| Abstract | The dimensional changes of dendritic and non-dendritic (stir-cast) Zn-Al alloy (ZA-27) were investigated during ageing at temperatures in the range 20–245 °C. The linear expansion of both dendritic and non-dendritic samples increased rapidly with ageing time after about 24 h at 95 °C. An initial normalization treatment led to a large initial growth of the alloy which increased further after prolonged ageing times (≥ 10⁴h). Accelerated ageing tests in the range 75–250 °C showed that increasing the ageing temperature decreased the long term linear expansion of the alloy. In comparing the behaviour of the two materials, the stir-cast material grew less than the dendritic alloy. Extensive SEM and TEM done on as-stir-cast and aged samples showed that the main changes during ageing occurred in the α (FCC) lamellae of the β→α+(Zn) eutectoid, the α phase at the primary α particles/β eutectoid interface and in the interparticle areas. In the α lamellae and at the interface, zinc precipitated whilst the τ′ phase precipitated in the interparticle areas. It is considered that the occurrence of this latter phase, rich in copper, is responsible for the growth of stir-cast material during ageing. |
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