On the Nature and Stability of Phases Present in a Rapidly Solidified Aluminium Alloy 7010 Containing Scandium

Abstract

The influence of Sc on the nature and stability of phases present in rapidly solidified (RS) Al alloy 7010 (hereafter termed base alloy) produced by melt spinning were examined and compared with the phases present in the same alloys when produced by the conventional ingot metallurgical (IM) route. Transmission electron microscopy (TEM) revealed that the second phases present in the IM alloys have an HCP structure but with varying lattice parameters. On the other hand, the second phase particles present in the base alloy when produced by rapid solidification (RS) route always have a quasi-crystalline, icosahedral structure. In the rapidly solidified 7010 + Sc alloy, however, the icosahedral phase is located only at the grain boundary triple point junctions, and the grain interfaces are populated by the second phase particles having an HCP structure. An analysis of the chemistry of the second phase particles revealed that the icosahedral phase is stabilized by certain minimum concentrations of Zn + Mg/Zn + Mg + Cu. The reason for the absence of the icosahedral phase at the grain interfaces of the rapidly solidified 7010 + Sc alloy is the partitioning of relatively lower concentrations of the major alloying elements into the second phases formed at those sites. Micro-hardness measurements revealed higher hardness values for the RS 7010 + Sc alloy in both as-melt spun as well as artificially aged conditions. These results are correlated with the microstructures developed in these materials.

Keywords:

• Aluminium alloy 7010
• Icosahedral phase
• Microanalysis
• Micro-hardness measurement
• Rapid solidification
• Sc addition
• Transmission electron microscopy

ACKNOWLEDGMENTS

The authors gratefully acknowledge the financial support by Defence Research and Development Organization (DRDO), Government of India.