Study On Directional Solidification Of Ni-Si Hypereutectic Alloy

Ni-Si eutectic in situ composite maerials have the effect of abnormal R(i.e.,with increasing temperature,the strength is not a continuous decline,but first increased and then decreased),high temperature oxidation corrosion and the dual characteristics of metals and ceramicsand other characteristics.In order to investigate the solidification law of the larger components,the solidification characteristics of Ni-Si hypereutectic at different solidification rates were studied.In this paper,the composition system of Ni-Si based hypereutectic in situ composites was designed,and the master alloy ingots of Ni-Si hypereutectic composites were prepared by Bridgman directional solidification technique.The microstructure,phase distribution and phase composition of Ni-Si hypereutectic solidification structure were studied by means of metallographic analysis,energy spectrum analysis and diffraction analysis;The crystal growth mechanism of Ni-Si hypereutectic composites;the effect of annealing temperature on metastable phase structure and the orientation relationship between Ni-Si hypereutectic structures were also studied.The microhardness and fracture toughness of Ni-Si hypereutectic alloy also were measured.The results show that with the increase of solidification rate,the grain size of directional solidification becomes more and more refined and the lamellar spacing decreases with the increase of solidification rate.Non-directional solidification tissue because of the heat flow direction is isotropic the formation is dendritic structure,in the initial stage of solidification,the preferred orientation of performance is weak and the structure is relatively coarse;with the heat gradually stabilized,the crystal growth direction along the optimal growth,the structure is relatively small and stable;growth in quenching interface always flat interface.Ni-Si hypereutectic structure consists ofmatrix ?-Ni,eutectic phase Ni3 Si,and metastable phase Ni31Si12.The Ni phase is a nonfaceted phase,Ni3 Si phase and Ni31Si12 phase are faceted phase,and the Ni3 Si phase mainly grows in the lamellar structure,while the Ni31Si12 phase mainly grows with irregular polygonal morphology.The presence of metastable Ni31Si12 makes the hardness and brittleness of Ni-Si hypereutectic alloy increase.After annealing at different temperatures,when the annealing temperature is 1100?,and insulation 30 min,the metastable phase is significantly reduced.The different solidification rate of the Ni-Si hypereutectic material's orientation relationship were not the same.The experimental results show that with the increase of solidification rate,the average hardness of Ni-Si hypereutectic composites is higher than that of pure Ni3 Si.The hardness of the materials with different eutectic modes of Ni-Si is from large to small in order to be hypereutectic,eutectic and hypoeutectic.The results show that the toughening effect of hypoeutectic is more obvious than that of eutectic and hypereutectic,the hypereutectic hardness is the largest,and the effect of brittleness is the worst.Compared with the arc smelting samples,the room temperature fracture toughness of after Bridgman directional solidification samples are increased at R=6?m/s and 9?m/s.At R=25?m/s and 40?m/s,due to the faster rate,the thermal activation process is inhibited,the dislocation at the crack tip of pile up,its not easy passivation,only extends along the crack tip of the maximum principal stress plane direction,so that the embrittlement tendency of the material becomes larger.