Microstructure And Properties Of Al₂O₃/TC4Brazed Joint Reinforced By In Situ Synthesized TiB Whiskers

In view of the excellent performance of ceramics, such as the high meltingpoint, high temperture resistance, corrosion resistance, radiation resistance, highfrequency and high voltage resistance, as well as insulation perfomances, it issignificant to join ceramics and metals characterized good ductility. Brazing isapplied widely to join ceramics and metals, because it has unique advantages forjoining great property difference materials and melting sensitively materials.However, it is necessary to avoid the residual stresses of joint caused by metallurgyincompatibility and physical properties mismatch. Combining with in situ synthesistechnology and brazing method, the method of in situ synthesizing TiB whiskers tostrengthen ceramic/metal brazed joints is proposed. The method can reduce theceramic/metal joints residual stresses, and improve the joints room-temperature andhigh-temperature shear strength. AgCuTi+B, CuTi+TiB₂and Cu+TiB₂fillersprepared with mechanical mixing are used to braze Al₂O₃to TC4respectively.Effect of TiB on joint microstructure and mechanical properties is investigated. Theformation mechanism of TiB during brazing is revealed.According to SEM, EDS, XRD and TEM, the microstructure of Al₂O₃/TC4joint with AgCuTi+B composite fillers is Al₂O₃/Ti₃（Cu,Al）₃O/Ti₂Cu+Ti₂（Cu,Al）/Ag（s.s）+TiCu+Ti（Cu,Al）+Ti₂Cu+TiB/（αTi）+Ti₂Cu/TC4. Excessive Bpowders or deficient Ti powders results in the presence of TiB₂in joint. As the Bincrease, TiB whiskers increase, while Ti₃（Cu,Al）₃O area remains nearly constant.As the Ti powders increase, Ti₃（Cu,Al）₃O area thickens. The area consist of Ti₂Cuand Ti₂（Cu,Al） thickens and then thins. The dissolution of TC4dissolved into liquidbrazing fillers decreases. As brazing temperature rises and holding time extends,Ti₃（Cu,Al）₃O reacts with Al₂O₃to form TiO. The dissolution of TC4dissolved intoliquid brazing fillers increases. Both Ti₂Cu and Ti₂（Cu,Al） increase, and then thearea consist of Ti₂Cu and Ti₂（Cu,Al） thickens.The microstructure of Al₂O₃/TC4joint using CuTi+TiB₂or CuTi+TiB₂composite fillers is Al₂O₃/Ti₃（Cu,Al）₃O+Ti₄（Cu,Al）2O/Ti₂Cu+Ti₂（Cu,Al）+Ti₃Al/Ti₂（Cu,Al）/Ti₂Cu+AlCu2Ti+TiB/（αTi）+Ti₂Cu/TC4. When TiB₂increases, TiBwhiskers increase. The area consist of Ti₂（Cu,Al） appears and becomes continuous,then moves to Al₂O₃. When Ti increases,（αTi）+Ti₂Cu appears in Ti₂Cu grainboundaries. The area consist Ti₂（Cu,Al） becomes continuous and thickens. AlCu2Tidecreases gradually, enen disappears. When brazing temperature rises and holdingtime extends, the area consist of Ti₃（Cu,Al）₃O and Ti₄（Cu,Al）2O thickens. The dissolution of TC4dissolved into liquid brazing fillers increases. Ti₂Cu increases,yet Ti₂（Cu,Al） increases at first and then decreases. The area consist of Ti₂（Cu,Al）gradually moves away from Al₂O₃. Ti₃Al increases in the grain boundaries of Ti₂Cu,while Ti₂（Cu,Al） and AlCu2Ti decrease.During brazing, B or TiB₂reacts with Ti to form TiB. TiB grows by B atomsdiffusing into TiB. TiB sizes and shapes are responsive to the kind and additiveamount of B source, brazing temperature and holding time. The size of TiBgenerated from B is smaller than that from TiB₂. When it increases, TiB changesfrom single to clusters, and its size increases at first and then decreases. In addition,the size and formation amount of TiB decrease when brazing temperature andholding time increase. When B or TiB₂content increases, the wetting and spreadingof brazing fillers on Al₂O₃decrease.According to Al₂O₃/TC4joints residual stresses caculated by finite elementsimulations, the highest residual tensil stress appears on the corner of Al₂O₃, and thehighest residual shear stress appears at Al₂O₃and brazing seam near Al₂O₃/brazingseam interface. With the increasing of TiB content, the highest residual tensil stressin Al₂O₃decreases, the highest residual shear stress in Al₂O₃and brazing seam nearAl₂O₃/brazing seam interface increases, and the equivalent residual stressesdistributed in Al₂O₃decrease, but that in brazing seam decrease. When joint fractureoccurs, TiB whiskers can make cracks deflect during propagation. For the elasticmodulus of TiB is higher than the brazing seam, TiB can transfer the modulus loadin joint, and move stresses at a crack tip to regions remote from the crack tip.Finally, the stress intensity at the crack tip decreases, and the joint shear strength isimproved. At room temperture, the maximum shear strength ofAl₂O₃/CuTi+TiB₂/TC4joints is143MPa, which is239%higher than that of theAl₂O₃/CuTi/TC4joints; the maximum shear strength of Al₂O₃/Cu+TiB₂/TC4jointsis97MPa, which is269%higher than that of the Al₂O₃/Cu/TC4joints; themaximum shear strength of Al₂O₃/AgCuTi+B/TC4joints is78MPa, which is81%higher than that of the Al₂O₃/AgCuTi/TC4joints. At high temperture, the maximumshear strength of Al₂O₃/Cu+TiB₂/TC4joints is40¹25%higher than that of theAl₂O₃/Cu/TC4joints. It can be attributed that TiB whiskers exist in joints, acted asceramic skeleton, owing to its high melting point and large aspect ratio.