Coarsening of τ′ precipitates during aging in a Zn-22 wt.% Al-2 wt.% Cu alloy

The coarsening process of τ′ precipitates was studied during the isothermal aging of a Zn-22 wt.% Al-2 wt.% Cu alloy by means of X-ray diffraction, scanning electron microscopy, and Vickers hardness. The initial microstructure consisted of a lamellar constituent of the η and α phases, and isolated regions of the ε phase, after solution treatment at 350 °C for 5 days and then furnace-cooled annealing. The X-ray diffraction results indicated that the τ′ phase was formed by the following four-phase reaction α+ε→η+τ′, after aging at 100, 200, and 250 °C. Additionally, the discontinuous precipitation η→η+α was also detected to occur after aging. The growth kinetics of the τ′ phase was observed to follow the LSW theory for diffusion-controlled coarsening in the samples aged at 250 °C, but the growth kinetics was slower in the samples aged at 100 and 200 °C. This fact was attributed to the simultaneous occurrence of the four-phase reaction and coarsening process of the τ′ phase.