A Phenomenological Mesoscopic Field Dislocation Mechanics (PMFDM) model is developed, extending continuum plasticity theory for studying initial-boundary value problems of small-scale plasticity. PMFDM results from an elementary space-time averaging of the equations of Field Dislocation Mechanics (FDM), followed by a closure assumption from any strain-gradient plasticity model that attempts to model effects of geometricallynecessary dislocations (GND) only in work-hardening. The specific lower-order gradient plasticity model chosen to substantiate this work requires one additional material parameter compared to its conventional continuum plasticity counterpart. The further addition of dislocation mechanics requires no additional material parameters. The model a) retains the constitutive dependence of the free-energy only on elastic strain as in conventional continuum plasticity with no explicit dependence on dislocation density, b) does not require higher-order stresses, and c) does not require a constitutive specification of a 'back-stress' in the expression for average dislocation velocity/plastic strain rate. However, long-range stress effects of average dislocation distributions are predicted by the model in a mechanistically rigorous sense. Plausible boundary conditions (with obvious implication for corresponding interface conditions) are discussed in some detail from a physical point of view. Energetic and dissipative aspects of the model are also discussed. The developed framework is a continuous-time model of averaged dislocation plasticity, without having to rely on the notion of incremental work functions, their convexity properties, or their minimization. The tangent modulus relating stress rate and total strain rate in the model is the positive-definite tensor of linear elasticity, and this is not an impediment to the development of idealized microstructure in the theory and computations, even when such a convexity property is preserved in a computational scheme. A model of finite deformation, mesoscopic single crystal plasticity is also presented, motivated by the above considerations.Lower-order gradient plasticity appears as a constitutive limit of PMFDM, and the development suggests a plausible boundary condition on the plastic strain rate for this limit that is appropriate for the modeling of constrained plastic flow in three-dimensional situations.

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FePt/iron oxide core/shell nanoparticles are synthesized by a two step polyol process with 1,2hexadecanediol as the reducing reagent. Monodispersed 2.6-nm FePt nanoparticles are first obtained by reduction of iron(III) acetylacetonate and platinum(II) acetylacetonate. These preformed FePt nanoparticles are then used as seeds and an iron oxide shell is formed in the second synthesis step. The role of the iron oxide shell on sintering of FePt nanoparticles is investigated. Annealing studies show that these FePt/ iron oxide core/shell structures are stable after annealing at 550 °C for 30 min at which 2.6-nm FePt nanoparticles without oxide shell coating start to sinter. Low-temperature magnetic hysteresis behavior of the annealed core/shell nanoparticles suggests exchange coupling between the magnetically hard FePt core and the magnetically soft iron oxide shell.

Monodispersed FePt nanoparticles are synthesized by reduction of iron(II) acetylacetonate and platinum(II) acetylacetonate with 1,2-hexadecanediol as the reducing reagent in the polyol process. As-prepared FePt nanoparticles are chemically disordered with fcc phase. Transmission electron microscopy (TEM) images show a self-assembled particle array with an average particle size of 3 nm and a standard deviation about 10%. The transformation from chemically disordered fcc to chemically ordered L10 phase is achieved by annealing at 650 degrees C for 30 min in Ar atmosphere where the oxygen level is less than 1 ppm. Magnetic hysteresis measurements show a coercivity of 9.0 kOe at 293K, and 16.7 kOe at 5 K for the annealed FePt nanoparticles.

The influence of Cu, Ag, and Au additives on the L1 0 ordering, texture, and grain size of FePt thin films has been examined. Lattice parameter data indicated that Au and Ag additives tended to segregate from FePt, but Cu alloyed with FePt. FePt films with Au or Ag additive showed 1-2 kOe higher coercivity values compared to a pure FePt film after annealing at 450°C and above for 10 min. The addition of at least 20 vol. % Cu to FePt boosted average coercivity values and increased ͑001͒/͑002͒ x-ray peak intensity ratios, suggesting an accelerated L1 0 ordering process for annealing temperatures exceeding 350°C. Decreasing the film thickness promoted ͑001͒ film texture in FePtϩ20% Cu films, but higher annealing temperatures were required to achieve large coercivity. Au and Ag limited the average grain size compared to a pure FePt film. Cu additive increased the average grain size and film roughness.

In Part I of this set of two papers, a model of mesoscopic plasticity is developed for studying initial-boundary value problems of small scale plasticity. Here we make qualitative, finite element method-based computational predictions of the theory. We demonstrate size effects and the development of strong inhomogeneity in simple shearing of plastically constrained grains. Nonlocality in elastic straining leading to a strong Bauschinger effect is analyzed. Low shear strain boundary layers in constrained simple shearing of infinite layers of polycrystalline materials are not predicted by the model, and we justify the result based on an examination of the no-dislocation-flow boundary condition. The time-dependent, spatially homogeneous, simple shearing solution of PMFDM is studied numerically. The computational results and an analysis of continuous dependence with respect to initial data of solutions for a model linear problem point to the need for a nonlinear study of a stability transition of the homogeneous solution with decreasing grain size and increasing applied deformation. The continuous-dependence analysis also points to a possible mechanism for the development of spatial inhomogeneity in the initial stages of deformation in lower-order gradient plasticity theory. Results from thermal cycling of small scale beams/films with different degrees of constraint to plastic flow are presented showing size effects and reciprocal-filmthickness scaling of dislocation density boundary layer width. Qualitative similarities with results from discrete dislocation analyses are noted where possible. ARTICLE IN PRESSwww.elsevier.com/locate/jmps 0022-5096/$ -see front matter r (A. Acharya).We discuss the convergence of approximate solutions with mesh refinement and its implications for the prediction of dislocation microstructure development, motivated by the notion of measure-valued solutions to conservation laws. r

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