Fracture of Ti-Al 3 Ti metal-intermetallic laminate composites: Effects of lamination on resistance-curve behavior

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I. INTRODUCTION

INTERMETALLICS are known to possess high compressive strength and stiffness, high oxidation resistance and melting temperatures, sometimes a low density, and, generally, good creep resistance.[1] However, owing to their ordered structure, intermetallics exhibit limited dislocation mobility leading to brittle fracture at low temperatures (KIC 2 MPa 1m for Al3Ti), thus limiting their use as structural components. Considerable research has gone into the development of structural intermetallics,[2] and one of the strategies for increasing the utility of intermetallics involves the toughening of the intermetallic with ductile reinforcements. Over the last two decades, composites with different ductile-reinforcement morphologies, which included particles, wires, and laminates, were developed, and the effect of these ductile reinforcements on the mechanical properties has been extensively investigated.[3–26] In spite of the classical tradeoff between higher stiffness/ strength or higher toughness, an optimal application-specific microstructural design can be achieved by incorporating the excellent properties exhibited by the individual components RAGHAVENDRA R. ADHARAPURAPU, Graduate Student, KENNETH S. VECCHIO, Professor, and FENGCHUN JIANG, Project Scientist, are with the Materials Science and Engineering Program, Department of Mechanical and Aerospace Engineering, University of California, San Diego, La Jolla, CA 92093-0411. AASHISH ROHATGI, Senior Scientist, is with GeoCenters Inc., Naval Research Laboratory Operations, Fort Washington, MD 20749-1340. Contact e-mail: [email protected] Manuscript submitted January 15, 2005. METALLURGICAL AND MATERIALS TRANSACTIONS A

in the right microstructure architecture. The development of metal-intermetallic laminate (MIL) composites is a step toward that direction. Previous studies on mollusk shells[27,28] have shown that hierarchical structures over various length scales, based on weak constituents (CaCO3 and organic binders), endow the final structure with highly optimized mechanical properties. Such biomimetic motivation has led to the development of MIL composites with damage-critical properties, such as specific fracture and fatigue properties (i.e., properties normalized with density) equal or superior to the component materials. The MIL composites do not possess a strict hierarchical structure as exhibited by the mollusk shells; instead, they are layered materials with discrete interfaces over the microscale. Nevertheless, such a biomimetic architecture at a single length scale has been shown to dramatically improve many properties such as fracture and impact behavior.[26] In order to use the beneficial properties of the intermetallic Al3Ti, such as its high compressive strength and stiffness, low density, and good creep resistance, while mitigating its detrimental brittle nature, it is necessary to reinforce the brittle phase with tougher ductile metals.[29] Various methods that have been developed to produce laminate composite structures include diffusion

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