Abstract
A DSC scan of a Cu-33.5Zn-4Sn (mass%) alloy shows two exothermic peaks at around 250 and 300°C. The first peak is associated with the precipitation of α1-plates with M9R structure, and the second peak is associated with the formation of γ precipitates. For the first peak, the α1-plates start to nucleate at grain boundaries and grow dendritically upon heating. The primary and secondary arms of the dendritic α1-plates grow in two preferred directions, namely, [110]B2 and [100]B2 of the β′ matrix. However, the stacking faults in the two arms are parallel and differ from those with the structure of the twin-accommodating variants. As the thickness and number of α1-plates increase, the hardness of the alloy increases significantly; however, the recovery strain reduces dramatically. Additionally, the γ precipitates nucleate heterogeneously at the interface between the α1-plate and the β′ matrix on heating above 300°C. The volume fraction of the γ precipitates increases with temperature, resulting in a reduction in the ductility of the alloy and a complete loss of its shape memory effect.
