The structure of cementite and its relation to ferrite

Abstract

A consideration of the orientation relationships which have been determined between cementite, ferrite, ε-carbide and austenite, indicates that the most frequently encountered orientation relationship between cementite and ferrite is fundamentally significant. It is consistent with the ε-carbide/cementite and ferrite/ε-carbide orientations. The austenite/cementite orientation is also compatible if ferrite (martensite) is formed as a transient intermediate step, and there is published evidence for this. An examination of the crystallography of the two phases, ferrite and cementite, in this orientation suggests a very simple explanation of the second structure as a modification of the first. The transformation mechanism implied requires that close packed rows of iron atoms in ferrite become “zig-zag” rows in cementite. The displacements are parallel to {112} ferrite planes. A second set of small displacements create reflection symmetry about these planes which are the ferrite twin planes. An analogy with the formation of the hexagonal zeta phases in some silver alloys is indicated.