Ken-ichi Hirano1 and Hitoshi Hori2
1Faculty of Engineering, Tohoku University, Sendai
The solvus curve of the G.P. zones in Al-Zn alloys has been determined by specific heat and electrical resistivity measurements.
By specific heat measurements, the reversion temperature for the G.P. zones has been determined to deduce the highest temperature for the G.P. zones to exist. It has been found that for a constant aging temperature the reversion temperature rises with aging time and finally becomes a constant temperature which depends on the aging temperature.
The electrical resistivity has been measured continuously during the cooling and heating of the specimen between the α phase region and room temperature. It has been found that, once the G.P. zones are formed, the electrical resistivity becomes higher than the value of homogenuous solid solution. The temperature for the initiation of G.P. zone formation during cooling coincides quite well with the highest temperature at which the G.P. zones are stable as determined by specific heat measurements.
The solvus temperature for the G.P. zones thus determined does not coincide with the previous results by other workers. The present results show that the solvus of the G.P. zones is located above the metastable α -α ' miscibility gap extraporated from the high temperature region where the α ' phase is stable. Further, it has been shown that the solvus of the G.P. zones in the concentrated alloys is also located above the stable α -α ' miscibility gap.
The present results suggest that the G.P. zones in this alloy are coherent precipitates of the α ' phase with a limited particle size.
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