Characteristics of lath martensite: Part II. The martensite-austenite interface
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I.
INTRODUCTION
IN spite
of the great importance of the structure of the martensite-parent interface in martensitic transformations, very little experimental work has been done on such interfaces. Theoretically, two different kinds of dislocations may exist in the martensite-parent interface.l'2 One kind of dislocation, termed secondary dislocation by Christian, 1 and coherency dislocation by Olson and Cohen, 2 transforms the parent lattice into the product lattice. The second kind of dislocations accommodate the misfit between the parent and product lattices, and in martensitic transformations they also accomplish the lattice invariant shear needed to establish a macroscopically invariant interface3'4 between the parent and product lattices. These dislocations are termed primary dislocations by Christian and anticoherency dislocations by Olson and Cohen. In this paper we prefer to use the more descriptive terms transformation dislocations and misfit dislocations for the secondary (coherency) and primary (anticoherency) dislocations, respectively. It is also considered5 that an irrational semicoherent martensitic interface must consist of steps on adjacent close packed planes. Since the misfit dislocations, contrary to transformation dislocations, have the same properties as lattice dislocations, they can be studied using conventional transmission electron microscopy techniques. This, of course, requires the spacing of the dislocations to be within the resolution of transmission electron microscopy. Although transformation dislocations have been observed for bcctwins, 6 fcc-twins, 7 and the fcc ~ hcp martensite transformation,8 they are not very likely to be resolvable for the fcc ~ bcc martensite transformation in Fe-alloys, because calculations9 show that the Burgers vector is small and the dislocations are very closely spaced. Because of this, and the fact that experimental information about misfit dislocations is more important than experimental information about transformation dislocations, as far as calculations of martensite crystallography using the phenomenological
B. E J. SANDVIK and C.M. WAYMAN, respectively, are Research Associate and Professor in the Department of Metallurgy and Mining Engineering, University of Illinois at Urbana-Champaign, Urbana, IL 61801. Manuscript submitted April 30, 1982.
METALLURGICALTRANSACTIONS A
theories 3'4 are concerned, we describe only the misfit dislocations below. An important property of martensite is that the martensiteparent interface must be able to move conservatively. According to the simplest model, a martensitic interface may consist of a single set of parallel misfit dislocations. 5'1~ Conservative movement of the interface then requires the Burgers vector of the dislocation to have a component normal to the interface, except when the dislocations are pure screws. When such an interface glides, the dislocations accomplish the lattice invariant shear of the phenomenological theories. More complex interface dislocation arrays also may comprise a semicohere
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