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.     

Multiple matrix cracking in a fiber-reinforced titanium matrix composite under high-cycle fatigue

An experimental study of multiple matrix cracking in a fiber-reinforced titanium alloy has been conducted. The focus has been on the effects of stress amplitude on the saturation crack density and the effects of crack density on hysteresis behavior. Comparisons have been made with predictions based on unit cell models, assuming the sliding resistance of the interface to be characterized by a constant interfacial shear stress. In addition, independent measurements of the sliding stress have been made using fiber pushout tests on both pristine and fatigued specimens. D.P. WALLS, Graduate Student, formerly with the Materials Department, University of California, Santa Barbara     

Off axis loading of silicon carbide fibers in a titanium aluminide matrix

Silicon carbide SCS-6 fibers in a Ti24Al + 11Nb matrix were subjected to off axis loading in a “thin-slice” pushout test, resulting in various combinations of shear, radial compression, and tension along the fibers as a function of orientation angle. The load necessary for debonding decreased as the orientation angle increased, whereas the average frictional sliding stress after 60 s of sliding remained relatively constant for orientation angles less than 30 deg. Analyses of the specimen bending stresses and of the contact stresses by finite element modelling and thin plate theory are presented.     

Effects of composite processing on the strength of sapphire fiber-reinforced composites

The current interest in tough, high-temperature materials has motivated fiber coating development for sapphire fiber-reinforced alumina composites. For this system, it has been demonstrated that the interfacial properties can be controlled with coatings which can be eliminated from the interface subsequent to composite consilidation. However, these fugitive coatings can contribute to the high temperature strength degradation of sapphire fibers. Such degredation,which compromises the composite strength and toughness, is the focus of the current investigation. It has been observed that, in some cases, by selecting appropriate composite processing conditions, such effects can be minimized. But overcoming fiber strength loss remains an important issue.     

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.     

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.     

烧结温度对反应烧结碳化硅组织与性能的影响

本文研究了1450、1550、1650℃不同烧结温度制备的反应烧结SiC材料的密度、硬度、抗弯强度、显微组织、显微硬度及断裂行为。结果表明:烧结温度对材料密度影响较小。低温反应烧结的SiC晶粒的晶体结构不够完整,存在亚晶界等缺陷,晶粒强度较低,烧结材料的硬度和抗弯强度较低。高温反应烧结的SiC晶粒的晶体结构完整性增加,晶粒强度较高,烧结材料的硬度和抗弯强度较高。因此为了提高反应烧结碳化硅的力学性能,应该适当提高烧结温度或延长烧结时间。     

The effect of matrix microstructure on the tensile and fatigue behavior of SiC particle-reinforced 2080 Al matrix composites

The effect of matrix microstructure on the stress-controlled fatigue behavior of a 2080 Al alloy reinforced with 30 pct SiC particles was investigated. A thermomechanical heat treatment (T8) produced a fine and homogeneous distribution of S′ precipitates, while a thermal heat treatment (T6) resulted in coarser and inhomogeneously distributed S′ precipitates. The cyclic and monotonic strength, as well as the cyclic stress-strain response, were found to be significantly affected by the microstructure of the matrix. Because of the finer and more-closely spaced precipitates, the composite given the T8 treatment exhibited higher yield strengths than the T6 materials. Despite its lower yield strength, the T6 matrix composite exhibited higher fatigue resistance than the T8 matrix composite. The cyclic deformation behavior of the composites is compared to monotonic deformation behavior and is explained in terms of microstructural instabilities that cause cyclic hardening or softening. The effect of precipitate spacing and size has a significant effect on fatigue behavior and is discussed. The interactive role of matrix strength and SiC reinforcement on stress within “rogue” inclusions was quantified using a finite-element analysis (FEA) unit-cell model.     

Cast microstructure and fatigue behavior of a high strength aluminum alloy (KO-1)

Microstructure and fatigue behavior were studied in sand-cast and end-chilled KO-1 high strength aluminum alloy. Secondary dendrite arm spacing and volume percent microporosity increase with distance from the chill, whereas volume percent inter dendritic nonequilibrium secondary phase decreases. Solution kinetics of the secondary phase depend on the dimensionless parameter [Dt/L ²], where:D is the diffusivity of copper in this alloy at the solution temperature,L is half the dendrite arm spacing, andt the solutionizing time. The fatigue life of the alloy at room temperature was measured in reversed bending on unnotched specimens at stress levels of 17,300 and 19,000 psi and was found to decrease with increasing distance from the chill. Unnotched solutionized specimens exhibited a longer fatigue life than as-cast specimens, even though crack growth studies showed that cracks grew more rapidly in the solutionized material. This would be attributed to a delay in crack initiation resulting from the decrease in the amount of microporosity and the rounding of micropores during heat treatment. Micropores and inclusions acted as sources of stress concentration for fatigue crack initiation. Cracks then usually grew transgranularly.     

Effects of interfacial strength on fatigue crack growth in a fiberreinforced Ti-alloy composite

The fatigue crack growth behavior of a Ti-6A1-4V composite with boron fibers was previously studied in the as-received and thermally exposed conditions. Fracture strengths of the composite, fiber, and interface were characterized together with fatigue crack growth rates and failure mechanisms. Utilizing the matrix and fiber properties as input, a recently proposed model was exercised to elucidate the effects of interfacial strength on crack growth rates in the composite. Comparison of experimental results with model calculations revealed that a weak fiber/matrix interface combined with a strong, high-modulus fiber led to interface debonding and crack deflection and produced the beneficial effects of increased threshold and reduced transverse crack growth rates. This paper is based on a presentation made in the symposium “Interfaces and Surfaces of Titanium Materials” presented at the 1988 TMS/AIME fall meeting in Chicago, IL, September 25–29, 1988, under the auspices of the TMS Titanium Committee.     

Creep rupture of a silicon carbide reinforced aluminum composite

The microstructure, texture, and whisker orientations in 6061 Al-20 wt pct SiC whisker composites have been examined using transmission electron microscopy and X-ray diffraction. Tension creep tests of the composite material have also been conducted in the temperature range 505 to 644 K (450 to 700 F). The steady state creep rate of the composite depends strongly on the temperature and applied stress. The stress exponent for the steady state creep rate of the composite is approximately 20.5 and remains essentially constant within the range of test temperatures. The activation energy is calculated to be 390 kJ/mol, nearly three times as high as the activation energy for self-diffusion of aluminum. No threshold stress was observed. Fracture surface examination using scanning electron microscopy shows that the composite fails by coalescence of voids in the aluminum matrix which originate at the aluminum-SiC interface. It is demonstrated that SiC paniculate composites are less creep resistant than SiC whisker composites.     

The effect of microstructure on the fatigue crack growth behavior of Age-Hardened high strength steels in a corrosive environment

Fatigue crack growth rates have been measured in laboratory air and in a 3.5 pct NaCl aqueous solution under cathodic polarization of -0.85 V(Ag/AgCl reference electrode) for various agehardened, high strength steels. At low ΔK values in air, the fatigue crack growth resistance of the underaged condition was improved compared to the overaged condition. This improvement can be explained with increased crack closure and slip reversibility. The fatigue crack growth resistance of material containing both coherent precipitates and incoherent precipitates with the matrix was similar to that of the material including only incoherent precipitates because of the restriction of inhomogeneous deformation by the incoherent precipitates. Corrosion fatigue crack growth resistance in the aqueous solution was dependent on aging conditions. The environmental sensitivity was greater in the underaged materials owing to hydrogen embrittlement. The environmental sensitivity of underaged materials containing coherent precipitates with the matrix was improved by the coexistence of incoherent precipitates. The decreased amount of strain to fracture due to hydrogen diffusing into plastic zone led to decreased crack closure.     

Physicomechanical and fatigue properties of titanium carbide base sintered carbides

Conclusions In strength properties KTS-1N alloy is close to the standard tungsten carbide base sintered carbides while in hardness it exceeds them. This material possesses high high-temperature strength, scale resistance, and cyclic resistance. As the result of the action on the material of periodically acting compressive loads of up to 4 GPa fracture occurs primarily in the plastic binder.Translated from Poroshkovaya Metallurgiya, No. 3(267), pp. 74–78, March, 1985.     

Effects of temperature on the fatigue crack growth behavior of cast gamma-based titanium aluminides

This article reports the results of an experimental study of the effects of temperature (25 °C, 450 °C, and 700 °C) on the fatigue crack growth behavior of three near-commercial cast gamma titanium aluminide alloys (Ti-48Al-2Cr-2Nb, Ti-47Al-2Mn-2Nb+0.8 pct TiB₂, and Ti-45Al-2Mn-2Nb+0.8 pct TiB₂). The trends in the fatigue crack growth rate data are explained by considering the combined effects of crack-tip deformation mechanisms and oxide-induced crack closure. Faster fatigue crack growth rates at 450 °C are attributed to the high incidence of irreversible deformation-induced twinning, while slower crack growth rates at 700 °C are due to increased deformation by slip and the effects of oxide-induced crack closure.