Complex metallic alloy

A quasicrystal (QC) is a type of solid that is well-ordered but not periodic. QCs are often associated with classically forbidden rotational symmetries, although strictly speaking, this is not necessary [1]. The discovery of QCs led to a refinement of the definition of a crystal as any soHd with an essentially discrete diffraction pattern. [2]. This transferred the definition of the concept from real to reciprocal space and in doing so broadened the scope of the term to encompass both periodic and quasiperiodic materials. Thus a QC is a nonperiodic crystalline material. 

An approximant is a periodic crystalline material that is closely related to a QC, both in chemical composition and in atomic structure. Often, an approximant contains the same clusters as those embedded in the QC of related composition [3]. 

This is a crystalline compound composed of (mainly) metallic constituents and has three distinguishing features (i) a large unit ceU, containing up to thousands of atoms (ii) the occurrence of well-defined clusters (often of icosahedral symmetry) and (iii) some disorder, essentially because the clusters do not fill Euclidean three-dimensional space. The approximants comprise a subset of complex metallic alloys (CM As). QCs can also be considered a subset, with a unit cell that is infinitely large in two or three dimensions. 

Surface and Interface Science Properties of Composite Surfaces Alloys, Compounds, Semiconductors, 

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M Liu, P Schmutz, S Zanna, A Seyeux, H Ardelean, G Song, A Atrens and P Marcus, Electrochemical reactivity, surface composition and corrosion mechanisms of the complex metallic alloy Al3Mg2, Corrosion Science, 52 (2010) 562-578. 

The micrograph shown in Fig. 1(a) was taken in 1998 in the framework of a transmission electron microscopy (TEM) study on the S28-phase in the system Al-Pd-Mn [1]. The 28-phase has very large lattice parameters and contains about 1500 atoms in its unit cell. According to the high degree of complexity of the structure, the material is referred to as a complex metallic alloy (CMA). 

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.

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