Effects of small addition of magnesium on initial aging rate of Al-Zn alloys

Abstract

The effects of addition of a small amount of Mg atoms on initial aging rate of Al-(3 or 5 at%) Zn alloys were investigated in order to clarify the mechanism that G. P. zone formation is remarkably suppressed by this addition. The specimens were composed of two sorts of alloy systems; Al-3 at%Zn-Mg and Al-5 at%Zn-Mg, in which Mg contents were systematically changed.In these studies, special attention was paid to the compositions of specimens whether they were two-phase or three-phase alloys at each aging temperature. Since the aging behavior would be different between two-phase and three-phase alloys, the two-phase alloys should be required for the quantitative studies of the above mechanism.The results obtained were as follows:(1)The compositions of Mg limit(between two-phase and three-phase alloys)in each alloy system were determined as a function of temperature by means of X-ray analysis and resistivity and calorimetric measurements. The results revealed that all specimens, which showed suppression phenomena in previous investigations, were surely three-phase alloys at each aging temperature.(2) It was confirmed that the aging curves of the two-phase specimens did not show any suppression in the initial aging rate in quite the same way as in Al-Zn binary alloys. Whereas, in three-phase specimens, the suppression was more marked with the increase of Mg content.(3) The suppression phenomena observed in three-phase alloys could be explained by the slow rate of formation of the 2nd G. P. zone, which would have appeared during the aging probably corresponding to θ (Mg2Zn11) stable phase.(4) As the results, the previous theories which explained the retardation of the initial aging rate in Al-Zn-Mg alloy system on the assumption of interaction between vacancy and Mg atom were proved to be erroneous.