Phase decomposition in extruded Zn-Al based alloy

Abstract

Ageing characteristics of an extruded eutectoid Zn-Al based alloy were investigated using X-ray diffraction and scanning electron microscopy techniques. The extruded alloy consisted of Al rich α phase and Zn rich η_E′ and ∈ phases. The original cast eutectoid Zn-Al alloy was extruded at 250 °C. Both supersaturated α_s′ and β_s′ phase decomposed during extrusion and appeared as fine and coarse lamellar structures. The η_E′ and ∈ phases particles formed in the original interdendritic region. It was found that two Zn rich phases η_E′ and ∈ decomposed sequentially during ageing at 170, 140 °C. The decomposition of the η_E′ phase occurred as a discontinuous precipitation in the early stage of ageing and the decomposition of the ∈ phase took place in a four phase transformation: α + ∈ → T′ + η in the prolonged ageing. Two typical morphologies of the decomposition of the Zn rich phases η_E′ and ∈ were distinctive in back-scattered scanning electron microscopy.