Plastic Deformation Of Fcc Metals Research Articles

Atomistic modeling of the coupling between electric field and bulk plastic deformation in fcc metals

A notable impediment in maintaining high electric fields in accelerating structures is the onset of breakdown events. While bulk mechanical properties of the materials are known to significantly affect the breakdown propensity, the underlying mechanisms coupling electric fields to bulk plastic deformation in experimentally relevant thermal and electrical loading conditions remain to be identified at the atomic scale. We present the results of large-scale molecular dynamics simulations (MD) to investigate a possible mode of coupling. Specifically, we consider the activation of Frank-Read sources, which leads to dislocation multiplication, under the combined action of biaxial thermal stresses caused by rf losses and surface tractions induced by electric fields. With the help of a charge-equilibration formalism incorporated in a classical MD model, we show that the creation of surface slipped steps can couple to electric fields in a way that enhances local stresses and facilitates further activation of existing dislocation sources. We quantify the possible enhancement of surface slip under typical microstructural parameters of annealed copper. We show that such a mechanism could potentially promote breakdown precursor formation at very high electric fields, but that its impact is limited at fields typical of the operation of accelerator structures. In this regime, thermal stresses caused by rf losses are expected to be the main drivers of plastic deformation.

The effect of grain boundary on the local incipient plastic deformation of fcc metals during nanoindentation

The effect of grain boundaries (GBs) on deformation mechanisms becomes increasingly important as the volume of deformation reaches the submicrometer and nanoscale. The current work investigates the impact of grain boundaries on the incipient plasticity of small-scale deformations of fcc metals. For this purpose, the behavior of single and bi-crystal Au thin films during nanoindentation are studied, using large-scale atomistic simulations. Various symmetric ⟨110⟩ tilt GBs with a wide range of misorientation angles are included to analyze the effect of GB geometry on the nanoscale plasticity mechanisms. Potentially, GBs can act as a source, sink, or obstacle for lattice dislocation, depending on their geometry, energy level, and distance from the deformation zone. The role of the heterogeneous nucleation and emission of dislocations from GBs on the plasticity and hardness of bicrystals is analyzed. According to our results, the intrinsic free volume involved in the GB region is associated with dislocation nucleation at the GB. The volume of the plastic zone generated beneath the tip and the way it grows is strongly dependent on the GB structure. Dislocation nucleation occurs predominantly in the early stages of indentation at GBs with a dissociated interface structural unit, before the interaction of lattice dislocation and GB. Coherent twin boundaries display the lowest effect on the hardness. Based on our results, there is a strong correlation between the interfacial boundary energy and its effect on the bicrystal hardness. GBs with lower interfacial energy offer a higher barrier against slip transmission and nucleation at the GB.

Beyond classical thermodynamics: Dislocation-mediated plasticity

It is suggested a thermodynamic framework to describe plastic deformation in FCC metals at low strain rates and temperatures when the major mechanism of deformation is dislocation motion. The theory involves new thermodynamic parameters: dislocation polarization tensor, stress resistance tensor, entropy and temperature of microstructure. They are linked by constitutive equations. Entropy and temperature of microstructure are associated with the rate of slip avalanches and average energy drop in slip avalanches, respectively. The theory is illustrated by comparison with experimental data.

Software for Mathematical Modeling of Plastic Deformation in FCC Metals

The question on the necessity of software implementation in the study of plastic deformation in FCC metals with the use of mathematical modeling methods is investigated. This article describes the implementation features and the possibility of using the software Dislocation Dynamics of Crystallographic Slip (DDCS). The software has an advanced user interface and is designed for users without an extensive experience in IT-technologies. Parameter values of the mathematical model, obtained from field experiments and accumulated in a special database, are used in DDCS to carry out computational experiments. Moreover, the software is capable of accumulating bibliographic information used in research.

Transitional grain boundary structures and the influence on thermal, mechanical and energy properties from molecular dynamics simulations

The thermo-kinetic characteristics that dictate the activation of atomistic crystal defects significantly influence the mechanical properties of crystalline materials. Grain boundaries (GBs) primarily influence the plastic deformation of FCC metals through their interaction with mobile dislocation defects. The activation thresholds and atomic mechanisms that dictate the thermo-kinetic properties of grain boundaries have been difficult to study due to complex and highly variable GB structure. This paper presents a new approach for modelling GBs which is based on a systematic structural analysis of metastable and stable GBs. GB structural transformation accommodates defect interactions at the interface. The activation energy for such structural transformations was evaluated with nudged elastic band analysis of bi-crystals with several metastable 0 K grain boundary structures in pure FCC Aluminium (Al). The resultant activation energy was used to evaluate the thermal stability of the metastable grain boundary structures, with predictions of transition time based on transition state theory. The predictions are in very good agreement with the minimum time for irreversible structure transformation at 300 K obtained with molecular dynamics simulations. Analytical methods were used to evaluate the activation volume, which in turn was used to predict and explain the influence of stress and strain rate on the thermal and mechanical properties. Results of molecular dynamics simulations show that the GB structure is more closely related to the elastic strength at 0 K than the GB energy. Furthermore, the thermal instability of the GB structure directly influences the relationship between bi-crystal strength, temperature and strain rate. Hence, theoretically consistent models are established on the basis of activation criteria, and used to make predictions of temperature-dependent yield stress at a low strain rate, in agreement with experimental results.

Macroscopic and Microscopic Descriptions of the Plastic Deformation of Fcc Metals over a Wide Range of Strain and Temperature

Macroscopic and Microscopic Descriptions of the Plastic Deformation of Fcc Metals over a Wide Range of Strain and Temperature T. Csanadia,∗, N.Q. Chinh, J. Gubicza and T.G. Langdon Department of Materials Physics, Eotvos University Budapest H-1117 Budapest, Pazmany P. setany 1/A., Hungary, Departments of Aerospace & Mechanical Engineering and Materials Science, University of Southern California Los Angeles, CA 90089-1453, USA, Materials Research Group, School of Engineering Sciences, University of Southampton Southampton SO17 1BJ, U.K.

Study of the True Nature of Microband Boundaries in Aluminum with 3D EBSD

There are two opposing theories regarding the nature of aligned dislocation boundaries generated during plastic deformation of FCC metals: (i) they are oriented along crystallographic planes, and (ii) their alignment is dictated by the macroscopic stress state during plastic deformation. 3D crystallographic orientation data were collected on a volume containing microbands in commercial purity aluminum, and 3D boundaries were reconstructed. Both types of alignment were found in local surface features.

Modelling plastic deformation of metals over a wide range of strain rates using irreversible thermodynamics

Based on the theory of irreversible thermodynamics, the present work proposes a dislocation-based model to describe the plastic deformation of FCC metals over wide ranges of strain rates. The stress-strain behaviour and the evolution of the average dislocation density are derived. It is found that there is a transitional strain rate (∼ 104 s-1) over which the phonon drag effects appear, resulting in a significant increase in the flow stress and the average dislocation density. The model is applied to pure Cu deformed at room temperature and at strain rates ranging from 10-5 to 106 s-1 showing good agreement with experimental results.

Activation volume analysis of plastic deformation in fcc materials using nanoindentation

ABSTRACTActivation volumes for plastic deformation of fcc metals are measured using nanoindentation. Materials include bulk, polycrystalline aluminum and α-brass, and sputtered copper and nickel thin films on silicon substrates. From a hardness measurement, V* is defined as 9kBT/∂H/∂ln , where H is the hardness and is a representative strain rate beneath the indenter. Data obtained using nanoindentation are consistent with those obtained using more conventional experiments based on uniaxial loading.

Enhanced modeling of hardening in crystal plasticity for FCC metals

A crystal plasticity model for large plastic deformation of FCC metals is extended in order to be able to predict grain size dependent effects. An aggregate of grains at a material point is considered, where each grain is subdivided into a single crystal interior section and several bi-crystals, which are assumed to represent the grain boundaries, each having the crystallographic orientations of their adjacent grains. The micro-macro interaction law is taken into account by a Taylor approach, which is modified for the bi-crystal elements: compatibility as well as stress equilibrium are met at their interface. Moreover, during loading, a plastic deformation difference between the grain core and the associated bi-crystal half arises, of which a plastic strain gradient can be determined, dependent on the grain size. To maintain compatibility of the lattice between the core and the boundary, a certain amount of geometrically-necessary dislocations (GNDs) is required. These dislocations form additional obstacles to the dislocation movement supporting the ongoing plastic deformation, and accordingly introduce enhanced slip system hardening. The well-known Hall-Petch relation, indicating the empirical influence of the grain size on the flow stress, is simulated numerically.

A hybrid finite element formulation for polycrystal plasticity with consideration of macrostructural and microstructural linking

A hybrid finite element formulation for the plastic deformation of FCC metals with anisotropy is outlined. Polycrystal plasticity theory is used to develop the constitutive response. The hybrid approach facilitates introduction of the microscale stress in the macroscopic statement of equilibrium. Convergence of the hybrid formulation is contrasted with that of a velocity-pressure formulation. It is demonstrated that the hybrid formulation is well suited for studies where significant spatial variations in constitutive response result from having only one, or a very few, crystal orientations represented in each finite element. A simulation of channel die compression is made with one crystal per finite element. The resulting texture evolution is compared with other texture evolution models and experimental data for cold rolled aluminum. It is demonstrated that the brass texture component, observed in the experimental data, is developed through shear deformations arising from grain-to-grain interactions.

The absence of steady-state flow during large strain plastic deformation of some Fcc metals at low and intermediate temperatures

A study of the plastic deformation of several fcc metals and alloys at large strains was conducted. The purpose of this study was to take a critical look at the assumption of steady-state flow at low and intermediate temperatures. For this purpose, large strain data were obtainedvia torsion testing of thin-walled tubes. The stress-strain results from these tests followed two distinct trends: at low temperatures, strain hardening continued at shear strains of 8; at higher temperatures strain softening occurred. Continued strain hardening was observed in pure nickel, nickel-cobalt solid-solutions, pure aluminum, and two aluminum alloys. A laminar arrangement of closely spaced dislocation walls arises at large strains and low temperatures, which differs from the well-recovered equiaxed subgrain structure observed at high temperature. Thus, it appears that dynamic recovery processes are not sufficient to establish a steady-state dislocation structure at low temperatures. Strain softening in nickel and nickel-cobalt at higher temperatures was attributed to dynamic recrystallization. In none of the large strain tests conducted was a steady-state flow stress, independent of strain, observed. Torsion test data were found to match data from steady-state tensile creep and compression tests. In the case of the large strain torsion tests of nickel, recrystallization occurred. This suggests that recrystallization and boundary migration can be important processes in creep.