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<p>The mechanical and stress corrosion behaviour of 7xxx aluminium thick-plate alloys are strongly influenced by a dual distribution of <i>η</i>-phase <i>quench</i>-induced and <i>age</i>-induced grain boundary precipitates (Q-GBPs and A-GBPs) that nucleate independently during cooling from solution treatment and artificial ageing, respectively. A novel cold intergranular fracture technique has been used to allow the interaction between these two precipitate distributions, formed on GB faces, in two alloys with different quench sensitivities, AA7050 and AA7085, to be more rigorously compared than previously possible. After a full T76 heat treatment, large widely spaced Q-GBPs are observed, between which high-density patches of small A-GBPs nucleate during subsequent artificial ageing. The A-GBP patches are separated from the Q-GBPs by distinct GB precipitate-free areas. Their respective distributions vary in a non-intuitive way with cooling rate. As the rate increases, the A-GBPs and patch area fractions reduce to a minima before rapidly increasing at very high cooling rates as Q-GBP nucleation becomes fully suppressed. This occurs because, although the size of the Q-GBPS GBPs reduces with cooling rate, there is a more dramatic increase in their nucleation density, which leads to a minima in the GB area where sufficient solute supersaturation is retained for precipitation during ageing. The same relative effects are seen in both alloys, but the Q-GBPs formed in AA7085 are significantly smaller and the density higher due to the lower solvus temperature.</p>

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Effect of Cooling Rate on the Heterogeneity of Grain Boundary η-Phase Precipitates in 7xxx Aluminium Alloys Following Artificial Ageing

  • 期刊论文

  • 出版模式：

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  • 发表日期：

    2025年7月10日

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  • Yichao Yao,
  • Ryan Euesden,
  • Matthew Curd,
  • Timothy Burnett,
  • Pratheek Shanthraj,
  • Philip B. Prangnell

摘要

The mechanical and stress corrosion behaviour of 7xxx aluminium thick-plate alloys are strongly influenced by a dual distribution of η-phase quench-induced and age-induced grain boundary precipitates (Q-GBPs and A-GBPs) that nucleate independently during cooling from solution treatment and artificial ageing, respectively. A novel cold intergranular fracture technique has been used to allow the interaction between these two precipitate distributions, formed on GB faces, in two alloys with different quench sensitivities, AA7050 and AA7085, to be more rigorously compared than previously possible. After a full T76 heat treatment, large widely spaced Q-GBPs are observed, between which high-density patches of small A-GBPs nucleate during subsequent artificial ageing. The A-GBP patches are separated from the Q-GBPs by distinct GB precipitate-free areas. Their respective distributions vary in a non-intuitive way with cooling rate. As the rate increases, the A-GBPs and patch area fractions reduce to a minima before rapidly increasing at very high cooling rates as Q-GBP nucleation becomes fully suppressed. This occurs because, although the size of the Q-GBPS GBPs reduces with cooling rate, there is a more dramatic increase in their nucleation density, which leads to a minima in the GB area where sufficient solute supersaturation is retained for precipitation during ageing. The same relative effects are seen in both alloys, but the Q-GBPs formed in AA7085 are significantly smaller and the density higher due to the lower solvus temperature.