Effect of fiber volume fraction on the fracture behavior of Nb-1 wt pct Zr/218W composites at elevated temperatures
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I. INTRODUCTION
THE increasing use of space-based applications ranging from satellite communications to the fabrication of the International Space Station requires the development of advanced power systems to provide constant and reliable sources of electrical power. It is anticipated that future National Aeronautics and Space Administration (NASA) missions would require tens to hundreds of megawatts of electrical energy, according to projections made in the late 1980s and early 1990s.[1–4] Current designs for advanced power systems call for spacebased, molten lithium–cooled nuclear reactors operating between 1350 and 1380 K, under stresses varying between 5 and 3 MPa, respectively, for a duration of 7 to 10 years.[2,3] These stringent design requirements suggest that refractory metals with a demonstrated compatibility with molten lithium be used for fabricating the pressure vessels, heat pipes, clad tubes, and other critical components.[5] Although the Nb-1 wt pct Zr alloy has been shown to be compatible with molten lithium,[6,7] its creep strength barely meets the aforementioned design requirements. However, Nb-1 pct Zr composites reinforced with strong, heavily cold-worked molybdenum or tungsten fibers have shown considerable improvements in elevated-temperature strength.[2,3,8–12] As expected, these studies reveal that the unidirectional and 1/215 deg angle– ply Nb/W and Nb-1 pct Zr/W composites possess much better creep- and stress-rupture properties than the unreinforced matrix, by as much as a factor of 10 to 30 at 1400 and 1500 K.[2,3,10–12]
S.V. RAJ, Materials Research Engineer, is with the NASA–Glenn Research Center at Lewis Fields, Cleveland, OH 44136. L.J. GHOSN, formerly Researcher with Case Western Reserve University, Cleveland, OH 44115, is Staff Technology Engineer with Eveready Battery Company, Inc., Westlake, OH 44145. This article is based on a presentation made in the Symposium “Mechanisms and Mechanics of Composites Fracture” held October 11–15, 1998, at the TMS Fall Meeting in Rosemont, Illinois, under the auspices of the TMS-SMD/ASM-MSCTS Composite Materials Committee. METALLURGICAL AND MATERIALS TRANSACTIONS A
However, these early investigations did not study the effect of the fiber volume fraction on the mechanical properties of the composites. These studies reported that the measured volume fraction (Vnc) of the fibers in the composite plates after consolidation varied between 43 and 52 pct, although the desired or nominal volume fraction (Vnf ) was 40 pct.*[8,9] In some instances, much larger variations were *Three volume fractions of fibers are described in the text. First, Vnf refers to the expected fiber volume fraction without considering contributions from the outer clad matrix layers. Second, Vnc refers to the experimental value measured from metallographic cross sections of composite specimens without including the thickness of the clad. Third, Veff refers to the measured fiber volume fraction, taking into account the thickness of the clad material.
observed, ranging from 30 to
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