| An investigation has been carried out for the precipitation process in dilute Al-Cu-Mg alloys containing 2.0 wt% Cu and l.l wt% Mg by the detailed measurement of changes in hardness and electrical resistivity during ageing at the various temperatures between 40˚and 300℃. The preprecipitation in this alloy has occurred in three different ageing stages; (1) G.P.B. zones formed during quenching and ageing for less than 1 minute known as the stage of "fast reaction", (2) G.P.B. zones formed in the early stage of ageing below 200℃ known as the stage of "slow reaction", and (3) G.P.B. (2) zones formed in the early stage of ageing above 250℃. The apparent activation energy for the formation of zones in the stage of slow reaction has been determined to be 0.25±0.03 eV and 0.24±0.01 eV, in the low and high ageing temperatures, respectively. The critical ageing temperature at which the rate of zone formation comes to maximum is found to be in the range of 70˚∼120℃. Precipitation of the metastable S` phase has been observed even at such a low ageing temperature as 40℃. Therefore, it seems that the precipitation of the S` phase depends upon mainly ageing time, and not so much upon ageing temperture. The equilibrium S phase has been shown to precipitate at the elevated ageing temperature above 200℃ on prolonged ageing. |
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