In-situ Laue diffraction to follow dislocation patterning during fatigue

Ainara Irastorza Landa
EPFL, 2016
EPFL thesis

Abstract

The macroscopic strength of metals is determined by the dislocation arrangements that are formed when dislocations slip in the crystal lattice in response to the applied stress. Despite the extensive research carried out, the transition from uniform to non-uniform dislocation structures is not yet fully understood. This information is however essential to support the development of computational models that aim to predict dislocation patterning. Lattice rotation caused by the presence of dislocations is, for instance, a parameter that is often not taken into account in computational models although it plays a role in the development of dislocation ensembles. Experimentally following in-situ dislocation patterning including involved lattice rotations at lengthscales comparable to those in simulations is not straightforward. In fact, the current available techniques that have the necessary spatial and angular resolution are either destructive (3D-EBSD) or very time consuming (3D X-ray Laue diffraction). In this dissertation we present a new experimental approach that allows following lattice rotation and dislocation ensembles time-resolved during deformation. The technique is based on X-ray Laue diffraction scanning in transmission mode, which provides a sub-micron spatial resolution in 2D and statistical information on orientation spread in the third dimension. The in-situ mechanical tests are performed at the microXAS beamline of the Swiss Light Source Synchrotron. The method has been applied to understand dislocation pattering of copper single crystals occurring in the earlier phases of low cycle fatigue. Samples with different crystal orientations (single crystal, coplanar and collinear) have been cyclically deformed up to a maximum of 120 cycles. A dedicated miniaturized shear-fatigue device compatible with Laue diffraction has been constructed for that purpose and a sample preparation based on picosecond laser ablation has been developed. The developed procedure analyzes the evolving dislocation microstructures in terms of lattice rotation, lattice curvature and geometrically necessary dislocation densities at lengthscales similar to those addressable in computational models. It sacrifices on the fully 3D aspect but it brings the time resolution required to validate ongoing simulations. It has been shown that the error made by integration depends on the dislocation network expected. For instance, the technique can be used to validate 3DDD models or density based dislocation dynamics models when applied in deformation geometries where the formation of 2D dislocation patterns are expected (e.g. vein-channel structure in single slip oriented fatigued samples). The greatest advantage of this technique is that the evolving regions are easy to follow due to the high rotational sensitivity and time resolution of the method. What is more, the influence of the initial microstructure can be evaluated and quantitative values can be provided. On the other hand, the principal drawbacks are related to the gauge thickness of the samples. Even if the technique can estimate the orientation spread along the third direction, the approach cannot physically resolve the integrated signal along the thickness. This is source of uncertainty when providing actual values of lattice curvatures and geometrically necessary dislocation densities. Another shortcoming is the sensitivity of the method to the energy distribution of the X-ray beam provided in the beamline, which determines the collected signal - the principal basis of the subsequent analysis and interpretation.

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https://infoscience.epfl.ch/record/221090?ln=en

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Ainara Irastorza Landa
EPFL, 2016
EPFL thesis

Abstract

Official source

https://infoscience.epfl.ch/record/221090?ln=en

About this result

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Nanocrystalline metals: transient testing during in situ X-ray diffraction and molecular dynamics

Zhen Sun

Nanocrystalline (NC) metals have attracted widespread interest in materials science due to their high strength compared to coarse-grained counterparts. It is well know that during uniaxial deformation, the stress-strain behaviour exhibits an extraordinary work-hardening followed by an early observation of constant flow stress. Information on possible deformation mechanisms have been gathered by extensive research combining in situ deformation experiments, electron microscopy observations and computational modelling. Generally, these mechanisms are categorized into two types: dislocation slip and grain boundary (GB) accommodation processes, as for instance, GB sliding based mechanisms and GB migration eventually coupled to the shear stress. However, the interplay between these mechanisms resulting in the constant deformation resistance is not fully understood. Transient testing has proven to be a suitable tool to gather information on the rate limiting deformation mechanisms that are activated during the deformation path. In particular in stress reduction experiments, after an intermediate/large stress drop thermally activated dislocation slip is suppressed so that other underlying mechanisms are brought into foreground during subsequent transient creep. Those mechanisms may play a minor role in the determination of the flow stress but might still be essential to the development of a constant deformation resistance. Within this thesis, transient testing is combined with in situ X-ray diffraction at the Swiss Light Source. Therefore, the transient responses are captured in terms of evolution of macrostrain as well as diffraction peak broadening. Since dislocation slip and GB accommodation have an opposite footprint on the peak broadening, the presence of these two types of mechanisms can be distinguished. Three electrodeposited NC materials with different grain sizes are investigated: two NC Ni batches and one NC Ni50Fe50 batch. The results reveal that the constant flow stress reached during uniaxial deformation of electrodeposited NC metals reflects a quasi-stationary balance between dislocation-based mechanisms and GB-mediated accommodation. The latter plays an important role in producing plastic strain and recovering the defects and internal stress. Depending on the magnitude of the stress drop, a non-monotonic behaviour of the diffraction peak width is observed, suggesting an alternation of mechanisms. Also, by comparing transient responses among different NC materials, the relative contributions of dislocation slip and GB accommodation mechanisms are discussed in terms of grain size and alloying. Finally, different magnitudes of stress reduction are carried out by molecular dynamics (MD) simulations with the aim to verify in situ experimental results and explore the mechanisms responsible for GB accommodation. MD simulations confirm that dislocation slip is reduced after a moderate stress drop, however can continue to operate after adaption of the GB structures by a variety of GB accommodation mechanisms explaining the non-monotonic behaviour of the peak broadening during transient creep.

EPFL2016

Metal fatigue during cyclic loading puts an endurance limit on most of today's technology. It impacts the reliability of metallic components used for transportation, electronic devices and energy production because fatigue failure can occur without any apparent forewarning. This phenomenon limits the lifetime of many industrially manufactured items, whose periodic replacement affects the cost of everyday products. The problem of predicting fatigue failure in metals has been addressed using the finite element method, which can provide the stress and strain state of the material in complex geometries. Engineering models describing the material behaviour have been used to predict the damage accumulation, but microstructure and material design require constitutive models at the micrometre length scale, incorporating the physical processes in the material. This involves the study of dislocation dynamics and the formation of dislocation structures, which have been recognized as the key phenomena affecting the macroscopic fatigue behaviour of metals. This problem is computationally challenging, first because of the large number of state variables required to describe the dislocation behaviour, and second because the formation of dislocation structures takes place only after many deformation cycles. In this project a continuum dislocation-based model, specific for cyclic fatigue at the micrometre length scale, is developed. The main novelty is the prediction of 3D dislocation structures starting from a random initial dislocation distribution. In single slip deformation, the characteristic length scale and shape of dislocation structures are predicted using only physical parameters, such as the stacking fault energy, and without any fitting procedure. The model is implemented in a crystal plasticity finite element solver, describing all the slip systems. Therefore, it is possible to model polycrystalline structures and to study the orientation of multiple slip dislocation structures with respect to the loading direction. Compared with existing models, the implementation of the dislocation junction formation mechanism in a continuum framework is a step forward. A collaboration with another PhD student, carrying out electron channeling contrast imaging experiments on fatigued 316 stainless steel, has provided a validation of the model by direct comparison of experimental and simulated dislocation structures close to the specimen surface. The crystal lattice rotation is another variable that can be extracted from finite element simulations. The developed model correlates the forming dislocation structures with the lattice rotation around the coordinate axes. This allows the comparison with Laue microdiffraction experiments carried out on copper single crystal specimens by another PhD student at the SLS, the synchrotron at Paul Scherrer Institut. The dislocation-based model can calculate experimental observables, such as the rotation components and rotation gradients, as a function of the simulated dislocation density in the specimen, which is not directly observable. This provides a validation of the constitutive equations at the micrometre length scale.

EPFL2016

The comparable order of magnitude between interatomic distances in a crystal and the wavelength of X-rays make X-ray crystallography the ideal analytical tool to gain insight into the structure of crystalline material, including biomolecules. Nevertheless, biomolecular crystallography has until now relied on the successful growth of single crystals of suitable size and quality. These remain the exception rather than the rule, since biomolecules often produce polycrystalline precipitate instead. Yet, an interest in making use of the once-discarded polycrystalline material, through the technique of powder diffraction, has only recently emerged. This can be accounted for by the information deficit which powder diffraction data suffers from in comparison with that of single crystal. The paucity of information in powder data stems from the compression of the three-dimensional reciprocal space onto the one-dimension of a powder pattern. In spite of this, powder diffraction holds the potential for application in biomolecular crystallography as is shown in the two different studies presented herein. Both studies were carried out with methods which do not rely on employing previously determined crystal-structures as molecular models. This therefore allowed the objective assessment of the quality of information that powder diffraction data can contribute to the structural investigation of biomolecules. In the first project, the traditional single-crystal structure-solution process is applied to data extracted from protein powder diffraction patterns measured on a synchrotron source. The use of models is avoided by employing the de novo phasing method of isomorphous replacement. With two protein test-cases, namely hen egg white lysozyme and porcine pancreatic elastase, it is demonstrated that protein powder diffraction data can afford structural information up to medium resolution. Indeed, a single isomorphous replacement analysis generated molecular envelopes accurately describing the crystal packing of both protein systems, while a multiple isomorphous replacement experiment, carried out only on lysozyme, revealed an electron density map in which elements of the secondary structure could be located. In fact, the resolution of the latter was discovered to be sufficient to determine the chirality of the protein molecule it represented. In addition to being encouraging, these results do not reflect the full potential of biomolecular powder diffraction, due, in large part, to the ultimately unsuitable nature of one type of phasing method, single crystal, being applied to another type of data, powder. An alternative approach to extract information from protein powder diffraction data is to employ powder-specific structure-solution techniques, such as global optimization methods. Although these methods make use of a starting "model", it is a molecular description of the system under study based on known chemical quantities rather than a related molecular configuration based on a previously determined crystal structure. Since their conception, global optimization methods have continuously been developed to enable the tackling of increasingly complex crystal structures. However, the immense complexity of biomacromolecules has kept proteins well out of reach of such methods. In an attempt to further reduce the gap separating the two levels of complexity, the second study reported herein puts forth the implementation of Ramachandran plot restraints into the algorithm of a global optimization method, i.e. that of simulated annealing. More specifically, the Ramachandran plot was approximated using a two-dimensional Fourier series which was subsequently expressed as a penalty function and incorporated into the search algorithm.

EPFL2011

Nanocrystalline metals: transient testing during in situ X-ray diffraction and molecular dynamics

Zhen Sun

EPFL2016

EPFL2011