Atomic site correspondence and surface relief in the formation of plate-shaped transformation products
- PDF / 1,324,928 Bytes
- 6 Pages / 598 x 778 pts Page_size
- 44 Downloads / 151 Views
I.
INTRODUCTION
O N E feature that has often been used to define a martensitic (displacive) transformation and, hence, to distinguish martensitic plates from those formed by a diffusional (reconstructive) transformation, is the presence of a surface relief effect. ~j The origin of the surface relief is due to the invariant plane strain associated with the martensitic transformation, t21 In contrast to this, Liu and Aaronson pl presented experimental evidence that a surface relief occurred with the formation of hexagonal close-packed (hcp) y (Ag2AI) precipitate plates in AI-Ag alloys; a transformation which is classically diffusional. Further, in a series of articles concerning the formation of bainite, t4-7] Dahmen [Tj presented a crystallographic analysis (using a lattice correspondence) to show that features such as an invariant plane strain and surface relief can be associated with diffusional transformations. In spite of these data, it is still not clear how a diffusional transformation, which proceeds by vacancy diffusion and in which there is no atomic correspondence between the phases, can produce a surface relief effect. [s'81 Hence, the idea that diffusional transformations exhibit a surface relief effect is not widely accepted at present. The purpose Of this article is to show that diffusional (reconstructive) transformations can exhibit a surface relief, not because there is a lattice correspondence (or an atomic correspondence) between the phases as in a martensitic (displacive) transformation, but because there is an atomic site correspondence across the interface. That is, in many diffusional transformations, there is continuity of atomic sites across the interface, resulting in only certain sites that atoms can occupy as they cross the interface. Thus, regardless of which atoms occupy particular atomic sites, continuity of atomic sites across an interface can preserve the shape deformation and produce a surface relief effect even in a diffusional transformation, as demonstrated in this article. The concept of an atomic site correspondence across an interface between two phases is directly related to the atomic mechanisms of the transformation, and this point is used to JAMES M. HOWE, Associate Professor, is with the Department of Materials Science and Engineering, University of Virginia, Charlottesville, VA 22903. This article is based on a presentation made at the Pacific Rim Conference on the "Roles of Shear and Diffusion in the Formation of Plate-Shaped Transformation Products," held December 18-22, 1992, in Kona, Hawaii, under the auspices of ASM INTERNATIONAL's Phase Transformations Committee. METALLURGICAL AND MATERIALS TRANSACTIONS A
develop the concept of atomic site correspondence in Section II.
II.
MECHANISMS OF ATOMIC S I T E CORRESPONDENCE
In order to develop the idea of an atomic site correspondence between two phases, the discussion begins with a simple example, namely, the face-centered cubic (fcc) ~ hexagonal close-packed (hcp) transformation found in both martensitic (Co-Ni all
Data Loading...
