Surface Relief of Phase Transformation Products of Supercooled Austenite

摘要

It is very important to investigate surface relief of phase transformation products of supercooled austenite. Recently, many works have been carried out on the observation of surface relief of bainite, martensite, pearlite by means of AFM, STM, and SEM. In this paper, the surface relief of transformation products of super-cooling austenite was studied. The results show the surface relief of bainite, martensite and pearlite are all tented(∧) or N(N) type, the height of surface relief is commonly about ten nanometer, and with flat slope. The surface reliefs of martensite haven't the special character of N (N) type. The relief patterns correspond to their own microstructure, respectively. The formation mechanism of surface relief is complex as aberrance of heat treatment. The phase transformation of supercooled austenite is all first order phase transformation, namely, bulk expansion occur when supercooled austenite transforms to pearlite, bainite, or martensite. Surface reliefs indicate the super-cooling austenite transform on the surface of specimen, the formation condition and transformation resistance for new phase is different on surface and in interior of specimen. The free limit of bulk expansion is larger in the direction perpendicularly to specimen surface, and ferrite, cementite, bainite ferrite, martensite are all expanse outward. Accordingly, the surface relief results from the heterogeneous bulk expansion of specimen surface during phase transformation, which is a commonly phenomenon during supercooed austenite transformation on surface of specimen, not belong to a special phase transformation yet.