In-Situ Growth of SiC in MoSi 2 by Melt Processing
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IN-SITU GROWTH OF SiC IN MoSi2 BY MELT PROCESSING DANIEL J. TILLY, J.P.A. LOFVANDER and C.G. LEVI Engineering Materials Department, University of California, Santa Barbara CA 93106 ABSTRACT The growth of SiC during solidification of Mo-Si-C melts was investigated to explore the potential for developing in-situ refractory reinforcements for MoSi 2 matrices. Volume fractions of up to 20% were readily incorporated by arc melting. Primary O3-SiC grows as equiaxed particles, plates and hopper crystals, whereas secondary SiC grows with ribbon, thin flake or whisker morphologies. The dominant facets of both primary and secondary SiC are of the (111) and {002) type, which are also characteristic of the equilibrium crystal shape. There is a clear orientation relationship between the phases wherein the close packed planes and directions of the C11b and B3 structures are parallel. INTRODUCTION Molybdenum disilicide (MoSi2) is attractive for high temperature structural applications owing to its high melting point (TM = 2300 K) and good oxidation resistance associated with the formation of a protective silica scale [1,2]. The ordered tetragonal C11b structure offers the potential of high strength and stiffness at elevated temperatures, but its intrinsic brittleness hinders the application of monolithic MoSi2 as a structural material. Compositing schemes based on ceramic fibers or ductile refractory metal reinforcements are under investigation to improve both the toughness and creep strength of MoSi2 [3-61. Thermal expansion mismatch between MoSi2 and the reinforcements may cause matrix cracking during processing or thermal cycling [7]. Introduction of SiC into MoSi2 has been successfully used to lower the thermal expansion coefficient of the matrix and suppress cracking in Mo-wire reinforced composites [6]. SiC whiskers and particulates have also been shown to improve the creep resistance of MoSi2 [2,8]. Thus, SiC additions to MoSi 2 are of significant interest, but often lead to less than optimal microstructures if implemented by powder blending approaches. This paper reports on a study aimed at growing SiC-reinforcements in-situ by solidification processing of Mo-Si-C melts of suitable compQsitions. EXPERIMENTAL TECHNIQUES The alloys investigated in this study were produced with high purity elemental components; 99.97% Mo, 99.995% Si and 99.9% C (all in weight percent). Alloys were prepared by arc-melting under an atmosphere of purified argon containing less then 0.1 ppb oxygen. The buttons, weighing approximately 15 g each, were flipped and remelted several times to ensure macroscopic chemical homogeneity. The materials were characterized by optical, scanning (SEM) and transmission (TEM) electron microscopy. Specimens were prepared for optical and SEM microscopy by standard grinding and polishing techniques; a solution consisting of 2 % hydrofluoric acid, 25 % nitric acid (by volume) and lactic acid making up the balance was used for deep-etching to reveal the SiC phase morphologies. SEM Mat. Res. Soc. Symp. Proc. Vol. 273.
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