Cast microstructure and dispersoid formation in spray deposited Al-Li alloys

Spray deposition has been used to increase the concentration of dispersoid forming elements in model Al-Li-TM alloys (where TM stands for transition metals). The cast grain structure and the composition and stability of the dispersoids formed after a range of precipitation treatments was investigated. Very fine, nearly uniform, grain structures were obtained in the sprayed billets. The smallest grain size produced was 13 mu m. The substantial grain refinement achievable by spray casting is partly owing to the high cooling rates, but was also found to be strongly dependent on the content and combination of the peritectic dispersoid forming elements, which increase the survival of dendrite fragments formed on impact of the semisolid metal droplets. It was found that materials with the highest peritectic forming element concentrations showed the finest grain size. Combinations of Zr and Ti gave the best grain refinement. The influence of Ti, V, and Sc on the composition and coarsening rates of the beta' Al-3(Zr,X) L1(2) dispersoids formed during post-spraying precipitation treatments was also investigated. The elements Ti and Sc were found to be soluble in the Al-3(Zr,X) L1(2) phase substituting for Zr in proportion to the average alloy composition. Titanium containing Al-3(Zr,X) L1(2)beta' phase dispersoids had the slowest coarsening rates, but their stability was reduced by the presence of Ti and they transformed to the equilibrium D0(23) phase after prolonged heat treatment. On aging, the precipitation behaviour of the delta' phase was also affected by the Ti additions. MST/4058.