High-resolution electron microscopy study of ledge structures and transition lattices at the austenite–martensite interface in Fe-based alloys

Abstract

In order to clarify how the lattices of austenite and martensite are connected at the interface on the atomic scale in the region within the width of the transformation twins or between slipped planes, a high-resolution electron microscopy study of the austenite–martensite interface was performed for Fe–23.0Ni–3.8Mn, Fe–30.5Ni–10Co–3Ti and Fe–8.8Cr–1.1C (mass %) alloys. Martensitic transformations occur at a low temperature in these alloys and exhibit various transformation characteristics, such as isothermal and athermal transformations, Kurdjumow–Sacks and Nishiyama orientational relationships, and a reversible mobile interface and a non-reversible interface in the reverse transformation. There were three important findings: (1) the (121)_f interface is made up of two component planes, i.e. the terrace plane (111)_f (//(011)_b) and the ledge (0

0)_f, and the average numbers of atom rows of [

01]_f (//[

1]_b) on these planes are 2.7–4.9 on (111)_f and 1.3–2.6 on (0

0)_f. These are different depending on the alloy, but the ratio of the atom rows of the former to the latter is about 2:1, in accordance with the observed habit plane of (121)_f; (2) the neighbouring (0

0)_f ledges are separated by about five to ten planes of (112)_b; and (3) transition lattices from fcc to bcc (or bct) exist at the interface and the thickness of this transition lattice region is 0.2–0.8 nm. It is reasonably assumed that screw dislocations with a Burgers vector of 1/2[

01]_f (//1/2[

1]_b) reside at the ledge of the interface, which produces the lattice invariant deformation when they move with the interface. The complete scheme for the relaxation of the strain at the interface is considered to be as follows. First, transformation twins are introduced to relax the strain at the interface on the macroscopic scale, in accordance with the phenomenological theory, then screw dislocations are brought in at the ledges to release the strain between each of the twin-related martensite crystals and the austenite, and, finally, transition lattices are generated at the interface to relax the strain on the most microscopic scale, i.e. at the atomic level.

Acknowledgements

The authors would like to acknowledge Dr. T. Kikuchi, National Institute for Materials Science, for his help in preparing the samples, and Prof. Y. Nakamura, Tokyo Institute of Technology, for his constant encouragement throughout the present work.

Notes

HREM could not reveal the existence of the transformation twins because the diffraction pattern of the twin is exactly the same as that of the matrix, since the incident beam is parallel to the ⟨ 111 ⟩_b twinning shear direction and, furthermore, the {112}_b twinning boundary is also parallel to the incident beam under the conditions for observation of the lattice images from direction A.