The formation of an intermediate structure during the dezincification of β Cu–Zn alloys and its relevance for the martensitic transformation

The phenomenological theories for martensitic transformation by Wechsler, Lieberman, Read and Bowles, Mackenzie describes well the crystallography and orientation relationship between austenite and martensite by postulating an undistorted habit plane. However, they do not give information on the actual path of the atomic displacements during the martensitic transformation. An atomistic model which attempts to do that and results in the same crystallography as in the phenomenological models has previously been described for the transformation from the bcc to the fcc structure by one of the authors. In this model the main shear occurs on the bcc (110)β <11̄0>β plane, to which a shuffle on an inclined plane is added. Because both steps could never be separated experimentally, the model remained speculative. It has now been possible to show that an intermediate structure which consists only of shears on (110)β <11̄0>β can be obtained by the dezincification of β phase Cu–Zn alloys in the form of coherent spherical particles with the 4H structure. Its appearance proves that this structure has a sufficiently low free energy to serve also as the first step in the martensitic transformation, in which its energy can be decreased further by the shuffles.