An investigation of the effects of ductile-layer thickness on the fracture behavior of nickel aluminide microlaminates
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I. INTRODUCTION
SINCE the pioneering work of Krstic et al.,[1,2] considerable effort has been made to utilize the concept of ductilephase toughening in the design of toughened composite systems.[3–22] Much of the early work was performed on composites reinforced with ductile particles[3–6] or fibers[4,6–11] which were chosen to promote toughening, largely by crack bridging.[3–11] The early studies showed that fiberreinforced composites generally promote greater levels of toughening than particulate-reinforced composites. However, subsequent work on ductile layer–reinforced composites[4,12–22] soon revealed that such composite architectures result in even greater levels of toughening than ductile fiber– reinforced brittle matrix composites.[4,6–11] The interest, in recent years, has, therefore, shifted toward an understanding of the effects of ductile-layer thickness on the fracture toughness of brittle matrix composites.[12–22] This article presents the results of fundamental studies of the effects of ductile-layer thickness on the fracture-initiation toughness and resistance-curve behavior of nickel aluminide microlaminates. Model composites reinforced with 20 vol pct of ductile vanadium layers (100, 200, and 400 mm thicknesses) or ductile Nb-15Al-40Ti layers (100, 500, and 1000 mm thicknesses) were used in this study. The ductile-phase reinforcements were selected due to their compatibility with NiAl, which was demonstrated in unpublished diffusioncouple studies by the authors in the same processing temperature regime (,1100 8C). The vanadium and Nb-15Al-40Ti were also investigated, since they exhibit essentially elastic– perfectly plastic behavior. Furthermore, the partially ordered B2 Nb-15Al-40Ti intermetallic[23] (all compositions quoted in atomic percent unless stated otherwise) layers exhibit attractive combinations of damage tolerance and oxidation M. LI, formerly Graduate Research Associate, Department of Materials Science and Engineering, The Ohio State University, is Process Engineer, Huffman Corporation, Lake Wylie, SC 29710. W.O. SOBOYEJO, formerly Associate Professor, Department of Materials Science and Engineering, The Ohio State University, Columbus, OH, is Professor, Princeton Materials Institute and the Department of Mechanical and Aerospace Engineering, Princeton University, Princeton, NJ 08544. Manuscript submitted June 15, 1999. METALLURGICAL AND MATERIALS TRANSACTIONS A
resistance in the intermediate-temperature regime (650 8C to 850 8C).[23,24] As in recent studies by Bloyer et al.[12,13] on similar layered composites, fracture-initiation toughness and resistancecurve behavior in the model composite systems are shown to improve with increasing layer thickness in the crackarrestor orientation. The intrinsic steady-state toughness values were also shown to increase with increasing ductilelayer thickness. However, the results indicate that fracture instability is more likely to occur at smaller crack extensions, as the ductile-layer thickness is increased. The initiation toughness, resistance-c
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