A recent publication reported a new observation on the variation of electrical resistivity of an Al-4 wt.-%Cu alloy that had been solution-treated at 530°C, directly quenched to and aged at a temperature T1 and then up-quenched 20°C to T2 (below the θ′ solvus). If sufficient time has elapsed at T1 a maximum is given in the resistivity vs. time at T2 plot. Experimental evidence led to the conclusion that the resistivity maximum is the manifestatfon of the resolution of a small fraction of the existing metastable precipitates (GP zones, θ″, θ′) when conditions are such that the operative sub-solvus line is crossed during up-quenching.
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