In this report, the thermal changing in the initial stages of the natural agehardening phenomenon taking place in several representative Al-alloys are investigated by the method of the differential thermal analysis. The results obtained are as follows :
(1) It is clear that natural agehardening in Al-alloys requires the absorption of heat.
(2) In accordance with thermal analysis, activation energy for agehardening of Al-4.23%Cu alloy is determined as 13143 cal/mol or 15862 cal/mol and Al-9.55%MgZn2 alloy as 17950 cal/mol.
(3) By addition of Mg, the velocity of agehardening in Al-Cu alloy is remarkably accelerated and the existence of very small amount of Mg (0.004%) is essentially concerved in the original phenomenon of Al-Cu alloy.
(4) The order of specific heat in Al-alloys is the precipitation, agehardening and quenching states.
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