The study of the effect of chromium alloying on the structure and properties of thermally sprayed quasicrystalline Al–Cu–Fe coatings is a topical task in materials science. Chromium, as an alloying element, can have a significant effect on the processes of crystallization and formation of the phase composition of coatings, which, in turn, determines their operational characteristics.
The introduction of chromium into the Al–Cu–Fe alloy can lead to a change in the melting temperature and viscosity of the melt, which affects the kinetics of spraying and the formation of the coating structure. In particular, chromium can help to reduce the grain size of the quasi-crystalline phase, as well as to form a more homogeneous structure.
Analysis of the microstructure of chromium-alloyed coatings shows the presence of finely dispersed inclusions uniformly distributed in the matrix. These inclusions can act as crystallization centers, promoting the formation of a denser and finer-grained structure.
This paper presents the results of a study of the structural evolution of Al–Cu–Fe and Al–Cu–Fe–Cr coatings formed by high-velocity thermal spraying using an oxygen-fuel mixture. The effect of adding Cr on the phase composition of Al–Cu–Fe coatings at different spraying temperatures is also considered.
It was found that the porosity of the Al–Cu–Fe and Al–Cu–Fe–Cr coatings obtained in this study is lower than that of similar coatings applied by plasma spraying. The data obtained indicate that high-velocity flame spraying allows the creation of Al–Cu–Fe coatings whose phase structure is similar to that of Al–Cu–Fe coatings obtained by plasma spraying, as described in the literature.
Al–Cu–Fe coatings consist of the crystalline phase β-AlFe and the quasicrystalline phase i-AlCu20Fe15, as well as oxidized versions of these phases. The introduction of chromium into Al–Cu–Fe alloys leads to the formation of coatings containing the crystalline phase θ-Al2Cu and two quasicrystalline phases: i1-Al80Cr13.5Fe6.5 and i2-Al13Cr3Cu4.
Formation of these icosahedral phases in Al–Cu–Fe–Cr alloys has not been previously recorded, although the existence of quasicrystalline approximants with a composition close to the compositions of the i1-Al80Cr13.5Fe6.5 and i2-Al13Cr3Cu4 phases has been demonstrated. Based on the results obtained, we conclude that the structure of the icosahedral phase is significantly stabilized by adding Cr to Al–Cu–Fe alloys.
Mechanical properties such as hardness and wear resistance are also affected by chromium alloying. Increasing the chromium content can lead to an increase in the hardness of the coatings, which is associated with strengthening of the solid solution and the formation of finely dispersed inclusions. The optimum chromium content must be selected based on the required performance characteristics of the coating, since excessive alloying can lead to brittleness and reduced adhesion to the substrate.
In conclusion, chromium alloying is an effective way to modify the microstructure and properties of thermally sprayed quasicrystalline Al–Cu–Fe coatings.
Author: Elina Huttunen-Saarivirta, Erja Turunen, Marke Kallio
Institute: Tampere University of Technology, Institute of Materials Science, P.O. Box 589, Fin-33101 Tampere, Finland, VTT Technical Research Centre of Finland, Surface Engineering and Laser Processing Department, P.O. Box 1703, Fin-02044, Finland, VTT Technical Research Centre of Finland, Materials and Chemicals, Hermiankatu 8 G, P.O. Box 16071, Finland-33101 Tampere, Finland