1Bay Zoltán Institute for Materials Science and Technology,
H-1116 Budapest, Fehérvári út 130.
Quasicrystals (QC) have been discovered 15 years ago in 1984. (1, 2) The surprising structures are still a challenge for the scientific community. The quasicrystals display medium to long range atomic order, while lacking the periodicity of crystals. The icosahedral phase, the first quasi-periodic phase discovered, and all the phases followed later could very likely not have been found without transmission electron microscopy (TEM) as only TEM is able provide crystallographic, compositional and morphological information on extremely small volume of material. At present quasicrystalline phases may be considered just as particular intermetallic compounds among others. The stability is generally discussed within the context of Hume-Rothery rules for alloys but it gives no answer to the question what compositions lead to the quasiperiodic order. (3) Recently a new theory seems to became more and more accepted, namely that by controlling the size and composition it is possible to have clusters which can simulate different atoms in the periodic table. (4) These clusters are stable and can serve as the building blocks for forming cluster materials much in the same way as atoms. It was proposed that quasicrystals can be described as an assembly of these clusters. The quasicrystal-community consists of scientists belonging to different branches of science. For material scientists the most important goal is the application of these new type of matter. For reasonable applications there is necessary not only a repeatable production process of the desired material, further the determination of the structure and composition, but it is also important to get acquainted with the important physical properties (hardness, plasticity, resistivity, heat diffusivity, optical characteristics etc.), further the so called service properties as friction, corrosion etc.
Preparation of quasicrystals
During the last years several thousands of papers, both theoretical and experimental have been published. The predominant part of them is dealing with theoretical and experimental structure determinations. In great many publications different metallurgical techniques for producing quasicrystalls are described. For the preparation of quasicrystals depending on the composition all the production methods relevant to metastable alloys and glasses have been applied:
rapid solidification of a liquid, sputter or vapor codeposition, sequential vapor or sputter deposition,
solid state reactions:
by multilayer deposition and heat treatment, by ionimplantation, by ion beam mixing, by mechanical alloying laser or electromelting of alloys or thin layers plasma spray deposition of differently prepared crashed or powdered alloys further convential casting.
Physical properties of quasicrystals
The physical properties research (5) on quasicrystals could be developed only after high quality quasicrystalline single grained samples became also available for some alloys. An important feature of the quasicrystals is that they have a pseudogap at the Fermi level, wich persists to the surface. (XPS, UPS).
The resisitivity (r ) is anomalously high exhibiting generally a negative temperature coefficient. The resistivity in the same alloy system is quite composition dependent. The resistivity of the i-phase is much higher than that of the amorphous phase of the same alloy. The resistivity decreases drastically with the decrease of sample quality.
The thermal diffusivity (a ) of quasicrystalline materials are generally lower than those values measured on the ordinary crystalline Al alloys. The temperature dependence is just the opposite as in case of the latters, when heated to high temperature the QC alloys show increasing diffusivity. The results strongly suggest that quasicrystals may be used as thermal insulating barriers. At larger temperatures they offer the further advantage of superplasticity.
Concerning magnetic properties no specific properties characteristic of quasicrystalline structure, different from those in amorphous and crystalline alloys have not been established yet.
The optical conductivity shows a large maximum in the near infrared while reflectance (R) of all stable and highly ordered icosahedral quasicrystals lies in the range 0.5-0.6 and is nearly independent of wavelength from about 20m m-300nm.(6)
Mechanical tests for bulk samples of thermodynamically stable quasicrystals revealed that considering extremely high hardness values (7), these materials are not specially brittle, the mechanical behavior is comparable to that of ceramic crystals.
The deformation mechanism of quasicrystals is of special interest due to their special atomic structure. The motion of dislocations in quasicrystals is extremly difficult, the quasicrystals are highly strained internally. Evidences for plastic deformation in the quasicrystals were found. (8) In some cases interphase gliding between the icosahedral grains and stress induced approximant crystals were considered to be the main deformation processes at high temperatures.
The applications of quasicrystals
The applications of quasicrystals became more realistic as before after the stable Al-Cu-Fe quasicrystals in 1987 were found. (9), but there are some possibilities for the utilisation of metastable phases as well.
Quasicrystals as constituents of composites.
Japanese researcher (10) developed i-phase precipitation hardened aluminium alloys having surprisingly high tensile strength (960-1320 Mpa) and good ductility at RT. The alloy compositions are Al-Mn-Ce with 4-6 at% Mn and 1-3 at% Ce. The high strength is achieved for melt-quenched ribbon samples in which very small i-phase particles (5-100 nm) are surrounded by thin (~10 nm) fcc aluminium layer.
Quasicrystals as thick coatings.
Coating of semi-finished components is one of the most rapidly growing area of materials science in the last decades. By thermal spaying process, Al-Cu-Fe based quasicrystalline alloys have been used as coating materials (11) to product soft substrates against abrasion, oxidation, etc. The beneficial properties of these materials are as partly already mentioned:
high hardness (12)
low friction coefficient
low thermal conductivity
good oxidation and corrosion resistance (13)
Plasma sprayed Al63Cu25Fe12 coatings prepared with gas atomized powders are chemically more uniform than by spraying crushed ingots. The coatings are resistant to oxidation at 500°C and 700°C in dry O2 atmosphere (14)
Quasicrystals as thin films used for solar selective absorbers are promising (15)
New techniques for the preparation of thin QC films also by hungarian researchers were developed (16) however for optical properties very thin (8-15 nm) Al-Cu-Fe films would be necessary, at present still not available.
Today to study quasicrystals is no longer only a fascinating field of scientific research quasicrystals are real materials having and waiting for industrial applications.
Copyright © 2000