In this paper, a comparative analysis of the mechanical properties of the quasicrystalline and nearby crystalline phases in the Al–Cu–Fe system is performed. Microhardness, brittleness, and deformation behavior of materials are considered. The analogy in the mechanical response between these structurally different phases is emphasized, despite significant differences in their atomic structure. Factors affecting mechanical characteristics, including atomic structure, composition, and the presence of defects, are analyzed.
Quasicrystals, discovered by Dan Shechtman in 1982, are materials that have a long-range order, but do not have the translational symmetry characteristic of ordinary crystals. Their structure is somewhere between crystalline and amorphous, which determines their unique physical and mechanical properties. Al–Cu–Fe alloys, in particular, are among the most studied quasicrystalline materials due to their relatively high stability and the possibility of producing large single crystals.
The crystal phases in the Al–Cu–Fe system adjacent to the quasicrystalline phase are of interest for comparison, since they have a similar elemental composition, but exhibit a classical crystal structure. A comparison of the mechanical properties of these phases makes it possible to identify the relationship between structure and mechanical response, as well as to understand the role of quasi-periodic order in determining mechanical behavior.
In this paper, we review the results of experiments on measuring microhardness, brittleness, and deformation behavior of quasicrystalline and crystalline Al–Cu–Fe phases. Microhardness was measured using the Vickers method under various loads. Brittleness was estimated by the crack resistance value determined by indentation of the indenter. The deformation behavior was studied by means of compression and three-point bending tests. Optical and electron microscopy, as well as X-ray diffraction analysis, were used to analyze the microstructure and phase composition.
Microhardness. Quasicrystalline Al–Cu–Fe phases exhibit high microhardness comparable to that of many intermetallic compounds. However, there is a significant dependence of the hardness on the load, which indicates the influence of structural defects and surface effects. Crystal phases tend to have a slightly lower hardness, but also show a load dependence.
Fragility. Quasicrystals are known for their brittleness at room temperature. Studies have shown that the quasicrystalline Al–Cu–Fe phase has a low crack resistance. It is assumed that the brittleness is caused by the complex structure of quasicrystals, which complicates plastic deformation and leads to stress concentration near defects. Crystalline phases also exhibit brittle fracture, but their crack resistance is slightly higher than that of the quasicrystalline phase.
Deformational behavior. Compression tests have shown that the quasicrystalline Al–Cu–Fe phase is deformed mainly elastically up to failure. Plastic deformation is practically absent. The crystalline phases show some plastic deformation before fracture, but the magnitude of plastic deformation is small. Analysis of the microstructure of samples after deformation revealed the formation of shear bands near the fracture site, which indicates a deformation mechanism associated with localized shear.
Despite significant differences in the atomic structure, the quasicrystalline and crystalline phases of Al-Cu-Fe exhibit a number of analogies in their mechanical behavior. Both phases have high hardness, brittleness, and limited ductility. This indicates that mechanical properties are largely determined by interatomic bonds and the presence of structural defects, and not only by the type of long-range order.
One explanation for this analogy may be the presence of a short-range order that is similar in both phases. Even in quasicrystals, there are local atomic configurations that resemble crystal structures. These local atomic configurations can have a significant effect on mechanical properties.
The composition of the alloy and the presence of structural defects significantly affect the mechanical properties of quasicrystalline and crystalline Al–Cu–Fe phases. A change in the ratio of Al, Cu, and Fe can lead to changes in the interatomic bonds and hardness of the material. The presence of defects, such as vacancies, dislocations, and grain boundaries, can serve as stress concentrators and contribute to the destruction of the material under lower loads.
A comparative analysis of the mechanical properties of quasicrystalline and crystalline Al–Cu–Fe phases has shown that, despite differences in structure, they have similar mechanical characteristics, such as high hardness, brittleness, and limited plasticity. This indicates that mechanical behavior is determined not only by the type of long-range order, but also by interatomic bonds, short-range order, and the presence of defects. Further research aimed at studying the effects of composition, defects, and microstructure will help to better understand the relationship between the structure and mechanical properties of quasicrystalline and crystalline materials.
Author: G. Laplanche, J. Bonneville, A. Joulain, V. Gauthier-Brunet, S. Dubois
The Institute: Institut P’ – Université de Poitiers, CNRS UPR 3346, ENSMA, Département de Physique et Mécanique des Matériaux SP2MI, Téléport 2, Boulevard Marie et Pierre Curie, BP 30179, 86962 Futuroscope Chasseneuil Cedex, France