Since Dehardening Phenomenon has been observed by Preston and others in 1929, several reports on the studies of physical properties of this phenomenon such as electric resistance, thermal and magnetic analyses, etc. have been published, and the mechanism of dehardening is said to be. that the solute atoms, which are under segregated state in solid solution by aging, will disperse back to quenched state with sudden heating at about 200° for a few minutes, (dehardening treatment), but Preston is the only one who recognized this change by X-rays. He carried out some experiments with single crystals of Al-Cu 4% alloy and pointed out that the streaks which appear on Laue-photographs during aging at room temperature will disappear by dehardening treatment at 200°, though several problems on the streaks still remain unexplained.
Generally it is difficult to detect the change during aging at room temperature of Al-Cu alloys by Debye-Scherrer method, and so the author studied the dehardening phenomenon at 250° with back reflection method with a polycrystalline specimen of Al-Cu 4% alloy after previous tempering at 150° for various periods up to 28 days and distinct change was observed on diffraction lines. In parallel to this X-ray study, changes in hardness and electric resistance were also measured. The results obtained are as follows:
(1) The amount of dehardening decreases with the period of previous tempering at 150°, and this decreased. amount corresponds to the amount of precipitation by previous tempering which was calculated from the change, of lattice constant. (Fig. 3).
From this result it is quite certain that precipitated solute atoms do not play a part in dehardening at 250°, and it is only segregated solute atoms which receive some changes at dehardening.
(2) The diffused diffraction line obtained by tempering at 150° for 10 days, when submitted to sudden heating at 250° for a few minutes, recovers the same sharpness as that of quenched state, at the point where softning occures, and lattice constant remains unchanged.
These X-ray results constitute some of the most important data to support the recent theory.
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