HREM study on the ledge structures, transient lattices and dislocation structures at the austenite-martensite and austenite-bainite interfaces in Fe-based alloys

Abstract

High-resolution electron microscopy (HREM) has been performed to know the atomic arrangement of the austenite-martensite interface and the austenite-bainite interface in Fe-based alloys. The alloys studied are Fe-23.0Ni-3.8Mn, Fe-8.8Cr-1.1C, Fe-30.5Ni-10Co-3Ti (mass %) for martensitic transformation and Fe-2Si-1.4C (mass %) for bainitic transformation. These alloys have various transformation characteristics depending on the alloy; for martensitic transformation, athermal and isothermal kinetics, the Kurdjumow-Sachs (K-S) and Nishiyama (N) orientation relationships, reversible and irreversible movement of the interface, and for bainitic transformation, upper bainite and lower bainite. All the interfaces observed had to be limited to {112} (macroscopically {225}) or very close to {112} because of the geometrical condition that the atom rows of f, b and b must be observed parallel to the interface, i.e., the edge-on orientation. The austenite-martensite interface is (121)f with the K-S orientation relationship of (111)f//(011)b and [101]f// [111]b, and the interface is basically composed of the terrace of (111)f and the ledge of (010)f, which have the average ratio of 2:1 for the number of atom rows of [101]// [111]b on these planes. This interface always accompanies the transient lattice region with the thickness of 0.4-1.0 nm, where the lattice changes continuously from fcc to bcc (or bct). No extra-half plane is observed at the (121)f interface over a large distance of 100-200 lattice planes. The interface for both the upper and lower bainites is close to (112)f with the N orientation relationship of (111)f//(011)b and [110]f//[100]b'. Contrary to the interface for martensite, this interface for bainite has many extra-half planes except when the interface is close to (112)f. The interface is basically made up of the terrace of (111)f//(011)b and the ledge of (011)b'//(112)f, and the height of the ledge is usually one or two atomic layers. No transient lattice region accompanies this interface. Transformation dislocations and a nucleating martensite are observed at the tip of growing martensite.