Microstructural Evolution And Crystal Growth Kinetics Of Undercooled Ni-Si Compound-Compound Eutectic Alloys

Eutectic alloys have found many applications in industries because of their virtues of low melting points and good castability. Due to the potential applications compound-compound type eutectic alloys have attracted more and more attention. It has been well documented that a morphological transition from regular lamellar eutectics to anomalous eutectics occurs to some eutectic alloys under large undercooling conditions. However, compound-compound type eutectic alloys have been little investigated with respect to this transition. Thus, it is necessary to perform more studies on this type of eutectic alloys.In the present thesis, Ni71.2Si29.8,Ni54Si46and Ni43.8Si56.2eutectic alloys were chosen as model alloys for an understanding of the mechanism of microstructural evolution in compound-compound type eutectic alloys. Bulk melts of the alloys were undercooled using the glass fluxing technique combined with superheating cycles. Their rapid solidification processes were in-situ observed using a high-speed camera. Crystal growth velocities in undercooled melts were determined using the three-dimensional computer animation technique of the recorded camera images. The microstructure of the solidified alloys was examined. The main conclusions are as follows.(1) The recalescence front during rapid solidification of Ni71.2Si29.8, Ni54Si46and Ni43.8Si56.2eutectic alloys always advanced from one side to the other side of the sample surface.(2) The cooling curves of recalescence process of Ni71.2Si29.8,Ni54Si46and Ni43.8Si56.2eutectic alloys showed three types, including single recalescence, separated double recalescence and continuous double recalescences. The phase selection phenomenon occurred to the Ni54Si46and Ni43.8Si56.2eutectic alloys during rapid solidification.(3) The crystal growth velocities of Ni71.2Si29.8,Ni54Si46and Ni43.8Si56.2eutectic alloys all rose with increasing undercooling. (4) The solidification microstructure of Ni71.2Si29.8,Ni54Si46and Ni43.8Si56.2eutectic alloys consisted of cellular regular eutectic for low undercooling, but consisted of anomalous eutectic structure for high undercooling, where the multi-angular particles of one phase were embedded in the matrix of the other phase. The solidification microstructures showed directionality.(5) It is suggested that the crystal growth mode in the Ni71.2Si29.8,Ni54Si46and Ni43.8Si56.2alloys transited from coupled growth of two phases to uncoupled dendritic growth of one phase under large undercooling conditions, and that remelting of single-phase dendrites led to the anomalous eutectic during recalescence.