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Phase selection and re-melting-induced anomalous eutectics in undercooled Ni–38 wt% Si alloys

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Abstract

The Ni–38 wt% Si alloy whose eutectic products are two stoichiometric intermetallic compounds (i.e., NiSi and NiSi2) was undercooled by the melt fluxing technique. After in situ observations of the recalescence processes using a high-speed camera and by electron back-scattering diffraction analysis of the solidification microstructures, the crystal growth velocities, phase selection, and microstructure evolutions were studied. Due to a growth-controlled mechanism, the primary phase changes from the NiSi to the NiSi2 phase at a critical undercooling ΔT ≈ 48 K. Even in the absence of the driving force of chemical superheating, the transition from regular eutectics to anomalous eutectics happens. The reason is that the single-phase dendrite of NiSi2 phase solidifies firstly and then the NiSi phase grows epitaxially to form an uncoupled eutectic-dendrite at high undercooling. The present work provides further experimental evidences for the dual origins of anomalous eutectics (e.g., uncoupled eutectic-dendrite growth during the recalescence stage and coupled lamellar eutectic growth at low undercooling during the post-recalescence stage) and is helpful for understanding of non-equilibrium phenomena in undercooled melts.

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Notes

  1. It should be mentioned that in recent work by Binder et al. [25], the high-speed video data with the help of POV-Ray were fitted assuming a reasonable overall shape of the growing phase (e.g., a spherical envelope) to obtain the growth velocity. Such a method should be more reliable than but is not adopted by the current work.

  2. This is similar to the work by Li et al [27], in which the Co–61.8 at.% Si eutectic alloy was undercooled by both an electromagnetic levitator and an electrostatic levitator. At low undercooling, only a single recalescence event can be found but the microstructure consists of primary CoSi phase and CoSi–CoSi2 eutectics. In subsequent work by Zhang et al. [11], the recalescence behaviors of undercooled Co–61.8 at.% Si eutectic alloy are much more complex; please see their Fig. 1. All these results indicate that the transformation process and the recalescence behaviors may not follow the one-to-one relationship.

  3. It should be noted that for coupled growth, the lamellar spacing of eutectic formed at high undercooling should be smaller than that formed at low undercooling. The present coarse eutectic is formed by uncoupled growth but not coupled growth. Therefore, it is not strange that the lamellar spacing of the present coarse eutectic formed at high undercooling is larger than that of the thin lamellar eutectic formed at low undercooling.

  4. Eutectic-dendrite can be defined as a dendrite on the whole, the solids of which are formed by eutectic solidification. There are two kinds of eutectic-dendrite according to the growth mechanism, i.e., by coupled and uncoupled eutectic growth. For the former, other alloy element should be added to the eutectic alloy or a negative temperature gradient should be improved to the eutectic interface to make the interface unstable to a dendritic morphology. This is the physical basis for the current eutectic-dendrite growth theory [2, 23, 28]. For the latter, the primary dendrite phase is followed by solidification of a second phase.

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Acknowledgements

The authors would like to thank the Natural Science Foundation of China (Nos. 51371149, 51134011 and 51431008), the China National Funds for Distinguished Young Scientists (No. 51125002), Huo Yingdong Young Teacher Fund (No. 151048), the Aeronautical Science Foundation of China (No. 2015ZF53066), the Free Research Fund of State Key Lab. of Solidification Processing (No. 92-QZ-2014) and the project of Shaanxi Young Stars of Science and Technology (No. 2015KJXX-10).

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  1. State Key Laboratory of Solidification Processing, Northwestern Polytechnical University, Xi’an, 710072, Shaanxi, China

    Cun Lai, Haifeng Wang, Qian Pu, Tingting Xu, Jinsong Yang, Xi Zhang & Feng Liu

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  1. Cun Lai
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  2. Haifeng Wang
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Correspondence to Haifeng Wang or Feng Liu.

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Lai, C., Wang, H., Pu, Q. et al. Phase selection and re-melting-induced anomalous eutectics in undercooled Ni–38 wt% Si alloys. J Mater Sci 51, 10990–11001 (2016). https://doi.org/10.1007/s10853-016-0312-y

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