Fatigue and fracture of damage-tolerant In Situ titanium matrix composites
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INTRODUCTION
INGOT metallurgy (IM) and rapid solidification processing powder metallurgy (RSP/PM) techniques have been used to produce in situ titanium matrix composites reinforced with TiB whiskers.[1–6] Such composites are particularly of interest due to their attractive combinations of low cost and improved mechanical properties, when compared to fiber-reinforced composites. The main driving force for the development of these in situ composites is the increase in modulus that can be achieved, without much loss of ductility, when the B levels are controlled. Furthermore, even a marginal increase in modulus is of significant interest with respect to the design of high-stiffness components for aerospace and automotive applications. In situ titanium matrix composites also have higher elevated-temperature strengths[2] and relatively low densities (;4.8 g/cm3) that make them attractive candidates for high-performance applications. The attractive combinations of mechanical properties have also attracted considerable interest from the Toyota Motor Corporation (Daza Nagakute, Japan), which is currently involved in the final stages of developing in situ titanium matrix composite exhaust and intake valves, connecting rods, and other automobile engine components.[6] The McDonnell-Douglas Corporation is also considering their potential use in the next generation of airplanes.[2] The in situ composites are also particularly attractive because the ingots and powders can be processed into a variety of complex shapes using conventional wrought-processing techniques (rolling, forging, and extrusion) as well as nearnet-shape processing techniques.[2] Such composite processing methods can also be achieved at low cost. The RSP of Ti-B alloys, with large undercoolings and high cooling rates, has been shown to be very effective for the production of in situ titanium composites which contain large volume fractions of reinforcing second-phase particles similar to the whiskers used in metal matrix composites.[1–6] The RSP-produced alloys have excellent compositional homogeneity, small grain sizes, and homogeneously distribS. DUBEY, Graduate Research Associate, and W.O. SOBOYEJO, Associate Professor, are with the Department of Materials Science and Engineering, The Ohio State University, Columbus, OH 43210-1179. R.J. LEDERICH, Principal Technical Specialist, is with McDonnell Douglas Corporation, St. Louis, MO 63166. Manuscript submitted December 5, 1996. METALLURGICAL AND MATERIALS TRANSACTIONS A
uted dispersoids, but are relatively expensive when compared to the IM materials. In the present study, the IM technique was used to prepare in situ composites. The composites were produced by alloying with B in the melt to produce a uniform dispersion of in situ rodlike titanium boride (TiB) whiskers. In order to assess the damage tolerance of the IM in situ titanium matrix composites with attractive combinations of basic mechanical properties, the fatigue and fracture properties of different in situ composites are examined in this articl
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