Formation of a TiB 2 -reinforced copper-based composite by mechanical alloying and hot pressing
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I. INTRODUCTION
A COPPER-BASED composite formed by dispersion strengthening with TiB2 is a leading candidate for applications where an excellent combination of high thermal and electrical conductivity and high-temperature mechanical strength are required.[1–5] Unlike the precipitation-strengthened copper alloys (such as Cu-Zr and Cu-Cr), Cu-TiB2 maintains its strength up to very high temperatures because of the excellent thermal stability of the TiB2 particles.[2,4,6] The thermodynamic stability of the TiB2 can also result in an extremely low residual content of Ti and B in the copper matrix, and, therefore, the composite exhibits a relatively high thermal and electrical conductivity.[1,4] Direct manufacture of Cu-TiB2 from Cu and TiB2 starting material does not yield composites with acceptable microstructural properties (e.g., fine and uniformly distributed TiB2 particles and strong interfacial bonding between the ceramic particles and the matrix).[4,6] In-situ formation TiB2 by reaction appears to be necessary, and several ways of doing this have been investigated.[4,6] Chrysanthou and Erbaccio[1] have studied in-situ formation of composites with up to 18 wt pct TiB2 by carbothermic reduction of B2O3 in the presence of Cu-Ti melts and by mixing Cu-Ti and Cu-B melts. In the first method, the wetting between the ceramic particle and the matrix was poor due to the evolution of CO gas during the carbothermic reduction of B2O3. Only a partial reaction between the two melts was obtained in the second method, since a continuous TiB2 layer prevented further mixing and, hence, further reaction between the two alloys. Poor ceramic particle distribution and coarse particle size are some of the major problems in those manufacturing processes, in which in-situ formation of ceramic particles occurs in liquid Cu alloys because of the density difference between TiB2 and the Cu matrix.[1,4] In this connection, Lee et al.[4] have reported an improved spray-forming process, in which the ceramic particles are formed after deposition of droplets rather than formed in the Cu-Ti-B melt as in S.J. DONG, Post-Doctoral Fellow, Y. ZHOU, Assistant Professor, and B.H. CHANG, Post-Doctoral Fellow, are with the Materials Engineering and Processing Group, Department of Mechanical Engineering, University of Waterloo, Waterloo, ON, Canada N2L 3G1. Y.W. SHI, Professor, is with the Department of Materials Engineering, School of Materials Science and Engineering, Beijing Polytechnic University, Beijing, People’s Republic of China 100022. Contact e-mail: [email protected] Manuscript submitted June 13, 2001. METALLURGICAL AND MATERIALS TRANSACTIONS A
conventional spray forming. This is very promising, since the size of the ceramic particles in Cu-TiB2 formed by this improved process (termed reactive spray forming) was much finer than that in conventional spray-formed composites. However, the coarse particles were observed along dendrite cell boundaries, as compared with much finer and more uniformly distributed particles in the matrix.[4] Biselli e
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