An investigation of dilute Al–Hf and Al–Hf–Si alloys

To meet the demand for extruded aluminium alloys tolerant of temperatures up to 600 °C, highly stable microstructures with the ability to maintain strength at such temperatures are required. Such alloys are presently not available commercially, however, by adding elements that form a homogeneous and dense distribution of thermally stable dispersoids, this may be achieved. A relatively untried element in this respect is hafnium (Hf), which may lead to a significant increase in the recrystallisation resistance/structural stability. By subjecting the alloy to a carefully chosen homogenisation treatment, fully coherent Al 3Hf-dispersoids may form. In the current investigation a binary Al–Hf alloy and a ternary Al–Hf–Si alloy have been investigated. It has been found that Si enhances the Al 3Hf-precipitation in these alloys. However, the results indicate that the Si-content should be kept below 0.15 wt.% in order to avoid the formation of AlHfSi-phases.