The study of phase transformations and stability in low-iron AlCuFe alloys is a complex task due to the diversity of possible crystalline and quasi-crystalline phases formed in this system. Low iron content radically changes the thermodynamic conditions determining the kinetics of phase transformations and the stability of the structures formed.
A key aspect is the analysis of the effect of melt cooling rate on the formation of phase composition. Rapid quenching is capable of fixing metastable states, including quasi-crystalline phases, which do not form during slower cooling. The study of these metastable phases requires the use of modern analytical methods, such as high-resolution transmission electron microscopy and synchrotron radiation diffraction.
The study of phase transformations and microstructures of AlCuFe alloys with copper concentrations in the range of 20–50 at.% and iron concentrations of 1–10 at.% was carried out on annealed samples. The methods of differential thermal and magnetothermal analysis, scanning electron microscopy, electron probe analysis and powder X-ray diffraction were used. Typical phase transformations classified as polymorphic, discontinuous sedimentary, quasi-binary eutectoid and ternary peritectic (U-type) were revealed.
A common feature of these transformations is the dissolution of a primitive cubic phase (CsCl type) followed by the formation of a face-centered orthorhombic phase. Identification of the crystal structures of the cubic and orthorhombic phases was carried out on the basis of powder diffraction data using the DICVOL indexing methods, intensity extraction, ab initio EXPO structure determination and Rietveld refinement (FULPROF).
It was found that the face-centered orthorhombic cell (a=8.1530(3), b=14.1370(4), c=10.0736(4), V=1161.0(7), Z=4) with the initial structural model ζ1-Al3Cu4 (space group Fmm2) at room temperature describes the diffraction pattern of the sample quenched at 620 °C quite well. The dimensions of the orthorhombic cell were refined by high-temperature X-ray diffraction in situ at temperatures of 463, 553 and 636 °C.
It is noted that these phase transitions are sensitive to heat treatment conditions and alloy composition, and can be suppressed by relatively slow cooling. It is found that metastable structures formed both by slow cooling and in cast alloys contain a cubic phase replacing the equilibrium orthorhombic phase.
Subsequent heat treatment allows us to study the phase stability of the obtained materials. Annealing at different temperatures stimulates diffusion processes and leads to a redistribution of elements, which in turn affects the stability of quasi-crystalline phases. It is important to note that the presence of small changes in the alloy composition, such as impurities or a slight change in the ratio of Al, Cu and Fe, can significantly affect the kinetics of phase transformations and the final phase composition.
Understanding these patterns opens up opportunities for the targeted creation of low-iron AlCuFe alloys with unique properties suitable for various technical applications. Further research in this area should be aimed at developing thermodynamic models that describe phase equilibria and transformation kinetics in this complex multicomponent system.
Author: Liming Zhang, Julius Schneider, Reinhard Lück
Institute:
Department of Earth and Environmental Sciences, Department of Crystallography, Ludwig Maximilian University of Munich, Theresienstrasse 41, D-80333 Munich, Germany
Max Planck Institute for Metal Research, Heisenbergstrasse 3, D-70569 Stuttgart, Germany