Hindawi Publishing Corporation Journal of Metallurgy Volume 2013, Article ID 628495, 12 pages http://dx.doi.org/10.1155/2013/628495 Research Article Columnar-to-Equiaxed Transition in Metal-Matrix Composites Reinforced with Silicon Carbide Particles Alicia E. Ares 1,2 and Carlos E. Schvezov 1,2 1 Faculty of Sciences, University of Misiones, 1552 F´ elix de Azara Street, 3300 Posadas, Misiones, Argentina 2 Member of Scientific Research Career (CIC) of the National Council of Scientific and Technical Research (CONICET), 1917 Rivadavia Street, 1033 Buenos Aires, Argentina Correspondence should be addressed to Alicia E. Ares; aares@fceqyn.unam.edu.ar Received 30 July 2013; Accepted 4 November 2013 Academic Editor: Menahem Bamberger Copyright © 2013 A. E. Ares and C. E. Schvezov. Tis is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Te present work is focused on the study of the efect of directional heat extraction on the silicon-carbide (SiC) distribution in zinc-aluminum matrix composites (MMCs) and on the columnar-to-equiaxed (CET) position in directionally solidifed samples. To this end, a ZA-27 alloy matrix was reinforced with ceramic particles of SiC and vertically directionally solidifed. Te cooling rates, temperature gradients, and interphase velocities were then measured, and their infuence on the solidifcation microstructure of the MMCs was analyzed. Te recalescence detected and measured during the equiaxed transition was of the order of 3.5 ∘ C to 1.1 ∘ C. Te values of the temperature gradients reached a minimum during the CET and were even negative in most cases (between −3.89 K and 0.06 K). Te interphase velocities varied between 0.07 mm/s and 0.44 mm/s at the transition. Also, the presence of ceramic particles in ZA-27 alloys afected the thermodynamic local conditions and the kinetics of nucleation, producing a fner microstructure. 1. Introduction A metal-matrix composite (MMC) is composite material with at least two constituent parts, with one being a metal. Te other material may be a diferent metal or another material, such as a ceramic or organic compound. Te matrix is the monolithic material into which the reinforcement is embedded and is completely continuous. Te reinforcement does not always serve a purely structural task but is also used to change physical properties such as wear resistance, friction coefcient, or thermal conductivity. Te reinforcement can be either continuous or discontinuous [1]. Te aspect ratio of the reinforcement is an important quantity, because the degree of load transfer from the matrix to the reinforcement is directly proportional to the reinforcement aspect ratio. Particle or short fber reinforced metals have a much lower aspect ratio, so they exhibit lower strengths than their continuous fber counterparts, although the properties of these composites are much more isotropic [1, 2]. Metal-matrix composites can be processed by several techniques. Some of these important techniques are liquid- state processes (casting or liquid infltration, squeeze cast- ing, or pressure infltration), solid-state processes (difusion bonding, deformation processing, powder processing, sinter- forging, and deposition techniques), in situ processes, and spray-forming of particulate MMCs [2]. Metal-matrix particulate composites such as SiC particle- reinforced aluminum can ofer a 50–100% increase in Young’s modulus over that of unreinforced aluminum, that is, the modulus being equivalent to that of titanium, but density that is about 33% lower. In general, ceramic reinforcements (fbers, whiskers, or particles) have a coefcient of thermal expansion lower than that of most metallic matrices. On the other hand, when the composite is subjected to a tem- perature change, thermal stresses are generated in both com- ponents [3]. Te development of composites obtained by solidifcation of alloys has made remarkable progress and their applications in automotive and aerospace industries have increased in recent decades. Among these applications, the most current one is the zinc and aluminum base. It is important for metallurgical processes to establish relationships between thermal parameters and solidifcation structures, because the solidifcation structure encompasses