Studies On Preparation And Properties Of Continuous SiC Fiber Reinforced Ti₂AlNb Matrix Composites

Continuous SiC fiber reinforced Ti₂AlNb matrix composites?SiC_f/Ti₂AlNb?have great application potential in high temperature components of aerodynamic systems.However,there are few reports on the preparation of SiC_f/Ti₂AlNb composites by matrix coated fibers method?MCF?,and the analysis of their microstructure,interfacial reaction and mechanical properties.In this paper,SiC_f/Ti₂AlNb composites were prepared by magnetron sputtering physical vapor deposition?PVD?and hot isostatic pressing?HIP?.The mechanism of the formation process on the interfacial reaction of composites influenced by the molding process,the original structure of the matrix alloy and the diffusion barrier coating were explored.The influence of interfacial reaction layer on the axial tensile properties of composites was revealed.The HIP temperature of SiC_f/Ti₂AlNb composites has an important influence on the degree of densification,the microstructure of the matrix,the thickness of the interfacial reaction layer and the residual stress of the material.Regulating the HIP temperature is an important means for the composite to obtain excellent room temperature axial tensile strength.The SiC_f/Ti₂AlNb composites with fully dense and suitable interfacial reaction layer thickness were prepared under the HIP process of 950°C/150 MPa/2 h,and the axial tensile strength at room temperature was 1515 MPa.Substrate bias process in magnetron sputtering PVD process has an important influence on the physical and micromechanical properties of the PVD-Ti₂AlNb alloy coating such as deposition rate,phase composition,grain size,composition,surface morphology,stress state,hardness and toughness.The high bias process induces the segregation of Nb element at the grain boundary,which promotes the amorphization of the Ti₂AlNb coating to form a uniform nanocrystalline-amorphous two-phase nanostructure with a Nb-rich amorphous shell encapsulating the Ti₂AlNb nanocrystal core.This structure contributes to the coating having both high hardness and good toughness.The original microstructure of the matrix alloy has an important influence on the matrix microstructure,interfacial reaction layer and axial tensile strength of the SiC_f/Ti composite.The formation of the interfacial reaction layer is a process in which the C element diffuses into the matrix to form reaction products,after undergoing two processes dominated by the reaction control and the diffusion control respectively.The fine grain size and a large content of?/B2 phase will cause C atoms to have more grain boundary diffusion paths and higher diffusion rates,thereby promoting the formation of reaction products and making the composite have a thicker reaction layer.The strong interfacial bonding strength in the appropriate range can increase the crack propagation paths,which is beneficial to improve the axial tensile strength at room temperature of the composite.SiC_f/C/Ti₂AlNb and SiC_f/C/B₄C/Ti₂AlNb composites with C-coating and C/B₄C duplex coating as diffusion barrier coatings were prepared.The C-and C/B₄C-dominated interfacial reactions in the SiC_f/C/Ti₂AlNb and the SiC_f/C/B₄C/Ti₂AlNb were explored,revealing two different reaction products sequences:The different-sized TiC and the coarse-grained?Ti,Nb?C+AlNb₃ for the SiC_f/C/Ti₂AlNb;and the fine-grained TiB₂+TiC,the needle-shaped?Ti,Nb?B₂/NbB+?Ti,Nb?C,the coarse-grained?Ti,Nb?C+AlNb₂ for the SiC_f/C/B₄C/Ti₂AlNb.Under the same consolidation conditions,the reaction product characteristics make the SiC_f/C/B₄C/Ti₂AlNb has a thicker interfacial reaction layer.When the interfacially debonded occurred between the pyrolytic C coating and the polycrystalline B₄C coating containing amorphous boron in SiC_f/C/B₄C/Ti₂AlNb,The SiC_f/C/Ti₂AlNb with thinner interfacial reaction layer has a higher interfacial bonding strength than SiC_f/C/B₄C/Ti₂AlNb.The room temperature tensile fractures of SiC_f/C/Ti₂AlNb exhibit more fiber debonding and small step fracture obviously,which make the axial tensile strength at room temperature of SiC_f/C/Ti₂AlNb composites higher.