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KINEMATICS AND KINETICS OF CRYSTAL PLASTICITY

A. S. Argon

in Strengthening Mechanisms in Crystal Plasticity

Crystal plasticity is overwhelmingly a consequence of the self similar translations of dislocations, which is viewed as a limiting form of more general shear transformations that also include ... More

Crystal plasticity is overwhelmingly a consequence of the self similar translations of dislocations, which is viewed as a limiting form of more general shear transformations that also include twinning and martensitic transformations. Unlike elastic deformation, which can be homogeneous down to the atomic scale, plastic deformation involving dislocation translations or other shear transformations are locally discrete and inhomogeneous. Plasticity can be viewed as homogeneous only when the discrete processes are homogenized over a representative volume element (RVE), large enough to represent quasi-smooth behavior. Thus, for the purpose of viewing plasticity as a continuum field theory, its applicability is limited to volume elements no smaller than the RVE over which processes have been homogenized. In this chapter, the essential kinematics of inelastic deformation is introduced broadly. This is followed by the development of dislocation line properties that are associated with plastic strain production and the expenditure of plastic work in crystal plasticity, including concepts of dislocation line tension, dislocation mass, forms of interaction of dislocations with applied stresses, with each other, with free surfaces, and inhomogeneities. The principles of thermally activated deformation processes are introduced and then applied to dislocation glide and overcoming of local obstacles by thermal assistance. The detailed developments of dislocation properties emphasize face centered cubic crystals, including a full complement of partial dislocation and their dislocation.Less

THE ROLE OF STRONG H-BONDS IN NATURE: A GALLERY OF FUNCTIONAL H-BONDS

Gastone Gilli and Paola Gilli

in The Nature of the Hydrogen Bond: Outline of a Comprehensive Hydrogen Bond Theory

Functional H-bonds are H-bonds which are significantly stronger than the surrounding ones and, for this reason, can play a specific role in the mechanism of action of important chemical or ... More

Functional H-bonds are H-bonds which are significantly stronger than the surrounding ones and, for this reason, can play a specific role in the mechanism of action of important chemical or biochemical processes. This chapter reports a preliminary collection of these bonds organized in a graphic gallery of cases with little discussion, a collection of themes which have already been, or deserve to be, investigated to unravel the true role played by the H-bond in natural systems. Themes treated include: RAHB-driven processes (prototropic tautomerism in heteroconjugated systems, secondary structure of proteins, and DNA base pairing); H-bond-controlled crystal packing; bistable H-bonds in functional molecular materials (ferro/antiferroelectric crystals, excited-state proton transfer); low-barrier charge-assisted H-bonds in enzymatic catalysis (the catalytic triad of serine proteases; and proton transmission in water chains (Grotthuss mechanism, gramicidine A channel, aquaporin channels).Less

Ferromagnetism in the h.c.p. phase

E. R. DOBBS

in Helium Three

The higher-density, hexagonal close packed (h.c.p.) crystal is expected to become ferromagnetic below about 10 microkelvin. This chapter shows clear evidence of its existence.