Microstructures and Properties of Refractory Metal-Silicide Eutectics
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MICROSTRUCTURES AND PROPERTIES OF REFRACTORY METAL-SILICIDE EUTECTICS B.P. BEWLAY, K-M. CHANG, J.A. SUTLIFF, and M.R. JACKSON G.E.-C.R.D, Schenectady, N.Y. 12301
ABSTRACT New materials with improved high temperature properties are required for higher efficiency gas turbines. One group of alloys which show promise in this area are the eutectic systems Cr-Cr3Si, Nb-Nb 3 Si, and V-V 3 Si. Crystals of these refractory metalsilicide eutectics were directionally solidified using Czochralski crystal growth in conjunction with cold crucible levitation melting. The crystals were examined using scanning electron microscopy and transmission electron microscopy. In the present paper, these systems are examined with respect to the directionally solidified eutectic microstructures, phase equilibria, crystallography and mechanical properties.
INTRODUCTION The present paper describes a study of three silicide strengthened directionally solidified eutectics, Cr-Cr 3 Si, Nb-Nb 3Si, and V-V 3Si. Eutectic composites based on refractory metals are presently under investigation as high temperature structural materials [1-7] because of their high melting temperatures (>1600*C), high temperature strengths and low densities, as shown in Table I. In these three eutectic systems, the first component is the refractory metal (R), and the second is the silicide (R3 Si). Phase volume fraction calculations from the assessed phase diagrams are shown in Table 1. One approach to increase the toughness of intermetallics is to generate a two phase microstructure of the intermetallic with a ductile phase. The role of eutectics in the development of ductile phase-intermetallic composites has recently been emphasized by Mazdiyasni and Miracle [7]. Directionally solidified eutectics containing a strong ductile phase with a strong intermetallic possess three advantages. First, there is intrinsic thermodynamic stability between the matrix and the reinforcement. Second, the microstructure can be influenced by solidification conditions. Third, component fabrication is generally easier than for synthetic composites. The Cr-Si binary phase diagram has been reviewed recently by Gokhale and Abbaschian [8, 9]; the eutectic is at 15 at.%Si (all compositions in the present paper are in at.%) and is between Cr (with 9.5 % Si in solid solution at the eutectic temperature) and Cr 3 Si (22.5 %Si). Si concentrations in Cr at the eutectic temperature as low as 2.3 %Si and as high as 10.7 %Si have been reported. In addition, Cr 3 Si compositions between 15 and 24 %Si have been published. The binary phase diagram of Nb-Si indicates a eutectic point at 18.7 %Si for Nb-Nb 3 Si eutectic at 1883°C [9]. The silicide phase is a line compound and undergoes eutectoid decomposition at 1720'C [1]. The solid solubility of Si in Nb at the eutectic temperature is -4.8 %Si. The V-Si binary phase diagram [10, 11] shows a eutectic reaction between V and V 3Si at 13 %Si and 1870'C. The solid solubility of Si in V at the eutectic temperature is 7 %Si. V 3Si has a homogeneity range between
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