In-situ observations of titanium metal-matrix composites under transverse tensile loading
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THE reinforcement of titanium alloys by unidirectional high-strength fibers leads to improved mechanical properties under loading parallel to the fibers. However, this gain is at the expense of properties in the transverse direction. The strength[1,2] and fatigue-crack-growth resistance[3–6] is much lower under loading normal to the fiber direction. Thus, the transverse properties are of concern for utilizing this type of material, since transverse stresses are present in cracked components even if the applied stress is along the longitudinal direction alone.[7] It has also become increasingly important to understand the capability of fiber matrix interfaces in supporting a transverse stress and the mechanism of interfacial failure in metal-matrix composites (MMCs) under cyclic and monotonic loading. Transverse properties under cyclic loading have been reported elsewhere.[3] Considerable work has also been carried out under monotonic transverse loading, where the interfacial properties and debonding mechanism[2,8–13] were investigated. It has been found that the transverse properties are strongly dependent upon the interfacial bonding condition, interfacial reaction, mismatch of thermal-expansion coefficients of the fiber and matrix material, and fiber spatial arrangement.[14,15] Attempts[14,15,16] have also been made to predict transverse failure mechanisms and properties by computer modeling, based on the mechanisms observed experimentally. However, most of the previous investigations were carried out after test interruption, which limits the number of observations for each testpiece. The mechanisms observed are also X. WU, Senior Research Fellow, C. COOPER, PhD Student, and P. BOWEN, Professor, are with the IRC in Materials for High Performance Applications/School of Metallurgy and Materials, The University of Birmingham, Edgbaston, Birmingham, B15 2TT, United Kingdom. Manuscript submitted August 27, 1999. METALLURGICAL AND MATERIALS TRANSACTIONS A
often applicable only to given stresses or a limited range of stresses. Here, the transverse response under monotonic loading has been investigated in detail by carrying out tests in a field-emission gun (FEG)–scanning electron microscope (SEM), which allows dynamic processes during loading to be monitored continuously and recorded directly, albeit at surface positions only. II. EXPERIMENTAL PROCEDURE Two materials are studied here. (1) A Ti-6Al-4V (wt pct) matrix reinforced with 8 and 21 pct volume fractions of Sigma 1140⫹ carbon-coated silicon carbide fibers (diameter of ⬇110 m), produced by DERA (Farnbrough, United Kingdom), made by a foil-fiber-foil hot isothermal processing (hipping) route at a temperature within the range of 890 ⬚C to 940 ⬚C for 2 hours, at a pressure of 100 MPa. (2) A Ti-21s (Ti-15Mo-2.7Nb-3Al-0.2Si, in wt pct) matrix reinforced with 35 pct SCS-6 silicon carbide fibers produced by Textron (Textron Specialty Material Division, Lowell, MA) hipped at a temperature of 900 ⬚C. The 8 pct 1140⫹/ Ti-6-4 composite was in a 12-ply form, with a plate th
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