High temperature structural study of decagonal Al-Cu-Rh quasicrystal.

The structure of d(ecagonal)-Al-Cu-Rh has been studied as a function of temperature by in-situ single-crystal X-ray diffraction in order to contribute to the discussion on energy or entropy stabilization of quasicrystals (QC) [1]. The experiments were performed at 293 K, 1223 K, 1153 K, 1083 K, and 1013 K. A common subset of 1460 unique reflections was used for the comparative structure refinements at each temperature. The results obtained for the HT structure refinements of d-Al-Cu-Rh QC seem to contradict a pure phasonic-entropy-based stabilization mechanism [2] for this QC. The trends observed for the ln func(I(T1 )/I(T2 )) vs.|k⊥ |^2 plots indicate that the best on-average quasiperiodic order exists between 1083 K and 1153 K, however, what that actually means is unclear. It could indicate towards a small phasonic contribution to entropy, but such contribution is not seen in the structure refinements. A rough estimation of the hypothetic phason instability temperature shows that it would be kinetically inaccessible and thus the phase transition to a 12 Å low T structure (at ~800 K) is most likely not phason-driven. Except for the obvious increase in the amplitude of the thermal motion, no other significant structural changes, in particular no sources of additional phason-related configurational entropy, were found. All structures are refined to very similar R-values, which proves that the quality of the refinement at each temperature is the same. This suggests, that concerning the stability factors, some QCs could be similar to other HT complex intermetallic phases. The experimental results clearly show that at least the ~4 Å structure of d-Al-Cu-Rh is a HT phase therefore entropy plays an important role in its stabilisation mechanism lowering the free energy. However, the main source of this entropy is probably not related to phason flips, but rather to lattice vibrations, occupational disorder unrelated to phason flips like split positions along the periodic axis.

High-entropy alloys in light transition metal systems

High-entropy alloys (HEAs) are a new class of alloys designed with the approach of maximization of configurational mixing entropy by increasing the number of constituents [1,2]. Alloys produced in such a way are reported for a variety of promising properties (high hardness and strength, wear resistance, magnetism etc.) [3]. However, origin of these properties (microstructure, phase content, element composition, thermal history) is not always clear. High mixing entropy in HEAs favours the formation of single-phase substitutional solid solutions at elevated temperatures with approximately equiatomic compositions and simple average crystal structures of either the cF4-Cu (fcc) or the cI2-W (bcc). Nevertheless, only a few element combinations produce truly single-phase materials. In order to search for new HEAs compositions samples in the systems Cr-Fe-Co-Ni-Al and Cr-Fe-Co-Ni-Mn were synthesized by arc melting and homogenized in tantalum ampoules at 1100 and 1300 °C for 2 weeks. DTA, X-ray diffraction and electron microscopy measurements were performed. Only samples with small Al content (~ 5 at.%) showed the single-phase microstructure. Their local atomic structure is under investigation.

Crystallographic data mining - A path to structure prediction in intermetallics?

"Many metals adapt very simple structure types, mainly sphere packings such as cubic and hexagonal close-packed, as well as simple body-centered cubic structures. In contrast, around 2 % of all intermetallic crystal structures exhibit unit cells containing 100 or more atoms [1]. These compounds are also termed ""complex intermetallics"" and, while the simplest packings approximate metal atoms as spheres and in doing so explain a number of frequent structure types, the existence of such intricate geometries is not immediately evident. From the analysis of a large number of intermetallic structures, we can recognize geometrical patterns (e.g., recurring building blocks) in order to find out more about their general building principles. Among complex intermetallics, two very distinct phenomena can be observed when inspecting the different structures. On one hand, chemically very diverse compounds can crystallize in rather similar structures, as was shown for a group of more than 40 complex face-centered cubic compounds [2]. On the other hand, small compositional changes within select intermetallic systems can yield a number of different structures, which can also belong to the class of complex intermetallics. A showcase system is found among Al-Cu-Ta, where a small range of the ternary phase diagram contains at least four different structures, all of them complex [3]. By gaining deeper knowledge of the factors influencing structure stabilization, we aim at a better understanding of the mechanisms that are responsible for the formation of intermetallic structures. Thus, we hope to contribute to the quest for explaining why certain intermetallic structures form, while countless other theoretically possible structures do not. Consequently, the prediction of intermetallic structures, especially in still largely unknown parts of multinary phase diagrams, will come into reach."