MECHANOCHEMISTRY AND MECHANICAL ALLOYING 2003 JOURNAL OF MATERIALS SCIENCE 39 (2 0 0 4 ) 5325 – 5331 Microstructure changes in TiB 2 -Cu nanocomposite under sintering Y.-S. KWON Research Center for Machine Parts and Materials Processing, University of Ulsan 680-749, Korea D. V. DUDINA ∗ , M. A. KORCHAGIN, O. I. LOMOVSKY Institute of Solid State Chemistry and Mechanochemistry SB RAS, Kutateladze 18, Novosibirsk, 630128, Russia E-mail: dudina dv@mail.ru; dudina@solid.nsc.ru Stability and growth of nanoparticulate reinforcements in metal matrix composites during heating are widely studied for dispersion-strengthened alloys, which contain several volume percent of reinforcing phase. When high volume content of nanoparticles distributed within a matrix is concerned results of particles aggregation and growth as well as crystallization mechanisms are not so evident. In this work microstructural evolution under sintering in metal-matrix composite TiB 2 -Cu with high volume content (up to 57%) of titanium diboride nanoparticles 30–50 nm in size was investigated. The nanocomposite powders were produced through synthetic method combining preliminary mechanical treatment of initial powder mixtures in high-energy ball mill, self-propagating exothermic reaction and subsequent mechanical treatment of the product. We focused on microstructure changes in TiB 2 -Cu nanocomposite consolidated by Spark-Plasma Sintering and conventional sintering and showed that in the former case fine-grained skeleton of titanium diboride is formed with connectivity between particles well established. In the latter case behavior of nanoparticles is surprising: at low temperatures fiber-like structures are formed while increasing temperature causes appearance of faceted crystals. These unusual results allow us to propose the direct involvement of nanoparticles in the processes of crystallization by moving as a whole in the matrix. C  2004 Kluwer Academic Publishers 1. Introduction Increased interest to properties of nanomaterials re- quires stability and evolution of nanostructure under a variety of conditions to be studied. This covers evalu- ation of stability under increasing temperature, changes during consolidation from powder state and processes involved in the performance of nanomaterial. In this paper we studied peculiarities of nanostructure evolution in TiB 2 -Cu nanocomposite. This system was chosen for investigations due to unique combination of the properties of constituent phases. TiB 2 exhibits high melting point, high hardness and in contrast to other ce- ramics it is thermally and electrically conductive. TiB 2 - Cu composites were obtained in a number of works [1–4] with the goal to produce dispersion-strengthened copper as potential material for electric contacts. Mate- rials combining high-temperature stability and thermal conductivity can be also obtained on the basis of TiB 2 - Cu system. In this case a side of a bar subjected to high temperatures is made of titanium diboride while the opposite one being cooled is made of copper, dis- tribution of elements through the bar being continuous ∗ Author to whom all correspondence should be addressed. [5]. Therefore, not only dispersion-strengthened alloys of this type are of interest but functionally gradient ma- terials as well. Methods of mechanical treatment in high-energy ball mills have shown promise in preparation of pow- der nanomaterials [6]. When compounds are obtained, which are formed through highly exothermic reactions it is efficient to use mechanical treatment together with conventional self-propagating high-temperature syn- thesis (SHS). We obtained TiB 2 -Cu nanocomposite combining self-propagating exothermic reaction with preliminary and subsequent mechanical treatment of the powders [7]. Conditions for SHS-reaction to pro- ceed are satisfied by high exothermity of titanium di- boride formation. SHS-reaction in this system gives no phases other than required (no Cu-Ti intermetallics or borides other than TiB 2 ). This kind of processing makes it possible to obtain very high concentrations (more than 50 vol%) of reinforcement particles of extremely small (nano) size. So, two tasks can be allotted: to determine conditions of compaction allowing for nanostructure retention and 0022–2461 C  2004 Kluwer Academic Publishers 5325