Interfacial Characteristics And Fatigue Behavior Of SiC Fiber-Reinforced Ti Matrix Composites

Due to their higher specific strength and stiffness compared to monolithic titanium alloys,SiC fiber-reinforced titanium matrix composites(SiC_f/Ti composites)are developed for advanced structural applications in future aerospace engine components.The fatigue behavior of SiC_f/Ti composites under various fatigue loading conditions must be investigated to ensure the high reliability of the structures during lifetime.Previous fatigue investigations concentrated mainly on titanium matrix composites reinforced by Textron SCS or DERA Sigma series of SiC fibers.Nonetheless,there is no information in the published literature relating to the fatigue behavior of titanium matrix composites reinforced by Chinese SiC fibers.In this paper,interface modification by means of proper fiber coatings and heat treatment methods was conducted to acquire potential approaches to improve fatigue properties of Chinese SiC_f/Ti composites.Meanwhile,effects of the characteristics of fiber/matrix interface on the fatigue behavior of Chinese SiC_f/Ti composites were systematically investigated.The matrix alloy used in this study was Ti-6Al-4V alloy.Chinese SiC fibers with C and C/Mo coatings were used as the reinforcement,respectively.SiC_f/C/Ti-6Al-4V and SiC_f/C/Mo/Ti-6Al-4V composite systems with six kinds of interfacial characteristics were obtained using foil-fiber-foil method,vacuum hot pressing and vacuum heat-treatment at 750 °C for 36 h and 100 h,respectively..The interfacial characteristics of SiC_f/C/Ti-6Al-4V and SiC_f/C/Mo/Ti-6Al-4V composite systems with different interfacial characteristics were charactered using scanning electron microscopy(SEM),X-ray energy dispersive spectrometer(EDS)and fiber push-out test.The effects of interfacial characteristics on the static tensile properties,fatigue life and fatigue crack growth rate of the composites were investigated.The fatigue damage initiation way and evolution process of the composites were investigated by means of the interrupted fatigue tests and SEM analysis.Furthermore,the effects of interfacial characteristics on fatigue crack propagation behavior of the composites were illustrated.SEM and EDS interface analysis results indicate that the interface phase sequence of the as-fabricated SiC_f/C/Ti-6Al-4V and SiC_f/C/Mo/Ti-6Al-4Vcomposites are SiC?C coating?TiC?Ti-6Al-4V and SiC?C coating?Mo coating?TiC??p+??Ti-6Al-4V,respectively.With the increase of the heat treatment time,the interface reaction of SiC_f/C/Ti-6Al-4V composites became more and more serious,which leads to significantly coarsening of a brittle TiC interfacial reaction layer and even the formation of some microporosities.As the heat treatment time is increased,the matrix of 750 °C/100 h heat-treated SiC_f/C/Mo/Ti-6Al-4V composite nearby the fiber is continuously ?-stabilized as a result of Mo element in the Mo coating diffusing into the Ti-6Al-4V matrix.As a result,the interface phase sequence of the 750 °C/100 h as-heated SiC_f/C/Ti-6Al-4Vcomposite is SiC?C coating?TiC????p+??Ti-6Al-4V.Mo coating consumption rate is greater than the rate of C coating consumption in the heat-treated composites.Furthermore,the introduction of Mo coating not only reduces the depletion rate of the C coating in the composites,but also avoids the decrease in the in-situ strength of SiC fiber in the heat-treated compositesThe push-out test results show that the interfacial shear strength of the as-fabricated SiC_f/C/Ti-6Al-4V and SiC_f/C/Mo/Ti-6Al-4V composites is 115.7 MPa and 129.3 MPa,respectively.As the heat treatment time increases,the corresponding interfacial shear strength of the SiC_f/C/Mo/Ti-6Al-4V composites sharply increases.However,the interfacial shear strength of the SiC_f/C/Mo/Ti-6Al-4V composites is slightly higher.The room temperature tensile test results indicate that the Young's moduli of the as-fabricated and heat-treated SiC_f/C/Ti-6Al-4V and SiC_f/C/Mo/Ti-6Al-4V composites are basically the same.The tensile strength and elongation of the SiC_f/C/Mo/Ti-6Al-4V composites are higher than those of the same processing SiC_f/C/Ti-6Al-4V composites.As the heat treatment time is increased,the tensile strength and elongation of the SiC_f/C/Ti-6Al-4V composites reduce gradually.However,the tensile strength and elongation of the SiC_f/C/Mo/Ti-6Al-4V composites are almost the same.This phenomenon agrees with the change in the SiC fiber in-situ strength in the composites with the increase of the heat treatment time.Fractographic analysis shows that the fatigue fracture surfaces of Chinese SiC reinforced Ti-6Al-4V composites consist of a fatigued region and a fast fracture region.The fraction of the fatigued region with respect to the total fracture surface decreases with the increase of applied maximum stress.Fatigue cracks mainly initiate at the machined defects of specimen surface such as the damaged fiber or micro-notch.Initiation of matrix fatigue cracks was occasionally observed within the bulk of the present composites such as the fiber/matrix interface and fiber touching locations.The fatigue test results show that the fatigue life of the as-fabricated SiC_f/C/Ti-6Al-4V composite decreases substantially in proportion to the increase in maximum applied stress.In the medium and high life range,the relationship between the maximum applied stress and cycles to failure of the as-fabricated SiC_f/C/Ti-6Al-4V composite in the semi-logarithmic system could be fitted as a linear equation: Smax/?=1.381-0.152×lgNf.Whether higher cyclic stress(Smax= 900 MPa)or low cyclic stress(Smax=600 MPa),the fatigue life of the SiC_f/C/Ti-6Al-4V composites with the increase of the heat treatment time sharply reduces.However,the fatigue life of the SiC_f/C/Mo/Ti-6Al-4V composites with the increase of the heat treatment time is gradually improved.The fatigue damage initiation way and the evolution process of the as-fabricated SiC_f/C/Ti-6Al-4V composite are closely related to the applied cyclic stress levels.At high cyclic stress level(Smax=1000 MPa),fiber cracking is the major fatigue damage initiation modes.After fibers fracture,fiber cracks and matrix cracks nearby the fractured fibers begin to connect and form a macroscopic propagating crack.At intermediate cyclic stress level(Smax=800 MPa),matrix crack initiation and propagation are the major modes of fatigue damage.Matrix cracks initiated from the ground surface edge or the cracked fiber/matrix interfacial reaction layer near the specimen edge propagate almost perpendicularly to the loading axis and most of the fibers remain intact and bridge the cracks.At low cyclic stress level(Smax=600 MPa),only partial interface debonding can be observed both between the C coating layer and the interface reaction layer and within C coating layer after the cyclic loading up to 106 cycles.The fatigue crack propagation behaviors of Chinese SiC fiber-reinforced Ti-6Al-4V composites are closely relative with the interfacial characteristics.Fatigue crack deflection and fiber bridging matrix fatigue crack in the 750 °C/100 h heat-treated SiC_f/C/Ti-6Al-4V composite during fatigue loading are hardly obvious because of higher interfacial shear strength,thick and brittle TiC interfacial reaction zone,and lower in-situ strength of fiber in this composite.However,the above phenomenon all occurred in the other Chinese fiber-reinforced composites.The ultimate tensile strength,elongation,fatigue life and fatigue crack growth resistance of SiC fiber-reinforced Ti-6Al-4V composites can be effectively improved by the introduction of the C/Mo double coatings combined with 750 °C heat treatment in comparison with C single coating.