Homogenization of Metal Matrix Composites by High-Pressure Torsion I. SABIROV, O. KOLEDNIK, and R. PIPPAN This article deals with the homogenization of metal matrix composites (MMCs) with inhomogeneous particle distribution by severe plastic deformation. In this study, Al6061-10 pct SiC and Al6061-20 pct Al 2 O 3 powder metallurgy (PM) MMCs with clustered particle distribution in the as-fabricated condition are subjected to high-pressure torsion (HPT) at room temperature. The evolution of the microstructure during HPT is investigated. It is shown that, in the two materials, two different types of particle clusters appear that behave differently during deformation. In MMCs with dense particle clusters, the process of declustering during HPT occurs through a mechanism of particle debonding from the surface of the clusters. On the contrary, in MMCs with diffuse particle clusters, the deformation of the clusters is mainly responsible for the homogenization; therefore, the strain necessary to obtain a homogeneous particle distribution can be predicted by the Tan and Zhang model (M.J. Tan and X. Zhang: Mater. Sci. Eng. A, 1998, vol. 244, pp. 80-85). I. INTRODUCTION AS is well known, the presence of particle clusters in metal matrix composites (MMCs) significantly degrades their formability and ductility. [1–5] Tan and Zhang [5] proposed, based on a geometrical model, that to attain a homogeneous particle distribution in MMCs, the theoretical mean inter- particle spacing should be equal or higher than the matrix powder size, as follows: [1] In Eq. [1], d m is the matrix powder size required to attain a homogeneous particle distribution in MMCs with a particle size d p and a particle volume fraction f. For example, for an MMC having a particle volume fraction f = 10 pct and a particle size d p = 50 m, a matrix powder size d m  37 m would be required (Eq. [1]). For MMCs with small particles, which have a high strength and good ductility, [1] however, a very small matrix powder would be required. But it is very difficult to fabricate such small powder particles from metals, such as aluminium, due to the formation of oxide layers, clus- ters of powder particles, etc. [6] By secondary deformation methods, such as rolling or extrusion, the required size d m can be enhanced. Tan and Zhang [5] proposed an estimation formula for the required matrix powder size, depending on the extrusion ratio or the reduction ratio during rolling, [2] where R is the extrusion ratio during a self-similar reduction and R' is the rolling ratio of a sheet. [5] However, for small d p  d m ca p 6 f b 1>3 - 1 d 1R 1 - R¿ d p  d m ca p 6 f b 1/3 - 1 d particle sizes and reasonable sizes of d m , this would require extremely high values of extrusion or rolling ratio. The purpose of this article is to show that a method involving severe plastic deformation can be applied for the homogenization of strongly clustered MMCs. Furthermore, the mechanism by which the process of declustering occurs during high-pressure torsion (HPT) of strongly clustered MMCs is investigated. Finally, the possibility of predicting the homogeneity of the particle distribution in MMCs after severe plastic deformation is also of interest. II. MATERIALS AND EXPERIMENTAL PROCEDURES A. Materials An Al6061-10 pct SiC MMC and an Al6061-20 pct Al 2 O 3 MMC are investigated. The chemical composition of the matrix material is given in Table I. The alumina particle size is between 1 and 5 m and the SiC particle size is between 0.3 and 1m. In both materials, there are a few single particles that are about 10 m in size or somewhat larger. The size of the matrix powder, d m , was 40 m for both MMCs. The extru- sion ratio, R, was 10. The composites were supplied in the form of rods with a diameter of 25 mm. Both materials exhibit an inhomogeneous particle distribution in the as-fabricated condition (Section III, Figures 2(a) and 5(a)). The Al6061- 10 pct SiC MMC contains mostly dense particle clusters (par- ticle clusters without matrix between individual particles). On the contrary, in the Al6061-20 pct Al 2 O 3 , a fraction of diffuse particle clusters (with some amount of matrix between indi- vidual particles) prevails; dense particle clusters are also observed. The microstructure of both materials in the as- fabricated condition is described in detail in Section III. B. High-Pressure Torsion For HPT, disks with a diameter of 8 mm and a thickness, t, of 0.8 mm are cut perpendicular to the extrusion axis (Figure 1). The specimens are subjected to HPT at a pressure of 5 GPa at room temperature. The number of turns, n, is METALLURGICAL AND MATERIALS TRANSACTIONS A VOLUME 36A, OCTOBER 2005—2861 I. SABIROV, Postdoctoral Fellow, O. KOLEDNIK and R. PIPPAN, Professors, are with the Erich Schmid Institute of Materials Science, Austrian Academy of Sciences, 8700-Leoben, Austria. Contact e-mail: sabirov@ unileoben.ac.at Manuscript submitted February 18, 2005.