MORPHOLOGICAL TRANSITION OF MARTENSITE ASRELATED TO Ni CONTENT IN A Fe-5Ni/Fe-10Ni DiFFUSION COUPLE

摘要

A diffusion couple composed of Fe-5%Ni and Fe-10%Ni alloys was carburized in a salt bath to 0.8%C through the entire thickness. The samples had a smooth Ni gradient ranging from 5 to 10% Ni. Samples were carburized at 900°C, and cooled in oil. A morphological transition in martensite and an increase in the volume fraction of retained austenite are clearly visible as the Nickei content rises towards 10%. Grain boundary formation of bainite is also visible in the Iower Nickei content regions. The formation of bainita ceases at an approximate Nickei content of 6.5%, and the scale transition of martensite (from very fine plate to large plate morphology) is more pronounced shortly afterwards, at approximately 7% Ni. An increase in the amount of retained austenite is also present as the nickei content increases, and the beginning of its appearance seems to be closely related to the change in martensite morphology.