The Influence of Casting Cooling Rate on the Microstructure and Phase Composition of Al–REM and Al–Zr–REM Alloys

The research examines the effect of cooling rate upon casting on the microstructure, phase composition, and hardness of aluminum alloys modified with zirconium and rare earth metals (REMs). Binary and ternary alloys, namely Al–Zr, Al–REM, and Al–Zr–REM (REM = Er, Y, Yb, Ce) with a total alloying element content of ~1.4–1.5 at % have been investigated. The alloys have been produced by mold casting and melt spinning, providing cooling rates in the range ~10–106 K/s. The results demonstrate that solidification leads to the formation of eutectics based on REM intermetallics and aluminum solid solution, as well as primary D023–Al3Zr phase crystals. X-ray phase and energy dispersive analysis have revealed the presence of Al3Yb, Al3Er, Al11Ce3, Al3Y, and Al4Y phases, depending on the alloy composition. Increasing the cooling rate from 10 to 103 K/s has been found to reduce the size of dendritic cells by a factor of 2–3, and the size of eutectic particles and primary Al3Zr crystals by 5–10 times. In addition, the hardness increases by 20–25%, though no evidence of increased nonequilibrium REM solubility has been observed. A fine quasi-eutectic structure consisting of REM aluminide particles uniformly distributed in the aluminum matrix forms at cooling rates of ~106 K/s. X-ray diffraction analysis indicates that the zirconium content in the aluminum solid solution can reach 1.2 at %. Moreover, an increase in nonequilibrium solubility has been detected for Er and Yb but not for Ce and Y.