Electron microscopy study of 2H and 18R martensites in Cu–11.92 wt% Al–3.78 wt% Ni shape memory alloy

The microstructures and the crystal structures of the β1′ and γ1′ martensite phases in Cu–11.92 wt% Al–3.78 wt% Ni shape memory alloy were studied by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Samples subjected to two different heat treatments were analysed. In the alloy, two kinds of martensite phases were observed depending on heat treatment, and it was seen that the annealing time is effective on forming of these phases. The change between the 18R and 2H stacking sequences was investigated by diffraction technique. Further experiments revealed that the β1′ martensite structure is actually of the M18R type (modified 18R) monoclinic one with its β angle very close to 90°.