Nature of Martensitic Phase Transition in β Phase Alloys

Electron diffraction study on the formation of the 9R type martensite in AgCd shows that the martensitic phase transition proceeds through the successive two processes. The first process is the symmetry change due to very small lattice distortion in the β 2 phase domains, which have characteristics of the embryos of the martensites. The close packed layers are not yet formed at this stage. This process can be caused from softening of a phonon mode localized in the β 2 phase domains. The second process is the change of the distorted lattice to the 9R type martensite without any symmetry change. Based on the results, nature of the martensitic transition in the β phase alloys is considered and a general model of the transition is presented.