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This paper studies the effects of high-pressure torsion (HPT) at ambient temperature on microstructural evolution and mechanical properties enhancement in pure copper. The aim is to introduce gradient microstructure, with various statistical distributions of grain size and grain orientations to examine their effect on strength and ductility. To this end, extruded cylindrical pure copper subjected to HPT for 1, 2, and 3-turns resulted in grain refinement down to the grain size of 500 nm. Combination of microhardness test and EBSD scans through the radial direction confirm the creation of a heterogeneous structure through the thickness and radial directions. The results demonstrate that increasing the shear strain leads to (1) ultra-fine grain (UFG) generation at deformed coarse-grain boundaries, (2) an increase in the fraction of recrystallized grains and high angle grain boundaries, and (3) a homogenous structure in the last step. A unique mixture has been obtained due to the particular shape of the anvils. The mixture included a chain of UFGs and coarse grains contain dislocations and subgrains. The highest level of gradient structure through the thickness was observed after 1-turn, which leads to the best combination of strength and ductility.

Mechanical properties that are considered to be deterministic in the macro-scale have been shown to be stochastic in the sub-micron length scale. The origin of such stochastic responses is not well understood. This work examines the potential influence of grain boundaries and grain orientations on the stochastic nature of pop-in and hardness measurement in annealed high purity polycrystalline Cu samples during low load nanoindentation. Statistical analysis on pop-in load and hardness showed that variations of these measurements depend on crystal orientations and is influenced by the indenter probe size. Analysis on the pop-in load statistics showed that pop-ins are likely initiate from an atomic sized precursor that leads to dislocation generation or expansion. Variation in hardness measurements near an arbitrary chosen grain boundary and the apparent grain boundary hardening effect observed may be related to the higher density of dislocations at and near the grain boundary.

Cu/Ni/Nb tri-layer and CuNi/Nb alloy multilayer films were annealed to examine the microstructural evolution and mechanical properties of three component laminated metallic nanostructures. Scanning transmission electron microscopy showed Ni x Nb y compounds forming on either side of the Nb layer in both tri-layer and alloy films. Post annealing nanoindentation showed all annealing conditions resulted in increased hardness, indicative of the Ni x Nb y intermetallic phase increasing the hardness of the films. The hardness of both films achieves a maximum hardness after annealing for 3 h at 300 °C. The hardness increase as the annealing temperature increases corresponds to a thicker Ni x Nb y layer at the incoherent boundary. This strengthening technique could be implemented in a variety of different multilayer systems to achieve age-hardenable multilayer metallic nanostructures.

Deformation twinning, a common and important plastic deformation mechanism, is the key contributor to the excellent combination of strength and ductility in twinning-induced plasticity (TWIP) steel. In the open literature, a significant amount of research has been reported on the microstructural characteristics of deformation twinning and its influence on the overall deformation behavior of TWIP steel. In this study, we examine the feasibility of a mechanism-based crystal plasticity model in simulating the microstructural level deformation characteristics of TWIP steel. To this end, a model considering both double-slip and double-twin is developed to investigate the stress–strain behavior and local microstructural features related to the formation and growth of micro-twins in low stacking fault energy (SFE) TWIP steel. The twin systems are described as pseudo-slips that can be activated when their resolved shear stress reaches the corresponding critical value. A hardening law that accounts for the interaction among the slip and twin systems is also developed. Numerical simulations for different mesh sizes and single crystal patch tests under different loading modes are carried out to verify the modeling procedure. Our simulation results reveal that, despite its simple nature, the double-slip/double-twin model can capture the key deformation features of TWIP steel, including twin volume fraction evolution, continuous strain hardening, and the final fracture in the form of strain localization.

Recent attention has been given to the proper treatment of the planar traction-free surfaces which typically bound a computational box in three-dimensional dislocation dynamics. This paper presents an alternative to the use of the finite-element method for this purpose. Here, to annul the tractions produced by a sub-surface dislocation segment on a finite-area free surface S , a combination of an image dislocation segment, and a distribution of N prismatic rectangular Volterra dislocation loops meshing S is utilized. The image dislocation segment, with the proper sign selection of the Burgers vector components, annuls the shear stresses, and the normal stress component is annulled discretely at N collocation points representing the centers of the loops. The unknowns in this problem are the magnitudes of the N Burgers vectors for the loops. Once these are determined, one can back calculate the Peach–Koehler force acting on the sub-surface segment and representing the effect of the free surface. As expected, the accuracy of the method improves as the loops continuously decrease in size.

In this work, we develop a framework coupling continuum elasto-viscoplasticity with three-dimensional discrete dislocation dynamics (micro3d). The main problem is to carry out rigorous analyses to simulate the deformation of single crystal metals (fcc and bcc) of finite domains. While the overall macroscopic response of the crystal is based on the continuum theory, the constitutive response is determined by discrete dislocation dynamics analyses using micro3d. Size effects are investigated by considering two boundary value problems: (1) uniaxial loading of a single crystal cube, and (2) bending of a single crystal micro-beam. It is shown that boundary conditions and the size of the computational cell have significant effect on the results due to image stresses from free-boundaries. The investigation shows that surface effects cannot be ignored regardless of the cell size, and may result in errors as much as 10%. Preliminary results pertaining to dislocation structures under bending conditions are also given.

In this work, we discuss a thermodynamic theory of plasticity for self-organization of collective dislocations in FCC metals. The theory is described by geometrical tensor quantities of crystal defect fields such as dislocation density tensor, representing net mobile dislocation density and geometrically necessary boundaries, and the incompatibility tensor representing immobile dislocation density. Conservation laws for the two kinds of dislocation density are formulated with dislocation products and interactions terms. Based on the second law of thermodynamics, we drive basic constitutive equations for the dislocation flux, production and interaction terms of dislocations. We also derive a set of reaction-diffusion equations for the dislocation density tensor and incompatibility tensor which describes the vein and persistent slip band (PSB) ladder structures. These equations are analyzed using linear stability and bifurcation analysis. An intrinsic mesoscopic length scale is determined which provides an estimate for the wavelength of the PSBs. The basic aspects of the model are motivated and substantiated by analyzing the stress fields of various possible dislocation configurations using discrete dislocation dynamics.

The internal structure of planar dislocation boundaries is explored through a coupling of experimental observations of extended geometrically necessary boundaries (GNBs) in deformed single crystals with a dislocation dynamics simulation. The internal dislocation structure of a GNB is approached first through standard diffraction contrast analyses to identify the boundary Burgers vectors in the transmission electron microscope (TEM). This result is placed in the context of a large series of boundaries by the calculation of boundary misorientation angle/axis pairs from Kikuchi pattern analysis in the TEM of orientations on either side of the boundaries. Of special interest are the boundary misorientation axes which together with the boundary normal, allow one to estimate the contribution of primary and secondary dislocations to boundary rotations. Selected boundaries are constructed using the experimental data, crystal plasticity analysis and Frank’s formula. The constructed boundaries are input into the dislocation dynamics code and allowed to equilibrate (relax). The internal stress field of the boundary is determined.

A numerical methodology to incorporate anisotropic elasticity into three-dimensional dislocation dynamics codes has been developed, employing theorems derived by Lothe [J. Lothe, Philos. Mag. 15 (1967) 353], Brown [L.M. Brown, Philos. Mag. 15 (1967) 363], Indenbom and Orlov [V.L. Indenbom, S.S. Orlov, Sov. Phys. Crystallogr. 12 (6) (1968) 849], and Asaro and Bamett [R.J. Asaro, D.M. Barnett, in: R.J. Arsenault, J.R. Beeler Jr., J.A. Simmons (Eds.), Computer Simulation for Materials Applications, Part 2. Nuclear Metallurgy, Vol. 20, p. 313]. The formalism is based on the stress field solution for a straight dislocation segment of arbitrary orientation in three-dimensional space. The general solution is given in a complicated closed integral form. To reduce the computation complexity, look-up tables are used to avoid heavy computations for the evaluation of the angular stress factor ( Σ ij ) and its first derivative term ( Σ ij ′). The computation methodology and error analysis are discussed in comparison with known closed form solutions for isotropic elasticity. For the case of Mo single crystals, we show that the difference between anisotropic and isotropic elastic stress fields can be, for some components of the stress tensor, as high as 15% close to the dislocation line, and decrease significantly away from it. This suggests that short-range interactions should be evaluated based on anisotropic elasticity, while long-range interactions can be approximated using long-range elasticity.