Research On Formation Mechanism And Mechanical Properties Of High Entropy Alloy Brazed ZrB₂-SiC/Nb Joint

ZrB2-SiC ceramic composites exhibit excellent properties,such as high chemical stability,high electrical and thermal conductivity and strong corrosion resistance,making them to be an ideal high temperature resistant material in supersonic flight,spacecraft and rocket propulsion.It is common to join ceramics with high melting point metals such as Nb to combine the properties of metal and ceramic together.Compared to diffusion welding,friction welding,fusion welding and other connection methods,brazing methods have the advantages of high reliability,stable joining performance,simple process and high processing accuracy.Traditional Ni-based high-temperature filler generates a large amount of brittle intermetallic compounds and residual stress at the interface,which significantly reduces the joint performance.The Ag-based filler becomes soft at high temperature and thus the high-temperature performance of the joint is also poor.The high temperature resistant CoFeCrNiCuTix high-entropy alloy,composed of solid solution structure,is used as the brazing filler in this study to braze the ZrB2-SiC ceramics and Nb alloy together under vacuum at different holding times.CoFeCrNiCu high-entropy alloy was as a filler metal for vacuum join ZrB2-SiC ceramics and Nb alloys.The research show that the typical interface structure of the brazed joint is ZrB2-SiC/Cr2B/(Cr,Fe)2B+FCC+Cr2B+Laves+ Cu(s,s)/Nb(s,s)/Nb.There is no effect of holding times(brazing temperature)on the types of interface products of brazed joints,but the volume fraction,size and thickness of the precipitated phases change with holding time.The amount of Nb alloy dissolved in the liquid filler and the amount of Cr enriched in the ZrB2-SiC ceramic determine the formation and evolution of the brazed joint structure.When the holding time is shorter or the brazing temperature is lower,the amount of Laves+FCC eutectic structure formed was less due to lack of dissolved Nb.They are mainly distributed on the side of Nb alloy and there was only 2%in the brazing joint.Short holding time(lower brazing temperature)also makes the interface reaction insufficient,leading to a double-layer(Cr,Fe)2B+Cr2B structure and a thickness of 30?m.As holding time(brazing temperature)increases,the thickness of Cr2B reaction layer on the ZrB2-SiC ceramic side increases and the amount of Laves phase in the joint increases with the increasing of Nb alloy in the liquid filler.With the holding time(brazing temperature)increased,more Nb alloy dissolves and the amount of Laves+FCC eutectic structure in the joint greatly increased,occupied most of the central area of the joint and the content of Nb in the brazing joint was about 16%.Long holding time(higher brazing temperature)allows the interface react sufficiently and the reaction layer on the ZrB2-SiC ceramic side is only composed of jagged Cr2B due to the substitution effect of B atoms on the C atoms in the carbide.The thickness reaction layer was increased to 50?m.With holding time(brazing temperature)increased,the solubility of Nb alloy was higher,and the Laves+FCC eutectic structure occupied most of the central area of the joint(the average content of Nb in the brazing joint is about 18%at this time).The growth of the jagged Cr2B layer toward the center of the joint was hindered and the thickness of the Cr2B layer was reduced to 35?m.The shear strength of the joint is determined by the thickness and shape of the reaction layer on the ceramic side interface and the shape and distribution of the Laves phase in the braze join.The highest shear strength can be obtained 216 MPa when brazed at 1160?and maintained for 60 min.The shear strength of the joint can reach 94MPa at a high temperature of 650?.This is own to the good mechanical properties of the eutectic structure(composed of hard Laves+soft FCC phase)at room temperature and high temperature and the strong bond of the jagged Cr2B layer and the matrix.In this case,the joint breaks along the ZrB2-SiC ceramic matrix.ZrB2-SiC ceramics and Nb alloy were brazed at 1160? for 60 min with CoFeNiCrCuTix high-entropy alloy filler.The study found that the typical interface structure of the joint is:ZrB2-SiC/Cr2B+(Ti,Nb)B/FCC+rich-Ti Laves+(Ti,Nb)B+Cu(s,s)/Nb(s,s)/Nb.The influence of Ti content on the interface structure and mechanical properties of ZrB2-SiC/Nb joint was systematically studied.It is found that the rich-Ti Laves phase was formed due to the addition of large atomic size Ti fill into the filler alloy or brazing joint,and its content increases with Ti content.The joint brazed by high-entropy alloys filler without Ti can be divided into a tooth-shaped Cr2B reaction layer and a central area composed of a eutectic mixed structure of FCC phase and rich-Nb lamellar Laves phase.Ti and Nb are mutual solid solution elements.The increase of Ti content in the joint makes the FCC phase and the rich-Nb lamellar Laves phase to transform into a big bulk Ti-rich Laves phase and the quadrilateral(Ti,Nb)B phase.The tooth-shaped Cr2B was disappeared.The residual stress generated in the joint during the brazing process tends to cause defects such as holes and microcracks in the bulk Ti-rich brittle Laves phase.Therefore,the normal temperature performance decreases from 216 MPa to 52 MPa.However,with the increase of Ti,the high-temperature mechanical properties of the joint first decrease,and then increase.It was mainly due to the formation of rich-Ti Laves phase and quadrilateral(Ti,Nb)B with excellent high-temperature mechanical properties.When brazing with CoFeCrNiCuTi1.5 filler,the high temperature performance of the joint reached 92%of its room temperature performance.