Fracture and Fatigue of Niobium Silicide Alloys
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Fracture and Fatigue of Niobium Silicide Alloys David M. Herman1, B.P. Bewlay2, L. Cretegny2, R. DiDomizio2, and John J. Lewandowski1 1 Case Western Reserve University, Department of Materials Science and Engineering, Cleveland, OH 44106-7204 2 General Electric Global Research, Schenectady, NY 12301, USA. ABSTRACT The fracture and fatigue behavior of refractory metal silicide alloys/composites is significantly affected by the mechanical behavior of the refractory metal phase. This paper reviews some of the balance of properties obtained in the alloys/composites based on the Nb-Si system. Since some of the alloy/composite properties are dominated by the behavior of the refractory metal phase, the paper begins with a review of data on monolithic Nb and its alloys. This is followed by presentation of results obtained on Nb-Si alloys/composites and a comparison to behavior of some other high temperature systems. INTRODUCTION Because of their high melting point, low density, high fracture toughness and very good properties at ultrahigh temperatures, Niobium-Silicon alloys have attracted recent attention [1] [2]. This class of alloys has very good strength retention, low creep rate at ultrahigh temperature and very good oxidation resistance up to 1300-1400°C. These alloys continue to be considered as one of the candidates for airfoils in gas turbines/aircraft engines to replace the currently used nickel-base superalloys which are limited to temperatures less than 1150°C. [1] [2] While the oxidation resistance of the monolithic refractory metals are generally poor, the intermetallic compounds of refractory metals with silicon(e.g. Nb,Mo) are of interest because of their high melting points, their increased oxidation resistance, and their relatively low densities [1,2]. In particular, alloys/composites in the systems Mo-Si-B-X (X=W,V,Cr,Nb,Al,Ge,Re) and Nb-Ti-Hf-Cr-Al-Si-Ge combine many of the attractive features of refractory metals and intermetallic compounds. Despite the potential of silicides for use as high temperature materials such as turbine airfoils or some hypersonic applications for service temperatures ranging from 1000-1400°C, relatively little is known about their structure and properties(in monolithic and composite form) at ambient or elevated temperatures. Some of the additional property requirements for the hottest parts of aircraft are provided elsewhere [3]. Predominant issues in the mechanical behavior of these advanced materials are their ambient and high temperature properties, including the strength, ductility, toughness, creep, fatigue performance, and oxidation resistance at intermediate and high temperatures. Recent works have successfully demonstrated the potential of improving the toughness of niobium silicide systems via compositing with ductile Nb-phases utilizing ingot casting and extrusion, directional solidification, as well as lamination via PVD or powder metallurgy techniques [3] [4] [5] [6]. Parallel efforts are being developed to improve the oxidation and creep resistance of
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