MARTENSITIC-TRANSFORMATION OF A NI-AL ALLOY .2. THEORETICAL TREATMENTS

The seven-layered martensite structure of the Ni-Al alloy reported in the preceding paper is investigated on a theoretical basis. We have developed a treatment of the martensite phase transformation within a site-independent scheme describing atomic fluctuations in a highly anharmonic lattice. From a microscopic viewpoint a pseudospin variable is introduced which stands for the amplitude of the local vibrational mode called the embryonic mode, and the martensite transformation is discussed as the ordering process of the pseudospins. Through a coarse-graining procedure, a semimacroscopic view has also been developed within the Ginzburg-Landau formalism. The results have been applied to analyze the observed neutron-diffraction spectra. In particular, the anomalous incommensurability of the spectra is understood by considering that the order parameter takes on two different values alternatively in space, which is interpreted in terms of the theoretically predicted crest-riding-periodon phase. © 1990 The American Physical Society.