Active modulation of competitive nucleation and eutectic growth for refractory Re_72.5W_27.5 alloy under electrostatic levitation state

The rapid eutectic growth of refractory liquid Re_72.5W_27.5 alloy was realized under electrostatic levitation state, which attained a maximum undercooling of 510 K (0.16T_E). The active modulation of competitive nucleation was achieved by triggered nucleation and spontaneous nucleation respectively. For triggered nucleation between 107 and 454 K undercooling, anomalous eutectic grew primarily from the liquid alloy in the first recalescence, which was followed by lamellar eutectic growth epitaxially in the second recalescence. However, for spontaneous nucleation in the undercooling range of 366–510 K, σ dendrite became the primary phase in the first recalescence. The solidification mode of remaining liquid varied from lamellar eutectic to only (Re) phase as alloy undercooling increased beyond the critical value of 426 K. Once liquid undercooling exceeded 474 K, only σ phase prevailed in the whole solidifying process. It was found that the growth velocities for anomalous eutectic and σ dendrite both displayed power laws versus liquid undercooling. But anomalous eutectic exhibited a faster growth kinetics than σ dendrite in their coexistent undercooling regime of 366–454 K. Furthermore, the Vickers hardness decreased with undercooling in the case of triggered nucleation, whereas it increased for spontaneous nucleation because the harder σ dendrite had larger volume fraction with enhanced undercooling. The maximum Vickers hardness attained 1679 HV corresponding to the highest undercooling.