Effects of Solidification Parameters on Lamellar Microstructures of Near Eutectic Cr-Cr 3 Si Alloys
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Effects of Solidification Parameters on Lamellar Microstructures of Near Eutectic Cr-Cr3Si Alloys H. Bei 1, E. P. George 1,2, G. M. Pharr 1,2 1 The University of Tennessee, Department of Materials Science and Engineering, Knoxville, TN 37996-2200 2 Oak Ridge National Laboratory, Metals and Ceramics Division, Oak Ridge, TN 37831-6093 ABSTRACT Directional solidification of Cr-Cr3Si eutectic alloys has been carried out using a high temperature optical floating zone furnace. Uniform and well-aligned lamellar structures were obtained over a fairly wide range of intermediate growth rates but not at very low or very high growth rates where degenerate and cellular structures, respectively, were obtained. The lamellar spacing was found to increase with decreasing solidification rate, in agreement with the Jackson-Hunt theory. In addition, for a fixed growth rate, the lamellar spacing was found to increase with increasing rotation rate. Lamellar structures could also be produced at off-eutectic compositions, but only for a limited range of growth conditions. The Cr-rich lamellae are effective in stopping indention cracks nucleated in the brittle Cr3Si phase. INTRODUCTION The intermetallic compound Cr3Si has been identified as a promising candidate for high temperature structural applications because of several desirable properties, e.g., high melting point, good oxidation resistance, high strength and hardness, as well as low density[1-3]. However, its application is limited by its intrinsic brittleness and low fracture toughness at temperatures below 1200 °C. Ductile phase toughened Cr3Si has been considered as a possible basis for a high-temperature composite where the strong, but brittle Cr3Si is combined with a ductile solid solution [2-4]. One important subset of this class of materials is in situ composites produced by directional solidification of eutectic alloys. Many eutectics, including the Cr-Cr3Si system, when properly grown, possess a uniform rod-like or plate-like (lamellar) structure. In the present work, a series of binary Cr-Si alloys with nominal silicon concentrations ranging from 15 to 16.05 at.% were directionally solidified using a high-temperature optical floating zone furnace under argon atmosphere. The structural morphologies, the effects of composition and growth condition on microstructure, and the relationship between the lamellar structure and mechanical properties of Cr-Cr3Si alloys were examined in this work. EXPERIMENTAL PROCEDURES Several Cr-Cr3Si alloys with compositions near the Cr-Cr3Si eutectic point were arc melted and drop cast in a cylindrical copper mold measuring 10 mm in diameter and 100 mm in length. All compositions in this paper are given in terms of at. %, unless otherwise stated. The chromium and silicon (>99.995% pure) starting materials were first mixed by arc melting, with the buttons flipped and re-melted five times to ensure good mixing before drop casting. Total weight losses after melting and casting were less than 0.2%. Even if all of this weight loss is
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