Dendrite growth velocity-undercooling relationships and microstructural evolution in highly undercooled dilute Cu-Sn alloys

A melt encasement (fluxing) technique has been used to systematically investigate the velocity-undercooling relationships in a range of dilute Cu Sn alloys, A maximum level of undercooling of 220 K was achieved in the Cu-1 wt% Sn alloy. For all the alloy systems, the growth velocity-undercooling relationships were smooth and could be described well by means of a power-law over the full range of undercoolings studied, Detailed microstructural studies were carried out on a Cu-3 wt% Sn alloy, The microstructures produced were predominantly dendritic although these structures became progressively more fragmented as the undercooling increased. Grain refinement was observed at the highest undercoolings (greater than 190 K in Cu-3 wt% Sn) but without the spherical substructure previously seen to accompany grain refinement in Cu-based alloys, Microstructural analysis using light microscopy, texture analysis and microhardness measurements reveals that recrystallisation accompanies the grain refinement at high undercoolings. Furthermore, at undercoolings between 110 K and 190 K, a high density of subgrains are seen within the microstructure which indicate the occurence of recovery, a phenomenon previously unreported in samples solidified from highly undercooled melts.