FINE-STRUCTURE AND FORMATION MECHANISM OF BAINITE IN STEELS

Scanning tunneling microscopy (STM), together with light optical micro scopy (LOM), scanning electron microscopy (SEM) and transmission elect ron microscopy (TEM), of ultra fine structure of bainitic ferrite (a) shows that subunits inside a bainitic plate consist of sub-subunits, a nd STM of the surface relief effect accompanying bainitic structure fu rther confirms the existence of sub-subunits. The multi-layer structur al constitution of bainite exhibits the typical self-embedded characte ristics, and the fractal multi-dimension has been determined to be 2.7 4. Moreover, STM shows that an individual relief corresponding to a ba initic plate is composed of a sheaf of miniature reliefs which seem to arise from the formation of bainitic platelets, such as subplates, su bunits and/or sub-subunits. The discovery of bainitic sub-subunits is helpful to give insight into transformation mechanism and interpret th e complex and variable morphology of bainite. Based on the findings re garding the multi-layer structure of bainite, it is proposed that bain ite forms via ledgewise growth which is essentially accompanied by sym pathetic nucleation, i.e., sympathetic-nucleation-ledgewise-growth (SN LG). In this paper the SNLG mechanism will be theoretically discussed and practically applied to explain the multi-layer structure of bainit e.