Microstructure and Properties of Laser-Deposited Ti6Al4V Metal Matrix Composites Using Ni-Coated Powder
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THE extreme conditions of the environment in space that involve, for example, thermal and ionizing radiation and thermal cycling, provide both a challenge and opportunity for material scientists and designers.[1] In such applications, particulate-reinforced metal matrix composites (MMCs) are of interest, given that their specific strength, stiffness, toughness, and coefficient of thermal expansion (CTE) can be tailored to attain low density and dimensionally stable structures. The strength and stability of the interfacial region between ceramic reinforcement particles and metal matrix govern the mechanical and physical response of MMCs. For example, the strength, stiffness, and resistance to environmental attack can be improved with a strong interface allowing for effective load transfer from the matrix to the reinforcement.[2] The failure modes can also be influenced by the characteristics of ceramic/ metallic matrix interface.[3,4] Failure processes that are initiated by interfacial debonding are likely to occur when a composite material with a weak interface is subjected to an applied stress. Commonly used ceramic reinforcements are not wetted by metal systems.[4] In fact, inspection of the scientific literature reveals that significant efforts have been devoted to the problem of interfaces in MMCs, and that the application of a coating onto the surface of the reinforcement has evolved B. ZHENG, Postdoctoral Research, Y. ZHOU, Associate Researcher, and E.J. LAVERNIA, Dean of College of Engineering, are with Department of Chemical Engineering and Materials Science, University of California, Davis, CA 95616. Contact e-mail: [email protected] J.E. SMUGERESKY, Senior Staff Member, is with Sandia National Laboratories, Livermore, CA 94551-0969. D. BAKER, CEO, is with Advanced Powder Solutions, Inc., Houston, TX 77095. Manuscript submitted August 17, 2007. Article published online March 21, 2008 1196—VOLUME 39A, MAY 2008
as one of the most effective approaches used to modify the interfacial structure to promote wettability.[5] One approach that is being considered is to encapsulate the ceramic particle with a metal coating, thereby mitigating the challenge of wetting at the metal-ceramic interface.[2,5] This approach effectively prevents the formation of voids or cracks at the metal-ceramic interface, and prevents clustering of the ceramic particles. A wide range of materials and methods has been used to apply coatings onto the surface of reinforcement particles and fibers. For example, if a metallic coating is applied onto a ceramic particle, a strong metallic bond can be formed between the coating metal and the matrix metal, while good bonding, formed by either mechanical interlocking or diffusion and chemical reaction between the reinforcement and the coating metal, may be achieved through the coating processes. In cases where the metal coating is reactive with the matrix, brittle intermetallic phases may form at the interface during processing, such that the composite properties are degraded. This problem can be avoide
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