Anomalous eutectics in intermediately and highly undercooled Ni-29.8at.%Si eutectic alloy

The transition from regular eutectics to anomalous eutectics is a well-known non-equilibrium phenomenon for undercooled eutectic alloys, the mechanism of which has been studied extensively but is still controversial. In this work, a Ni-29.8at.%Si eutectic alloy, as an ideal modeling system whose eutectic products are two stoichiometric intermetallic compounds, whose rapid solidification does not suffer any phase-selection and whose transformation path does not include any solid-state phase transformation, was undercooled by a melt fluxing technique and observed in-situ by a high-speed camera. A common transition from uncoupled eutectics to anomalous eutectics and a unique transition from uncoupled coarse lamellar-eutectics to anomalous eutectics were found at intermediate and high undercooling, respectively. The formation of coarse lamellar-eutectics is highly related to the abrupt increase of growth velocity at ΔT ≈ 149 K, which is accompanied by a transition from dual-recalescence to single-recalescence. An electron back-scattering diffraction analysis shows that epitaxial growth of the second δ-Ni2Si phase on the primary γ-Ni31Si12 phase follows a particular eutectic orientation relationship. Anomalous eutectics were concluded to be formed by uncoupled eutectic growth with the reduction of interfacial energy but not chemical superheating as the driving force.