We review microstructures and properties of metal matrix composites produced by severe plastic deformation of multiphase alloys. Typical processings are wire drawing, ball milling, roll bonding, equal-channel angular extrusion, and high-pressure torsion of multiphase materials. Similar phenomena occur between solids in frictional contact such as in tribology, friction stir welding, and explosive joining. The resulting compounds are characterized by very high interface and dislocation density, chemical mixing, and atomic-scale structural transitions at heterointerfaces. Upon straining, the phases form into nanoscaled filaments. This leads to enormous strengthening combined with good ductility, as in damascene steels or pearlitic wires, which are among the strongest nanostructured bulk materials available today (tensile strength above 6 GPa). Similar materials are Cu-Nb and Cu-Ag composites, which also have good electrical conductivity that qualifies them for use in high-field magnets. Beyond the engineering opportunities, there are also exciting fundamental questions. They relate to the nature of the complex dislocation, amorphization, and mechanical alloying mechanisms upon straining and their relationship to the enormous strength. Studying these mechanisms is enabled by mature atomic-scale characterization and simulation methods. A better understanding of the extreme strength in these materials also provides insight into modern alloy design based on complex solid solution phenomena.
In this overview of steel-based composites, consideration is given to conventional metal-matrix composites, in which steel is combined with another metal, ceramic, or polymer. In addition, we define fully steel composites, in which both components of the structure are developed within the steel. These approaches are integrated by discussing a series of macroscopic, mesoscopic, and microscopic examples. This review provides an integrated view of steel composites and allows modeling of the mechanical response to be considered both at the continuum level and in terms of dislocation mechanisms depending on the length scale and the degree of mechanical contrast between the constituent phases. In the context of fully steel composites, consideration is given to static systems in which the volume fraction of the strengthening phase is constant and the length scale is varied by heat treatment or imposed plastic strain. Moreover, we discuss dynamic systems in which a phase transition occurs concomitantly with plastic strain, resulting in an increase in the density of planar barriers that control the plasticity. A discussion of classical works that describe materials such as Damascus steels is used as a template to consider a variety of ways of producing ultrahighstrength steel composites. Examples of applications are cited and linked to the important issue of developing appropriate fabrication methods for the production of current and future steel composites. 213 Annu. Rev. Mater. Res. 2010.40:213-241. Downloaded from www.annualreviews.org by University of Sussex on 10/10/12. For personal use only.
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