Misorientation data from Electron Backscatter Diffraction (EBSD) is often used to identify strain localisation and quantify plastic strain at the microstructural scale. However, the exact relationship between local plastic strain and misorientation and how it changes at the grain and sub-grain level has not been studied in detail. We have used high resolution digital image correlation (HRDIC) to measure plastic strain at the sub-micron scale on the surface of a nickel superalloy strained to 2%. The strain values have been correlated to different misorientation measures at the grain and subgrain scale, over several hundreds of grains. We show that although the grain mean plastic strain is positively correlated to the lattice misorientation, there is a large scatter in the correlation, which depends on the misorientation measure used. There is also essentially nocorrelation between the magnitude of grain strain and grain orientation derived parameters like the Schmid factor and the Taylor factor, largely due to deformation bands at the mesoscale that are not crystallographic. At these strain levels, the relationship between misorientation and plastic strain is affected by the differences in how slip (discontinuous) and lattice rotation (continuous) develop, by local grain interactions and the development of transgranular strain localisation. It is therefore effectively not possible to quantify plastic strain within individual grains using EBSD derived misorientation values alone, although some measures of misorientation are more appropriate than others if there is an understanding of the underlying local plastic phenomena. Whereas slip is localised in slip bands, the misorientation varies smoothly in a manner that is only weakly spatially correlated to the slip. These fifindings have implications for the modelling of the deformed state of polycrystalline metals at the microstructural scale using continuum mechanics.

dislocations on basal or prismatic planes and {11-22} compression twins are commonly activated in deformed Titanium (Ti). In the present work, their interactions are investigated by both crystallographic analysis and atomistic simulations. For a three-dimensional {11-22} twin, we firstly analyze seven possible twin boundaries (TBs) bonding two low index planes in matrix and twin. Next, we focus on the two lower energy boundaries,{11-22}M/T || {11-22}T/M coherent twin boundary (CTB) and {-12-11}M/T || {-12-11}T/M . Depending on dislocation character and boundary type, we define four types of interactions between dislocations and these TBs. Further, we predict possible dislocation reactions on/across TBs using crystallographic analysis according to the deformation compatibility and the change in elastic energy, such as twinning/detwinning of the primary twin, slip transmission and secondary twinning, for each type of interaction. Molecular dynamics (MD) simulations are then conducted for all interactions under pre-selected loadings in order to explore the dynamic process associated with each of these interactions and examine the predicted reactions. MD simulations predict that the interaction between dislocations and some facets can lead to the formation of secondary twins and dislocations on basal or prismatic planes in twins, and reveal the possibility of forming and dislocations in twins. Moreover, some of the possible reactions take place on lateral TBs other than CTBs.

By systematically investigating the relaxation behavior of a model metallic glass based on the extensive molecular dynamics (MD) simulations combined with the dynamic mechanical spectroscopy method, a pronounced ultra-low temperature peak on the loss modulus spectrum was discovered for the first time in MD simulations. It was found that the relaxation peak occurs at a much lower temperature than the typical temperature for the conventional β relation peak as reported in the literature. According to the atomic displacement analysis, we unravel that the reversible atomic motions, rather than the thermal vibrations or local structural rearrangements, mainly contribute to this relaxation peak. We further identify the atomic level mechanism of this fast relaxation process by characterizing the local geometrical anisotropy. Furthermore, by tracing the dynamic behaviors of these “reversible” atoms, we demonstrate the intrinsic hierarchy of the relaxation modes, which are triggered by atomic vibrations and gradually developed to include the reversible and irreversible atomic movements. Our findings provide an important piece of the puzzle about the holistic picture of the rich relaxation processes in metallic glasses.

Nanocrystalline materials are known to possess excellent radiation resistance due to high fraction of grain boundaries that act as defect sinks, provided they are microstructurally stable at such extreme conditions. In this work, radiation response of a stable nanocrystalline Cu-Ta alloy is studied by irradiating with 4MeV copper ions to doses (close to the surface) of 1 displacements per atom (dpa) at room temperature (RT); 10 dpa at RT, 573 and 723 K; 100 and 200 dpa at RT and 573 K. Nanoindentation results carried out for samples irradiated till 100 dpa at RT and 573 K show exceptionally low radiation hardening behavior compared to various candidate materials from literature. Results from microstructural characterization, using atom probe analysis and transmission electron microscopy, show a stable nanocrystalline microstructure with minimal grain growth and a meagre swelling in samples irradiated to 100 dpa (~0.2%) and 200 dpa at RT, while no voids in those at 573 K. This radiation tolerance is partly attributed to the stability of Ta nanoclusters due to phase separating nature of the alloy. Additionally, the larger tantalum particles are observed to undergo ballistic dissolution at doses greater than 100 dpa and are eventually precipitated as nanoclusters, replenishing the sink strength, which enhanced material’s radiation tolerance when exposed to high irradiation doses and elevated temperatures.

In this work, we propose a series of Object Kinetic Monte Carlo simulations complemented by an analytical model that allows rationalizing a certain number of experimental facts related to the growth of high purity, recrystallized zirconium alloys under irradiation. Our vision of the phenomenon rests essentially on vacancy diffusion anisotropy (with faster diffusion in the basal planes than perpendicular to them) that is necessary to lead to the formation of layers of prismatic interstitial dislocation loops parallel to the basal plane. The acceleration of the deformation under irradiation and this localization of the damage are strongly connected. An analytical model developed using the concepts of difference of anisotropic diffusion between vacancies and interstitials makes it possible to account for the observed phenomena.

A transmission electron microscopy (TEM) specimen of a titanium aluminide (TiAl) alloy was irradiated in-situ, at the IVEM-TANDEM facility, with 1 MeV Kr ions to a maximum fluence of 1.25×10^19 ions m^−2 at room temperature. The irradiated microstructure was then investigated ex-situ using advanced analytical TEM in combination with TEM image simulations and molecular dynamics (MD) simulations. The TEM examination showed that dot-like defects first formed in both the α2-Ti3Al and γ -TiAl phases of the irradiated microstructure. With increasing irradiation dose planar defects formed and propagated within the γ phase, and most of the planar defects were accompanied by the dot-like defects. The TEM image simulations and MD irradiation simulations revealed that the origins of the dot-like and planar defects were interstitial clusters and stacking faults, respectively, and large interstitial clusters were surrounded by dislocation loops. The interstitial clusters formed and grew in the irradiated microstructure due to much faster diffusion of interstitials than that of vacancies at room temperature. Local stress concentrations increased near large interstitial clusters and lamellar interfaces, which resulted in the nucleation and propagation of stacking faults. Moreover, the configurations of the lamellar interfaces in the start microstructure were found to play an important role in the formation and accumulation of the radiation induced interstitial clusters and stacking faults at room temperature.

In a previous manuscript, large scale 3D atomistic simulations were used to study the interaction between a curved dislocation with a dominant screw character and a Coherent Twin Boundary (CTB) for three FCC metals (Al, Cu and Ni) using 6 embedded-atom method (EAM) potentials. Under both uniaxial and multiaxial stresses, both transmission mechanism and critical transmission stress differ from the results reported in 2D simulations. Transmission mechanisms were found to be material dependent, and non-glide (Escaig) stresses have a profound effect on the transmission. In order to provide input for constitutive equations for mesoscopic approaches, the previous results are used to inform analytic models that incorporates mechanisms observed in the atomistic simulations. For Al, the Fleischer mechanism of cross-slip at the twin boundary is used whereas for Cu and Ni, Escaig mechanism of cross-slip at the twin boundary is invoked.For Cu and Ni the transmission stress is determined by the critical stress required to grow the cross-slip nucleus in the adjacent grain, as opposed to growth of cross-slip nucleus in the twin boundary. As a result, the critical transmission stress in these materials is almost athermal. The analytic, mechanistic model reproduces the atomistic simulation results for the Escaig stress dependence of the transmission stress as well as its dependence on a shear component in the CTB fairly well, within 10% for the Escaig stress dependence.

Hydrogen has a significant impact on the formation of vacancies, clusters and voids in palladium and other metals. The formation of vacancy-hydrogen complexes in bulk Pd and at ∑3 and ∑5 grain boundaries was investigated using first-principles calculations and thermodynamic models. Equilibrium vacancy and cluster concentrations were evaluated as a function of temperature and hydrogen partial pressure based on the Gibbs energy of formation including vibrational and configurational entropies. Vacancies were found to be significantly stabilized by association with interstitial hydrogen, leading to enhanced concentrations by several orders of magnitude. Vacancy clusters were further stabilized at grain boundaries, with equilibrium concentrations reaching site saturation for clusters comprising up to three vacancies.Nanovoids were investigated based on Wulff constructions from calculated surface energies of the (0 0 1) and (1 1 1) terminations as a function of temperature and coverage of hydrogen adsorbates. The most stable termination changed from (1 1 1) in vacuum to (0 0 1) in H2, and the surface energies were lowered due to hydrogen adsorbates. Consequently, voids were also stabilized in the presence of hydrogen. Coalescing of vacancies into nanovoids was found to be thermodynamically unfavorable due to the loss of configurational entropy. It was therefore concluded that enhanced concentrations of vacancies and clusters does not directly cause the formation of voids. The formation of voids in Pd-based membranes was discussed in terms of microstructural characteristics, and strain due to chemical expansion and plastic deformation.

Measurements of the local tetragonality in Fe-C martensite at microstructural length-scale through pattern matching of electron backscatter diffraction patterns (EBSPs) and careful calibration of detector geometry are presented. It is found that the local tetragonality varies within the complex microstructure by several per cent at largest and that the scatter in the axial ratio is increased at higher nominal carbon content. At some analysis points the local crystal structure can be regarded as lower symmetry than simple body centred tetragonal. A linear relation between the nominal carbon content and averaged local tetragonality measured by EBSD is also obtained, although the averaged axial ratio is slightly below that obtained from more classical X-ray diffraction measurements.

In this study we investigate Zn liquid metal embrittlement at Fe grain boundaries (GBs) byemploying ab initio density functional theory (DFT) simulations. We consider three different GBs and two common alloying elements in steels (Si and Al) andevaluate how the interplay of the elements affects Zn enrichment and embrittlement of the GBs. Our simulations show that Zn has stronger tendency toenrich at both GBs and surfaces than Al and Si. By increasing the amount of Al and Si in the bulk, Zn can be reduced at both GBs and surfaces. This isquantified for a large temperature range by the newly introduced replacementpotency, which shows that compared to Al, Si has a larger potency to decreasethe amount of Zn at GBs and surfaces. Regarding GB embrittlement, our resultsdemonstrate that Si a small positive, Al a small negative, and Zn a strong negative effect on GB cohesion. In combination with Zn both Al and Si are ableto somewhat reduce the detrimental effect of Zn.