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I.

INTRODUCTION

THERE have been considerable efforts in developing new high-temperature materials for gas turbine engine applications. These efforts have been motivated by the desire to increase the thrust to weight ratio by increasing the operating temperature of the engine. Since Ni-based superalloys are currently used at about 0.8 to 0.9 of the melting point, any increase in the engine operating temperature requires new structural materials that can withstand the higher temperature requirementsY~ Several Nb-based in situ composites have been considered as possible high-temperature materials for the future. The compositions of these Nb-based materials generally contain silicon,t2~ chrominum,[7-'] aluminum,[7.~2.~31 and, among other alloying elements, titanium.t8.9,H,~31They are often referred to as in situ composites because they can be processed through composition controlled to exhibit a twophase microstructure containing a significant amount of a metallic solid-solution phase in a brittle intermetallic matrix. Such a two-phase microstructure resembles those of ductile-phase-toughened composites obtained by phase mixing[~4,~5,16] and hence the name in situ composites. In situ composites based on the NbsSi3fNb system have been studied extensively, notably, by Dimiduk and coworkers at the Wright Patterson Air Force Base. t2,3,41Some work on Nb3A1/Nb has been reported in the literature,t7.~21 Additional work on this system or its variants appears to continue at several laboratories. There also are substantial activities in the Cr2Nb/Crt~~ and Cr2Nb/Nb systems,t7-9,181 as well as ternary variants of the Cr2Nb/Nb system with Ti additions,t8,9,~8~Interest in the Cr2Nb/Nb systems was motivated primarily by the high creep resistance of Cr2Nb at elevated temperatures, t19-221Being a Laves phase with a C 15 structure at temperatures below 1650 ~ Cr2Nb exhibits low fracture resistance at ambient temperature, tT~ As a re-

KWAI S. CHAN, Staff Scientist, is with the Southwest Research Institute, San Antonio, TX 78238-5166. Manuscript submitted October 19, 1995. 2518--VOLUME 27A, SEPTEMBER 1996

sult, two-phase systems containing Cr2Nb and Nb, i.e., in situ composites, are thought more desirable from the fracture-resistance point of view. The rationale is based on the anticipation that the Nb solid-solution phase may impart ductility and toughness through one or more ductile-phase toughening mechanisms, which include crack bridging, blunting, and among others, crack deflection,t23~In addition to alloy development,t2,7.8,'] phase transformation,t18] deformation,[3'733-2~ and fracture s t u d i e s , [2-9A2,17] substantial efforts also have been made to improve the oxidation resistance[241 of Nb-based alloys and in situ composites. Extensive studies of the fracture resistance of the NbsSi3/Nb system have demonstrated that the fracture resistance of this class of in situ composite can be enhanced by the ductile Nb phase through crack bridging.t2-6j Fractographic examination of the fracture surface, however, indicates that cle