Mechanisms, models, and simulations of metal-coated fiber consolidation
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I. INTRODUCTION
CONTINUOUS fiber reinforced titanium matrix composites are being developed for a variety of applications in gas turbine engines and other aerospace structures because of their very high specific stiffness and strength in the fiber direction.[1] For example, a Ti-6Al-4V/Sigma-1240 (SiC) composite with a 44 pct SiC fiber volume fraction has an axial Young’s modulus of more than 200 GPa, an ambient temperature fracture strength of about 1800 MPa, and a 600 8C creep rupture life of several thousand hours at 1000 MPa.[2] One approach for manufacturing composites of this type uses either a sputtering or electron-beam physical vapor deposition (PVD) technique to evaporate and condense the metal matrix on the fibers.[3,4] Composite components can then be formed by aligning the coated fibers in a metal canister that is subsequently evacuated, sealed, and consolidated at an elevated temperature by either hot isostatic pressing (HIP), vacuum hot pressing, or rolling.[4–7] Recent HIP consolidation experiments using in situ density sensors reveal that densification of Ti-6Al-4V vaporcoated fibers occurs at a very high densification rate.[5] As a result, complete densification can be accomplished at much lower temperatures and pressures than is needed for similar composition titanium alloy powders.[4,5] Two factors might be responsible for this anomaly. First, recent studies of a similar PVD Ti-6Al-4V alloy revealed a greatly enhanced low pressure superplastic formability because of its fine (30 to 100 nm) grain size.[3] However, rapid grain coarsening during elevated temperature consolidation can reduce the R. VANCHEESWARAN, Graduate Student, is with the MBA Program, Cornell University, Ithaca, NY 14853. J.M. KUNZE, Engineer, is with Triton Systems Inc., Chelmsford, MA 01824. D.M. ELZEY, Research Assistant Professor, Department of Materials Science and Engineering, and H.N.G. WADLEY, Edgar A. Starke Research Professor and Associate Dean for Research, School of Engineering and Applied Science, are with the University of Virginia, Charlottesville, VA 22903. Manuscript submitted May 19, 1999. METALLURGICAL AND MATERIALS TRANSACTIONS A
significance of this process. The second factor is the observation of large cusp-shaped pores that were created by the metal-fiber contacts. Although these shrink in dimension, they retain a cusp shape throughout the consolidation process.[6] Recent micromechanical analyses of the creep collapse of cusp-shaped pores has indicated that their collapse rate can be as much as an order of magnitude greater than that of their spherical pore counterparts.[8] These observations are of significant consequence to the design of composite fabrication processes. High performance can only be achieved with this class of composites when complete densification of the coated fibers is accomplished while simultaneously avoiding significant bending of fibers (a result of fiber-fiber crossovers usually present in a random packing of aligned fibers[4]) and excess growth of reaction product layers at the fi
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