二段時効処理について

抄録

Two-steps ageing treatment, as the author calls it, is a heat-treatment to change ageing temperature during its process. As the number and the stable size of the precipitate or pseudo-precipitate differs with the temperature of ageing the distribution of the precipitate or pseudo-precipitate can be changed by such treatment within a wide range, and thus the properties of the alloy at its final state can be changed; in some cases they may be inferior to the corresponding properties of the same alloy when normal ageing saturates, whereas in other cases when the condition is adequate, superior properties may be obtained. It is of tremendous interest from the industrial stand point to try to improve the properties of high strength aluminium alloys by means of such treatment, to say nothing of its theoretical interests in the study of ageing.
Various kind of treatment belonging to this type, which have been hitherto carried out by the author and other workers, can be classified into two main groups, 1) two-stage treatment, where the alloy is quenched into the temperature within the upper range of its ageing temperature, kept there for a suitable period, then cooled to the room temperature without completing ageing and will be aged once more at the room temperature, and 2) split ageing or two-steps ageing treatment in a narrow sense of the word, where the alloy is once quenched and brought to the room temperature and then aged primary at a temperature followd by the secondary ageing at another temperature.
A brief survey of the former report by late Dr. Imai and the author concerning the 1) treatment (1949) has been made with some further discussions. Then 2) treatment has been considered based on the experimental results obtained by the author this time and that of other workers. The most interesting results of the author's experiment, from the indstustrial stand point has been shown when Al-Mg₂Si alloy was once aged at the higher temperatures above its normal ageing temperature (to avoid the effect of partial reversion at the initial stage of secondary ageing) for a short period, followed by room temperature ageing and then aged at the normal temperature; 20min, at 190°C gave the best results and the properties thus finally obtained was found to be superior to the properties when the alloy is normally quenched and aged. This will also serve as an aid to solve the problem of preventing the deterioration of strength when room temperature ageing might be included between quenching and tempering, which is often the case in industrial practices.