Phase decomposition in extruded Zn-Al based alloy

Ageing characteristics of an extruded eutectoid Zn-Al based alloy were inve stigated using X-ray diffraction and scanning electron microscopy technique s. The extruded alloy consisted of Al rich alpha phase and Zn rich eta(E)' and epsilon phases. The original cast eutectoid Zn-Al alloy was extruded at 250 degrees C. Both supersaturated alpha(s)' and beta(s)' phase decomposed during extrusion and appeared as fine and coarse lamellar structures. The eta(E)' and epsilon phases particles formed in the original interdendritic region. It was found that two Zn rich phases eta(E)' and epsilon decomposed sequentially during ageing at 170, 140 degrees C. The decomposition of the eta(E)' phase occurred as a discontinuous precipitation in the early stage of ageing and the decomposition of the epsilon phase took place in a four phase transformation: alpha + epsilon --> T' + eta in the prolonged ageing. Two typical morphologies of the decomposition of the Zn rich phases eta(E) ' and epsilon were distinctive in back-scattered scanning electron microsco py. (C) 1999 Kluwer Academic Publishers.