Effect of Ag additions on the microstructure and phase transformations of Zn-22Al-2Cu (wt.%) alloy

Abstract

The relationship between microstructure and mechanical properties is studied for eutectoid Zn-22Al (wt.%) alloys modified with Cu and Ag. Three alloys with a Cu content of 2 wt.% and varying amounts of Ag were cast and hot-extruded. Different microstructural characteristics were induced by heat treatments: natural aging, artificial aging and furnace cooling. Structural and microstructural characterizations were carried out with X-ray diffraction and scanning electron microscopy. Mechanical properties were determined by tensile testing. Dilatometry was used for determining the effects of composition on the transformation points. The addition of Ag increased the ε phase fraction and provided solid solution strengthening, improving the mechanical strength and reducing ductility. Ag additions also displaced the eutectoid reaction to higher temperatures. The microstructure of the matrix has proven to have a strong impact on mechanical properties. The naturally aged specimens presented the highest ductility and tensile strength; however, these properties are severely affected by aging. Lamellar microstructures present the lowest ductility and values of tensile strength between those of the natural and artificially-aged specimens.

Investigation of microstructural change and damping behaviour of Zn–27Al–1Cu alloy in different aging periods

“…This structural change is evident from the difference between the microstructure of the alloy aged for 2.5 h and the alloy aged for 5 h given in Figure 4. The microstructure of the as-cast alloy formed as a result of solidification and the microstructural changes after heat treatment are compatible with previous studies on similar Zn-Al based alloys (Yang et al, 2013;Chen et al 2015;Jeshvaghani et al, 2016;Movahedi et al, 2019;Ferreira-Palma et al, 2021).…”

“…After aging, especially in the early stages, the supersaturated β phase decomposed into the irregularly cellular structure through discontinuous reaction: β→ α+η+ε. The progressive stage of aging activates the four-phase transformation (α+ε→T′+η) and initiates the formation of the T′ phase, where T′ is the phase with higher stability than the ε phase (Chen et al, 2015;Ferreira-Palma et al, 2021). The microstructure of the alloy aged for 2.5 hours and aged for 5 hours are given in Figure 4(c-d) and Figure 4(e-f), respectively.…”