Soaking the annealed Al-Cu alloy or duralumin in the nitre-bath heated at 540° or 500° respectively, the solution-velocities of various precipitates in the alloys have been determined rontgenographically. The effect of the rate of dissolution of the precipitates on the aging properties of the alloys has also been investigated. The results obtained may be summarised as follows:
(1) The dissolution of CuAl₂ at 540° in aluminium takes place at first very rapidly; for instance, it is 80% with the 3 minutes-soaking and nearly 90% with the 10 minutes-soaking.
Although no marked difference in hardness of the alloys after aging has been detected, the alloys soaked for a short time in the nitre-bath showed the tendency to harden rapidly during the aging, compared with those soaked for a longer time.
(2) In the case of duralumin, the solution-velocities of various precipitates, namely, CuAl₂, Mg₂Si, Al₁₃ Cu₇Mg₈, AlMn₆, etc., seemed to differ with each other; it is certain that the solution-velocity of Mg₂Si is smaller than that of CuAl₂.
In order to dissolve the precipitates almost completely into a solid solution, the duralumin should be soaked at least for 20 minutes at 500_??_510°