The Effect of Twins on the Mechanical Behavior of Boron Carbide
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TION
B4C is used widely in abrasives, wear-resistant components, and armor applications because of its high hardness and low density. Hot pressing boron carbide (B4C) powder is the commercial technique used to form personnel armor plates and components for various applications. It is possible for B4C components formed by the hot-pressing technique to reach nearly full theoretical density and achieve high mechanical performance. The alternative technique of forming B4C compacts, as described by Matsumoto et al.,[1] is by slip casting, sintering, and hot isostatically pressing (HIP) the powder. The mechanical properties of these slip-cast and HIP B4C materials were reported[2] to be as good as the hot-pressed B4C materials. However, the different processing routes could affect the microstructure, and the effect of microstructure on the mechanical and ballistic response is an important and common question when characterizing armor materials. Studies investigating the effect of the processing route on the mechanical properties of alumina used for ballistic applications have been conducted by Badmos et al.[3] In their research, it was found that alumina with the same composition but produced by dry pressing had higher hardness and lower toughness than those produced by slip casting. The aim here is to evaluate the mechanical properties and microstructure of the slip cast, sintered, and HIP B4C (from now referred to as slip cast B4C) and compare these properties with those of hot-pressed B4C. In an effort to determine the effect of microstructure on TOMOKO SANO, Materials Engineer, and CHARLES L. RANDOW, Mechanical Engineer, are with the Weapons and Materials Research Directorate, Materials and Manufacturing Science Division, U.S. Army Research Laboratory, Aberdeen Proving Ground, MD 21005. Contact e-mail: [email protected] Manuscript submitted December 14, 2009. Article published online December 1, 2010 570—VOLUME 42A, MARCH 2011
mechanical performance, simulations comparing the elastic response of structures (with and without the presence of growth twins) to quasi-static loading will be used to evaluate the differences between the two B4C types.
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EXPERIMENT
A commercially available, hot-pressed B4C and a research grade slip-cast B4C were obtained. The hotpressed B4C sample is the armor-grade reference benchmark material and has a density of 2.49 g/cm3, or 98.8 pct of the theoretical density. The slip-cast B4C was measured by the Archimedes method to have a density of 2.50 g/cm3, or 99.2 pct of the theoretical density. The hot-pressed and slip-cast B4C were also analyzed in a previous work. Chen et al.[4] analyzed the microstructure and intergranular precipitates of the equivalent hot-pressed B4C, and Sano et al.[5] evaluated the highstrain-rate response of the slip-cast B4C. The samples were polished on the Struers Rotopol automatic polisher with diamond slurries of decreasing diamond abrasive size at each polishing step until reaching 0.25 lm. The samples were then polished with 0.02 lm colloidal silica. The poli
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