Abstract
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 fcc-fct 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}/⟨0\bar11⟩ 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 analysis, it was found that the tweed contrast is caused by the static displacement of R=δ{011}⁄⟨0\bar11⟩, 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.
