Evolution of dendrite morphology of a binary alloy under an applied electric current: An in situ observation

The application of electric current during metal solidifica-tion process has been tried since 1960s, and it was found thatthe electric current can modify the process of nucleation andevolution of microstructure. Primary goal of those studieswas for the grain refinement by applying dc 1–3 , alternat-ing current ac 4 , electric current pulse ECP 5–7 . Sig-nificant refinement efficiency on eutectic or noneutectic mi-crostructure has been achieved by an applied electric currentfor low melting Sn-Pb Bi alloy, middle melting Al-Cu al-loy, even for high melting steel and iron. It is well knownthat an electric current usually impacts on a solidifying meltthrough a series of effects: electromigration, electricity-to-heat, electromagnetic force, etc. Based on these effects, anumber of various refining mechanisms have been estab-lished: for example, the improvement of interface stabilityand the specialty of current crowding 2 , the change of sol-ute diffusion and the selectivity of electric current 4 , thebreaking off dendrite arm 5 , the enhancement of nucleationrate 6,7 , etc. Nevertheless, each of them is hard to be fullyconfirmed. Sometimes, confliction occurs to each otheramong them for explaining a same phenomenon. Influencemechanism of electric current on solidifying alloy is stillcontroversial.A confirmed fact is, however, all the studies mentionedabove were done by analyzing microstructure/macrostructureafter complete solidification or interrupting solidification byquenching. As a result, the evolution of grain morphologyduring solidification cannot be