Formation of Al₃Sc Dispersoids and Associated Strengthening

The exceptional strengthening potential of scandium additions to aluminum was first discovered in the 1960s. The impact that scandium additions have on the alloys’ properties varies with parameters of the thermo-mechanical process and alloy composition. The benefits from scandium come from the various influences it has on the alloys’ microstructure which can be summarized as four main effects, grain refinement, increased recrystallisation temperature, dispersoid strengthening and nucleation of strengthening phase. The strength improvement from Sc additions is mainly provided by the formation of fine dispersions of Al₃Sc L1₂ dispersoids. The level of strengthening depends on the size and number density of these dispersoids. Temperatures in the range of 250 to 350 °C are usually preferred to nucleate a high number of small Al₃Sc. The use of higher temperatures will most commonly generate coarser particles with less strengthening potential. This paper reviews the heat treatment strategies to generate fine dispersions of dispersoids, the origins of dispersoid strengthening, the impact of strengthening in different alloy series and finally the effect of adding Sc in combination with Zr.