A molecular dynamics study of grain boundary structures and their impacts on dislocation nucleation mechanisms

It is well known that grain boundaries play a significant role in determining the mechanical properties of polycrystalline materials. The study of the relationships between grain boundary (GB) structure and its associated deformation mechanisms is therefore of importance. This thesis investigates GB structures and GB energies over a wide range of GB misorientation angles and it investigates their influences on deformation mechanisms. In the first part of this thesis, 1184 molecular statics (MS) simulations were conducted to study the structures and energies of the GBs of Cu and Al, taking into account their 66 tilt axes and various misorientation angles. The effects of GB tilt axis and misorientation angle on GB structure and GB energy were systematically investigated. In the second part of this thesis, molecular dynamics (MD) simulations were performed to study the deformation mechanisms of symmetric tilt GBs with tilt axes of [0 0 1] and [1 1 0] in Cu under both ‘free’ and ‘constrained’ boundary conditions. The results indicate that stress states can have a profound effect on dislocation mechanisms. Dislocation nucleation was found to be independent of intrinsic GB structures. An automatic analysis of MD simulations provided detailed information on the dislocation nucleation and emission of GBs. The results in this thesis contribute to our understanding of GB structure, GB energy and the dislocation nucleation mechanism. The GB energy data obtained in this thesis will be included in a crystal plasticity finite element model to improve predictions of texture evolution and grain refinement during plastic deformation

Evaluation of Mechanical Properties of Σ5(210)/[001] Tilt Grain Boundary with Self-Interstitial Atoms by Molecular Dynamics Simulation

Grain boundary (GB) can serve as an efficient sink for radiation-induced defects, and therefore nanocrystalline materials containing a large fraction of grain boundaries have been shown to have improved radiation resistance compared with their polycrystalline counterparts. However, the mechanical properties of grain boundaries containing radiation-induced defects such as interstitials and vacancies are not well understood. In this study, we carried out molecular dynamics simulations with embedded-atom method (EAM) potential to investigate the interaction of Σ5(210)/[001] symmetric tilt GB in Cu with various amounts of self-interstitial atoms. The mechanical properties of the grain boundary were evaluated using a bicrystal model by applying shear deformation and uniaxial tension. Simulation results showed that GB migration and GB sliding were observed under shear deformation depending on the number of interstitial atoms that segregated on the boundary plane. Under uniaxial tension, the grain boundary became a weak place after absorbing self-interstitial atoms where dislocations and cracks were prone to nucleate

Atomistic Simulation of Hydrogen Embrittlement of Grain Boundaries in Metals

It has been known for about a century that hydrogen contamination causes severe degradation in the mechanical properties of metals. This phenomenon is generally termed as ‘hydrogen embrittlement’ (HE). In this thesis, the underlying mechanisms behind HE phenomenon were elucidated on an atomic scale. The H segregation at various grain boundaries (GBs) and its influence on the structure, mechanical properties, deformation mechanisms and failure response of GBs were examined by atomistic simulations. First, H segregation at various GBs was studied in this thesis. The results indicated that H segregation properties were very sensitive to GB structures. The effects of H atoms on the mechanical behaviour and plastic deformation of GBs were then examined. It was shown that H atoms modified the behaviour of dislocation nucleation and caused the yield stress of dislocation nucleation to increase or decrease. Different deformation mechanisms were directly responsible for this modification. In addition, H segregation increased the critical shear stress and impeded the coupled GB motion, irrespective of the GB structures. During GB migration, H vacancy clusters cannot grow, which suggests that the coupled GB motion may help to resist H-induced intergranular embrittlement. The role of H atoms in changing the interaction of dislocations with GBs was also investigated. Several interaction mechanisms such as dislocation transmission, nucleation and reflection were reported for different glide planes and GB structures. Segregated H atoms transformed these interaction mechanisms into ones involving dislocation absorption for most of GBs. This disordered the atomic structure of GBs and established a local stress state, which promoted the ultimate failure of GBs due to the formation of vacancies

The effect of Ag, Pb and Bi impurities on grain boundary sliding and intergranular decohesion in Copper

We investigate the changes in grain boundary sliding (GBS) and intergranular decohesion in copper (Cu), due to the inclusion of bismuth (Bi), lead (Pb) and silver (Ag) substitutional impurity atoms at a $\Sigma$5 (0 1 2) symmetric tilt grain boundary (GB), using a first-principles concurrent multiscale approach. We first study the segregation behavior of the impurities by determining the impurity segregation energy in the vicinity of the GB. We find that the energetically preferred sites are on the GB plane. We investigate the intergranular decohesion of Cu by Bi and Pb impurities and compare this to the effect of Ag impurities by considering the work of separation, $W_s$ and the tensile strength, $\sigma_t$. Both $W_s$ and $\sigma_t$ decrease in the presence of Bi and Pb impurities, indicating their great propensity for intergranular embrittlement, whilst the presence of Ag impurities has only a small effect. We consider GBS to assess the mechanical properties in nanocrystalline metals and find that all three impurities strongly inhibit GBS, with Ag having the biggest effect. This suggests that Ag has a strong effect on the mechanical properties of nanocrystalline Cu, even though its effect on the intergranular decohesion properties of coarse-grained Cu is not significant.Department of Energy, SciDac [grant number ER-25788], Intel Corporation, Graduate School of Excellence MAINZ, IDEE project of the Carl Zeiss FoundationThis is the author accepted manuscript. The final version is available from Taylor & Francis via http://dx.doi.org/10.1080/14786435.2016.121736

Plasticity of Nanotwinned Aluminum and Aluminum Alloy

Nanotwinned (nt) metals have been intensively studied and has shown unique mechanical properties, including high strength and high ductility. Although twins can be introduced into face-centered-cubic (fcc) metals by annealing (annealing twins), deformation (deformation twins) and growth (growth twins), most of these twinned metals have low stacking fault energy (SFE). The twinnability of fcc metals remains largely controlled by their SFE. Consequently, the high SFE of Al typically prohibits the formation of twins in aluminum (Al). This dissertation focuses on the introduction of several innovative strategies that can introduce high density growth twins in Al and Al alloys and study the influence of twinnability on strengthening and plastic deformation of these twinned alloys. The growth twins were observed in a polycrystalline Al thin film fabricated by magnetron sputtering. And the twin formation mechanism was discussed in a thermodynamic view. Then, we show that high-density twin boundaries can be introduced in Al films by tailoring the texture of the films without any seed layers. Transmission Kikuchi diffraction and transmission electron microscopy studies on (111), (110) and (112) textured Al films. Epitaxial Al (112) film has the highest density of ITBs, because the twin variants (335) and (535) are separated by Al (102) islands, promoting the formation of ITBs. The smaller domain size can thus be achieved by introducing HAGBs into the twinned bicrystal structure to inhibit the abnormal growth of single variant. Furthermore, twin boundaries in Al appear to be stronger barriers to dislocations than conventional high angle grain boundaries. Besides tailoring the twin structure by changing the growth orientation, alloying has been used in an Al matrix. The high strength epitaxial AlMg alloy has been fabricated with a high density twinned structure. The strong ITB barriers play an important role to strengthen the film. Combined with the solid-solution strengthening, the calculated flow stress correlated well with the experimental data. The knowledge derived from this study may facilitate the design of high-strength, light-weight, and ductile Al alloys

Low stress creep of copper and some aluminium and magnesium alloys at high and intermediate homologous temperatures.

Creep behavior of high purity copper under low stress has been investigated in tension for bamboo grain-structured wires 25 -500 pm diameter and foils 0.4 and 0.6 mm thick and, in bending, for foils 100 and 250 pm thick. Additionally, creep behavior of polycrystalline 7075 aluminium and AZ61 magnesium alloy has been investigated under low applied tensile stress. The conditions explored feature diffusional creep and related mechanisms expected to be operative at the high temperatures and low stresses involved. The creep and surface profile of copper (>99.99%) wires has been investigated close to their melting temperature (0.93 Tm) under stresses up to 0.35 MPa for which strain rate varied linearly with stress. For the thinnest wires (diameter 25- 125 pm), the strain rate was about twice that expected from Nabarro-Herring diffusional creep theory and between one and two orders of magnitude larger than expected from Harper-Dorn creep. For 500 pm diameter wire, the measured rate was initially near to Harper-Dorn prediction but became constant only at longer durations at a level about five times lower than this. The lower rates were about 1.5 times that expected from diffusional creep. The surface profile observations indicated a small contribution of grain boundary sliding to the creep process when grain boundaries were not closely perpendicular to the stress. The observed effect of grain aspect ratio on the creep rate is shown to provide better correlation with theory. Tensile creep tests were carried out on OFHC copper foils at 850°C and 990°C in the stress range 0.1-0.6 MPa. The stress exponent for creep was found to be close to 2 and measured rates were about two orders of magnitude faster than expected from diffusional creep. Slip lines, approximately 30 pm apart, were observed on the surface after creep. The creep process in these foils under tensile loading is ascribed to glide of dislocations controlled by the rate of generation of dislocations at Bardeen-Herring sources about 30 pm apart. The creep tests in bending (which are novel) were carried out at 950°C in cantilever configuration loaded under self weight. The measured profile of the crept foils confirmed the linear dependence of strain rate on stress with final curvature 7-13 times lower than predicted from diffusional creep theory. A hundred nanometer thick alumina coating was applied to some copper foils prior to creep exposure. The associated localization of strain at grain boundaries was found to result in fracture of a 100 nm thick alumina coatings there at extremely low applied stress and overall strain. Tensile creep test of thermomechanically treated 7075 aluminium alloy of initial grain size 48 (am at <5MPa and 350 to 410°C showed a stress exponent close to 1. After correcting for grain growth to 79 pm during the test, the creep rates were within a factor of two of those expected for Nabarro-Herring creep. The creep rates were found to be lower for longer test durations evidently due to grain growth at test temperature and thus indirect evidence for dependence of N-H creep rate on grain size was obtained. True activation energy for creep was found to be close to 165 kJ/mol comparable to the aluminium self diffusivity. For AZ61 magnesium alloy at 250 to 346°C, and stresses upto 6 MPa, Bingham type behaviour was observed with threshold stress decreasing with increasing temperature. The corresponding activation energy for creep was 106 ± 9 kJ/mol comparable with that expected for grain boundary self diffusion in magnesium with the resulting values of grain boundary diffusivity closely matching those obtained previously for Coble creep in pure magnesium. Grain elongation in the direction of the application of tensile stress was observed also to be consistent with operation of Coble creep. Strain rate versus stress for both these materials are shown to be continuous with published results for superplastic flow under comparable conditions

Effects of minor Cu and Mg additions on microstructure and material properties of 8xxx aluminum conductor alloys

The effects of minor Cu (0–0.29 wt%) and Mg (0–0.1 wt%) additions on the microstructure, electrical conductivity, mechanical, and creep properties of 8xxx aluminum conductor alloys were studied. The microstructure evolution was investigated using an optical microscope and the electron backscattered diffraction technique. The creep property was characterized by the primary creep strain and the minimum creep rate during creep deformation. The results demonstrated that additions of minor Cu and Mg reasonably improved the ultimate tensile strength but slightly reduced electrical conductivity. The addition of Cu remarkably decreased the primary creep strain but had a negligible effect on the minimum creep rate, leading to a beneficial effect on the short-term creep resistance but no advantage to the creep resistance under the long-term creep process. The minor addition of Mg greatly reduces both the primary creep strain and minimum creep rate, resulting in a significant and effective improvement in the creep resistance