Processing and mechanical properties of magnesium-lithium composites containing steel fibers
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I.
INTRODUCTION
HEAVILY deforming a mixture of two immiscible, ductile phases can produce sheet or wire composites with exceptionally high strengths.[1–15] These deformation-processed metal-metal composites (DMMC) are often prepared by ingot or powder metallurgy processes. The phases are deformed until the sheet or wire has a fiber-reinforced composite microstructure with highly elongated filaments. At high levels of deformation, the thickness and spacing of the filaments can be on the order of 10 nm and the ultimate tensile strength (UTS) can exceed 2000 MPa.[1,2,3] Models have attempted to correlate the high strength of these DMMC materials with observed microstructural features such as dislocation densities or filament size and spacing.[3–10] The mechanism(s) responsible for the high strengths has been debated,[16–20] but it has been clearly shown that the strength increases as the size and spacing of the filaments decreases.[1–15] Most DMMC alloys studied have consisted of a facecentered cubic (fcc) matrix with approximately 20 vol pct of a body-centered cubic (bcc) refractory metal reinforcing phase. The most thoroughly studied systems are the fcc/bcc Cu-X alloys, where X 5 Nb, Ta, W, Cr, or Fe. The fiber morphology that develops in Cu-based DMMC wire vs sheet has long been understood to be highly dependent on the deformation textures of the matrix and fibers. In Cu20Nb wire, the Nb develops a ^110& fiber texture that limits deformation of Nb filaments to plane strain.[1,38] However, the fcc Cu matrix can accommodate the restricted deformation mode of the Nb filaments because it has many active J.A. JENSEN, who obtained his Ph.D. from the Department of Materials Science and Engineering, Iowa State University, is Senior Research Engineer, Caterpillar Inc., Peoria, IL. L.S. CHUMBLEY is an Associate Professor, Department of Materials Science and Engineering, Iowa State University, and Scientist, Ames Laboratory (DOE), Ames, IA 50011. Manuscript submitted April 8, 1997. METALLURGICAL AND MATERIALS TRANSACTIONS A
slip systems. In rod material, these deformation textures result in highly elongated, ribbon-shaped filaments that are folded around the deformation axis and are believed to be ideal for hindering dislocation motion. In sheet material, the ribbon-shaped Nb filaments exhibit a lamellar microstructure (parallel to the rolling plane) that is somewhat less effective at hindering dislocation motion; therefore, the strength of sheet material is generally lower than wire material at comparable levels of deformation.[21] Other binary systems have been studied, although less extensively, to find DMMC materials with better specific strength, specific stiffness, and high-temperature stability.[22–27] Confirmation of the importance of texture on the DMMC microstructure and properties was found in studies of Ti-20Y wire. The hexagonal close-packed (hcp) Ti and hcp Y phases both developed a ^1010& fiber texture, which is comparable to a ^110& fiber texture in bcc metals,[1] in that deformation is limited to plane str
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