Microstructure And Mechanical Properties Study Of Si₃N₄ Ceramic And TiZrHfNbMo High Entropy Alloy Brazed Joint

Si3N4 ceramic has the characteristics of high melting point,high strength,high hardness,and excellent corrosion resistance.It is the most potential ideal material for missile nose radome.However,due to the inherent brittleness of Si3N4 ceramics,it is difficult to machine and shape,which greatly limits its application and development in actual production.TiZrHfNbMo high-entropy alloy is solid solution strengthened by five elements of titanium,zirconium,hafnium,niobium and molybdenum to form a high-temperature high-entropy alloy.TiZrHfNbMo high-entropy alloys have the comprehensive mechanical properties of ultra-high strength,ultra-high hardness,high stability,good high-temperature oxidation resistance,and excellent high-temperature softening resistance,which makes they have a good application prospect in the field of welding.This paper used AgCuTi powder filler and CNTs reinforced AgCuTi composite filler to braze TiZrHfNbMo/Si3N4 joints to study the effects of brazing temperature,holding time and CNTs content on the microstructure and mechanical properties of the brazing joint interface.When using AgCuTi filler to braze TiZrHfNbMo/Si3N4 joints,the typical joint interface phases was TiZrHfNbMo/TiCu(Hf,Zr)NbMo+Cu4Ti/Ag(s,s),Cu(s,s),CuTi2/CuTi+Ti5 Si3/Si3N4.With the continuous increase of brazing temperature,the number of brazing joint interface structure increased,the phase volume fraction increased,the thickness of the reaction layer between the brazing filler metal and the base metal became wider.AgCuTi reacted with Si3N4 to generated CuTi2/CuTi+Ti5Si3 intermetallic compound,which continuously increased to form a reaction layer.The mechanical properties of the TiZrHfNbMo/Si3N4 joint brazed using AgCuTi filler at the brazing temperature of 840℃ for holding time 60 min was the best and the average shear strength of it was 86 MPa.With AgCuTi brazed TiZrHfNbMo/Si3N4 shear sample was fractured on the Si3N4 ceramic side,and the fracture surface was flat and typical brittle fracture.Using different CNTs content reinforced AgCuTi composite filler brazing TiZrHfNbMo/Si3N4,the typical joint interface structure translated from TiCu(Hf,Zr)NbMo,Cu4Ti,Ag(s,s),Cu(s,s),CuTi,CuTi2,Ti5Si3,TiN to TiZrCuNbMo,Ag(s,s),Cu(s,s),TiC,Ti5Si3,TiN.In the brazing of TiZrHfNbMo/Si3N4 joints with CNTs-reinforced AgCuTi composite filler,the introduction of C in the CNTs-reinforced AgCuTi composite filler resulted in the formation of TiC phase in the TiZrHfNbMo/Si3N4 joint.TiC particle dispersion strengthening and particle reinforcement improved the shear performance of the welded joint.The strength of the TiZrHfNbMo/Si3N4 joint brazed with AgCuTi-1.0wt.%CNTs composite filler was up to 120 MPa.When brazing TiZrHfNbMo/Si3N4 with AgCuTi-1.5wt.%CNTs composite filler,due to the nano-agglomeration effect of CNTs,the TiC of the brazing joint changed from particles to bulk.And the uneven distribution of TiC in the joint bulk led to the residual stress of the joint to increase.Therefore,the strength of the TiZrHfNbMo/Si3N4 joint brazed with AgCuTi-1.5wt.%CNTs composite filler was greatly reduced.Brazing TiZrHfNbMo/Si3N4 joints with different CNTs content-enhanced composite filler were all brittle fractures.The AgCuTi-1.0wt.%CNTs composite brazing TiZrHfNbMo/Si3N4 joint phases were TiZrCuNbMo,ZrCu,CuTi,Ag(s,s),Cu(s,s)and Ti(s,s)at the different brazing temperature for 60 min holding time.As the brazing temperature increased,the phase volume fraction and distribution of the weld phase changed from small to large and from distribution to concentrated distribution.With the increase of the brazing temperature,the strength of the TiZrHfNbMo/Si3N4 joint brazed with AgCuTi-1.0wt.%CNTs composite filler first increased and then decreased.When the brazing temperature was 840℃ and the holding time was 60 min,the mechanical properties of the brazed joints were best and the shear strength of it was as high as 120 MPa.The fracture modes of TiZrHfNbMo/Si3N4 joints were brittle fractures at all brazing temperatures.The typical joint phases of AgCuTi-1.0wt.%CNTs composite filler brazing TiZrHfNbMo/Si3N4 were(TiZrCu)2HfNbMo,Cu4Ti,Ag(s,s),Cu(s,s),Ti(s,s),Ti5Si3,TiN at the brazing temperature of 840℃ for the different holding time.With the extension of the holding time,the Zr and Ti elements in TiZrHfNbMo diffused into the composite brazing filler metal,so that the phase of the composite brazing filler metal and the highentropy alloy reaction layer changed from(TiZrCu)2HfNbMo to the TiZrCuNbMo high-entropy phase.And at the Si3N4 ceramic side,increasing amount of Ti(s,s),Ti5Si3,and TiN phase led to the broadening of the reaction layer.AgCuTi-1.0wt.%CNTs composite filler brazing TiZrHfNbMo/Si3N4 joint strength first increased and then decreased at brazing temperature of 840℃ for different holding time.When the brazing temperature was 840℃ and the holding time was 60 min,the shear strength of the TiZrHfNbMo/Si3N4 joint was 120 MPa.AgCuTi-1.0wt.%CNTs composite filler brazing TiZrHfNbMo/Si3N4 joint fractures were typical brittle fractures at brazing temperature of 840℃ for the different holding time.