Microstructure And Property Of Ni-Ni₃Si In Situ Composite By Bridgman Directional Solidification

Ni3Si intermetallic compound has been mainly used in the areas of the high temperature environment. Due to its high strength, low fracture toughness, high temperature creep resistance as well as abnormal R effect etc. However, the engineering application of the Ni3 Si compound has been limited by its poor ductility, fracture toughness, and creep resistance at high temperature. In this paper Ni3 Si intermetallic compound was combined with ductile metal Ni during directional solidification by eutectic reaction, and the comprehensive mechanical properties of materials has been improved. The Bridgman directional solidification technology was used to prepare Ni-Ni3 Si eutectic and Ni-Ni3 Si hypoeutectic composite material at different solidification rate By using metallographic technique, electron microscope technology, image processing technology, and other analysis method, microstructure of the Ni-Si eutectic(hypoeutectic), phase composition and Ni-Ni3 Si eutectic solidification interface of phase relations in every phase of evolution law, organization and material mechanical properties, etc were analyzed.Firstly, microstructure of the Ni-Ni3 Si eutectic was studied, Ni-Ni3 Si eutectic composites displayed regular lamellar eutectic structure at the solidification rate V=6.0~40.0 ?m/s and the lamellar spacing was firstly decreased then increased with the increase of the solidification rate. Ni-Ni3 Si eutectic was composed of ?-Ni, ?1-Ni3 Si and metastable Ni31Si12 phase. The adjustments of lamellar spacing were also observed during the directional solidification process, which was consistent of the minimum undercooling criterion. The transitions from planar interface to cellular, then to dendritic interface, and finally to cellular interface morphologies with increasing velocity were observed with the increase of the solidification rate and the undercooling. The relationship of the Ni-Ni3 Si eutectic was(11—1—Ni//(1—11)Ni3Si and [1 1 0]Ni//[2 1 1]Ni3Si and the phase interface was a semi-coherent boundary.Secondly, microstructure and crystal growth of the Ni-Ni3 Si hypoeutectic was studied. Pseudo eutectic were obtained under the high undercooling condition; Ni-Ni3 Si eutectic was composed of ?-Ni, ?1-Ni3Si; The supercooling is evaluated according to the supercooling criterion, and the theory calculation is consistent of the experimental results well. Moreover, the interface growth temperature of the ?-Ni phase and eutectic phase are calculated in terms of BH model. It shows that the entirely coupled eutectics could not be obtained. The theoretical calculation results are consistent with the experiment results. With the increase of the solidification rate, the primary dendrite arm spacing is decreased and the microstructure is refined.Finally, mechanical properties of the Ni-Ni3 Si eutectic were studied. Micro-hardness of the Ni-Ni3 Si eutectic was firstly decreased then increased by the increase of the amount of the metastable Ni31Si12 phase; The tensile curve showed brittle charater of the Ni-Ni3 Si eutectic; The fatigue strength of the alloy was measured to be 520 MPa by four-point bending. Fatigue cracks were initiated from the Ni-Ni3 Si eutectic phase, and that the Ni matrix fractured in a cleavage fashion. It was caused by supersaturated Si in the Ni matrix.