Study On The Interfacial Diffusion Behaviors Of SiC Continuous Fiber Reinforced Ti-Matrix Composites

Due to its high specific strength and specific modulus, SiC continuous fiberreinforced Ti alloy-matrix composites (TMCs) can be used for aero-engines. Theinterfacial reaction taking place during the composite consolidation and hightemperature service leads to the formation of some brittle compounds distributed inseveral layers at the interface. The brittle interfacial reaction products become thecrack origination that makes the interface lose its stability and the ability oftransfering the applied load.Interfacial reaction is a reaction diffusion problem. Due to quite a few of diffusionelements and the factors influenced relating to the reaction diffusion, as well as thecomplicated structure of interfacial reaction products, it is very difficult in solvingdiffusion coefficients. In order to study the formation sequence of the interfacialreaction products and the diffusion path between SiC fiber and Ti alloying matrix,TA1, Ti6Al4V, Superα₂ and Ti₂AlNb were used as the matrices, which werereinforced by different SiC continuous fiber. The composites were fabricated and heattreateded under different conditions, and the morphologies and elemental distributionsas well as components of interfacial reaction zone were analysed with the help ofSEM, TEM, EDS and XRD methods. According to Fick's second diffusion law andGuass solving method in a semi-infinite diffusion couple for ternary systems, thediffusion coefficients and concentration distribution of the relative reaction elementswere obtained, and the micro-diffusion mechanism was discussed.From SiC fiber to matrices, the formed interfacial phase sequences are to beSiC|Ti₃SiC₂|Ti₅Si₃C_x|TiC_x+Ti₅Si₃C_x|Ti for SiC/TA1 composite, SiC|Ti₃SiC₂|Ti₅Si₃C_x|TiC|Ti₃Si|Ti6Al4V+TiC for SiC (no C coating)/Ti6Al4V composite,SiC|Ti₃SiC₂|Ti₅Si₃C_x|TiC|Ti6Al4V for SiC (C coating)/Ti6Al4V compositeafter the whole C coating being consumed completely, and SCS-6 SiC|(Ti,Nb)C|(Ti,Nb)₅(Al,Si)₃C_x|(Ti,Nb)C|(Ti,Nb)₃(Si,Al)|Superα₂ (Ti₂AlNb)+Ti₃AlC forSCS-6 SiC/Ti-Al intermetallic matrix composites.For Ti matrix composite reinforced with SiC fiber containing C coating, the growthof interracial reaction layers follows a role of parabolic rate before the C coating isconsumed, while it doesn't meet the same rule after the C coating is consumedcompletely. For the composites reinforced with SiC fiber without C coating, all thegrowth of interfacial layers follows the parabolic rate. The activation energy Q^k incomposites is in the following order: Q_TA1^k＜Q_Ti6Al4V^k＜Q_Ti600^k＜Q_Superα2^k, so SCS-6 SiC/Ti-Al intermetallic matrix composite has higher interface compatibility and stability.The calculated diffusion profiles of reactive elements are well fitted to the data measured. According to the crystal growth characteristics of interfacial reactionproducts which are controlled by diffusion, Ti₃SiC₂ and TiC have the layer-shape, theequiaxed-shape, respectively. C atoms diffuse through the reaction layer by interstitialdiffusion mechanism, and Si atoms diffuse through the reaction layer mainly byvacancy diffusion mechanism. While in Yi₃SiC₂ layer which is fine microstructure, theboundary diffusion is mainly the diffusion mechanism. C and Si atoms have theminimum diffusion coefficients in TiC_x layer, so the reactive elements diffusingthrough the TiC_x is a dominant determining step in interfacial reaction progress ofcomposite.Based on Koher's ternary solution model and Miedema's model for calculating theheats of formation of binary solution, the activity coefficients and activity interactioncoefficients in Ti6Al4V, Ti600, Superα₂ and Ti₂AlNb are obtained. According tothermodynamical factors in diffusion as well as the self-diffusion coefficients ofalloying elements, the further step is to get the inter-diffusion coefficients. Theactivity coefficients of Ti in this four alloys and intermetallics are in the followingorder:γ_{Ti in Ti6Al4V}＞γ_{Ti in Ti600}＞γ_{Ti in Superα2}＞γ_{Ti in Ti2AlNb}, and the inter-diffusioncoefficients of the main alloying element, Al, are (?)_{Al in Ti600}＞(?)_{Al in Ti6Al4V}＞(?)_{Al in Superα2}＞(?)_{Al in Ti2AlNb}.The diffusion bonding in fabricating Ti matrix composites was studied by the helpof fabricating TA1/Ti6Al4V diffusion couples and heat-treating in vacuum underdifferent conditions. According to the inter-diffusion coefficients calculated of thealloying elements as well as the numerical solution of the phenomenological equation,the theoretical diffusion profiles of alloying elements and Ti can be obtained. Themeasured data is matched well with the data calculated, so it can predict thedistribution of the diffusion atoms at the diffusion bonding interface.The interface reaction, the formation of interfacial phases and the actionmechanism of TiB₂ diffusion barrier coating in SCS-6 SiC/TiB₂/Superα₂ compositewere studied. It is shown that although TiB₂ can prevent C from diffusion, it reactswith Ti to form TiB. Until TiB₂ layer was consumed completely, the formed products,TiB, can't prevent the diffusion of C, Si, Ti and the alloying elements, and theinterface reaction is accelerated remarkably. From SCS-6 SiC fiber to Superα₂ matrix,the interfacial reaction products are SCS-6 SiC |TiC+Ti₅Si₃|TiC|Ti₃Si|TiB|Ti₃AlC|Superα₂. The diffusion direction of B atoms in TiB₂ is normal to theconsuming direction of TiB₂ coating, while it is parallel to the growing direction ofTiB, and the diffusion coefficients in TiB is much larger than that in TiB₂.