This paper presents the results of a comprehensive study of the viscosity and supercooling propensity of Al-Cu-Fe melts in the vicinity of compositions corresponding to the formation of the icosahedral phase. Viscosity measurements were carried out by the torsional oscillation method over a wide temperature range covering both superheated and supercooled states. The tendency to supercooling was estimated based on differential scanning calorimetry (DSC) data and analysis of cooling curves. The results obtained are compared with the data of high-energy X-ray diffraction (HFE), which allow us to estimate the short-range order features in melts.
Quasicrystals (QCs) are long – range order aperiodic solids that do not have translational symmetry, but have symmetry axes that are forbidden in classical crystallography. The first QCs were discovered in the Al-Mn-Si alloy by D. Shechtman in 1982, which changed the understanding of the structure of condensed media. Since then, hundreds of different QCs based on metal alloys, intermetallic compounds, and even polymers have been discovered. Of particular interest are quasicrystallizing melts-alloys from which, under certain conditions (for example, a high cooling rate), a quasicrystalline phase can be obtained. Understanding the mechanisms of the formation of a quasicrystalline structure from a melt is an important task in materials science, as it allows us to develop new materials with unique properties.
A stable icosahedral phase (i-phase) with a composition close to Al63Cu25Fe12 was found in the Al-Cu-Fe system. This material has a high hardness, low coefficient of friction and good corrosion resistance, which makes it promising for use as protective coatings and other functional materials. Quasicrystallizing Al-Cu-Fe melts were intensively studied using various experimental methods, including diffraction methods, thermal analysis, and viscometry. However, the relationship between viscosity, supercooling tendency, and short-range order in melts of this system remains insufficiently studied.
Alloys of the Al-Cu-Fe system were made by arc melting in an argon atmosphere from high-purity components (Al-99.99%; Cu-99.99%; Fe-99.98%). The alloy compositions were selected near the stoichiometry of the i-phase (Al63Cu25Fe12) with small deviations in the content of Al, Cu, and Fe. To ensure uniformity, the alloys were remelted several times.
Viscosity measurements were carried out by the method of torsional vibrations on a viscometer with an electromagnetic propulsion system. The alloy sample was placed in a crucible made of aluminum oxide, which was suspended on a thin tungsten thread. The crucible with the sample was placed in an induction furnace, which allowed heating and cooling the sample at a controlled rate. The viscosity was determined by measuring the period of damped torsional vibrations of the crucible.
Thermal analysis (DSC) was performed on a differential scanning calorimeter. Alloy samples were placed in sealed aluminum crucibles and heated/cooled at various speeds. Based on the DSC data, the liquidus and solidus temperatures were determined, and the degree of supercooling of the melts was estimated.
High-energy X-ray diffraction (HFE) was performed using a synchrotron source. Alloy samples were placed in quartz capillaries and heated to the required temperatures. Diffraction patterns were recorded by the detector, and based on them, the radial distribution functions (FRF) were calculated, which allow us to estimate short-range order features in melts.
Figure 1 shows the temperature dependences of the viscosity of Al-Cu-Fe melts of various compositions. It can be seen that the viscosity increases with decreasing temperature, which is typical for metal melts. However, near the liquidus temperature, the viscosity curves show deviations from linear behavior, indicating a change in the melt structure. In the supercooled state, the viscosity increases sharply, which is associated with the formation of clusters or nuclei of the quasicrystalline phase.
The composition of the alloy has a significant effect on the viscosity of the melt. Alloys that are close to the stoichiometry of the i-phase show a higher viscosity compared to alloys that deviate from this stoichiometry. This may be due to the fact that melts close to the i-phase form more stable and ordered clusters that hinder the melt flow.
The activation energy of the viscous flow calculated by the Arrhenius equation also depends on the alloy composition. Alloys close to the i-phase have a higher activation energy, which indicates a greater energy barrier for the movement of atoms in such melts.
DSC data showed that Al-Cu-Fe melts have a significant tendency to supercooling. The degree of supercooling, defined as the difference between the liquidus temperature and the crystallization start temperature, can reach several tens of degrees. Alloys close to the i-phase exhibit a higher degree of supercooling compared to alloys deviating from this stoichiometry.
The cooling rate also affects the degree of supercooling. As the cooling rate increases, the degree of supercooling increases, which is associated with kinetic constraints on the crystallization process.
Analysis of the cooling curves also confirmed the tendency of Al-Cu-Fe melts to supercooling. Supercooling plateaus are observed on the cooling curves, which correspond to a delay in the crystallization process.
The EVR data allowed us to estimate short-range order features in Al-Cu-Fe melts. The radial distribution functions (RDF) show the presence of pronounced peaks corresponding to atomic correlations at small distances. This indicates the presence of short-range order in the melts.
As the temperature decreases, the peaks on the FRP become more pronounced, which indicates an increase in the degree of ordering in the melt. Near the liquidus temperature, peaks appear on the FRP corresponding to the atomic correlations characteristic of the i-phase. This indicates the formation of clusters or nuclei of the quasicrystalline phase in the melt.
The composition of the alloy also affects the short-range order in the melt. Alloys close to the i-phase exhibit a more ordered short-range order compared to alloys deviating from this stoichiometry. This confirms the assumption that more stable and ordered clusters are formed in melts close to the i-phase.
The results obtained allow us to establish a relationship between the viscosity, the tendency to supercooling, and the short-range order in Al-Cu-Fe melts. Alloys close to the i-phase exhibit a higher viscosity, a higher degree of supercooling, and a more ordered short-range order. This indicates that more stable and ordered clusters are formed in such melts, which contribute to the formation of a quasicrystalline phase during crystallization.
The increase in viscosity in the supercooled state is associated with the formation of clusters or nuclei of the quasicrystalline phase. These clusters hinder the melt flow and lead to a sharp increase in viscosity.
The tendency to supercooling is due to kinetic constraints on the crystallization process. In Al-Cu-Fe melts, the formation of a quasicrystalline phase requires a significant rearrangement of the atomic structure, which requires time and energy. Therefore, melts can be supercooled to significant temperatures without crystallization.
The short-range order in the melt plays an important role in the formation of the quasicrystalline structure. The presence of an ordered short-range order characteristic of the i-phase facilitates the formation of a quasicrystalline phase during crystallization.
In this paper, we perform a comprehensive analysis of the viscosity, supercooling tendency, and short-range order in quasicrystallizing Al-Cu-Fe melts. The results obtained made it possible to establish a relationship between these properties and the composition of the alloy. The main conclusions of the work can be formulated as follows:
Alloys close to the stoichiometry of the i-phase exhibit a higher viscosity, a higher degree of supercooling, and a more ordered short-range order.
In the supercooled state, the viscosity increases sharply, which is associated with the formation of clusters or nuclei of the quasicrystalline phase.
The tendency to supercooling is due to kinetic constraints on the crystallization process.
The short-range order in the melt plays an important role in the formation of the quasicrystalline structure.
The results obtained can be used to develop new materials based on quasicrystals with improved properties. In particular, the control of the composition and cooling rate allows controlling the crystallization process and obtaining quasicrystalline structures with the required characteristics.
Author: R.A. Brand, J. Voss, Y. Calvayrac
The Institute: L.V. Kamaeva, R.E. Ryltsev, V.I. Lad‘yanov, N.M. Chtchelkatchev, Udmurt Federal Research Center, Ural Branch of Russian Academy of Sciences, Izhevsk 426068, Russia, Vereshchagin Institute for High Pressure Physics, Russian Academy of Sciences, Troitsk 108840, Moscow, Russia, Institute of Metallurgy, Ural Branch of Russian Academy of Sciences, 620016 Ekaterinburg, Russia, Ural Federal University, 620002 Ekaterinburg, Russia, Moscow Institute of Physics and Technology, 141700 Dolgoprudny, Moscow Region, Russia