METALLURGICAL AND MATERIALS TRANSACTIONS A VOLUME 28A, AUGUST 1997—1667Effects of Microstructure on the Strength and FatigueBehavior of a Silicon Carbide Fiber-Reinforced TitaniumMatrix Composite and Its ConstituentsW.O. SOBOYEJO, B.M. RABEEH, Y. LI, Y.C. CHU, A. LAVRENTENYEV,and S.I. ROKHLINThe results of a systematic study of the effects of microstructure on the strength and fatigue behaviorof a symmetric [0/90] 2s Ti-15A1-3Cr-3A1-3Sn/SiC (SCS-6) composite are presented along with rel-evant information on failnure mechanisms in the composite constituents, i.e., the interface, fiber, andmatrix materials. Damage micromechanisms are elucidated via optical microscopy, scanning electronmicroscopy (SEM), and nondestructive acoustic emission (AE) and ultrasonic techniques. Compositedamage is shown to initiate early under cyclic loading conditions and is dominated by longitudinaland transverse interfacial cracking. Subsequent fatigue damage occurs by matrix slip band formation,matrix and fiber cracking, and crack coalescence, prior to the onset of catastrophic failure. However,the sequence of the damage is different in material annealed above or below the b solvus of the Ti-15-3 matrix material. Mechanistically based micromechanics models are applied to the prediction ofthe changes in modulus induced by fatigue damage. Idealized fracture mechanics models are alsoemployed in the prediction of the fatigue lives of smooth specimens deformed to failure at roomtemperature. The article highlights the potential to develop mechanistically based predictive modelsbased on simplified mechanics idealizations of experimental observations.I. INTRODUCTIONT HE strong interest in the development of improvedtransonic and hypersonic aerospace vehicles has stimulatedconsiderable research on the development of light weight,high temperature materials, with the potential to replaceexisting nickel- and titanium-base alloys. [1,2] Some of thiseffort has led to the identification of fiber-reinforced tita-nium matrix composites as candidate materials for appli-cations in the intermediate temperature regime between 5007C and 6507C. Titanium matrix composites (TMCs) havebeen considered mainly as a result of their attractive com-binations of high temperature strength and stiffness, as wellas their creep resistance in the potential service temperatureregime. Unfortunately, however, fiber-reinforced TMCshave been shown to have only limited fatigue [3–14] and frac-ture [15,16,17] resistance. Also, many of the TMCs have inher-ently anisotropic properties and worse mechanicalproperties than their matrix alloys, especially in the trans-verse orientation. This is in spite of the significant levelsof crack-tip shielding that have been shown to occur inW.O. SOBOYEJO, Associate Professor, and Y. LI, PostdoctoralResearch Fellow, Department of Materials Science and Engineering, andS.I. ROKHLIN, Professor, Department of Industrial, Welding and SystemsEngineering, The Ohio State University, Columbus, OH 43210-1179.B.M. RABEEH, formerly Graduate Research Assistant, Department ofMaterials Science and Engineering, The Ohio State University, isDepartment Head, The Military Technical College, Cairo, Egypt. Y.C.CHU, formerly Graduate Research Associate, Department of Industrial,Welding and Systems Engineering, is Research Engineer, Philips TaiwanDivision, Taipei, Taiwan, People’s Republic of China. A.LAVRENTENYEV, formerly Graduate Research Associate, Departmentof Industrial, Welding and Systems Engineering, The Ohio StateUniversity, is Scientist, United Technologies Research Center, EastHartford, CT 06108.Manuscript submitted November 1, 1996.TMCs. [12,13] There is, therefore, a need for an improvementin the basic understanding of failure mechanisms in TMCs.Previous studies conducted on TMCs with various ar-chitectures have revealed that a complex sequence of dam-age is associated with failure under monotonic [15,16,17] andcyclic [13,14] loading. In particular, recent high resolutiontransmission electron microscopy studies [18,19] have shownthat the layered interfacial microstructure that exists be-tween the fiber and the matrix has a highly complex struc-ture. This layered interface, which consists predominantlyof titanium carbides (TiC and Ti 2 C), has also been shownto promote early nucleation of fatigue damage at the fi-ber/matrix interface. [14] Furthermore, although many of theTMC systems under consideration have metastable beta ti-tanium matrices that may undergo phase transformations inthe anticipated service temperature regime, the effects ofmicrostructure on the mechanical properties of TMCs arenot fully understood. [14,18–23] There is, therefore, a need forfurther studies of the effects of microstructure on the me-chanical properties of TMCs. There is also a need for non-destructive inspection techniques and micromechanics-basedmodels for the prediction of fatigue life and the effects offatigue damage in systems fabricated from TMCs.The results of a systematic study of the effects of micro-structure on the strength and micromechanisms of fatiguein a Ti-15V-3Cr-2Al-3Sn/Sic (SCS6) composite are re-ported in this article. Nondestructive ultrasonic/acousticemission and visual techniques are also used to study themicromechanisms of tensile and fatigue damage in the com-posite and its constituents. These include an assessment ofmatrix, fiber, and interfacial damage at room temperature.The shielding contributions from the observed crack bridg-ing mechanisms are assessed using fracture mechanicsmodels. A fracture mechanics-based method is also pre-sented for the estimation of fatigue life. The current article
