Degradation of residual strength in SCS-6/Ti-15-3 Due to fully reversed fatigue

Little attention has been given to residual strength degradation in titanium matrix composites (TMCs) after exposure to fatigue loading. To address this problem, fatigue tests on SCS-6/Ti-15-3 were performed to investigate the fatigue life and residual strength behavior of TMCs with different fiber volume fractions. Results indicate that fiber volume fraction seems to have an effect on both of these quantities. Lower fiber percentages result in a material where the characteristics of the matrix, such as hardening or cracking, play a much larger role in the composite response. Fatigue lives were not affected by fiber volume fraction at higher strain ranges, but lower fiber volume fractions resulted in shorter fatigue lives at lower strain values. Also, a slight increase in residual strength occurred up to 75 pct of fatigue life, for the lower-fiber volume fraction material. Despite these distinctions between specimens with different fiber contents, all specimens tested retained the majority of their strength prior to failure. This article is based on a presentation made in the symposium “Fatigue and Creep of Composite Materials” presented at the TMS Fall Meeting in Indianapolis, Indiana, September 14–18, 1997, under the auspices of the TMS/ASM Composite Materials Committee.     

Strength degradation of SiC fiber during manufacture of titanium matrix composites by plasma spraying and hot pressing

Titanium matrix composites (TMCs) reinforced with Sigma 1140+ SiC fiber have been manufactured by a combination of low pressure plasma spraying (LPPS spray/wind) and simultaneous fiber winding, followed by vacuum hot pressing (VHP). Fiber damage during TMC manufacture has been evaluated by measuring fiber tensile strength after fiber extraction from the TMCs at various processing stages, followed by fitting of these data to a Weibull distribution function. The LPPS spray/wind processing caused a decrease in mean fiber strength and Weibull modulus in comparison with as-received fibers. A number of fiber surface flaws, primarily in the outer C layer of the fiber, formed as a result of mechanical impact of poorly melted particles from the plasma spray. Coarse feedstock powders promoted an increase in the population of fiber surface flaws, leading to significant reduction in fiber strength. The VHP consolidation promoted further development of fiber surface flaws by fiber bending and stress localization because of nonuniform matrix shrinkage, resulting in further degradation in fiber strength. In the extreme case of fibers touching, the stress concentration on the fibers was sufficient to cause fiber cracking. Fractographic studies revealed that low strength fibers failed by surface flaw induced failure and contained a large fracture mirror zone. Compared with the more widely investigated foil-fiber-foil route to manufacture TMCs, LPPS/VHP resulted in less degradation in fiber strength for Sigma 1140+ fiber. Preliminary results for Textron SCS-6 fiber indicated a much greater tolerance to LPPS/VHP damage.     

Materials characterization of silicon carbide reinforced titanium (Ti/SCS-6) metal matrix composites: Part I. Tensile and fatigue behavior

Flexural fatigue behavior was investigated on titanium (Ti-15V-3Cr) metal matrix composites reinforced with cross-ply, continuous silicon carbide (SiC) fibers. The titanium composites had an eightply (0, 90, +45, -45 deg) symmetric layup. Fatigue life was found to be sensitive to fiber layup sequence. Increasing the test temperature from 24 °C to 427 °C decreased fatigue life. Interface debonding and matrix and fiber fracture were characteristic of tensile behavior regardless of test temperature. In the tensile fracture process, interface debonding between SiC and the graphite coating and between the graphite coating and the carbon core could occur. A greater amount of coating degradation at 427 °C than at 24 °C reduced the Ti/SiC interface bonding integrity, which resulted in lower tensile properties at 427 °C. During tensile testing, a crack could initiate from the debonded Ti/SiC interface and extend to the debonded interface of the neighboring fiber. The crack tended to propagate through the matrix and the interface. Dimpled fracture was the prime mode of matrix fracture. During fatigue testing, four stages of flexural deflection behavior were observed. The deflection at stage I increased slightly with fatigue cycling, while that at stage II increased significantly with cycling. Interestingly, the deflection at stage III increased negligibly with fatigue cycling. Stage IV was associated with final failure, and the deflection increased abruptly. Interface debonding, matrix cracking, and fiber bridging were identified as the prime modes of fatigue mechanisms. To a lesser extent, fiber fracture was observed during fatigue. However, fiber fracture was believed to occur near the final stage of fatigue failure. In fatigued specimens, facet-type fracture appearance was characteristic of matrix fracture morphology. Theoretical modeling of the fatigue behavior of Ti/SCS-6 composites is presented in Part II of this series of articles. This article is based on a presentation made in the symposium entitled “Creep and Fatigue in Metal Matrix Composites” at the 1994 TMS/ASM Spring meeting, held February 28–March 3, 1994, in San Francisco, California, under the auspices of the Joint TMS-SMD/ASM-MSD Composite Materials Committee.     

On the toughening of intermetallics with ductile fibers: Role of interfaces

The role of fiber debonding and sliding on the toughness of intermetallic composites reinforced with ductile fibers is examined. The toughness is shown to be a function of the matrix/fiber interface properties, residual stresses and the volume fraction, size and flow behavior of the fibers. Mechanical testing and in situ microstructural observations were carried out on a Ti-25at.%Ta-50at.%Al intermetallic matrix reinforced with W-3Re fibers. The fibers were coated with a thin oxide layer in order to induce debonding and prevent interdiffusion between the fiber and the matrix. The ductility, high strength and debond characteristics of coated tungsten-rhenium fibers promote a large increase in toughness. However, the mismatch in thermal expansion coefficients is the source of large residual tensile stresses in the matrix that induces spontaneous matrix cracking. Matrix cracking and composite toughness are examined as a function of the interfacial properties, residual stresses and properties of the fiber.     

Prediction of creep-rupture life of unidirectional titanium matrix composites subjected to transverse loading

Titanium matrix composites (TMCs) incorporating unidirectional fiber reinforcement are considered as enabling materials technology for advanced engines which require high specific strength and elevated temperature capability. The resistance of unidirectional TMCs to deformation under longitudinally applied sustained loading at elevated temperatures has been well documented. Many investigators have shown that the primary weakness of the unidirectional TMC is its susceptibility to failure under very low transverse loads, especially under sustained loading. Hence, a reliable model is required to predict the creep-rupture life of TMCs subjected to different transverse stress levels over a wide range of temperatures. In this article, we propose a model to predict the creep-rupture life of unidirectional TMC subjected to transverse loading based on the creep-rupture life of unidirectional TMC subjected to transverse loading based on the creep-rupture behavior of the corresponding fiberless matrix. The model assumes that during transverse loading, the effective load-carrying matrix ligament along a row of fibers controls the creep-rupture strength and the fibers do not contribute to the creep resistance of the composite. The proposed model was verified using data obtained from different TMC fabricated using three matrix compositions, which exhibited distinctly different types of creep behavior. The results show that the creep-rupture life of the transverse TMC decreases linearly with increasing ratio of the fiber diameter to the ply thickness. The creeprupture life is also predicted to be independent of fiber spacing along the length of the specimen.     

Design,analysis and application of an improved push-through test for the measurement of interface properties in composites

An improved fiber push-through test has been designed and used to obtain new information about interfaces in composites consisting of matrices of a Ti alloy and borosilicate glass, both reinforced with SiC fibers. Interpretation of these results is accomplished through an analysis of coupled debonding and push-through, followed by push-back. The sliding stress is found to vary with push-out distance and to be substantially reduced in the vicinity of a fatigue crack in the Ti matrix composite. These effects are attributed to asperity wear, matrix plasticity and fragmentation of the fiber coating around the debonded interface. Reseating effects on push-back have been demonstrated, but have been found to diminish as the relative fiber-matrix displacement increases. Fiber roughness has been identified as an important aspect of interface sliding.     

Aspects of fracture in particulate reinforced metal matrix composites

The tensile deformation and fracture behaviour of the aluminium alloy 6061 reinforced with SiC has been investigated. In the T4 temper plastic deformation occurs throughout the gauge length and the extent of SiC particle cracking increases with increasing strain. In the T6 temper strain becomes localised and particle cracking is more concentrated close to the fracture. The elastic modulus decreases with increasing particle damage and this allows a damage parameter to be identified. The fraction of SiC particles which fracture is less than 5%, and over most of the strain range the damage controlling the tensile ductility can be recovered, indicating that other factors, in addition to particle cracking are important in influencing tensile ductility. It is suggested that macroscopic fracture is initiated by the SiC particle clusters that are present in these composites as a result of the processing. The matrix within the clusters is subjected to high levels of triaxial stress due to elastic misfit and the constraints exerted on the matrix by the surrounding particles. Final fracture is then produced by crack propagation through the matrix between the clusters.     

Determination of residual stresses in thin sheet titanium aluminide composites

Residual stresses in Ti₃Al/SiC composites have been measured using two methods. The compressive residual stresses in the fibers were inferred from measurements of the change in their length when the matrix was entirely removed by etching. The stresses were found to vary substantially from fiber to fiber. The longitudinal and transverse stresses in the matrix were measured by X-ray diffraction. Repeated measurements were made as the outer layer of alloy was removed by electropolishing as far as the first row of fibers. In one composite of lower fiber volume fraction, the matrix stresses were thus found to be approximately uniform throughout the specimen. In a higher volume fraction material, on the other hand, the matrix stresses increased significantly with depth from the outer surface: the longitudinal matrix stresses among the fibers were found to be about 60 pct larger than they were on the specimen surface. The implications of these measurements for processing and reliability of thin sheet titanium aluminide composites are discussed.     

Fatigue crack growth resistance of unidirectional fiber-reinforced titanium metal-matrix composites under transverse loading

The transverse fatigue crack growth resistance of unidirectional 8 and 35 pct 1140+/Ti-6-4 fiber-reinforced composites has been investigated. It has been found that, at a low fiber volume fraction, the transverse fatigue crack growth resistance of metal-matrix composites (MMCs) is improved with respect to the monolithic matrix alloy. This occurs because “holes” from debonded interfaces can trap the crack and reduce the average fatigue crack growth rates by periodically increasing the effective crack-tip radius. However, an increase of fiber volume fraction from 8 to 35 pct decreases the fatigue crack growth resistance dramatically, due to the significant increase of the frequency of interaction and coalescence between the main crack, the debonded interfaces, and microcracks.     

Transverse strength of metal matrix composites reinforced with strongly bonded continuous fibers in regular arrangements

The composite limit flow stress for transverse loading of metal matrix composites reinforced with a regular array of uniform continuous fibers is calculated using the finite element method. The effects of volume fraction and matrix work hardening are investigated for fibers of circular cross section distributed in both sqyare and hexagonal arrangements. The hexagonal arrangement is seen to behave isotropically with respect to the limit stress, whereas the square arrangement of fibers results in a composite which is much stronger when loaded in the direction of nearest neighbors and weak when loaded at 45° to this direction. The interference of fibers with flow planes is seen to play an important role in the strengthening mechanism. The influence of matrix hardening as a strengthening mechanism in these composites increases with volume fraction due to increasing fiber interaction. The results for a power law hardening matrix are also applicable to the steady state creep for these composites. The influence of volume fraction on failure parameters in these composites is addressed. Large increases in the maximum values of hydrostatic tension, equivalent plastic stain, and tensile stress normal to the fiber-matrix interface are seen to accompany large increases in composite strength.     

Micromechanical modeling of unidirectional continuous sigma fiber-reinforced Ti-6Al-4V subjected to transverse tensile loading

A micromodeling analysis of unidirectionally reinforced Ti-6-4/SM1140+ composites subjected to transverse tensile loading has been performed using the finite-element method (FEM). The composite is assumed to the infinite and regular, with either hexagonal or rectangular arrays of fibers in an elastic-plastic matrix. Unit cells of these arrays are applied in this modeling analysis. Factors affecting transverse properties of the composites, such as thermal residual stresses caused by cooling from the composite processing temperature, fiber-matrix interface conditions, fiber volume fraction, fiber spacing, fiber packing, and test temperature are discussed. Predictions of stress-strain curves are compared with experimental results. A hexagonal fiber-packing model with a weak fiber-matrix interfacial strength predicts the transverse tensile behavior of the composite Ti-6-4/SM1140+ most accurately.     

12%SiCp/Al复合材料制备工艺及力学性能研究

对碳化硅颗粒进行表面氧化酸洗处理，采用粉末冶金加热挤压工艺制备了12％SiCp／Al（体积分数）复合材料。利用金相显微镜和电镜对微观组织进行了观测，拉伸试验测试复合材料的力学性能。试验结果表明：SiC颗粒在铝基体中分布比较均匀；T6热处理条件下12％SiCp／Al复合材料的屈服强度和抗拉强度分别约为472．4MPa、525．7MPa，伸长率为6．5％，弹性模量为92．7GPa。     

Effects of sic content on fatigue crack growth in

An experimental study has been conducted with the purpose of examining the fatigue crack growth characteristics of cast aluminum alloy matrix composites reinforced with different vol- ume fractions of silicon carbide particles. Particular attention has been paid to developing com- posite microstructures with similar matrix aging condition, precipitation, matrix strength, reinforcement particle size distribution, and interfacial characteristics but with different con- trolled amounts of reinforcement particles. Fatigue crack growth experiments have been con- ducted using constant stress amplitude methods with a fixed load ratio as well as constant K_max control involving a varying load ratio. The development of crack closure and the microscopic path of the crack through the composite microstructure are monitored optically and using the electron microscope in an attempt to examine the mechanisms of fatigue fracture. The results indicate that an increase in SiC content results in the suppression of striation formation in the ductile matrix. Although ductile matrix failure involving the formation of striations in the low SiC content composite or of void growth in the high SiC content composite is evident, the results also show that fracture of the reinforcement particles plays a significant role in dictating the rates of fatigue crack growth. Detailed quantitative analyses of the extent of particle fracture as a function of the reinforcement content have been performed to elucidate the mechanistic origins of fatigue resistance. The propensity of particle fracture increases with particle size and with the imposed value of stress intensity factor range. While discontinuously reinforced metal- matrix composites with predominantly matrix cracking are known to exhibit superior fatigue crack growth resistance as compared to the unreinforced matrix alloy, the tendency for particle fracture in the present set of experiments appears to engender fatigue fracture characteristics in the composite which are inferior to those seen in the unreinforced matrix material. Particle fracture also results in noticeable differences in the microscopic fracture path and causes a reduction in crack closure in the composites as compared to that in the matrix alloy. The results of this work are discussed in light of other related studies available in the literature in an attempt to develop a mechanistic perspective on fatigue crack growth resistance in metal-matrix composites.     

Fatigue behavior of SiC reinforced Ti(6AI-4V) at 650 °C

Axial, low cycle fatigue properties of 25 and 44 fiber vol pct SiC/Ti(6Al-4V) composites, measured at 650 °C, were compared with the fatigue properties of unreinforced Ti(6Al-4V) at the same temperature. A prior study of the fatigue behavior of this composite system at room temperature indicated that the SiC fiber reinforcement did not provide the anticipated improvement of fatigue resistance of this alloy. At 650 °C, the composite fatigue properties degraded somewhat from those at room temperature. However, these properties degraded more for the unreinforced matrix at 650 °C with the result that the composite fatigue strength was two to three times the fatigue strength of the matrix alloy. The reasons for this reversal are discussed in terms of crack initiation at broken fibers and residual matrix stresses.     

Cracking mechanisms in thermally cycled Ti-6AI-4V reinforced with SiC fibers

A titanium alloy (Ti-6A1-4V) reinforced with continuous SiC fibers (SCS-6) was thermally cycled between 200 ‡C and 700 ‡C in air and argon. The composite mechanical properties deteriorate with an increasing number of cycles in air because of matrix cracks emanating from the specimen surface. These cracks also give oxygen access to fibers, further resulting in fiber degradation. The following matrix cracking mechanisms are examined: (1) thermal fatigue by internal stresses resulting from the mismatch of thermal expansion between fibers and matrix, (2) matrix oxygen embrittlement, and (3) ratcheting from oxide accumulating within cracks. Matrix stresses are determined using an analytical model, considering stress relaxation by matrix creep and the temperature dependence of materials properties. Matrix fatigue from these cycli-cally varying stresses (mechanism (1)) cannot solely account for the observed crack depth; oxygen embrittlement of the crack tip (mechanism (2)) is concluded to be another necessary damage mechanism. Furthermore, an approximate solution for the stress intensity resulting from crack wedging by oxide formation (mechanism (3)) is given, which may be an operating mech-anism as well for long cracks. S.H. THOMIN, formerly Graduate Student, Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, MA     

Tensile and Fatigue Behavior of Aluminum Oxide Fiber Reinforced Magnesium Composites: Part I. Fiber Fraction and Orientation

The mechanical behavior of commercially pure magnesium reinforced with FP aluminum oxide fibers has been studied as a function of fiber fraction and orientation. Test specimens included material of two different volume fractions of fiber and four different fiber orientations. Axial properties were dependent on the fiber content and generally followed the rule of mixtures. Of the off-axis properties, only the elastic modulus exhibited a significant dependence on fiber content. Off-axis loading resulted in large reductions in both the tensile and fatigue properties. The reductions coincided with a change in fracture morphology from fracture across fibers during axial loading to fracture along the fiber direction for off-axis loading. A weak fiber/matrix interface was found to be responsible for the drop in tensile properties, and a combination of a weak matrix and a weak fiber/matrix interface were responsible for the reduced fatigue resistance.     

Effective elastic moduli of fiber-matrix interphases in high-temperature composites

This article describes a theoretical model and an experimental method for determination of interphasial elastic moduli in high-temperature composites. The interphasial moduli are calculated from the ultrasonically measured composite modulivia inversion of multiphase micromechanical models. Explicit equations are obtained for determination of interphasial stiffnesses for an interphase model with spring boundary conditions and multiphase fiber. The results are compared with the exact multiphase representation. The method was applied to ceramic and intermetallic matrix composites reinforced with SiC SCS-6 fibers. In both composites, the fiber-matrix interphases include approximately 3-μm-thick carbon-rich coatings on the outer surface of the SiC shell. Although the same fiber is used in both composite systems, experimental results indicate that the effective interphasial moduli in these two composite systems are very different. The interphasial moduli in intermetallic matrix composites are much greater than those in ceramic matrix composites. After taking the interphase microstructure into account, we found that the interphasial moduli measured for the intermetallic matrix composites are very close to the estimated bulk moduli of the pyrolytic carbon with SiC particle inclusions. Our analysis shows that the lower effective interphasial moduli in the reaction-bonded Si₃N₄ (RBSN) ceramic matrix composites are due to imperfect contact between the interphasial carbon and the porous matrix and to thermal tension forces which slightly unclamp the interphase. Thus, measured interphase effective moduli give information on the quality of mechanical contact between fiber and matrix. Possible errors in the interphasial moduli determined are analyzed and the results show that these errors are below 10 pct. In addition, the use of the measured interphasial moduli for assessment of interphasial damage and interphase reactions is discussed.     

Materials characterization of silicon carbide reinforced titanium (Ti/SCS-6) metal matrix composites: Part II. Theoretical modeling of fatigue behavior

Flexural fatigue behavior was investigated on titanium (Ti-15V-3Cr) metal matrix composites reinforced with cross-ply, continuous silicon carbide (SiC) fibers. The titanium composites had an eightply (0, 90, +45, -45 deg) symmetric layup. Mechanistic investigation of the fatigue behavior is presented in Part I of this series. In Part II, theoretical modeling of the fatigue behavior was performed using finite element techniques to predict the four stages of fatigue deflection behavior. On the basis of the mechanistic understanding, the fiber and matrix fracture sequence was simulated from ply to ply in finite element modeling. The predicted fatigue deflection behavior was found to be in good agreement with the experimental results. Furthermore, it has been shown that the matrix crack initiation starts in the 90 deg ply first, which is in agreement with the experimental observation. Under the same loading condition, the stress in the 90 deg ply of the transverse specimen is greater than that of the longitudinal specimen. This trend explains why the longitudinal specimen has a longer fatigue life than the transverse specimen, as observed in Part I. This article is based on a presentation made in the symposium entitled “Creep and Fatigue in Metal Matrix Composites” at the 1994 TMS/ASM Spring meeting, held February 28–March 3, 1994, in San Francisco, California, under the auspices of the Joint TSM/SMD/ASM-MDS Composite Materials Committee.     

离心铸造活塞用SiC颗粒增强铝基复合材料的制备及组织研究

采用机械搅拌法制备了体积分数为20％的SiC颗粒增强铝基复合材料。研究了材料的制备工艺以及SiC颗粒的预处理对微观组织的影响。     

In-situ observations of titanium metal-matrix composites under transverse tensile loading

The transverse stress-strain behavior of several titanium metal-matrix composites (TiMMCs) has been studied in-situ. Debonding of 1140+/Ti-6-4 composites occurs over a range of stresses. The sharpness of the first “knee” is affected by the fiber volume fraction and by the relative moduli of the matrix regions and the reinforced composite. It has been observed that debonding occurs mainly at the interface between two sublayers of carbon/carbon coatings in 1140+/Ti-6-4 composites and mainly at the interface between the carbon/reaction zone in the as-processed and peak-aged 35 pct SCS-6/Tiβ21s composites. At surface positions, this process starts at very low stresses (≥50 MPa) from the positions with sharp changes of curvatures (or undulations), voids, or debris at the periphery of the interface. Cracking of the outermost carbon sublayer and of the reaction zone in the 1140+/Ti-6-4 composites and the reaction zone in the SCS-6/Tiβ21s composites occurs during elastic deformation of the matrix. This has been directly observed in a field-emission gun (FEG)-scanning electron microscope (SEM) under incremental loading. Although these cracks are arrested and blunted by the matrix material, they cause local stresses and, thus, stimulate local plastic deformation of the matrix and subsequent development of a second knee on the stress-strain curve. The in-situ observations are discussed in terms of the effects of fiber volume fraction and fiber type on the loci and dynamic processes of interfacial debonding, cracking of carbon coatings and reaction zones, and plastic deformation of the matrix.