Unconstrained and constrained tensile flow and fracture behavior of an Nb-1.24 At. Pct Si alloy
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INTRODUCTION
A number of recent and current research studiest1-~51 are focusing on the development of ductile-phase-toughened intermetallic-based composite systems for use in advanced aerospace systems at temperatures from 1000 ~ to 1600 ~ Two distinct types of systems are being explored: (1) composition classes in which selected intermetallic compounds (silicides, aluminides, and Cr-based) are in thermodynamic equilibrium with refractory-metal/3-phase alloys (these may be termed as natural composites) IH~I and (2) synthetic or engineered composites that consist of an oxidation-resistant intermetallic matrix and a ductile refractory phase (e.g., MoSi2-Nb). I~2-151 These latter composites are not in thermodynamic equilibrium and require kinetic barriers to the reactions occurring between the constituents. The concept underlying both types of systems is to exploit the potentially favorable high-temperature mechanical properties (i.e., strength, stiffness, and creep resistance of intermetallic phases) while circumventing the lack of ductility and low fracture resistance of these phases by ductile-phase toughening. One example of the first type of system that has received some attentionl~-Tj is based upon Nb-Si-phase equilibria and consists of composites between the intermetallic NbsSi3 phase and the terminal solid solution (Nb) phase. The phase diagram indicates the existence of a wide two-phase field, which is (kinetically) stable up to 1660 ~ ~21 The compound NbsSi3 is essentially a line compound, and the terminal Nb phase has a low-Si solubility that varies slightly with temperature up to 1500 ~ Recent experimental results indicate that the two phases are thermochemically and morphologically M.G. MENDIRATTA, Director, Materials and Processes Division, and R. GOETZ, Research Engineer, are with UES, Inc., Dayton, OH. D.M. DIMIDUK, Research Scientist is with Wright Laboratories, WL/MLLM, Wright-Patterson AFB, OH. J.J. LEWANDOWSKI, Professor, is with the Department of Metallurgy and Materials Science, Case Western Reserve University, Cleveland, OH. Manuscript submitted December 27, 1994. METALLURGICALAND MATERIALSTRANSACTIONS A
stable, at least for 100 hours at 1500 ~ Measurements of mechanical properties of these composites in the wrought condition have indicated high-tensile strength retention to 1500 ~ reasonable creep resistance, and good toughness at room temperature, t3,4,J6,~71 The room-temperature mechanical properties--specifically, toughness--of the (Nb)/NbsSi3 composites are principally governed by the deformation and fracture behavior of the Nb phase. 13-5,16,181 Although the degree of toughening is interpreted within recent theories involving bridging of cracks by the constrained "ductile" Nb phase, E19'2~ the metallographic and fractographic observations of the fractured composite specimens have shown complexities in physical processes that are not fully accounted for by these theories. In particular, these theories do not discriminate between a variety of fracture modes and micromechanisms observed
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