The balance of mechanical and environmental properties of a multielement niobium-niobium silicide-based In Situ composit
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I.
INTRODUCTION
The application of refractory metals in hot stages of turbine engines has been attractive for many years.[1] Developments based on Nb and Mo alloys have offered significant gains in temperature capability due to their high melting temperatures and, for the refractory metals, relatively low densities. However, oxidation resistance has remained a major barrier to their application, even in the coated condition. Intermetallics offer great potential for high-temperature (.1000 7C) structural applications, but the intrinsic toughness of most intermetallics is too low for aircraft engine applications. However, combining intermetallic materials with metallic second phases has recently been shown to provide composites with promising toughness.[2,3] The most promising composites rely on ductile metals to provide room-temperature toughening and on intermetallics that coexist in equilibrium with those metals to provide the high-temperature strength. In this arena, Nb-Si in situ composites that consist of a Nb-based solid solution with Nb3Si and/or Nb5Si3 silicides have been shown to have great potential because of their attractive balance of high- and low-temperature mechanical properties.[4–9] Unfortunately, the composites from binary Nb-Si alloys have very poor oxidation resistance, and, generally, elements added to these composites to improve oxidation resistance compromise high-temperature mechanical properties.[4] The present article describes an analogous in situ composite generated from a more complex Nb-Ti-Hf-Si-Al-Cr alloy; this composite offers an improved balance of mechanical properties and oxidation. B.P. BEWLAY and M.R. JACKSON, Staff Metallurgists, are with the Corporate Research and Development Center, General Electric Company, Schenectady, NY 12301. H.A. LIPSITT, Professor, is with the Department of Mechanical and Materials Engineering, Wright State University, Dayton, OH 45435. Manuscript submitted June 7, 1996. METALLURGICAL AND MATERIALS TRANSACTIONS A
This composite consists of high-strength, high-melting temperature Nb-based silicides together with a high melting temperature, modest strength, high-toughness Nb-based metallic phase. The composition of the alloy that was directionally solidified to generate this composite was Nb24.7Ti-8.2Hf-2.0Cr-1.9Al-16.0Si in atomic percent (Nb-15.47Ti-19.19Hf-1.35Cr-0.68Al-5.88Si in weight percent); all compositions are given in atom percent throughout the remainder of this article. Throughout this article, this composite will be referred to as the metal and silicide composite (MASC). This composition was based on the binary Nb-16 pct Si composition;[7,10] Ti and Hf were added to improve the oxidation resistance.[11] Subramanian et al.[4] and Jackson et al.[11] reported that the oxidation resistances of both the single-phase silicide and the metallic phase were improved by partial substitution of Ti for Nb. The elements Cr and Al also improve the oxidation resistance of both the metallic and silicide phases. Titanium was also added to improve the int
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