Relation between ageing processes and compositions of (α+T) type Al-Zn-Mg alloys Mainly, Studies on measurements of specific heat

• Published in: Journal of Japan Institute of Light Metals Vol. 22; no. 1; pp. 62 - 72
• Main Authors: SUZUKI, Hisashi, KANNO, Motohiro, ASAMI, Shigenori, HORIYA, Takao
• Format: Journal Article
• Language: Japanese
• Published: The Japan Institute of Light Metals 30.01.1972
• ISSN: 0451-5994; 1880-8018
• DOI: 10.2464/jilm.22.62

The relation between the ageing processes and various compositions of two-phase (α+T) Al-Zn-Mg alloys were studied mainly by specific heat measurements and also by electric resistivity measurements and transmission electron microscopy. The results obtained were as follows: (1) There were observed two sorts of endothermic change and one exothermic change on the specific heat-temperature curves (S-T curves) in all the specimens aged at low temperatures. However, the detailed shapes of curves considerably varied according to the compositions of specimens. The changes in the shapes of resistivity-temperature curves corresponded well with those in the shapes of S-T curves in each specimen. (2) The changes in S-T curves due to the compositions were mainly explained by the following facts. The amount of G. P. zones formed during pre-ageing was much larger, but the amount reversed during heating was much smaller in the specimens of higher zinc content; and the conversion of GP. zone→η' or η'→η was much easier in these specimens. (3) In particular, a complicated shape (with small endothermic changes) was observed in the exothermic part of S-T curves in zinc rich specimens. It was found that the above endothermic changes were owing to the dissolution of the previous phase into matrix at the end of a certain transformation in the process of continuous precipitation. The changes in S-T curves of the specimens aged at high temperatures were also discussed in detail. (4) The above results were in good agreement with those obtained by electron micrographs. In addition, interesting knowledge was acquired on the nucleation of T phase in η phase, on abnormalities of S-T curves in zinc rich specimens, etc.