Cellular microstructure and heterogeneous coarsening of δ′ in rapidly solidified Al-Li-Ti alloys

Abstract

Four melt-spun Al-Li-Ti alloys with ∼2 wt% lithium and 0.10 to 0.35 wt% titanium have been obtained and heat-treated at 473 K for up to 1000 h. Rapid solidification gives rise to a matrix with titanium in solid solution which drastically alters the δ′ coarsening rate. While TEM studies of samples aged for short times show a homogeneous distribution of metastable δ′ phase, as ageing time is increased, and depending on the ribbon section, three different microstructures can be distinguished: (i) on the wheel side, the δ′ distribution is homogeneous; (ii) intermediate regions show δ′ particles delineating cells with narrow walls; (iii) on the gas side, δ′ particles delineate “circular” cells. A higher titanium content in the cell centres than on cell walls has been determined. The coarsening rate of δ′ in microstructure (i) above is slower than in binary Al-Li alloys. Cellular microstructures (ii) and (iii) show the preferential coarsening of δ′ particles on the walls, which is faster the higher the titanium concentration. Taking into account the fact that the partition coefficient of titanium in aluminium in the peritectic region is > 1, an explanation of δ′ phase evolution is given which leads to the conclusion that the effect of titanium in solid solution is to retain vacancies, restricting lithium diffusion.