2006 Volume 47 Issue 9 Pages 2427-2433
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, B2 and 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.