Interfacial Dislocation Arrays Research Articles

Elasticity theory of a thin bicrystal distorted by an interfacial dislocation array parallel to the free surfaces

Abstract Arrays of intrinsic and extrinsic boundary dislocations located along the boundary parallel to the free surfaces of a bicrystalline foil are treated using anisotropic elasticity theory. The dislocations are equispaced along the boundary, and their elastic fields interact with the free surfaces. The problem is solved by using a method proposed previously by the author (Bonnet 1981) for periodic elastic fields. It is reduced to the inversion of a 24 × 24 array of linear equations; if one crystal is thick (semi-infinite), this array reduces to 18°18. The stability of the arrays of dislocations is discussed, and the elastic energy stored in a foil containing an array of intrinsic boundary dislocations is calculated.

Electron microscopy of interfacial dislocations

It is becoming increasingly apparent that many fundamental properties of interfaces are affected by the presence of regular arrays of interfacial dislocations. The transmission electron microscope is the most suitable means for observing such dislocations. An outline of the various experimental methods available for characterizing interfacial dislocation structure will be presented, and a critical comparison made of their effectiveness. Various examples of interfacial dislocation analysis will be shown to emphasize the points made above.

Structure of crystalline interfaces

The structure of crystalline interfaces, as observed by transmission electron microscopy, is reviewed with emphasis on the similarity of grain and interphase boundaries of the dislocation type. Small-angle grain boundaries and low misfit interphase boundaries between similar crystal structures largely condense their mismatch into arrays of interfacial dislocations having Burgers vectors in common with dislocations located in the bulk crystals. Large-angle grain boundaries near certain misorientations corresponding to good fit between the abutting grains contain dislocations with Burgers vectors which are not found in the bulk crystal. Partially coherent interphase boundaries between quite dissimilar crystals, for example, f.c.c. and b.c.c., may also contain such dislocations. Principally, because of the difficulties involved in the acquistion of interfacial dislocations, dislocation interphase boundaries, in particular, usually do not have the minimum energy structure.

Observations concerning the β→αm massive transformation in Cu-Zn alloys

The occurrence and progress of the massive transformation in copper-rich Cu-Zn alloys, have been studied during quenching and heating by metallographic and electron-microscopic techniques. The transformation occurred predominantly at compositions within the single-phase α field in samples containing a diffusion gradient, and in two-phase samples containing both the α and β phases. αm particles produced during quenching from the β-phase field have been found to grow massively following a rapid reheating into the α-phase field. Equilibrium α, or Widmanstatten α particles, have not been observed to continue by massive growth, nor did they act as preferred sites for nucleation of the massive phase. These observations are consistent with a growth model suggested recently by Karlyn, Cahn, and Cohen.3 However, in samples rapidly quenched from the β phase, the onset of the massive transformation occurred in the two-phase field some 10° to 20°C above the equilibrium a-phase boundary. This observation is not consistent with a model that requires the predominant growth of the massive phase to occur within the α single-phase field, and includes delay times that increase on approaching the α/(α + β) phase boundary. No low index orientation relationship was found to exist across the faceted interfaces between the massive and the retained β(β′) matrix. In addition, no regular interfacial dislocation arrays were evident at these boundaries. These observations add further support to the notion that the advancing interface is structurally comparable to a high-angle, incoherent boundary.