The effects of low temperature aging treatments (80-200℃) and high temperature aging treatments (200-400℃) were studied by resistivity measurements, microhardness and X-ray technique.
For low temperature agings, martensitic transformation temperature increased with increasing aging time, followed by reaching saturated values. Increased aging temperature depressed saturated martensitic transformation temperatures. It is suggested that the saturation was associated with $DO_3$ ordering in the Cu-Zn-Al alloy.
It has been also found that transformation temperatures are reversibly dependent on aging temperatures. The apparent activation energy for the low temperature aging process was about 95±5 KJ/mole. It seems that the low temperature aging process was associated with the change in the degree of order of austenite phase. The phenomena has been interpreted theoretically by using Inden's model of long-range order.
For high temperature aging process, the apparent activation energy, which obtained from the time dependences of hardness and electrical resistance in the range 200℃ to 400℃, was about 120 KJ/mole. When aged at temperatures above 300℃, $β_1$ austenite phase was transformed into α and retained $β_1$ phase. On the other hand, below 300℃ an unidentified metastable phase was only observed.
