Design of multiscalar metallic multilayer composites for high strength, high toughness, and low CTE mismatch
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INTRODUCTION
IN designing materials for structural application, materials scientists normally must make tradeoffs in strength and toughness to achieve optimal mechanical properties. These types of tradeoffs are common across all classes of materials. One particularly successful method by which these tradeoffs can be controlled is through combination of materials with complementary properties in a composite. For example, high-strength ceramics can be reinforced with high-toughness metallic phases in order to ameliorate the inherent lack of ductility of the ceramic. Similarly, metal-matrix composites often include ceramic whiskers to increase their strength. The volume fractions and morphologies of the different phases may be chosen in order to achieve optimal property balance for specific applications. While it is the differences between the component materials that make a composite "work," these differences often cause considerable difficulties and limit applicability. Potential problems include chemical incompatibility, coefficient of thermal expansion (CTE) mismatch, and poor interfacial adhesion. ll3 While chemical incompatibility and adhesion problems may be addressed through methods such as adding a thin layer (coating) of another material between the two phases, CTE mismatch is an inherent diffic u l t y - both for materials processing and high-temperature application. The goal of the present report is to outline the design of a novel type of material that has the potential to combine both high strength and toughness without sustaining large CTE mismatches. A logical approach in developing high-strength/hightoughness multilayers or microlaminates is to use ceramic layers separated by relatively thin metallic layers. Recent studies L2,3jindicate that the failure mechanism in D.J. SROLOVITZ, Professor, S.M. YALISOVE, Assistant Professor, and J.C. BILELLO, Professor, are with the Department of Materials Science and Engineering, University of Michigan, Ann Arbor, M1. Manuscript submitted July 13, 1994. METALLURGICALAND MATERIALSTRANSACTIONSA
these materials depends sensitively on the toughness of the interfaces between the brittle and ductile materials. When the interfaces are weak, a crack that develops within the ceramic will propagate along the interface, resulting in significant debonding and some toughening. In the more favorable, tough interface case, the cracks from the ceramic are arrested at the interface. In this case, the metal deforms, and the crack may renucleate on the other side of the metal layer. This can lead to the formation of multiple cracks. In this manner, the ceramic/metal multilayer composite can sustain significant strain while accumulating a large degree of damage. Unfortunately, the difference in thermal expansion between metals and ceramics is normally too large to make this class of composites viable for high-temperature applications. Therefore, we are forced to look for an alternate material for either the strengthening or toughening phase. Ideally, the two materials should be
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