The mechanisms for spontaneous grain refinement in undercooled Cu–O and Cu–Sn melts

A melt encasement (fluxing) technique has been used to systematically study the velocity–undercooling relationship and microstructural development in samples of Cu–O and Cu–3 wt.% Sn at undercoolings up to 250 K. Both systems experience spontaneous grain refinement at high undercooling, but there are sufficient microstructural differences to suggest the mechanisms operating are different. In Cu–Sn grain refinement appears to be a consequence of recrystallisation, whereas in Cu–O a dendrite fragmentation process may have occurred.