Influence of Hf and Er on the Microstructure Evolution during Heat Treatment of Al–Mg Alloys with Combined Sc–Zr Microalloying

A study of the effect of the hafnium and erbium additives on the formation of a microstructure in the aluminum alloys with a high magnesium content, which are alloyed with small additives of scandium and zirconium, is described. Two high-magnesium alloys were selected for the study: 1580 (Al–Mg–Sc–Zr) and 1590 (Al–Mg–Sc–Zr–Er–Hf). The latter, in addition to scandium and zirconium, additionally contains hafnium and erbium. The alloys were subjected to heat treatments at temperatures of 370 and 440°C with exposure in the range from 2 to 100 h, after which the microhardness was measured and the fine microstructure was studied by the transmission electron microscopy and atom probe tomography methods. The investigation results allowed us to establish that nanoparticles of a much smaller size are formed in the 1590 alloy than in the 1580 alloy. This causes higher microhardness values in the 1590 alloy. The main explanation for this phenomenon is that, when zirconium and hafnium are added together, a denser shell is created around nanoparticles than when only zirconium is added. This shell thermally stabilizes nanoparticles and prevents them from increasing in size. As a result, when scandium leaves the solid solution, it is mainly involved not in the growth of already formed nanoparticles, but in the appearance of new ones. The presence of erbium has been detected only in large intermetallides. This is due to the fact that owing to its low solubility in an aluminum solid solution, a large amount of erbium is released during the metal crystallization in the form of primary intermetallides; therefore, it does not then participate in the formation of nanoparticles.