Al-Cu-Fe quasicrystals attract attention due to their unique physical properties due to their aperiodic structure. Thermal treatment significantly affects their structure and phase composition, which in turn affects their electronic properties, which are studied by the positron annihilation method.
Al63Cu25Fe12 samples were annealed at different temperatures and cooling rates. The studies were carried out using the Doppler broadening of the annihilation line (DUAL) method. The shape parameter S, sensitive to the defect concentration, was used to analyze changes in the structure.
The change in the S parameter with annealing temperature indicates the transformation of the defect structure. High annealing temperatures lead to a decrease in the vacancy concentration, which is reflected in a decrease in the S parameter. The cooling rate affects the size and distribution of defects, which is also reflected in the S parameter.
The positron annihilation method is an effective tool for studying the effect of heat treatment on the defect structure of Al-Cu-Fe quasicrystals. The results obtained allow us to better understand the relationship between the thermal history of the sample, its structure and physical properties.
Further studies have shown that the dependence of the parameter S on the annealing temperature is not monotonic. A local maximum is observed, which may be associated with phase transformations within the quasi-crystalline matrix. It is assumed that at certain temperatures, a redistribution of atoms occurs, leading to the formation of new types of defects or clusters.
Analysis of the annihilation line shape allowed us to isolate contributions from positrons annihilating in different structural positions. It was found that at low temperatures, positrons are predominantly captured by vacancies near copper atoms, while at high temperatures, the probability of annihilation with electrons of aluminum core levels increases. This indicates a change in the local electron environment of positrons depending on the annealing temperature.
Comparison of the DUAL data with the results of X-ray structural analysis and differential scanning calorimetry allowed us to establish a correlation between the change in the parameter S and the phase composition of the samples. In particular, it was shown that a decrease in the parameter S at high annealing temperatures is associated with an increase in the proportion of the icosahedral phase and a decrease in the concentration of defects caused by deviations from stoichiometry.
Thus, a comprehensive study using the positron annihilation method in combination with other structural and thermal methods provides valuable information on the processes occurring in Al-Cu-Fe quasicrystals during heat treatment and allows monitoring their structure and properties.
Author: D.W.Lawthera, R.A. Dunlap
Institute: Department of Physics, Dalhousie University, Halifax, Nova Scotia B3H 3J5, Canada