Mechanical properties of molybdenum alloyed liquid phase-sintered tungsten-based composites
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Mechanical Properties of Molybdenum Alloyed Liquid PhaseSintered Tungsten-Based Composites PRESTON B. KEMP and RANDALL M. GERMAN Tungsten-based composites are fabricated from mixed elemental powders using liquid phase sintering, usually with a nickel-iron matrix.m During sintering, the tungsten undergoes grain growth, leading to microstructure coarsening that lowers strength but increases ductility. Often the desire is to increase strength at the sacrifice of ductility, and historically, this has been performed by postsintering deformation. Swaging a 93W-5Ni-2Fe alloy with 15 to 20 pct area reduction gives a 1000 to 1200 MPa tensile strength, 70 HRA hardness, and 15 to 20 pct fracture elongation.t2.3] However, swaging precludes net shaping, an advantage inherent to powder metallurgy. There has been considerable research on alloying to adjust the as-sintered mechanical properties to match those of swaged alloys. Prior reports cover many additions, seemingly including much of the periodic table. Unfortunately, many of the modified alloys proved disappointing, largely due to degraded strength at the tungsten-matrix interface. Strength improvements characteristically occur through grain growth inhibition and solid solution strengthening.t4.5,6~ Of these modified alloys, the molybdenum-containing systems exhibit a promising combination of properties, cost, and processing ease. For example, the 82W-8Mo-7Ni-3Fe alloy gives a yield
PRESTON B. KEMP, Design Engineer, is with Michelin Americas Research & Development Corporation, Greenville, SC 29602. RANDALL M. GERMAN, Brush Chair Professor in Materials, is with the Department of Engineering Science and Mechanics, The Pennsylvania State University, University Park, PA 16802-8609. Manuscript submitted January 19, 1995. METALLURGICAL AND MATERIALS TRANSACTIONS A
strength that is 34 pct higher than the equivalent 90W-7Ni3Fe alloy (from 535 to 715 MPa) but with a 33 pct decrease in fracture elongation (from 30 to 20 pct elongation). This article reports on experiments geared to promoting improved properties in the W-Mo-Ni-Fe alloys. However, unlike the prior research which maintained a constant Ni + Fe content and varied the W:Mo ratio, this study considers the Mo:(Ni + Fe) ratio effect for 82, 90, and 93 wt pct W. Prior work on W-Mo-Ni-Fe sintered materials is summarized in Table I. tl,7 121These W-Mo-Ni-Fe compositions were fabricated from mixed elemental powders, mostly with a Ni:Fe ratio of 7:3. Pressed compacts were sintered at temperatures near 1500 ~ for times typically near 30 minutes (press and sinter). A few reports used powder injection molding with similar sintering temperatures. Reported mechanical properties included yield strength (~rr), ultimate tensile strength (~rv), elongation to failure (el), and Rockwell A hardness. In a few cases, the reports gave the grain size (G). The compilation of prior results shows a wide range of properties. Strength and ductility are inversely related, high strengths result in low ductilities, and high ductilities occur with low s
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