1,018 research outputs found
<a> prismatic, <a> basal, and <c+a> slip strengths of commercially pure Zr by micro-cantilever tests
No full textSlip strengths of <a> basal, <a> prism, and <c+a> pyramidal systems in commercially pure zirconium have been determined using micro-cantilever testing. A range of single crystal cantilevers 0.5 µm to 10 µm wide, oriented for single slip were prepared using focused ion beam (FIB) machining and subsequently deflected using a nanoindenter. The critical resolved shear stress (Ïcrss) was found by fitting a crystal plasticity finite element model to the experimental load-displacement data for these micro-bending tests. All the three slip systems in alpha-Zr show a marked size effect in bending described well by CRSS(W)=Tau0 + AWn, where W is the cantilever width, Tau0 is the CRSS at the macro scale and n=-1. The exponent, n, of near -1 is in good accord with hardening caused by the back stress generated by dislocations piling up at a diffuse barrier caused by the reduction of stress near the neutral axis. The macro scale CRSS values were used to successfully simulate deformation of a conventional macroscopic compression test
HR-EBSD Measurements near Twins in Zicaloy-2
No full textHR-EBSD measurements made on a Zircaloy-2 sample deformed to 2.7% in tension to generate multiple deformation twins.
HR-EBSD measurements were made using A Bruker HReFlash detector mounted on a Zeiss Merlin SEM. Sample preparation used Ar ion beam milling on a Gatan PECS II system as a final step.
The strain, stress and dislocation density variations available here were then calculated using in-house Matlab code (XEBSD) described in the following publications:
High resolution electron backscatter diffraction measurements of elastic strain variations in the presence of larger lattice rotations
TB Britton and AJ Wilkinson
Ultramicroscopy, (2012), vol. 114, 82-95
doi:10.1016/j.ultramic.2012.01.004
Measurement of residual elastic strain and lattice rotations with high resolution electron backscatter diffraction
TB Britton and AJ Wilkinson
Ultramicroscopy, (2011) vol. 111, 1395-1404
doi:10.1016/j.ultramic.2011.05.007
Determination of elastic strain fields and geometrically necessary dislocation distributions near nanoindents using electron back scatter diffraction
AJ Wilkinson and D Randman
Philosophical Magazine, (2010), vol. 90, 1159-1177
doi:10.1080/14786430903304145
Crystal Plasticity Analysis of Micro-Deformation, Lattice Rotation and Geometrically Necessary Dislocation Density
FPE Dunne, R Kiwanuka, AJ Wilkinson
Proc. Royal Society A, (2012), vol. 468, 2509-2531
doi:10.1098/rspa.2012.0050 Stress fields close to twin tips and the associated local neighbourhoods of a hexagonal close-packed (HCP) polycrystal were studied in details. For this purpose, a coarse grain textured Zircaloy-2 sample was firstly strained uniaxially in a macroscopic direction that favours tensile twin formation. The sample was then unloaded and residual elastic strains and lattice rotations measured using the high-resolution electron backscatter diffraction (HR-EBSD) technique. Measured elastic strain maps of various clusters of grains including parent and twin pairs were then analysed. Stress, dislocation density, and their associated concentrations close to twin tips, within twins, in the immediate neighbouring grain, at the intersection of two twins, and within parent grains were investigated. It is shown that the stress field at the twin tips varies as a function of local neighbourhood. High stress, lattice rotation, and dislocation density concentrations were generally observed close to twin tips both within twins and within the immediate neighbouring grains. It is shown dislocation density concentration is maximum at the intersection of two twins which can potentially provide susceptible site for crack nucleation
PIN, BLACK ANGUS BEEF
No full textImage shows a souvenir pin from the 2002 Olympic Winter Games. The pin consists of an official Certified Angus Beef logo attached to the official Olympic logo, inside an elliptical, blue and black field
Review of Mechanisms of strength and hardening in austenitic stainless 310S steel: Nanoindentation experiments and multiscale modeling
No full textThis Zenodo record is a permanently preserved version of a PREreview. You can view the complete PREreview at https://prereview.org/reviews/7371559.
A review of preprint arXiv:2205.03050 [v1] (https://doi.org/10.48550/arXiv.2205.03050)
"Mechanisms of strength and hardening in austenitic stainless 310S steel: Nanoindentation experiments and multiscale modeling"
F. J. Domínguez-Gutiérrez, K. Mulewska, A. Ustrzycka, R. Alvarez-Donado, A. Kosińska, W. Y. Huo, L. Kurpaska, I. Jozwik, S. Papanikolaou, and M. Alava
Reviewer: Angus J Wilkinson
27 November 2022
Overview
The paper presents interesting experimental and molecular dynamics (MD) simulation studies on nanoindention of a face centred cubic Fe-Ni-Cr stainless steel 310S. 310S is high in both Cr (~25wt%) and Ni (~20wt%) compared to other austenitic stainless-steel grades and finds application in areas where high temperature environmental degradation is a concern. A strength of the work is that the high quality of both the nanoindentation experiments and the MD simulations.
The nanoindentation experiments were undertaken with a Berkovich tip with loads in the range 0.25 to 10 mN, so that shallow indents below 200 nm depth resulted. Data for even the smallest <50nm, 0.25nM) indents appeared to be of good quality indicating careful experimentation on difficult measurements. Results from repeat tests are given to indicate noise levels and scatter in material response.
The MD simulations of the 310S alloy were undertaken using LAMMPS with an embedded-atom method (EAM) potential and models with Fe, Ni and Cr atoms in an initially randomised substitutional solid solution. In line with other literature a fixed layer furthest from the indented surface, and a thermostatic layer allowing for heat dissipation were included with the model which was initially equilibrated at 300 K. Given that repeat simulations were conducted for multiple orientations the models size was kept as large as reasonable possible and consisted of a total of 8.5-9 million atoms.
The most significant challenge for the work is in making a strong connection between the experiments and simulations when computational resource prohibits using a larger model, while experimental uncertainties are more marked for smaller indents. The dilemma is perhaps made most evident by comparing the 10 nm tip radius and 5 nm maximum indent depth used in the simulations with the smallest experimental indents of a little under 50 nm. A more fundamental barrier to direct comparison is that the simulations are for tip radius of 10 nm, while the experiments are for much larger value (~100 nm seems likely from load-displacement data though the actual value is not quoted). The larger tip radius in the experiments provides access to much larger indentation strains than is possible in the simulations, while larger strain gradients are in place for the simulations. Finally, there is a large difference in loading rate (and therefore deformation rate).
Reading the preprint provoked the following comments and questions some of which might be useful to the authors.
Main Points
· Given the challenges above in making direct comparisons the conclusion of excellent agreement between experiment and simulation should perhaps be softened, similarly the work itself does not deal with high temperature behaviour, or effects of irradiation so conclusions regarding the suitability of the alloy for nuclear applications seem out of scope.
· The attempt to estimate GND density is interesting, especially as the strain gradients are extremely high for the simulations, and quite a bit lower for the experiments. Typically, a length scale needs to be set in describing how the total dislocation density is split between GND and SSD densities, but this has not been made explicit here. For the MD simulations it may be that all dislocations have been taken to contribute to the GND density, but it is not clear what volume term has been used (text around eq 12-13 suggests contact diameter may be the indicative lengthscale). For the Ma-Clarke model how was the shear strain calculated (the Ma-Clarke paper was for Berkovich rather than spherical indents)? No details are given for the calculation of GND density from the EBSD map, but the characteristic lengthscale is likely markedly larger than for the MD simulation, and the effects of this should be discussed. A little more detail in methodologies should be given for all of this analysis. Fig 11 c) is the only figure where both simulation and experimental data are shown directly on the same plot. Ma & Clarke (and subsequently Nix & Gao) used both SSD and GND densities as contributions to a Taylor hardening expression to link strain gradients to indentation size effects. Have the authors thought of extended their analysis to see if this can consistently link the markedly lower hardness values seen for deeper experimental indents with the much higher hardness reported for MD simulations?
· Some interesting dislocation density-based laws are introduced in eq 8 to 10. These are fit to the MD simulation results for dislocation density in fig 8a. It would be good to state the dislocation mean free path and annihilation constants obtained. As with the point above can a Taylor hardening model then be used to connect to hardness values and then provide a link to the experimental data. [In passing the text describing eq 10 refers to grain size though perhaps indent size is more relevant here].
· Results from simulations and experiments show relatively little anisotropy in either indentation modulus or hardness (eg figs 4b, fig 5, fig 8b) – it seems odd then to state in the conclusions that "…310S indicates anisotropic properties…".
Minor Points
· Caption on fig 4 swaps parts (a) and (b)
· Scale bars should be added to fig 6, fig 7 a and b, fig 9, fig 11a and b, and fig 12
· Fig 9b – the 5nm and unload images seem to be identical though difference are talked about in the main text.
Angus J Wilkinson
27 November 202
Corrigendum to “A mechanistic study of the temperature dependence of the stress corrosion crack growth rate in SUS316 stainless steels exposed to pressurized water reactor primary water” [Acta Mater. 114 (2016) 15–24]
No full textThe authors regret that one of the main contributors to this paper was unintentionally omitted from the author list in the final version of the manuscript. The complete author list should read: Martina Meisnara, Arantxa Vilalta-Clementea, Michael Moodya, Angus J. Wilkinsona, Koji Ariokab, Sergio Lozano-Pereza,∗ The authors would like to apologise for any inconvenience caused
Strong Grain Neighbour Effects in Polycrystals- Data set
No full text<p>3D-XRD measurements are for a commercially pure zirconium (CPZr) sample deformed in-situ to 1.2% strain.</p>
<p>HR-EBSD measurements are for the same deformed sample after unload.</p>
<p>Crystal Plasticity Finite Element (CPFE) simulation was done on the measured micro-structure and results of the NHS model are provided.</p>
<p>HR-EBSD and CPFE results are generated using in-house codes.</p>
<p>Details of the 3D-XRD codes are provided in the following link:</p>
<p>https://sourceforge.net/p/fable/wiki/Home/</p>
<p> </p>
<p>The work and further analysis of the results are described in:</p>
<p>"Strong Grain Neighbour Effects in Polycrystal" Published in Nature Communications, DOI: <strong>10.1038/s41467-017-02213-9</strong></p>
<p> </p>
<p>Other relevant papers:</p>
<p>Abdolvand, H., Majkut, M., Oddershede, J., Wright, J., Daymond, M. R., “Study of 3-D Stress Development in Parent and Twin Pairs of a Hexagonal Close-Packed Polycrystal: Part I- In situ Three-Dimensional X-ray Diffraction Measurement”, Acta Materialia, July 2015, Vol 93, Page 246-255.</p>
<p> </p>
<p>Abdolvand, H., Majkut, M., Oddershede, J., Wright, J., Daymond, M. R., “Study of 3-D Stress Development in Parent and Twin Pairs of a Hexagonal Close-Packed Polycrystal: Part II- Crystal Plasticity Finite Element Modeling”, Acta Materialia, July 2015, Vol 93, Page 235-245.</p>
<p> </p>
<p>Abdolvand, H., Majkut, M., Oddershede, J., Schmidt, S., Lienert, U., Diak, B., Withers, P. J., Daymond, M. R., “On the Deformation Twinning of MgAZ31B: a Three-Dimensional X-ray Diffraction Experiment and Crystal Plasticity Finite Element Model”, International Journal of Plasticity, July 2015, Vol 70, Page 77-97.</p>
<p> </p>
<p>Gong, J., Britton, B. T., Cuddihy, M. A., Dunne, F. P. E. & Wilkinson, A. J. <a> Prismatic, <a> basal, and <c+a> slip strengths of commercially pure Zr by micro-cantilever tests. Acta Mater. 96, 249–257 (2015).</p>
<p> </p>
<p>Poulsen, H. F. An introduction to three-dimensional X-ray diffraction microscopy. J. Appl. Crystallogr. 45, 1084–1097 (2012)</p>
<p> </p>
<p>Wilkinson, A. J., Meaden, G. & Dingley, D. J. High-resolution elastic strain measurement from electron backscatter diffraction patterns: New levels of sensitivity. Ultramicroscopy 106, 307–313 (2006)</p>
<p> </p>
HR-EBSD Measurements near Twins in Zicaloy-2
No full text<p>HR-EBSD measurements made on a Zircaloy-2 sample deformed to 2.7% in tension to generate multiple deformation twins. <br />
HR-EBSD measurements were made using A Bruker HReFlash detector mounted on a Zeiss Merlin SEM. Sample preparation used Ar ion beam milling on a Gatan PECS II system as a final step. <br />
The strain, stress and dislocation density variations available here were then calculated using in-house Matlab code (XEBSD). The work and further analysis of the results is described in</p>
<p><em>Assessment of residual stress fields at deformation twin tips and the surrounding environments</em>, Abdolvand & Wilkinson, Acta Materialia (2016)<br />
dx.doi.org/10.1016/j.actamat.2015.11.036</p>
<p> </p>
<p> </p>
<p>The analysis methods implement in XEBSD are described in the following publications:</p>
<p>High resolution electron backscatter diffraction measurements of elastic strain variations in the presence of larger lattice rotations <br />
TB Britton and AJ Wilkinson <br />
Ultramicroscopy, (2012), vol. 114, 82-95 <br />
doi:10.1016/j.ultramic.2012.01.004</p>
<p>Measurement of residual elastic strain and lattice rotations with high resolution electron backscatter diffraction <br />
TB Britton and AJ Wilkinson <br />
Ultramicroscopy, (2011) vol. 111, 1395-1404 <br />
doi:10.1016/j.ultramic.2011.05.007</p>
<p>Determination of elastic strain fields and geometrically necessary dislocation distributions near nanoindents using electron back scatter diffraction <br />
AJ Wilkinson and D Randman <br />
Philosophical Magazine, (2010), vol. 90, 1159-1177 <br />
doi:10.1080/14786430903304145</p>
<p>Crystal Plasticity Analysis of Micro-Deformation, Lattice Rotation and Geometrically Necessary Dislocation Density <br />
FPE Dunne, R Kiwanuka, AJ Wilkinson <br />
Proc. Royal Society A, (2012), vol. 468, 2509-2531 <br />
doi:10.1098/rspa.2012.0050</p>
The interactions between slip band, deformation twins and grain boundaries in commercial purity titanium
No full textThis thesis apply High Resolution Electron Back Scatter Diffraction (HR-EBSD) technique to a variety of microstructure features and their interactions in pure h.c.p polycrystals. By correlating high quality Kikuchi patterns with a reference pattern, the relative state and distribution of strain, stress, and geometrically necessary dislocation (GND) density can be obtained with high strain sensitivity (10-4) and angular resolution (10 radian). This technique is companied by a further investigation of subsurface features using Differential Aperture X-ray Micro-diffraction (DAXM) technique. The two technique have shown excellent agreement in capturing the magnitude and distribution of stress and GND. Stress field and GND distribution induced by slip band and grain boundary interactions, including blocked slip band with no observable slip transfer in SEM and slip transfer, were characterised. It was found that some blocked slip bands lead to high and localised stress concentration in the neighbouring grain while others did not, and no stress concentration were correlated with transferred slip bands. These three categories of interactions were rationalised using a slip transfer criteria (called LRB criteria) by investigating the geometric alignments between the impinging slip system and all possible slip systems in the neighbouring grain. The level of stress concentration were quantified into a stress intensity factor K, following the Frank, Eshelby, and Nabarro (FEN) model. It was found that the level of stress intensity correlates well with the number of dislocations within the pile up plane. The slip band and grain boundary interaction case that led to the highest magnitude of stress intensity factor was further investigated using DAXM experiments. The 3D data set informed us additional information hidden below the sample surface. The distribution of stress concentration in 3D is a ribbon conforming to the line of intersection between slip plane and grain boundary. Stress intensity factor calculation along this ribbon have shown large variations which led to a concern that sometimes 2D results might not be conclusive. For example, if damage is observed in sample surface, there is a possibility that large populations of damage already exist below sample surface as a result of the stress fluctuations. The level of stress concentration and distribution measured by both HR-EBSD and DAXM agree with each other and 3D lattice rotation gradient used in DAXM GND calculation was found to affect the range of GND distribution and how fast it decays away from grain boundary. Twinning is a deformation mechanism in HCP metal that is equally important as dislocation slip. The stress concentrations associated with twin propagation, approaching grain boundary, and thickening were characterised using HR-EBSD, from which the calculated stress tensor were used to generate a local Schmid factor (LSF) map. It was found that during twin propagation, local positive shear provides a favourable LSF condition that promote twin tip extension while supress it from thicken. When twin tip is approaching the grain boundary, the positive shear stress field no longer favour twin propagation, a narrow positive LSF field still exist at the tip of twin, promoting it to grow thick. During propagation and thickening process, the LSF seem to only affect the tip of twins and therefore these processes are possibly tip controlled
The effect of pattern overlap on the accuracy of high resolution electron backscatter diffraction measurements
No full textData examining the spatial resolution of electron backscatter diffraction. EBSD patterns were obtained from a Zr alloy (zircaloy-4) using a Bruker eFlashHR detector at full 1600x1152 pixel resolution on an Zeiss Auriga SEM at 20keV and 10nA.
Cross correlation analysis carried out within Matlab was used to measure the degree of pattern overlap and its effects on errors in strain measurements. High resolution, cross-correlation based, electron backscatter diffraction (EBSD) measures the variation of elastics trains and lattice rotations from a reference state. Regions near grain boundaries are often of interest but overlap of patterns from the two grains could reduce accuracy of the cross-correlation analysis. To explore this concern, patterns from the interior of two grains have been mixed to simulate the interaction volume crossing a grain boundary so that the effect on the accuracy of the cross-correlation results can be tested. It was found that the accuracy of HR-EBSD strain measurements performed in a FEGSEM on zirconium remains good until the incident beam is less than 18nm from a grain boundary. A simulated microstructure was used to measure how often pattern overlap occurs at any given EBSD step size, and a simple relation was found linking the probability of overlap with step size
In situ full-field characterisation of strain concentrations (deformation twins, slip bands and cracks)
No full textThis thesis has developed novel methods to characterise the deformation field, in and ex situ, of strain concentrations (i.e., deformation twins, slip bands and cracks) using diffraction methods to map the local elastic deformation field, and calculate elastic strain energy release rate (J-integral) and stress intensity factors (SIFs) to parametrise the field under conditions of small-scale yielding.
To calculate the J-integral from an elastic strain field, the field was integrated to equivalent displacement to use the finite element formulation for high accuracy. The method was validated against two and three dimensional synthetic crack fields and then applied to strain fields measured using synchrotron energy-dispersive X-ray diffraction (EDXD) for a fatigue crack propagating in the heat affected zone (HAZ) of a welded bainitic steel. The integrated displacement field was then used to calculate the J-integral and mode I, showing a good agreement with the standard analytical solution and results obtained using displacement fields of the same crack measured using Digital Image Correlation (DIC). The parametrisation via the J-integral and SIFs was extended to use the finite element solver’s equivalent domain integration (EDI), and anisotropic elastic and elasto-plastic material properties.
The high-resolution electron backscatter diffraction technique (HR-EBSD) was employed to map the elastic strain field. The effect of the unknown deformation conditions at the electron backscatter reference pattern (EBSP0) were investigated to select an optimum EBSP0. The developed analytical techniques were then applied to study deformation twinning, slip band and crack local fields, in situ. First, an age-hardened duplex stainless steel (DSS) sample was deformed in tension, promoting plastic deformation by deformation twinning in the ferrite phase. The local in-plane strain fields ahead of a loaded deformation twin were measured, in and ex situ, and decomposed to the opening mode I and in-plane shear mode II SIFs. The analysis showed that the increase in twin lateral thickness correlates with mode I, while the elastic recovery when the load was removed was mainly in mode II.
By estimating the EBSP depth resolution using Monte Carlo (MC) simulation, the analysis was extended to the third dimension and applied to study, in situ, intragranular slip bands in the ferrite phase of an age-hardened DSS. The integrated displacement fields gave information about the in- and out-of- plane movement of the surface, which was decomposed to the three dimensional stress intensity factors, KI,II,III. This showed that constraint of the topological changes due to out-of-plane shear induces additional tensile stresses, and the ratio of mode II to mode III depends on the direction of the slip band Burgers vector relative to the observed surface.
Finally, a simplified method that did not rely on FE solvers to calculate the J-integral and SIFs was derived and used to investigate mixed-mode cleavage crack propagation in (001) single silicon crystal. The mixed-mode crack field was consistent with a constant maximum potential energy release rate (MPERR) criterion for crack propagation and the expected cleavage toughness of silicon
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