Regularities of the influence of microarc oxidation of aluminum alloys on the phase-structural state of the formed oxide coatings and the peculiarities of γ-AL2O3 → α-AL2O3 polymorphic transformation during their annealing

Abstract

The influence of the technological parameters of microarc oxidation on the regularities of the phase-structural state of coatings formed on D16 aluminum alloys (the main alloying element is Cu) and AMg3 (the main alloying element is Mg) and the effect of annealing in the temperature range 600-1280 °C on the γ-Al2O3 → α-Al2O3 phase transformation are investigated. It was found that in the coatings formed during microarc oxidation in a complex (alkaline-silicate) electrolyte, three main phase-structural states are revealed: γ-Al2O3, α-Al2O3 and mullite (3Al2O3·2SiO2). The conditions of electrolysis allowing the formation of a twophase state (γ-Al2O3 and α-Al2O3) on alloys of both types have been determined. It has been established that alloying elements of the AMg3 alloy provide in MAO coatings a higher stability of the γ-A12O3 structure in comparison with the analogous state in MAO coatings on D16 alloy. High-temperature annealing of MAO coatings made it possible to reveal a more complete phase transformation on D16 alloy, the structural basis of which is the appearance of tetragonality in the defective cubic lattice of the γ-Al2O3 phase. Annealing of MAO coatings stimulates the γ → α transformation with the greatest dynamics of change in the coatings obtained on D16 alloy. At the highest annealing temperature of 1280 °C (for 60 min) as a result of γ→α transformation, the relative content of the α-Al2O3 phase in the coating is 89 % (coating obtained on D16 alloy) and 30 % (coating obtained on AMg3 alloy). A model of the polymorphic γ-Al2O3 → α-Al2O3 transformation in aluminum oxide is proposed, based on the ordering of the metal cationic subsystem in octahedral and tetrahedral internodes and the enhancement of this process upon the weakening of the "metaloxygen" bond as a result of the replacement of Al ions by Cu ions that have a low binding energy with oxygen. A correlation between the relative content of the α-Al2O3 phase and the hardness of the MAO coating was found. With the highest content of the α-Al2O3 phase, the hardness reaches 16000 MPa.