Fiber Reinforced Titanium Matrix Composites Research Articles

Editorial

Application of diffusion barriers in composite materials is discussed from the point of view of thermodynamic and kinetic stability of the reinforcement/matrix interface. Two types of interface protective coating, i.e. inert diffusion barriers and reactive diffusion barriers can be distinguished. The application of one coating or the other depends on the type of damage the material suffers as a result of the interface reactions. A double-coating concept for SiC fiber-reinforced titanium matrix composites (TMC) is discussed from the viewpoint of this classification of diffusion barriers. Application of the C/TiB2 coating system for SiC fiber-reinforced TMC is considered in detail, and some approaches to improve the performance of this coating were suggested and examined experimentally with TiB2 coatings produced by magnetron sputtering deposition from a TiB2 target. The effect of the composition and the microstructure on barrier properties of TiB2 layers was examined. A crystalline microstructure with relatively large grains and of stoichiometric composition was found to be optimal for TiB2 protective coatings in SiC fiber-reinforced TMCs. A strong impact of layer stoichiometry on the diffusion of metal atoms through a crystalline TiB2 layer was found. However, stoichiometry did not significantly affect the diffusion of carbon. Crystalline TiB2 of proper stoichiometry can provide excellent protection for carbon coated (SCS6) SiC fibers in titanium aluminide based metal matrix composites (MMC).

Fatigue in selectively fiber-reinforced titanium matrix composites

Many applications of the Ti alloy matrix composites (TMCs) reinforced with SiC fibers are expected to use the selective reinforcement concept in order to optimize the processing and increase the cost-effectiveness. In this work, unnotched fatigue behavior of a Ti-6Al-4V matrix selectively reinforced with SCS-6 SiC fibers has been examined. Experiments have been conducted on two different model panels. Results show that the fatigue life of the selectively reinforced composites is far inferior to that of the all-TMC panel. The fatigue life decreases with the decreasing effective fiber volume fraction. Suppression of multiple matrix cracking in the selectively reinforced panels was identified as the reason for their lack of fatigue resistance. Fatigue endurance limit as a function of the clad thickness was calculated using the modified Smith-Watson-Topper (SWT) parameter and the effective fiber volume fraction approach. The regime over which multiple matrix cracking occurs is identified using the bridging fiber fracture criterion. A fatigue failure map for the selectively reinforced TMCs is constructed on the basis of the observed damage mechanisms. Possible applications of such maps are discussed.

Fatigue crack initiation and multiplication of unnotched titanium matrix composites

Fatigue crack initiation and multiplication of the unnotched SCS-6 silicon carbide fiber-reinforced titanium matrix composites with different matrix and interfacial properties have been investigated experimentally and analytically. Ti–15V–3Al, Ti–6Al–4V, and Ti–22Al–23Nb were chosen as matrix materials. The initiation and propagation of each individual matrix crack as a function of fatigue cycles and applied stress levels were carefully monitored. The statistical distribution of crack growth rates in each composite has been constructed and analyzed. The evolution of normalized matrix crack density and stiffness reduction of these composites under fatigue loading also has been characterized. A modified shear-lag model, coupled with the strain-life equation and a fiber bridging model were used to predict the fatigue crack initiation life, matrix crack growth rate, normalized matrix crack density, and residual stiffness of the composites. The predicted fatigue properties correlated well with experimental results.

Transverse tensile behavior of SiC-fiber/ Ti–6Al–4V composites—1. experimental results

Under transverse tension (loads normal to the fiber axis), fiber/matrix debonding in fiber-reinforced titanium-matrix composites results in reductions in specimen stiffness and ultimate strength, and increased loading of the matrix. The interface structure and chemistry have a strong influence on the debond stress and therefore contribute significantly to the transverse composite behavior. In the present study a cruciform specimen geometry was employed over a range of specimen thicknesses to investigate the remote stress levels where debonding initiates in Ti–6Al–4V composites containing SCS-6 (C+Si coating), SCS-0 (no coating), and Trimarc 1 (carbon coating) SiC fibers. The different surface conditions of the fibers resulted in a range of remote interface debond stresses with SCS-0 being the strongest and Trimarc 1 being the weakest. Multiple-fiber, single-ply specimens of the three different fibers revealed that the remote debond stress did not vary for SCS-6 and Trimarc 1 compared to single-fiber tests, but were slightly lower for SCS-0. Part 2 of this study presents analytical and numerical modeling studies of the local stress state at the interface in relation to the applied stress, and discusses them with regard to these experimental results.

Model-based optimization of consolidation processing

The performance of fiber reinforced titanium matrix composites (TMC) made by consolidation of spray deposited monotapes is strongly influenced by the processing conditions used. This high temperature consolidation step must simultaneously increase the relative density while minimizing fiber microbending/fracture and the interfacial reaction product layers at the fiber–matrix interface. These three microstructural variables have conflicting dependencies upon the consolidation process variables (temperature, pressure and time), and it has been difficult to experimentally identify process pathways that lead to composites of acceptable quality (where the fiber damage and the reaction layer thickness are kept below some bounds, while matrix porosity is eliminated). Here, models for predicting the microstructure's dependence upon process conditions (i.e. the time varying temperature and pressure) are combined with consolidation equipment dynamics to simulate the microstructure evolution. We then introduce the idea of process failure surfaces and show how a model predictive control algorithm, is able to design `locally' optimal process cycles that minimize fiber damage, reaction product layer thickness and porosity. As an example, we explore the planning of process schedules that process failure surfaces for several TMC systems.

Model-based simulation of the consolidation processing of metal coated fibers

The consolidation of metalized ceramic fibers by hot isostatic pressing (HIP) offers an attractive route for the manufacture of continuous fiber reinforced titanium matrix composites (TMCs). The performance of this class of composites is controlled by the fiber/matrix properties and by the composite's relative density, the grain size of the matrix alloy and the interfacial reaction layer thickness at the fiber–matrix interface after processing. Recently developed process models that predict the evolution of these microstructural attributes during consolidation processing are summarized and combined with consolidation equipment dynamics to simulate the microstructural evolution of PVD Ti–6Al–4V coated Sigma 1240 (SiC) monofilaments during HIP. For this system, diffusion accommodated grain sliding is the dominant densification mechanism for process cycles that preserve the refined grain size of the PVD metal coating.

Application of diffusion barriers in composite materials

Application of diffusion barriers in composite materials is discussed from the point of view of thermodynamic and kinetic stability of the reinforcement/matrix interface. Two types of interface protective coating, i.e. inert diffusion barriers and reactive diffusion barriers can be distinguished. The application of one coating or the other depends on the type of damage the material suffers as a result of the interface reactions. A double-coating concept for SiC fiber-reinforced titanium matrix composites (TMC) is discussed from the viewpoint of this classification of diffusion barriers. Application of the C/TiB 2 coating system for SiC fiber-reinforced TMC is considered in detail, and some approaches to improve the performance of this coating were suggested and examined experimentally with TiB 2 coatings produced by magnetron sputtering deposition from a TiB 2 target. The effect of the composition and the microstructure on barrier properties of TiB 2 layers was examined. A crystalline microstructure with relatively large grains and of stoichiometric composition was found to be optimal for TiB 2 protective coatings in SiC fiber-reinforced TMCs. A strong impact of layer stoichiometry on the diffusion of metal atoms through a crystalline TiB 2 layer was found. However, stoichiometry did not significantly affect the diffusion of carbon. Crystalline TiB 2 of proper stoichiometry can provide excellent protection for carbon coated (SCS6) SiC fibers in titanium aluminide based metal matrix composites (MMC).

EFFECT OF MATRIX ALLOY ON LONGITUDINAL TENSILE BEHAVIOUR OF FIBRE REINFORCED TITANIUM MATRIX COMPOSITES

Fiber reinforced titanium matrix composites (Ti MMCs) are of great interest for use in aeronautical gas-turbine engines because of their improved specific strength and stiffness and elevated temperature performance compared to monolithic Ti alloys. The Defence Evaluation and Research Agency is studying Ti MMCs for use in gas-turbine compressor discs. This report investigates the effect of the matrix alloy on the longitudinal tensile behavior of Ti MMCs reinforced with SM1140+ SiC fiber.

An investigation of the effects of interfacial microstructure on the fatigue behavior of a four-ply [75]4 continuous silicon carbide (SCS-6) fiber-reinforced titanium matrix composite

The effects of interfacial microstructure/thickness on the strength and fatigue behavior of a model four-ply [75]4 Ti-15V-3Al-3Cr-3Sn/SiC (SCS-6) composite are examined in this article. Interfacial microstructure was controlled by annealing at 815 °C for 10, 50, or 100 hours. The reaction layer and coating thickness were observed to increase with increasing annealing duration. Damage initiation/propagation mechanisms were examined in as-received material and composites annealed at 815 °C for 10 and 100 hours. Fatigue behavior was observed to be dependent upon the stress amplitude. At high stress amplitudes, the failure was dominated by overload phenomena. However, at all stress levels, fatigue crack initiation occurred by early debonding and matrix deformation by stress-induced precipitation. This was followed by matrix crack growth and fiber fracture prior to the onset of catastrophic failure. Matrix shear failure modes were also observed on the fracture surfaces in addition to fatigue striations in the matrix. Correlations were also established between the observed damage modes and acoustic emission signals that were detected under monotonic and cyclic loading conditions.

Interface effects on crack deflection and bridging during fatigue crack growth of titanium matrix composites

The effect of the interface on the crack deflection and crack bridging behavior of continuous fiber-reinforced titanium matrix composites has been investigated using three interfaces with significantly different mechanical characteristics. Each of these composites exhibited stress ranges in which fiber bridging was present and stress ranges in which stable fiber bridging was not present. The fatigue crack growth rate for all composites, even for the ones that did not exhibit fiber bridging was significantly below that of the matrix. This phenomenon, believed to be an effect of elastic crack shielding, was most significant for composites with the strongest interfacial bond. Interface failure ahead of the crack tip and its influence on the local stress intensity factor is believed to be responsible for the decrease in the shielding effect of low strength interfaces. Interface debonding was observed in all three composites, and damage to the interface ahead of the crack tip was seen in two of the three composites. A stress-based criterion for predicting debonding appeared to effectively explain the crack deflection behavior for the three composites. Evidence of crack deflection even for the strongest interface suggests that there is scope to increase the interface bond strength in SiC/Ti-alloy system for improved transverse properties without compromising the fatigue life.

Materials characterization of silicon carbide reinforced titanium (Ti/SCS-6) metal matrix composites. II. Theoretical modelling of fatigue behaviour : Chiang, K.T., Loh, D.H., Liaw, P.K. and Diaz, E.S. Metall. Mater. Trans. A. (Dec. 1995) 26A (12), 3249–3256

Under transverse tension (loads normal to the fiber axis), fiber/matrix debonding in fiber-reinforced titanium-matrix composites results in reductions in specimen stiffness and ultimate strength, and increased loading of the matrix. The interface structure and chemistry have a strong influence on the debond stress and therefore contribute significantly to the transverse composite behavior. In the present study a cruciform specimen geometry was employed over a range of specimen thicknesses to investigate the remote stress levels where debonding initiates in Ti–6Al–4V composites containing SCS-6 (C+Si coating), SCS-0 (no coating), and Trimarc 1 (carbon coating) SiC fibers. The different surface conditions of the fibers resulted in a range of remote interface debond stresses with SCS-0 being the strongest and Trimarc 1 being the weakest. Multiple-fiber, single-ply specimens of the three different fibers revealed that the remote debond stress did not vary for SCS-6 and Trimarc 1 compared to single-fiber tests, but were slightly lower for SCS-0. Part 2 of this study presents analytical and numerical modeling studies of the local stress state at the interface in relation to the applied stress, and discusses them with regard to these experimental results.

Infrared transient-liquid-phase joining of SCS-6/β21S titanium matrix composite

Fiber-reinforced titanium matrix composites (TMCs) are among the advanced materials being considered for use in the aerospace industry due to their light weight, high strength, and high modulus. A rapid infrared joining process has been developed for the joining of composites and advanced materials. Rapid infrared joining has been shown not to have many of the problems associated with conventional joining methods. Two models were utilized to predict the joint evolution and fiber reaction zone growth. TMC, 16-ply SCS-6/{beta}21S, has been successfully joined with total processing times of under 2 min utilizing the rapid infrared joining technique. The process utilizes a 50 C/sec ramping rate, 17-{micro}m Ti-15Cu-15Ni wt % filler material between the faying surfaces; a joining temperature of 1,100 C; and 120 sec of time to join the composite material. Joint shear strength testing of the rapid infrared joints at temperatures as high as 800 C has revealed no joint failures. Also, due to the rapid cooling of the process, no poststabilization of the matrix material is necessary to prevent the formation of a brittle omega phase during subsequent use of the TMC at intermediate temperatures, 270 to 430 C, for up to 20 h.

A method for measuring residual strains in fiber-reinforced titanium matrix composites

A new method for measuring the residual fiber strains in fiber-reinforced titanium matrix composites has been developed. The method involves selectively etching the matrix over a prescribed length of composite and subsequently measuring the extension of the relaxed fibers relative to neighboring fibers that are still embedded within the matrix material. The extensions are measured using confocal microscopy. The method is demonstrated on three unidirectionally reinforced composites with varying fiber volume fractions. The effects of specimen tilt and fiber splaying following dissolution on the measured fiber extensions are analyzed. The residual fiber strains are rationalized on the basis of the thermoelastic properties of the constituents through a concentric cylinder model. The current method can be applied to other metal matrix composites reinforced with large diameter (monofilament) fibers, provided the matrix can be selectively etched.

Effects of cladding on the tensile properties of titanium matrix composites

The effects of monolithic titanium cladding on the tensile properties of fiber-reinforced titanium matrix composites (TMCs) have been studied. The work is motivated by the realization that most TMC components currently under development will be clad with monolithic titanium alloys in order to prevent damage to the fibers during component manufacture and in service. The focus has been on the elastic modulus, the yield strength, and the failure stress and strain. Experiments have been conducted on panels with two different clad thicknesses as well as a panel without cladding. The experimental results have been compared with predictions of models based on an effective (average) fiber volume fraction. The correlations between the experiments and the model predictions indicate that these properties are insensitive to the spatial distribution of fibers.

Finite-element analysis of selectively SiC fibre-reinforced titanium composites

Finite element analysis has been applied to selectively SiC fibre reinforced titanium matrix composites. The influence of thermal residual stress on the behaviour of clad titanium fibre composites with initial unbridged sharp cracks under external loading has been investigated. Through-thickness stress distributions have been obtained for the composites as a function of loading configuration, fibre volume fraction, cladding thickness and composite thickness. It has enabled a quantitative understanding of the premature delamination of clad titanium fibre composites ahead of mode I crack that has been observed experimentally. Potential solutions to the problem have been suggested based on finite element predictions of the variation of tensile through-thickness stress close to the transition boundary between cladding material and composite as a function of fibre volume fraction and, especially, with thickness of the cladding layers.

Infrared infiltration and properties of SCS-6/Ti alloy composites

Liquid infiltration processes are seldom used in the fabrication of titanium matrix composites primarily because of severe interfacial reactions during processing. To minimize fibre-matrix interfacial reactions, a rapid infrared manufacturing (RIM) process has been developed. In this study, SCS-6 fibre reinforced titanium matrix composites were fabricated by the RIM process. Experimental results indicate that composites prepared by the RIM process exhibit small interfacial reaction zones and superior mechanical properties compared to diffusion bonded composites. Interfacial reactions during processing in this composite system have been investigated. Microscopic analyses showed that the carbon-rich coating present on SCS-6 fibres partially dissolved in the alloy during infiltration without forming a continuous reaction product. A physical model has been proposed to explain the dissolution mechanism of the carbon-rich coating. The dissolution rate of the coating at the processing temperature of 1300 °C was calculated to be 1.4×10−6 cm s−1.

Fatigue life prediction of fiber-reinforced titanium matrix composites

A micromechanical model was developed to predict the fatigue life of SiC fiber-reinforced titanium matrix composites. The propagation of matrix cracks was modeled by calculating the stress concentration factor at the matrix crack tip in a fiber bridged center matrix crack. The residual stiffness was then predicted by using the partial-crack shear-lag model, and the post-fatigued tensile strength was predicted by using the load carrying capacity of fibers proposed previously for brittle matrix composites. Finally, the catastrophic failure and fatigue life of the composite was determined by Monte Carlo simulation. An integrated computer simulation code was developed to simulate the evolution of fatigue damage, degradation of mechanical properties, and to predict fatigue life. The predicted matrix crack propagation rates, residual stiffness, residual tensile strength, and fatigue life were also correlated with experimental results.

High temperature deformation and fracture of a fiber reinforced titanium matrix composite

The longitudinal tensile properties of a SiC fiber-reinforced Ti alloy have been investigated over the temperature range of 20–600°C. Experiments have been conducted at both constant strain rate and constant stress. The experimental results have been compared with predictions of models based on fiber fragmentation, matrix flow and creep, and load transfer via interfacial friction. The models predict trends that are similar to the experimental ones, though they generally overestimate the tensile strength and ductility and the creep rupture time. The possible origins of such discrepancies are briefly discussed.

Interface Effects on the Tensile and Fatigue Crack Growth Behavior of Fiber-Reinforced Metal Matrix Composites

ABSTRACTThe effects of the interface on the tension and fatigue crack growth behavior of fiber-reinforced titanium matrix composites were studied using single-ply and single-fiber SiC/Ti-6Al-4V mini-composites with different SiC fibers and coatings. Attention was focused on fiber failure mechanisms in the absence of a matrix crack (tension loading), and on fatigue crack deceleration mechanisms. In contrast to established models of tensile failure, local load sharing behavior was observed for very weak interfaces, with plasticity playing a major role in enhancing stress concentration effects. The approach included statistical evaluation of fiber breaks, their locations, and comparisons between single-fiber and single-ply composites. Under fatigue loading, crack deflection behavior was found to be consistent with a strength based interface debonding model. Crack shielding through modulus mismatch was found to have a significant effect on crack growth rates, independent of bridging conditions, with a stronger interface performing better under such conditions. The ramifications of the different mechanisms on interface optimization for longitudinal and transverse properties of composites are indicated.

An evaluation of fiber-reinforced titanium matrix composites for advanced high-temperature aerospace applications

The current capabilities of continuous silicon-carbide fiber-reinforced titanium matrix composites (TMCs) are reviewed with respect to application needs and compared to the capabilities of conventional high-temperature monolithic alloys and aluminides. In particular, the properties of a first-generation titanium aluminide composite, SCS-6/Ti-24Al-11Nb, and a second-generation metastable beta alloy composite, SCS-6/TIMETAL 21S, are compared with the nickel-base superalloy IN100, the high-temperature titanium alloy Ti-1100, and a relatively new titanium aluminide alloy. Emphasis is given to life-limiting cyclic and monotonic properties and to the influence of time-dependent deformation and environmental effects on these properties. The composite materials offer a wide range of performance capabilities, depending on laminate architecture. In many instances, unidirectional composites exhibit outstanding properties, although the same materials loaded transverse to the fiber direction typically exhibit very poor properties, primarily due to the weak fiber/matrix interface. Depending on the specific mechanical property under consideration, composite cross-ply laminates often show no improvement over the capability of conventional monolithic materials. Thus, it is essential that these composite materials be tailored to achieve a balance of properties suitable to the specific application needs if these materials are to be attractive candidates to replace more conventional materials.