Some considerations on isothermal aging curves of aluminum alloys

Isothermal aging curves of various aluminum alloys have been often plotted against logarithm of aging time. However, the discussion on the shape and slope of these curves is usually done without taking serious considerations on the time scale. In this paper, some significant differences between the logarithmic and the linear plotting were discussed. The results obtained were as follows:(1) Even in the situation where the aging occurs monotonously with an increasing rate, which itself decreases as the aging proceeds, the aging curve can take a complicated form when the logarithmic time scale is adopted; it can have an incubation period, abrupt acceleration, a two stage change, etc, depending on the increasing rate.(2) It has been found that if most of the aging curves, reported previously, are replotted in the linear time scale, the shapes of these curves become very simple, and the complicated features seen in the logarithmic expression disappear.(3) It is proposed, therefore, that the various previous considerations on the shape of the aging curve in the logarithmic expression should be reexamined.