THE AGING EFFECT ON CU-ZN-AL SHAPE MEMORY ALLOYS WITH LOW CONTENTS OF ALUMINUM

The effects of various quenching and aging treatments including direct quenching (DQ), up-quenching (UQ), and step-quenching in a water bath at 100-degrees-C (SQWB) and in an oil bath at 100-degrees-C (SQOB) have been studied on two Cu-Zn-Al shape memory alloys (SMA's) with the aluminum content less than 3.8 wt pct. The structures of two heats at room temperature are parent (alloy A) and martensite phases (alloy B), respectively. It is found that the rapid rate of cooling after solution treatment is an essential requirement for a Cu-Zn-Al SMA with low aluminum content; otherwise, a less rapid rate of cooling during the quenching process causes the precipitation of alpha phase, which has been observed in SQWB specimens. The temperature of the martensitic transformation (M(s)) is found to be increased with both the size of ordering domain and the degree of ordering. Nevertheless, a stable and sound shape memory property has been obtained on specimens of either UQ or SQOB. Furthermore, the periods of isothermal aging required to initiate precipitation of either alpha or alpha-1 phase have been examined at various temperatures; then the time-temperature-transformation (T-T-T) diagrams are constructed. From the calculation of the activation energy for precipitation of alpha-1, the shape memory life expectancy of alloy B with 3.14 wt pct Al content has been determined to be only 41 days when operating at a temperature as small as 100-degrees-C. For this reason, it is suggested that a suitable operating temperature of Cu-28.2Zn-3.14Al SMA should be less than 80-degrees-C.