Bainite transformation and the diffusional migration of bainite/austenite broad interfaces in Fe-9%Ni-C alloys

Abstract

The processes of bainitic structure formation in Fe-9%Ni-C alloys have been investigated by means of transmission electron and hot stage optical microscopies. The bainite laths formed initially with sharp surface reliefs grow in thickness direction rather slowly with surface rumpling-type reliefs. A pearlitic layer often forms in the carbon-enriched region between the bainitic ferrite laths. The pearlitic ferrite nucleates epitaxially on or grows directly from an adjacent bainitic ferrite lath. Such pearlite colonies always exhibit the Isaichev ferrite/cementite orientation relationship with the [010] θ‖<111> α‖<110> γ near invariant line between three phases on the bainite habit plane. In the typical upper bainite with cementite layer between ferrite laths, a cementite/ferrite orientation relationship with the (100) θ≈‖{110} α bainite habit plane was recognised in addition to the (011) α‖( 1 ̄ 03) θ habit. The general feature of cementite precipitation in such upper bainite laths is that the near invariant [010] θ‖<111> α‖<110> γ direction lies on the bainite habit plane.