Morphological Study of the Isothermal Martensitic Transformation of Iron Alloys

Abstract

Morphological study of the isothermal martensitic transformation of Fe-25-7%Ni-2.76%Cr, Fe-25.4%Ni-2.17%Mn, and Fe-25.3%Ni-4.52%Mo alloys with very few content of carbon and nitrogen has been carried out and the following results were obtained.
(1) The typical isothermal martensite of these three iron alloys transformed at temperature range from room temperature to about-100°C were a lath type martensite having tangleddislocations.
(2) AT gretaly lower temperature than-100°C, the martensitic transformation with burst-like mode occured.
The martensite transformed in such manner had many twins other than dislocations, and it can be considered that Georgiyeva et al had regarded improperly such kind of martensite as the isothermal one.
(3) The dislocations in the isothermal martensite lath were less closely tangled than that in the so-called packet type athermal martensite.
(4) Some regions having different image contrasts were observed in the isothermal martensite laths by transmission electron microscopy. It is supposed that those implies a little change of the orientation during the transformation.
(5) The lattice orientation relationship between the isothermal martensite and the austenite was analysed approximately as the so-called K-S relationship.(6) Some highly dislocated region was observed in the austenite surrounding the isothermal martensite. That region was extending into a characteristic direction.
(7) These results drawn from this study give important informations about the interrelation among three types of γ→α' marteneite transformation in iron alloys.