Research On Toughening Mechanism Of Unidirectionally Solidified Ni-25at%Si Fabricated By Electron Beam Floating Zone Method And Its Brazing With Ti600

In this thesis,the in-situ toughened Ni-25at%Si alloy was prepared by electron beam floating zone method firstly,and then the in-situ toughened Ni-25at%Si alloy was brazed with Ti600.The commercial finite element software Marc was used to calculating the temperature field of electron beam refining process.And the electron beam refining process was controlled based on the temperature distribution which was varied with the process parameters.The microstructure of in-situ toughened Ni-25at%Si alloy and the brazed joint of Ni-25at%Si and Ti600 alloys were analyzed by optical microscope,scanning electron microscope,transmission electron microscope,X-ray diffraction and laser scanning confocal microscope.The mechanical properties of in-situ toughened Ni-25at%Si alloy and its brazed joints were evaluated by compressive strength,fracture toughness,shear strength,nanoindentation and microhardness,respectively.The electron beam refining process of Ni-25at%Si can be divided into 2 stages.The first stage featured the forming of a stable floating zone and the second stage featured the moving of the stable floating zone.The experimental results showed that it was difficult to balance the surface tension and gravity during the melting process due to the low surface tension of Ni-25at%Si alloy.The heat capacity of Ni-25at%Si alloy bar was limited,which easily led to the heat accumulation inside the melting zone during the refining process,resulting in the dynamically changing of surface tension and decreased the stability of the melting process.Based on the inherent relationship among temperature,surface tension and the stability of the melt zone,the temperature field of the floating zone was quantitatively analyzed by numerical simulation and the process parameters for maintaining the stability of the floating zone were obtained.Then the "beam gradient attenuation" technique was put forward to eliminate the effect of heat accumulation in refining process and obtained stable smelting process.The analysis of microstructures of Ni-25at%Si showed that three different zones could be obtained in the refined specimen,namely the transition zone,directionally solidified zone and quenching zone.When the refining rate was less than 10mm/h,the refined specimen showed fully lamellar structure,consisting of Ni solid solution,Ni₃₁Si₁₂and Ni₃Si.Otherwise,a non-fully lamellar structure was formed with a large amount of Ni₃₁Si₁₂2 dendrite.The two kinds of typical microstructures obtained in this study were different with the microstructure of Ni-25at%Si obtained in equilibrium condition.The formation mechanism of fully lamellar Ni-25at%Si alloy was explained by non-equilibrium thermodynamic calculation and the maximum temperature growth criterion.The analysis of mechanical properties of Ni-25at%Si showed that the specimens obtained by 10mm/h owned the optimal comprehensive properties with the compressive strength of 2576 MPa,the fracture toughness of 35.9 MPa·m^1/2.The deformation of Ni-25at%Si alloy showed an extraordinary improvement from 0 to 8%.The fracture toughness of the fully lamellar structure obtained at low refining rate was significantly higher than that of the non-fully lamellar structure obtained at high refining rate.The crack bridging,crack deflection and interface debonding were responsible for the improvement of fracture toughness of Ni-25at%Si.The research on the weldability of Ni-25at%Si/Ti600 showed that several IMCs formed in the Ni-25at%Si/Ti600 brazing joint.The continuous Ti₂Ni layer of the joint obtained by low process parameters deteriorated the mechanical properties of the joints.The joints obtained by high process parameters showed a large number of cavities at the Ni-25at%Si side,which was harmful to the brazing quality.The calculation of the residual stress showed that the maximum residual stress appeared at the Ti600 side.Ti-Zr-Ni-Cu+B composite solder was used to break the continuity of Ti₂Ni layer at low process parameters and reduce the residual stress at the Ti600 side to further improve the weld quality.The average shear strength of the brazing joints obtained by 1213K/10 min was improved from 44 MPa to 84 MPa after adding B element in the Ti-Zr-Ni-Cu solder.Cavity-free and crack-free brazing joints with higher shear strength were obtained.The influence of B element on the interface structure was analyzed from kinetics.Additionally,the strengthening mechanisms of B element were analyzed by calculating the residual stress and analyzing the fracture morphology of the sheared joints.