DEFORMATION INSTABILITIES, POLYCRYSTALS, FLOW IN METALS WITH NANOSTRUCTURE, SUPERPOSITION OF STRENGTHENING MECHANISMS, AND TRANSITION TO CONTINUUM PLASTICITY
The chapter starts with a consideration of processes that establish and control dynamic dislocation fluxes, their creation where they have been initially not present, often requiring over-stress leading to prominent yield phenomena and factors that govern establishment of mobile dislocation densities. This is followed by consideration of dynamic flux instabilities referred to as Portevin-LeChatelier effects leading to jerky flow. A related phenomenon that is considered is the dynamic overshoot instability at very low temperatures. Also discussed is the elevation of plastic resistance in polycrystals above that of unconstrained single crystals. Such compatible plastic flow among grains of different orientation usually results in grains of different plastic resistance and internal back stresses that manifest themselves in reduced plastic resistance when the deformation direction is reversed. This phenomenon, known as the Bauschinger effect, is discussed. Grain size-dependent rises in plastic resistance, known as the Hall-Petch effect, due to compartmentalization of strain hardening in grains of decreasing size is considered. This is pursued further to polycrystals of nanometer scale. The chapter considers a number of interactions of individual mechanisms of strengthening with each other, and demonstrates that such interactions are generally complex and their superposition can rarely be described by simple additivity. In the well defined cases when several mechanisms can co-exist in the same precipitate, the superposition of mechanisms occurs via the superposition of the individual force distance curves for the different mechanisms. The chapter discusses the peculiarities of some of these interactions. Finally, the chapter concludes with a discussion of the conditions of transition between dislocation mechanics and continuum plasticity.
Keywords: yield phenomena, mobile dislocation densities, dislocation fluxes, Portevin-LeChatelier effects, jerky glide, cryogenic temperatures, polycrystal plasticity, Bauschinger effect, Hall-Petch effect, strengthening mechanisms
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