Icosahedral phases, which possess noncrystalline long-range order, have attracted considerable attention due to their unique properties, which are different from those of ordinary crystals and amorphous materials. In particular, the formation of icosahedral-phase-based quasicrystals is observed in Al–Cu–Fe–Be alloys, which opens up new prospects for the creation of materials with improved characteristics.
Studies show that adding beryllium to the Al–Cu–Fe alloy stabilizes the icosahedral phase, expanding its region of existence in the phase diagram. This is due to the fact that beryllium, having a small atomic radius, effectively fills the voids in the icosahedron structure, thereby reducing the energy of the system and promoting the formation of stable quasicrystals.
The effect of beryllium (Be) addition and cooling rate on the solidification process of the icosahedral (i) phase in standard Al62−xBexCu25.5Fe12.5 (x=0, 1, 3, 5, 7 at.%) cast alloys was studied using X-ray diffraction, differential thermal analysis, scanning electron microscopy and energy-dispersive X-ray spectroscopy. It was found that the introduction of beryllium transforms the mechanism of i-phase formation from a peritectic reaction to primary crystallization. The percentage content of the i-phase increased from 45% at x=0 to 90% at x=7. The data obtained indicate that the substitution of aluminum with beryllium favors an increase in the ability of the i-phase to form quasicrystals (QFA).
Two criteria are proposed for identification and evaluation of the characteristics of the i-phase formed as a result of the peritectic reaction. These parameters can be represented through the relative quasicrystalline transition temperature Trq=Tp/Tl and the relative supercooling ΔTr=ΔTlp/Tl, where Tl is the liquidus temperature, Tp is the peritectic peak temperature, and ΔTlp=Tl−Tp is the solidification interval of the primary phase.
The formation of quasicrystals in Al–Cu–Fe–Be alloys occurs as a result of a complex process involving nucleation and growth of the icosahedral phase. At the initial stage, nuclei of the quasicrystalline structure are formed, which then grow, absorbing the surrounding matrix. The growth rate and final size of the quasicrystals depend on many factors, including alloy composition, temperature, and cooling rate.
Quasicrystals in Al–Cu–Fe–Be alloys exhibit unique physical properties such as high hardness, low friction coefficient, and corrosion resistance. These properties are due to the aperiodic structure of quasicrystals, which prevents dislocation propagation and ensures high strength of the material. Due to their unique characteristics, icosahedral-phase quasicrystals in Al–Cu–Fe–Be alloys find application in various fields, including protective coatings, medical implants, and high-performance catalysts.
Author: G.SSong, E Fleury, SM Lee, WT Kim, DH Kim
Institute: Yonsei University, Center for Non-Crystalline Materials, Department of Metallurgical Engineering, 134, Sincheon-dong, Seodamun-gu, Seoul 120-749, South Korea, Korea Institute of Industrial Technology, Incheon, South Korea, Cheongju University, Department of Physics, Cheongju, South Korea