al-Cu phase diagram at low temperatures. However, the homogenized alloys showed most of the phases predicted by the equilibrium diagram. The hardness of alloys increases with the Cu content because of the presence of Cu-containing phases such as, the θ and τ phases in both alloys. The hardness of the homogenized alloys was lower than that of the as-cast alloys as a result of the elimination of the dendritic structure.