| TZM molybdenum alloy shows unique properties including high erosion resistance,high-temperature stiffness,excellent creep strength,and good conductivity of heat and electricity.Zr C particle reinforced tungsten matrix composite(ZrC_p/W composite)possesses excellent high temperature mechanical properties as excellent high temperature strength,high elastic modulus,thermal stability,and moderate thermal expansion.Both the materials are widely applied in aerospace systems and nuclear energy fields.However,the TZM alloy will be recrystallize at about 1200oC,and the processing technology of the ZrC_p/W composite is complex and difficulty,which will limit the application of the two materials.To overcome the influence of the differences in physical,chemical and functional structures between the two materials,it is necessary to connect the two dissimilar materials together to broaden their high-temperature applications.Brazing is chosen to bonding dissimilar materials since it offers relatively small thermal effect on substrates,simple operation and good repeatability.Ti-based and Ni-based Ti-Ni eutectic brazing alloy were used to wetting and brazing TZM alloy to the ZrC_p/W composite,the nano-indentation properties of the phases in the reaction zones were studied.According the nano-indentation properties results,the composition ratio of the Ti-Ni alloy was optimized to produce a high plasticity factor phase of Ti Ni,which improved the performance of brazing joint.The wettability test on the TZM molybdenum alloy and ZrC_p/W composite were carried out with Ti-Ni eutectic filler metal.Ti-Ni eutectic alloy have good wettability on TZM alloy and ZrC_p/W composite.The wetting angles of Ti-24 Ni and Ti-61 Ni on the TZM and ZrC_p/W are 3.6o,1.8o,4.2o and 48o,respectively.Active element Ti can improve the wettability and spreadability of the Ti-Ni alloy on the substrates,when the content of element Ti turns higher,the wettability and spreadability of the Ti-Ni alloy on the substrate are obviously improved.TZM/Ti-Ni/ZrC_p/W brazed joint are divided into three reaction zones: the reaction zone I close to the TZM alloy consists Ti-Mo solid solution,striped Ti-Ni phase or Ti-Mo solid solution phase;the reaction zone III adjacent to the ZrC_p/W composite mainly consists W(s),Ti(Zr)C particles and Ti-Ni-Zr alloy compounds;the reaction zone II is formed by the residual Ti-Ni-Zr alloy phase during the cooling process.With the increase of brazing temperature or holding time,the reaction zone I becomes rough,the river-like stripes becomes larger and more,meanwhile,the elements proportion in Ti-Mo solid solution changes;the reaction zone II becomes narrower or disappears eventually,and the proportion of elements in Ti-Ni-Zr alloy phase in reaction zone II changes,furthermore,more and more W particles and(Ti,Zr)C particles entering into this zone;reaction zone III increases with brazing temperature.The micro-morphology and phase composition of brazed joints affect the mechanical properties and the fracture path,fracture characteristics of the brazed joints.The wider reaction zone II which contains more brittle phases will reduce the shear strength of the brazed joints,and the fracture mostly occur in this zone.When reaction zone II becomes thinner or disappears eventually,and the brittle phases content turns less,the brazed joints shear strength increases,and the fracture locates on the TZM side near the interface with the reaction zone I.However,when the reaction zone I becomes rough,and the river-like stripe phase becomes coarser and larger,and even some holes or micro-cracks appear in the reaction zone I,then the brazed joints shear strength decreases,and the fracture generally locates in the reaction zone I.To analyze the influence of different Ti-Ni-Zr alloys on the properties of brazed joints,nano-indentation technology was used to study the nano-indentation hardness and plasticity factor of of the different alloy phases in the brazing seam: Ti Ni3,(Ti,Zr)Ni2,Ti Ni2,Ti2 Ni phases have higher nanohardness and lower plasticity factor,while Ti Ni phase has the lowest hardness and the highest plasticity factor.By adjusting the element ratio of Ti-Ni filler metal,50Ti-50 Ni filler metal was selected to braze TZM alloy and ZrC_p/W composite.When brazing temperature was 1340oC and holding for 10 minutes,the maximum room temperature shear strength of the brazed joint was 146.1 MPa,which can reach the 96.8% of the TZM.The maximum value of high temperature shear test is 118.4MPa.In order to further analyze the interfacial reaction mechanism between Ti-Ni and Mo,Zr C and W,pure Ti and pure Ni were used in the experiments to react with TZM,Zr C and W.It has been found that Mo and Ti are easily mutually dissolvable to form Ti-Mo solid solution.C atom and Zr atom in Zr C unit cell can diffuse into Ti-Ni solution when contact with Ti at high temperature.Zr C can react with Ni to form liquid phase when the brazing temperature over 1200oC;C atom in liquid Ti-Ni solution can react with Ti to form Ti C;Zr atom in Ti-Ni solution can form alloy compounds;W can form solid solution with Ti and Ni,but it is hard to take on the reaction.The formation mechanism of interface structure of TZM/Ti-Ni/ZrC_p/W brazing joint is summarized through the analysis of reaction interface between the brazing alloy and the substrate: when the brazing alloy melts and wets the substrates,the brazing alloy diffuses into the substrates;the brazing alloy diffused into TZM forms river-like stripes Ti-Mo solid solution or Ti-Ni phase in the reaction I;Mo atom dissolves into the liquid brazing alloy and form Ti-Mo solid solution layer;the Ti-Ni alloy destroys the connection of the Zr C and W particles and makes the Zr C and W particles enter into the liquid alloy;Ti C particles are formed by atom C and atom Ti,Ti-Ni-Zr alloy compounds are formed when some Zr atoms diffused into the Ti-Ni alloy during the cooling process;the residual liquid alloy begins to cool and form the reaction zone II and reaction zone III. |
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