Ａｌ‐４％Ｃｕ‐Ｃｄ合金における中間相の析出挙動

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In Al-Cu-Cd alloys, Cd as an additional element would affect the secondary defect structure of Al-Cu binary alloys, and on the other hand, the Cd rich phase formed at the relatively early stage of aging. Therefore, the study on the precipitation behavior of the intermediate phase (θ') was carried out in Al-4wt%Cu-Cd alloy system with various Cd contents up to 0.114wt%, in relation to the defect structure and the Cd rich phase. Specimens were solution-treated at 520°C, quenched and finally aged at 200 and 250°C for various times.The results obtained were as follows.(1) The defect structure in this alloy system also consisted mainly of dislocation loops, which were finer and denser with increasing Cd content.(2) The θ' precipitates first appeared on dislocation loops in ternary alloys as well as in the binary alloy. But in ternary alloys, θ' precipitated on Cd rich phase formed in the matrix, resulting in the denser distribution of θ' in ternaly alloys (with increasing Cd content) compared with that in the binary alloy.(3) However, in ternary alloys, it was observed that Cd rich phase particles did not easily grow and only large ones were seen to act as the sites of θ' precipitate. The slow growth rate of those phases would explain the above fact that θ' precipitated preferentially on dislocation loops.(4) These phenomena as to θ' precipitate in this alloy system were in contrast to those in Al-Cu-Sn alloy system, already reported by the authors. In other words, it would be clear that the aging rate at elevated temperature was lower in Al-Cu-Cd alloys than in Al-Cu-Sn alloys, in which all Sn rich phase particles were available for θ' precipitate with the disappearance of preferential precipitation of θ' on dislocation loops.(5) The width of the precipitate free zone of θ' corresponded well with that of the loop free zone in the binary alloy, but not in ternary alloys since θ' formed on Cd rich phase particles within the loop free zone.