Generation of metallic nanocomposites by severe plastic deformation A. Bachmaier and R. Pippan* In this article, the potential to fabricate composites with ultrafine grained structures, as well as composites with a nanostructure, by different severe plastic deformation methods, is reviewed. A broad spectrum of diverse composites produced by severe plastic deformation methods exist which include metal–metal composites, metal matrix composites and amorphous metal matrix composites. Furthermore, the influence of the strain path and initial structure on the final composite material is outlined. Keywords: Accumulative roll bonding, Composite, Equal channel angular pressing, High-pressure torsion, Nanocomposite, Severe plastic deformation Introduction Some of the general principles of severe plastic deformation (SPD) have been known for 60 years and some knowledge is reported to have existed since ancient times. 1 Great scientific interest in these techniques started about 20 years ago which was mainly triggered by development of new techniques and the improvement of the properties obtained by SPD processing. Several reviews deal with the background, the basic principles of SPD techniques, the processing of pure metals and metallic alloys and the fundamental properties intro- duced by SPD. 2–6 Briefly summarised, large strains (true strains >10) are applied to a material under high hy- drostatic pressure during SPD which induces significant grain refinement without changing the overall dimen- sions of the specimen. The microstructure obtained after SPD is often termed ultrafine-grained (UFG) and the materials processed by SPD are often reported to have unique properties not accessible by other techniques. The exact definition of the terms UFG and SPD itself and a list of requirements that should be met during SPD are given in several reviews, for example, in Ref. 2. Therefore, the fundamental principles of these methods are outlined only briefly in the following section and the processing of pure metals and metallic alloys by SPD is only summarised shortly in the present review. The focus in this review is a new approach towards the use of different SPD methods for the generation of composites or even nanocomposites. A composite is a material which consists of two or more distinct phases or constituents with different physical or chemical properties separated by an inter- face. Examples for a classical composite material are ceramic particles or carbides mixed in a metallic matrix. In general, composite materials can have a ceramic, metallic or a polymeric matrix. The properties, for example, mechanical properties, of the matrix are improved by adding constituents which are often denoted as reinforcements or reinforcement phases. A classical alloy with a microstructure consisting of two phases produced by solidification methods from the melt is usually not denoted a true composite material. 7 Nevertheless, we will treat them as composite materials in this review because the behaviour is the same. One major goal is to synthesise composites in bulk form with a uniform microstructure and a homogeneous distribution of the reinforcing constituents in the matrix on the micrometre or even nanometre scale. Processing nanocomposites with traditional techniques such as powder extrusion, hot isostatic pressing, powder injec- tion moulding or sinter forging has always been a challenge due to issues in maintaining the nanostructure in the composite while simultaneously obtaining a fully dense bulk material. 8 Most of these traditional techni- ques include the following composite production steps: consolidation of powders or nanopowders (ceramic, metallic or their mixtures) into a form and sintering at elevated temperatures at which the powder particles can inter diffuse and bond satisfactorily. Due to the sintering treatment, grain growth can often not be avoided and the nanostructure is lost during processing. SPD offers a new processing alternative to conven- tional composite production methods. With SPD tech- niques, it is possible to obtain massive bulk composite materials which often have a nanoscaled microstructure even after processing. The reason for this is that the starting material, independent of whether it is bulk or powder, is in most cases micrometre sized instead of nanometre sized and grain refinement is achieved directly during processing. The microstructural refine- ment is homogenous through the whole sample and in the case of consolidation of powders, the amount of porosity can be reduced to values of nearly zero. The article is divided into the following sections: in the section on ‘SPD methods’, the different SPD methods are described briefly; in sections on ‘Deformation of single-phase materials’ and ‘Deformation of multiphase materials’, the deformation of single-phase materials Erich Schmid Institute of Materials Science, Austrian Academy of Sciences, Jahnstrasse 12, 8700 Leoben, Austria *Corresponding author, email reinhard.pippan@oeaw.ac.at ß 2013 Institute of Materials, Minerals and Mining and ASM International Published by Maney for the Institute and ASM International DOI 10.1179/1743280412Y.0000000003 International Materials Reviews 2013 VOL 58 NO 1 41