The formation of a directionally solidified non-oxide eutectic surface layer based on the 75 mol% B4C and 25 mol% TiB2 eutectic composition has been developed using a continuous-wave, high-power, CO2 laser. To prevent the oxidation of nonoxide compounds and to reduce thermal stress formation, an atmospherically controlled furnace capable of back-heating samples up to 1100°C during the laser processing has been employed. The effect of the laser scan rate on the eutectic microstructure formation is investigated. It is found that at relatively slow laser scan rates, ∼2–4 mm/s, the formation of a colony-type eutectic microstructure with submicrometer scale features is observed. At higher heating rates up to 42 mm/s, disordered eutectic grains with nanometer-scale TiB2 lamellae form. The dependence of the TiB2 interlamellar spacing on the crystallization rate in the laser solidified B4C–TiB2 eutectic has the same general trend as in the eutectic produced by a floating zone method. However, this method allows the production of much smaller microstructural length scales, on the order of 100 nm, as a consequence of the high crystallization velocity.
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Document Type: Research Article
Department of Materials Science and Engineering, Pennsylvania State University, University Park, Pennsylvania 16802
Applied Research Laboratory, Laser Processing Division, Pennsylvania State University, University Park, Pennsylvania, 16802
Publication date: 2008-11-01