Grain refinement of aluminium using niobium additions

Abstract

An investigation has been made into the behaviour of niobium as a grain-refining agent in aluminium. It was found that highly effective refinement could be obtained at moderate overall Nb levels of around 0·1 wt-% and above. These satisfactory results were obtained, however, when inoculation was carried out using additions of a master alloy (Al–5Nb), which contained about 5 vol.–% of Al3Nb intermetallic particles embedded in the matrix. Other forms of addition were found to be less effective. The effect of holding time and mould material was also investigated. It is argued that some characteristics of these, and other previously published results, may be satisfactorily explained by the hypothesis that intermetallic particles introduced during inoculation are slow to dissolve and often survive to act as nucleating substrates. This eliminates the necessity for reprecipitation which may also be kinetically slow in some cases.