Stability and stabilization of 2H martensite in Cu-Zn-Al single crystals

The stabilization of the 2H martensitic phase in Cu - Zn - Al single crystals with an electron concentration e/a = 1.53 was investigated. This orthorhombic 2H martensite was first induced from the cubic beta phase by the direct beta -> 2H or the indirect beta -> 18R -> 2H transformations. On loading the 2H martensite, a transition without hysteresis is observed at a stress which was denoted sigma(T1). It was found that this stress is associated with a change in the behaviour of the 2H martensite. A high stabilization of the 2H martensite, around 300 K, is only obtained if an ageing is performed at a stress above sigma(T1). Additionally, the stresses of the transformation to another martensitic phase, called 18R(2), were found to be constant when the value of sigma(T1) is below the retransformation stress. The 2H martensite and its behaviour on ageing were studied by dilatometry, calorimetry, mechanical testing, optical microscopy and transmission electron microscopy (TEM). Models accounting for the stabilization of the 2H martensite on ageing are proposed.