The influence of coarsened perovskite carbide K particle in austenite on martensite transformation and the orientation relationship of K and ferrite phases formed by the grain boundary reaction in Fe-6.83wt.%Al-1.66wt.%C heated at 800℃ and 850℃ respectively have been studied by X-ray diffraction, optical and transmission microscopies.
The K phase was isomorphously precipitated first by spinodal decomposition in austenite when cooled to 850℃ after 4 hr. solution treatment at 1200℃. Then the precipitate size increased and morphology changed from the cuboid to raft to plate shape on further aging at 850℃ most likely due to the strain effect generated by precipitates. The presence of coarsened K particles from 300Å up to 0.1μm in diameter had severe effect on martensite morphology. The usual planar martensite austensite interface was not observed. Also, the martensitic substucture was markedly effected by the presence of coarsened K particle in austenite. Coarsened K particles acted as barrier to formation of transformation twins and hence twins not continuous. Only twins segmented were observed. Martensite was formed with tow-variant $\{112}_b$ type transformation twins about 100Å in width. Martensite evolved from austenite without aging shows the martensite morphology composite lamellar formation of two variants. The lamellae are alternated by two variants whose c-axe are perpendicular to each other in such a way as to lessen the strain effect of Bain distortion that has been generated during martensite transformation.
Seperate from the morphological development, K and α phases with lamellar morphology were formed by grain boundary reaction in specimens which were heated at 800℃ and 850℃ after quenching from 1200℃ to water. And it was found that the orientation relationship between K and α phase formed with lamellar morphology correspond to Kurdjmov-Sach's orientation relationship.
