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I.

INTRODUCTION

THE fracture properties of ductile-phase reinforced laminated brittle-matrix composites have been studied in some detail over the past decade with the objective of improving their crack-growth resistance for aerospace structural applications requiring reduced weight and increased mechanical performance. Such research has generally focused on the constrained deformation behavior of the ductile second phase and the matrix-reinforcement interfacial properties,[1–6] although effects of reinforcement volume fraction,[7] laminate orientation,[7,8,9] and microstructural scale of the layers[7,10–14] have all been examined. For example, laminates based on the Nb/Nb3Al and Nb/ or TiNb/g-TiAl systems have been reported to show toughnesses exceeding 10 MPa=m (e.g., compared to KIc values of 1 MPa=m for Nb3Al[10]), which result from the significant plastic energy dissipation in large bridging zones that are typically observed behind the crack tip. Despite efforts to improve the toughness of brittle-matrix laminates, little attention has been paid to their performance under cyclic loading. This can be critical for intermetallicmatrix composites because cyclic fatigue loading often promotes subcritical crack growth in the reinforcements themselves, thereby diminishing the bridging zone in the crack wake and correspondingly reducing the fatigue crackgrowth resistance of the composite.[8] Such behavior has been observed in ductile metal reinforced brittle-matrix

composites using particulate, fiber, and disc-shaped reinforcements.[7,8,15,16] Specifically, for ‘‘laminate-like’’ composites of TiNb disc reinforced g-TiAl composites, it was found that the orientation of the composite had strong influence on fatigue performance.[7,8] When the discs were aligned in the crack-divider or edge orientation, the fatigue properties of the composite were inferior to the unreinforced matrix, whereas with the discs aligned in the crackarrester or face orientation, the composite displayed marginally better fatigue resistance (Figure 1). However, apart from this study, reports of the effect of microstructure and composite orientation on fatigue crack-growth resistance in ductile-metal reinforced brittle-matrix laminates have not been available. Accordingly, the current work addresses the influence of reinforcement morphology, orientation, and metal layer thickness on the fatigue crack-growth resistance of laminated Nb-reinforced Nb3Al intermetallic composites. The study focuses on three laminate layer thicknesses (at a nominally constant volume fraction of 20 pct) in both the divider and arrester orientations. Results are compared with previous examinations of in situ particulate-reinforced Nb/Nb3Al composites to show the effectiveness of coarserscale, high aspect-ratio reinforcements in promoting crackgrowth resistance. II.

EXPERIMENTAL PROCEDURES

A. Laminate Processing D.R. BLOYER, Postdoctoral Researcher, and R.O. RITCHIE, Professor, are with the Materials Sciences Division, Lawrence Berkeley National Laboratory, and Departme