Loading rate and test temperature effects on fracture of In Situ niobium silicide-niobium composites
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I.
INTRODUCTION
DEVELOPMENT of high-temperature intermetallics has been driven by promising combinations of low density with elevated temperature mechanical properties. The transition metal silicides have captured recent attention for applications exceeding 1273 K. m In particular, the transition (T) metal silicides with the 5:3 stoichiometry (e.g., TsSi3) were shown to lay on the envelope occupying the highest melting temperatures and lowest densities of intermetallics formed from over 20 crystal structure types. 12] Some of these materials (i.e., those based on Zr, Nb, Mo, Hf, Ta, W, and Re) have melting temperatures above 2500 K. Their excellent phase stability is marked by their narrow range of stoichiometry and melting temperatures exceeding those of the pure elements from which they are formed. 13,41Of these 5:3 silicides, NbsSi3 has the highest melting temperature (2753 K) among those with a density below that of nickel-based superalloys (7.2 vs 8.5 g/cm3). Inherently low ambient temperature fracture resistance (1 to 3 MPa'~/m)15,61 and inadequate ductility limit the use of NbsSi 3 in critical structural applications, in spite of evidence of high-temperature strength, t7,sl stiffness, and creep resistance.t8] Among extrinsic techniques, composite addition of ductile "reinforcement" has emerged as a potent method to augment the room-temperature fracture resistance of brittle intermetallics. The thermodynamic stability 16,1~ and me-
JOSEPH D. RIGNEY, formerly Graduate Student, Department of Materials Science and Engineering, the Case School of Engineering, Case Western Reserve University, and Postdoctoral Research Associate, the Materials Department, Oxford University, Oxford, OX1 3PH England, is Postdoctoral Research Associate, ALCOA Technical Center, Alcoa Center, PA 15069. JOHN J. LEWANDOWSKI, Professor, is with the Department of Materials Science and Engineering, the Case School of Engineering, Case Western Reserve University, Cleveland, OH 44106. Manuscript submitted December 14, 1995. 3292--VOLUME 27A, OCTOBER 1996
chanical compatibility of NbsSi3 and terminal Nb(ss-Si) phases imply that various processing routes are available to create composite structures. Laminates made from Nb5Si3 disks and Nb foils has demonstrated the toughening possible using such approaches.ts.gJ In addition, the large twophase field in the Nb-rich end of the equilibrium Nb-Si phase diagramtlOl (Figure 1) indicates the potential for using in situ eutectic reactions in composite manufacture of select compositions. Composites produced from arc melting Nb10 to 16 at. pct Si alloys and subsequent thermomechanical processing have realized toughnesses as high as 25 MPa~/-m while maintaining high-temperature strengths. 171 Coupled growth of Nb-Si and Nb-Si-Ti microcomposites using directional solidification has also produced structures exhibiting increased toughness values, tm It is well known that changes in applied strain rate and test temperature influence deformation (yield and flow) and fracture (plastic rupture vs cleavage) of
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