Comparison of interfacial structure-related mechanisms in diffusional and martensitic transformations

Abstract

Some central problems in understanding the similarities of and the differences between ledgewise martensitic and ledgewise diffusional growth are examined. Martensitic growth can be described in terms of a lattice correspondence and a plane undistorted by the shear transformation. Diffusional growth can be similarly described in some cases but not in others. On the basis of the Sutton-Balluffi definitions of glissile and sessile boundaries, only misfit dislocations (on terraces or risers) or orthogonal sets of disconnections provide a truly sessile interface. When closely spaced structural ledges (now termed "structural disconnections") are present during diffusional growth, they must have been glissile in the formation of a local equilibrium structure during the initial stages of growth. Once they are in local equilibrium and evenly spaced, however, they can only move synchronously because of their local strain interaction. Under these circumstances, extrinsic sources of growth ledges are required to move such interfaces in a diffusional manner. During martensitic growth, however, disconnections in the form of transformation dislocations can move freely in a synchronous manner. Also, on this basis, the apparent (undistorted) habit plane is generally useful in deducing the transformation mechanism during martensite formation, but is only occasionally so during diffusional growth, where only the terrace plane is generally useful.

Edge-to-edge matching of lattice planes and coupling of crystallography and migration mechanisms of planar interfaces

“…It is now well understood that the continuous migration of a twin boundary, as an entity, in its normal direction is energetically unfavorable, and that this normal migration of the twin boundary occurs via the successive nucleation and lateral gliding of ledges in the twin plane. Such ledges have been termed "transformation dislocations," [11][12][13][14] "growth ledges," [15,16] "disconnections," [17,18] and "Moiré ledges." [1] Existing experimental observations using high-resolution transmission electron microscopy indicate that the height of the ledges formed on twin boundaries is consistent with that of Moiré ledges, i.e., 3d {111}f .…”

“…Note that these disconnections are different from those (transformation) disconnections formed on low-index interfaces, as they are now an intrinsic part of the interface structure, and only the synchronous motion of these disconnections can maintain the orientation and the structure of the interface. These disconnections are thus structural disconnections (or structural ledges), [16] rather than transformation disconnections, growth ledges, or transformation dislocations. (Ledges that are not an intrinsic part of the interface structure are defined as transformation dislocations, per References 11 sin a ϭ s…”

Discussion on Strain Accommodation Associated with Formation of LPSO Structure

“…1,3) Later, more general concept, i.e., transformation disconnection, was proposed. 4,5) Figure 1 shows schematically the atomic motion in front of a transformation disconnection. 6) In a diffusionless/ displacive transformation (i.e., martensitic transformation), the motion of a single atom can be described by small displacement from the matrix lattice site (M) to the product lattice site (P), as is schematically shown in Fig.…”

Pattern formation during diffusion limited transformations in solids

“…Here, we mention that the anisotropies of surface tension and interfacial mobility are generally more pronounced in solid-solid transformations, which often leads to faceted interfaces. Even more, high-temperature and bainitic transformation can proceed by the motion of transformation disconnections (defects with ledge/dislocation character), by the motion of growth ledges (defects with pure ledge character or with negligible dislocation content), or by a mixture of these [9,10]. Nevertheless, in the present work, we discuss neither the role of anisotropy nor these (discrete) defect structures but concentrate on the role of elastic effects; the main reason for this simplification is the striking result that steady state dendrite-like growth is even possible without anisotropy of surface tension due to elastic effects, in contrast to diffusion-limited solidification with only rough surfaces [11].…”

“…Recently, Greenwood et al used phase field methods to investigate twodimensional dendritic structures in the solid state in the presence of competitive interactions between surface energy and elastic anisotropy [18]. They predict a transition from surface dominated growth along the [10] direction to an elastically driven growth along the [11] direction by changes in the elastic and surface anisotropy and supersaturation. However, there is a still substantial lack of both experimental and theoretical investigations concerning the growth kinetics of the emerging patterns.…”