Metadislocation

Defects as shown in Fig. 1(a) are the subject of the present chapter. They are referred to as metadislocations, and occur in numerous structurally complex metallic materials. The concept of metadislocations addresses a central problem in the plasticity of materials with large lattice parameters In these materials, conventional dislocation-based deformation mechanisms are prone to failure. This is a direct consequence of the elastic strain energy, which, per unit length of dislocation, is given by... 

Fig. 1. High-resolution transmission electron micrographs, (a) A metadislocation in the complex metallic alloy (CMA) phase C28-Al-Pd-Mn [1] and (b) an edge dislocation in BaTiOs [2] (courtesy of C.L. Jia), both imaged in end-on orientation. The arrows indicate the dislocation cores.

Metadislocations, the subject of the present chapter, are among the conceptually most stunning types of defect discovered in CMAs. Their construction is closely... 

Phason lines and phason planes can assume a dual function in s-type phases. On the one hand, they are structural defects, for instance in the phases Sg-Al-Pd-Mn and -Al-Pd-Mn. On the other hand, phason lines and phason planes can arrange regularly, forming a related e-type phase with larger c-lattice constant, and hence become elements of a new ideal structure. Phason lines and phason planes are pivotally connected to metadislocation formation and movement, as well as in phase transitions and formation of e-phases. 

The overall appearance of the metadislocation resembles a dislocation in a simple metal but the differences are obvious the apparent extension of the strain field of the metadislocation is larger by more than one order of magnitude, and the associated phasons are not inserted hal lanes like those of an edge dislocation in a simple metal, but consist of a locally transformed area. Historically, the first observed metadislocations in E2s-Al-Pd-Mn were interpreted as defects in a structure of defects (Section 3.2) and were, therefore, termed metadislocations [1]. 

Fig. 17. Group of metadislocations in 28-Al-Pd-Mn. (a) At lower magnification in two-beam Bragg contrast using the (006) reflection for imaging close to the (120) axis and (b) close to the (010) zone axis, (c) Imaged under bright-held Laue conditions at the (010) zone axis, (d) Boxed area in (c) at higher magnihcation. Scale bars. 100 nm (a, b, c) and 10 nm (d).

Fig. 18 depicts a metadislocation in Eg-Al-Pd-Mn. The matrix structure entirely consists of flattened hexagons in alternating orientation. In this example, the six associated phason halfplanes stretch out to the right-hand side. 

First characterizations of the Burgers vector of metadislocations were carried out by means of Bragg-contrast analysis [39]. Their Burgers vector direction was determined as parallel to the [001] direction. Since the line direction is parallel to the [010] direction, metadislocations are pure edge dislocations. 

Fig. 19. Metadislocations in a -Al-Pd-Mn at a low magnification, (a) Under bright-fleld Lane conditions along the (010) zone axis, (b) Under two-beam bright-field conditions using the (006) reflection imaged...

Fig. 20. Tiling representation of the metadislocation in Fig. 1(a). The metadislocation core is represented by a dark-gray tile. Six phason halfplanes are terminated at the right-hand side of the core.

The tiling representation shown includes non-essential features of the metadislocation, and in this sense it is held at a low degree of abstraction. Fig. 21(a) is a tiling representation of the metadislocation in the S28-structure at a higher degree of abstraction. The curvature of the phason planes is neglected but the essential features of the metadislocation, that is, the core tile, the six inserted phason halfplanes, and the surrounding S28-structure are present. 

Fig. 21. Tiling representation at a higher degree of abstraction, (a) Metadislocation in the E28-structure.

Because metadislocations are connected to a number of associated phason halfplanes, their Bnrgers vector cannot be determined by means of a regular Burgers circuit [40]. The phason planes are not an element of the ideal structure of the Eg-phase, and hence a comparison circuit for the metadislocation in ideal Eg cannot be performed. The same holds for the metadislocation in E2s, since the associated slab of Ee is not present in the ideal E28-strnctnre. 

Fig. 22. (a) Basis vectors in the (010) plane, (b) Burgers vector determination by a circuit around the tile representing the core of a metadislocation with six associated phason halfplanes. 

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