| Continuous carbon fiber reinforced silicon carbide composite(Cf/SiC)is a kind of structural materials for high-temperature applications because of its high specific strength,excellent fracture toughness,and superior strength at elevated temperatures.Its brittleness and poor machinability,however,are the key issues that hinder the wider applications of Cf/SiC.Thus,most such applications in the aerospace industry require joining Cf/SiC to ductile metals,especially to Nb,which has excellent high-temperature strength and low price.In this work,we designed three Ti-based brazing alloys for brazing Cf/SiC and Nb based on phase diagrams.The(Ti,Nb)solid solution was in-situ formed in the three brazing layers,which increased the toughness of the joint.The joints with excellent mechanical properties were fabricated.The joints of Cf/SiC and Nb brazed with Ti-Zr-Cu-Ni amorphous alloy mainly consisted of the brittle(Ti,Zr)2(Cu,Ni)intermetallic with the Zr2Cu structure.The CTE of Zr2Cu is 26.0×10-6K-1,which has a large mismatch with Cf/SiC.Because of the poor deformability of the(Ti,Zr)2(Cu,Ni)intermetallic,the thermal stress in joints cannot be well released,resulting in the cracks in the joints.Based on the analysis of phase diagrams,Ti-Zr-Cu-Ni/Ti composite brazing alloys were designed to shift the compositions of the brazing alloys from the intermetallic primary phase region to the Tiss primary phase region,which was beneficial for increasing Tiss content.Furthermore,the structure of(Ti,Zr)2(Cu,Ni)shifts from Zr2Cu-tytpe to Ti2Ni-type.The CTE of Ti2Ni is 6.10×10-6K-1,which is well matched with parents materials.With the increased brazing temperature,the Nb content in the joint increased.The Nb element that dissolved into the brazing layer suppressed the eutectoid transformation of?-Ti to form?-Ti+(Ti,Zr)2(Cu,Ni)and increased the quantity of the(Ti,Nb)ss phase in the brazing layer.The typical microstructure of the joint was(Cf/SiC)/(Ti,Zr)C/(Ti,Nb)ss+(Ti,Zr)2(Ni,Cu)/?-(Ti,Nb)ss/Nb.The highest shear strength was 121 MPa at a brazing process parameter of 1000?for 15min,which increased by 278%compared with the joints brazed by Ti-Zr-Cu-Ni amorphous alloy.To quantificationally adjust the proportion of(Ti,Nb)ss in the brazing layer and improve the performance of(Ti,Nb)ss at elevated temperatures,we designed Ti-Zr-Cu-Ni/Nbnp composite brazing powders.When the Nbnp addition is 12 wt.%,the brazing layer consists of the?-(Ti,Nb)ssand intermetallic.The proportion of?-(Ti,Nb)ss increased when the Nbnp addition increased.The optimum brazing process parameter for attaining the highest shear strength of 125 MPa was 1000?for 20min.The typical microstructure of the joint was(Cf/SiC)/(Ti,Zr)C/?-(Ti,Nb)ss+(Ti,Zr)2(Ni,Cu)/?-(Ti,Nb)ss/Nb.And the high-temperature shear strength of the joints at 600?reached 113 MPa,which increased by 36%compared with the joints brazed by Ti-Zr-Cu-Ni/Ti composite alloy.The high-temperature shear strength at 800?was 105MPa.It is inconvenient to increase additional steps during the assembling process by adding Ti foil or Nb nano-powders to Ti-Zr-Cu-Ni.A novel brazing alloy with a ductility-strength dual-phase microstructure was designed for solving this problem.Three Nb-Ti Cu alloys were fabricated by the arc melting method,which consists of bcc-Nb and Ti Cu.The melting behaviors of Nb-Ti Cu alloy were investigated.The melting process of Ti Cu and bcc-Nb are separated.The typical microstructure of the joint was(Cf/SiC)/Ti C/Cu4Ti3+bcc-(Nb,Ti)ss/Nb.The highest shear strength was 139MPa at a brazing process parameter of 1050?for 15 min,which increased by 341%compared with the joints brazed by Ti-Zr-Cu-Ni amorphous alloy,14.9%compared with the joints brazed by Ti-Zr-Cu-Ni amorphous alloy,and 11.2%compared with the joints brazed by Ti-Zr-Cu-Ni amorphous alloy,respectively. |
PDF Full Text Request