In Fe-Pd alloys containing about 30 at% palladium, fcc-fct thermoelastic transformation exists, and the fct martensite phase is further transformed non-thermoelastically to the bet martensite phase. The morphology and structure of the martensitic transformation have been extensively studied by means of X-ray diffractometry, optical and electron microscopy. The change in surface relief associated with the fcc-fct martensitic transformation was investigated in detail by using single crystals which were made by the Bridgman method. The concentration dependence of their Ms temperatures was determined by using polycrystalline specimens, and it was shown that the fct phase is a stable low temperature one. In the cooling process, faint band regions appeared dispersedly along the {101 } traces and grew with jerky motion. In the heating process, the fcc austenite regions partially arised from the fct martensite regions. The temperature hysteresis of the fcc-fct transformation was about 10K, and cyclic treatment between cooling and heating showed a good reversibility of the process. Under the in situ electron microscope observation, plate type structures having the {101 } trace appeared, though their size was two orders of magnitude smaller than that of fct bands observed by optical microscopy.In addition, it was confirmed that the fcc austenite containing palladium less than 25 at% is transformed to bcc martensite, and that the bcc martensite was clearly distinguished from the bet one.

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The mechanism of thermoelastic fcc-fct martensite transformation of Fe-Pd alloys containing about 30 at%Pd was studied by optical and transmission electron microscopy. It was found that the fccfct transformation was not accompanied with internal twins in the beginning of the transformation, but they were introduced during the growth process of the fct martensite in which the degree of tetragonality increased. In the fcc austenite phase, the remarkable tweed contrast consisting of {011}/<011> striations and accompanying diffuse streaks perpendicular to them was observed, and lamellae appeared along the tweed contrast with cooling under the in situ electron microscope observation. The lamellae were identified not with the fct martensite variant plates but with thin {011} internal twins of the fct martensite, and the tweed contrast disappeared in the inside of lamellae. From the microscopical and diffraction and the origin of the static displacement is the formation of the very thin platelets of martensite nuclei. At the transformation temperature, the enhancement and extension of the tweed contrast occurred leading to the fct martensite phase. Taking account of the results that the tweed contrast persistently remained in the fct martensite except in the inside of internal twins, the variant mosaic structure model was proposed and the mechanism of the fcc-fct thermoelastic martensite transformation was successfully explained.

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