Precursor Phenomena at Martensitic Phase Transition in Fe-Pd Alloy. I. Two-Tetragonal-Mixed Phase and Crest-Riding-Periodon

Precise X-ray diffraction study has been carried out in the vicinity of fcc-fct transition in Fe-Pd alloys in order to investigate the pretransitional phenomena at martensite transformations. Using a high-resolution X-ray spectrometer with fine temperature control unit, the temperature variation of the profiles of Bragg reflections were measured. A new intermediate phase was discovered in a narrow temperature range between the austenite (fcc) and the martensite (fct) phase. Its structure is characterized by a “two-tetragonal-mixed” phase, and interpreted as a coherent mixture of two tetragonal lattices with different tetragonalities. The origin of the stability of this intermediate phase has been investigated. It may be interpreted in terms of the previously postulated “crest-riding-periodon” phase.