Abstract
Some specific structures of intermetallic alloys, like approximants of quasicrystals, have their unit cells and most of their atoms located on a periodic fraction of the nodes of a unique {\bb Z}-module [a set of the irrational projections of the nodes of a (N > 3-dimensional) lattice]. Those hidden internal symmetries generate possible new kinds of defects like coherent twins, translation defects and so-called module dislocations that have already been discussed elsewhere [Quiquandon et al. (2016). Acta Cryst. A72, 55-61; Sirindil et al. (2017). Acta Cryst. A73, 427-437]. Presented here are electron microscopy observations of the orthorhombic phase NiZr - and its low-temperature monoclinic variant - which reveal the existence of such defects based on the underlying {\bb Z}-module generated by the five vertices of the regular pentagon. New high-resolution electron microscopy (HREM) and scanning transmission electron microscopy high-angle annular dark-field (STEM-HAADF) observations demonstrate the agreement between the geometrical description of the structure in five dimensions and the experimental observations of fivefold twins and translation defects.
Highlights
The present article is the experimental continuation of a search to identify possible new defects in structures where the atoms, in addition to being periodically distributed, are located on a long-range-ordered subset of the nodes of a Z-module.The phase diagram of the binary system (Ni, Zr) presents a congruent solidification point at 1533 K for the equiatomic composition Ni50Zr50 close to a eutectic transformation, on the Zr-rich side, with a precipitation of Zr67Ni33 at 1295 K
In order to check the validity of our previous predictions from the Z-module description, we prepared samples of NiZr for high-resolution TEM investigations in high-resolution electron microscopy (HREM) and scanning transmission electron microscopy high-angle annular dark-field (STEM-HAADF) modes
The STEMHAADF and STEM bright-field (STEM-BF) observations have been performed on an FEI Titan Themis 200 [Center for Nanosciences and Nanostructures (C2N), Marcoussis, France]
Summary
The present article is the experimental continuation of a search to identify possible new defects in structures where the atoms, in addition to being periodically distributed, are located on a long-range-ordered subset of the nodes of a Z-module. Very to the case of quasicrystals (see Shechtman et al, 1984), this allows us to reformulate the ideal structure of NiZr by embedding it in a five-dimensional space using atomic positions with five indices for the ðx; yÞ description, in addition to the scalar two-valued spin-like index Æ representing the z coordinate Æ1=4. This new configurational five-dimensional Euclidian space corresponding to the ðx; yÞ plane decomposes as. As defined in a previous article (Sirindil et al, 2017) we call this kind of structure a Z-module-based alloy
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