Research On The Growth And Reinforcing Mechanisms Of In Situ TiB Whiskers In ZrB₂-SiC Ceramic Joints

The ZrB2-SiC ceramics (ZS) can be applied in hypersonic flight as heat shields due to their unique combination of high melting point, low density, good electrical/thermal conductivities and high ablation resistance. Practical applications in advanced aerospace fields invariably require ZS to be integrated with themselves or with metal to constitute large or complex components. Thus, the development of robust joining technologies of ZS is critical. In this paper, an in-situ TiB whisker reinforced joining method is developed by using ZrB2as B source. The microstructure of joints and reaction mechanism are analyzed, the growth behavior of TiB during joining process is discussed, and the mechanical properties of joints are evaluated by finite element simulations and mechanical tests.The thermodynamic model is established to assess the feasibility of in situ reaction of TiB. The result shows that the activity of Ti plays a key role in the progress of in situ reaction. In Ag-Cu-Ti filler system, effects of Ti content and brazing temperature on the in situ reaction are investigated. It is found that the SiC decomposes first in brazing liquid, producing Ti3SiC2and Ti5Si3. TiB whiskers are produced successfully at880oC with15wt.%Ti. The precipitation of AgCu4Zr accelerates decomposition of ZrB2, which finally increases amount of TiB. The ZS is brazed to TC4alloy by in situ TiB reinforced joining method. Owning to the miscibility gap in Ag-Cu-Ti system, brazing liquid divides into Ag rich liquid and Ti-Cu rich liquid. The latter promotes in situ reaction and results in an TiB whisker array on ZS interface at880oC. The typical microstructure of the joint is ZS/TiB+Agss/Ti2Cu+AgCu4Zr/Ti2Cu+α-(Ti,Al)ss/TC4.In order to investigate in situ reaction in solid state, ZS is diffusion bonded to Nb with Ti interlayer. Experiments show that initial TiB whiskers are disordered at900oC. A continuous reaction layer composed of (Ti,Zr)5Si3,(Ti,Zr)2Si and β-(Ti,Zr,Si)ss is produced by reactions between (Ti,Zr)ss and SiC. This layer prevents formation of dense TiB. The TiB whisker array is formed at1100oC. On the Nb side, the ductile β-(Ti,Nb)ss is produced due to mutual diffusion between Ti and Nb, which increase the toughness of joints.The TiB whiskers with3-D distribution and various orientations can realize effective stress accommodation in the joint. The aim of the second part of this paper is to prepare3-D distribution TiB reinforced joints. It is found that the decomposition of ZrB2is faster than SiC in Ti rich liquid. The layer by layer detachment of TiB array results in3-D distribution of TiB in seam. Due to the sever decomposition of ZrB2, complex intermetallic compounds (IMCs) are produced, which embrittle the joint. In order to overcome this problem, another Ti-Ni filler system with low activities of Ti is designed. When using Ti/Ni/Ti stacking foils as filler alloy brazing at1050oC for10min, TiB whiskers with parallel/vertical cross distribution are produced in the seam. The typical interfacial microstructure of the joint is: ZS/TiB+(Ti,Zr)3Si+β-(Ti,Zr,Si)ss+Ti2Ni/Ti2Ni+TiNi. When the Ti content is low, filler alloy experiences transient liquid procedure, the produced TiB is short, and the matrix of seam is TiNi. When the Ti content is high, the dwell time of filler liquid is prolonged, the produced TiB is long and big, and the matrix of seam is Ti2Ni. As brazing temperature exceeds1080oC, reverse reaction of TiB happens, which results in TiB2produced in the seam.The vacancy diffusion mechanism is used to explain the growth of TiB in solid state. Due to the block of reaction layer, Ti atoms occupy sites at the tip of TiB crystal easily. Simultaneously, vacancies of B atoms are produced and diffuse along the Z-shape chain of B ([010] direction) to the bottom of TiB crystal, where B atoms released from ZrB2fill vacancies. The kinetics based on Fick’s second law is calculated. The coefficient of B atom diffusion in TiB crystal is D=1.311×10-14m2/s. The relationship between the length of TiB and joining time is x0.891t. In liquid state, TiB grows by stacking on (100) plane. Stacking faults along [100] are developed easily, which results in Bf/B27coherent boundaries in TiB crystals. The non-equilibrium crystallization of TiB in liquid alloy leads to a twin-whisker structure.The stress distribution around single TiB whisker is simulated by finite element method (FEM) and the modulus and coefficient of thermal expansion (CTE) of TiB reinforced regions are evaluated by theories of material mesomechanics. Results show that the orientation and volume fraction of TiB have a great effect on the mechanical properties of reinforced regions. Shear tests are employed to evaluated joint strength. At room temperature, the strength of ZS/ZS joints reinforced by TiB whisker array reaches146MPa (15wt.%Ti), a33%increase over the one using ordinary Ag-Cu-Ti filler alloy. At800oC, the shear strength of ZS/Nb joints with Ti foil and ZS/ZS joints with Ti/Ni/Ti stacking foils (65at.%Ti) reaches83MPa and67MPa, respectively, indicating excellent high temperature mechanical properties. The deflection effect of TiB on cracks increases the fracture energies of joints.