Abstract
A new observation is reported on the variation of the electrical resistivity of a quenched Al–4·07 wt.-%Cu alloy that has been aged at T 1 (in the range 110–200° C) and then up-quenched about 20° C to T 2. If sufficient time has elapsed at T 1a maximum is attained in the relation between resistivity and time at T 2 with a height commonly < 5 × 10–9 Ω cm. It is concluded that the resistance maximum arises from re-solution of intermediate precipitates which may be G.P.[1], “or”, when conditions are such that the operative solvus line is crossed during up-quenching. It is difficult to perform a meaningful quantitative analysis of the data in terms of the position of the operative solvus because the resistivity measurements cannot yield the vital information about the local solute concentration round the small proportion of intermediate precipitates, estimated to involve about 0·3% of the total copper content of the alloy, that revert into solution. No re-solution of could be observed by hot-stage transmission electron microscopy at temperatures below 300° C during up-quenching under conditions that gave rise to an electrical resistivity maximum.
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