High-resolution Synchrotron X-ray Tomography Research Articles

Effects of applied pressure and residual heat quenching on the mechanical and thermal properties of cast Al–Mg–Zn–Mn–Fe alloys

In this study, the effects of applied pressure (AP) and residual heat quenching (RQ) on the mechanical properties and thermal conductivities of Al-4.6Mg-3.6Zn-0.86Mn-0.48Fe alloys were investigated based on intensively microstructure analysis by scanning electron microscope (SEM), high resolution transmission electron microscopy (HRTEM) and synchrotron X-ray tomography (SXT). The results indicated that AP and RQ did not change the type of intermetallic; instead, they reduced the length and quantity of lath-like Al6(Fe,Mn) and significantly refined the grain by increasing the cooling rate. Moreover, η′ precipitation particles were enriched near the grain boundaries, which was beneficial for the yield strength of the alloy; the AP + RQ sample exhibited the best mechanical properties of the ultimate tensile strength, yield strength and elongation of 269 MPa, 164 MPa and 11.4 %, respectively, which were 19 %, 17 %, and 83.8 % higher than those of the gravity cast (GC) sample. Moreover, the thermal conductivity of the AP + RQ sample did not show significant changes, which were attributed to the reduction in solid solute caused by the increased volume fraction of η′ precipitates, thus improving the thermal conductivity; however, the formation of numerous semi-coherent interfaces between η′ and the matrix led to increased lattice distortion, resulting in electron scattering.

In situ X-ray tomography study on internal damage evolution of solid propellant for carrier rockets

Nitrate Ester Plasticized Polyether (NEPE) propellant has been widely used for carrier rockets. However, the propellant is usually subjected to complex loads, changing the microstructures, which eventually affects its structural integrity. To elucidate the damage mechanism of the propellant, an in situ tensile testing rig with a maximum strain rate of 0.1 s−1 was developed with high-resolution synchrotron X-ray tomography. The internal microstructure and the resultant damage evolution have been characterized quantitatively. The mechanical behaviour of sampled propellant was extracted from the dynamic mechanical analysis. In addition, numerical simulation in terms of the spatial geometry of filler particles was performed in the framework of the micromechanical approach. To visualize the debonding process, a cohesive-zone model was employed to trace the debonding at the filler/matrix interface and then the damage evolution was characterized according to the cohesive behaviour. The results can assist in correlating the cohesive behaviour due to local heterogeneity with the macroscopic mechanical response for the NEPE propellant.

Hoop Strain Measurement During a SiC/SiC Ceramic Composite Tube Burst Test by Digital Volume Correlation of X-Ray Computed Tomographs

BackgroundSilicon carbide-fibre/silicon carbide matrix (SiC/SiC) composites are candidate materials for accident tolerant fuel cladding in light water and advanced gas cooled nuclear fission reactors. The mechanical and damage behaviour of ceramic composites is sensitive to the composite geometry, the microstructure and the loading state. Reliable test methods are needed to investigate the subcritical damage that affects hermetic properties and strength, and this requires precise measurements under loading states that are representative of operating conditions.ObjectiveThe objective was a novel methodology to measure the deformation of an internally pressurised ceramic composite tube.MethodsA burst test of an internally ground SiC/SiC (filament wound and braided) ceramic composite tube, pressurized by radial expansion of a compressed elastomer insert, was observed in situ by high resolution (synchrotron) X-ray tomography. The full field three-dimensional displacements were measured by digital volume correlation, with a precise rotation correction applied to obtain the relative radial and circumferential displacements of the tube wall for the first time.ResultsThe hoop strain, and its spatial variations, were determined as a function of the applied hoop stress and showed ovalisation and barreling of the tube. The quantity of subcritical matrix cracking increased with the tensile hoop strain, but the critical crack that caused rupture was not at the location of maximum tensile strain.ConclusionPrecise measurements of the deformation during the burst test found non-uniform hoop strains that caused a non-uniform distribution of subcritical cracking, which could influence the hermetic properties and strength.

Fossilized cell structures identify an ancient origin for the teleost whole-genome duplication

Teleost fishes comprise one-half of all vertebrate species and possess a duplicated genome. This whole-genome duplication (WGD) occurred on the teleost stem lineage in an ancient common ancestor of all living teleosts and is hypothesized as a trigger of their exceptional evolutionary radiation. Genomic and phylogenetic data indicate that WGD occurred in the Mesozoic after the divergence of teleosts from their closest living relatives but before the origin of the extant teleost groups. However, these approaches cannot pinpoint WGD among the many extinct groups that populate this 50- to 100-million-y lineage, preventing tests of the evolutionary effects of WGD. We infer patterns of genome size evolution in fossil stem-group teleosts using high-resolution synchrotron X-ray tomography to measure the bone cell volumes, which correlate with genome size in living species. Our findings indicate that WGD occurred very early on the teleost stem lineage and that all extinct stem-group teleosts known so far possessed duplicated genomes. WGD therefore predates both the origin of proposed key innovations of the teleost skeleton and the onset of substantial morphological diversification in the clade. Moreover, the early occurrence of WGD allowed considerable time for postduplication reorganization prior to the origin of the teleost crown group. This suggests at most an indirect link between WGD and evolutionary success, with broad implications for the relationship between genomic architecture and large-scale evolutionary patterns in the vertebrate Tree of Life.

3D short fatigue crack closure behavior in Ti-6Al-4V alloy investigated using in-situ high resolution synchrotron X-ray tomography

In-situ synchrotron X-ray microtomography has been employed to investigate the three-dimensional (3D) closure behavior of short fatigue crack in Ti-6Al-4V alloy. At a spatial resolution level of 1 µm, tomography datasets were taken at two short crack fronts to observe the time evolution behavior of closure (sometimes called 4D imaging). Owing to the high spatial resolution level, the complex 3D crack morphology consisting of deflections, facet paths and crack branching/overlap caused by the 3D variation in the microstructural features were readily observed together with the occurrence of crack closure. This study separated the contributions of near-tip and behind-tip crack closure and observed their behavior with crack growth. It was deduced that the evolution of behind-tip closure with increase in crack length predominantly affects the overall closure behavior in the short crack regime. Crack front closure behavior variation was found to be attributable to the existence of different crack path morphologies which showed varied degree of fracture surface roughness due to Mode II and Mode III displacements. The relationship between closure, crack tip opening displacement range (ΔCTOD) and crack growth rate are also discussed.

Refinement and Experimental Validation of a Vacancy Model of Pore Annihilation in Single-Crystal Nickel-Base Superalloys During Hot Isostatic Pressing

Initially as-cast and homogenized single-crystals of nickel-base superalloy CMSX-4 were subjected to hot isostatic pressing at temperature of 1288 °C. Two series of experiments were performed: under the same pressure of 103 MPa but with different durations, between 0.5 and 6 h, and under different pressures, between 15 and 150 MPa, but for the same time of 0.5 h. The porosity annihilation was investigated metallographically and by high resolution synchrotron X-ray tomography. The obtained experimental results were compared with the predictions of the vacancy model proposed recently in our group. In this work the model was further refined by coupling with X-ray tomography. The model describes the evolution of the pore arrays enclosed in the 3D synchrotron tomograms during hot isostatic pressing and properly predicts the time and stress dependences of the pore annihilation kinetics. The validated model and the obtained experimental results can be used for selecting the optimal technological parameters such as applied pressure and processing time.

Influence of nanovoids in the hydrogen embrittlement fracture of Al–Zn–Mg–Cu alloys

Al–Zn–Mg alloys are representative high-strength aluminum alloys, but hydrogen embrittlement hinders further strengthening. The role of nanovoids in the hydrogen embrittlement of a hydrogen-charged Al–Zn–Mg–Cu alloy is examined using high-resolution imaging-type synchrotron X-ray tomography. Although nanovoids are initiated during deformation, their growth and coalescence behaviors are limited. Hydrogen partitioning analysis indicates that this is because most of the formed vacancies are not stabilized because of the low hydrogen occupancy. Furthermore, 3D/4D clustering analyses of nanovoids reveal that they do not aggregate on a specific plane to initiate a crack, and that linkage between nanovoids is less likely to occur.

Probing the 3D molecular and mineralogical heterogeneity in oil reservoir rocks at the pore scale

Innovative solutions have been designed to meet the global demand for energy and environmental sustainability, such as enhanced hydrocarbon recovery and geo-sequestration of CO2. These processes involve the movement of immiscible fluids through permeable rocks, which is affected by the interfacial properties of rocks at the pore scale. Overcoming major challenges in these processes relies on a deeper understanding about the fundamental factors that control the rock wettability. In particular, the efficiency of oil recovery strategies depends largely on the 3D wetting pattern of reservoir rocks, which is in turn affected by the adsorption and deposition of ‘contaminant’ molecules on the pores’ surface. Here, we combined high-resolution neutron tomography (NT) and synchrotron X-ray tomography (XRT) to probe the previously unobserved 3D distribution of molecular and mineralogical heterogeneity of oil reservoir rocks at the pore scale. Retrieving the distribution of neutron attenuation coefficients by Monte Carlo simulations, 3D molecular chemical mappings with micrometer dimensions could be provided. This approach allows us to identify co-localization of mineral phases with chemically distinct hydrogen-containing molecules, providing a solid foundation for the understanding of the interfacial phenomena involved in multiphase fluid flow in permeable media.

Investigating the Mechanical Characteristics of Bone-Metal Implant Interface Using in situ Synchrotron Tomographic Imaging.

Long-term stability of endosseous implants depends on successful bone formation, ingrowth and adaptation to the implant. Specifically, it will define the mechanical properties of the newly formed bone-implant interface. 3D imaging during mechanical loading tests (in situ loading) can improve the understanding of the local processes leading to bone damage and failure. In this study, titanium screws were implanted into rat tibiae and were allowed to integrate for 4 weeks with or without the addition of the growth factor Bone Morphogenetic Protein and the bisphosphonate Zoledronic Acid. Samples were subjected to in situ pullout using high-resolution synchrotron x-ray tomography at the Tomcat beamline (SLS, PSI, Switzerland) at 30 keV with 25 ms exposure time, resulting in a total acquisition time of 45 s per scan, with a 3.6 μm isotropic voxel size. Using a custom-made loading device positioned inside the beamline, screws were pulled out with 0.05 mm increment, acquiring multiple scans until rupture of the sample. The in situ loading protocol was adapted to ensure short imaging time, which enabled multiple samples to be tested with short loading steps, while keeping the total testing time low and reducing dose deposition. Higher trabecular bone content was quantified in the surrounding of the screw in the treated groups, which correlated with increased mechanical strength and stiffness. Differences in screw implantation, such as contact between threads and cortex as well as minor tilt of the screw were also correlated to the mechanical parameters. In situ loading enabled the investigation of crack propagation during the pullout, highlighting the mechanical behavior of the interface. Three typical crack types were observed: (1) rupture at the interface of trabecular and cortical bone tissues, close to the screw, (2) large crack inside the cortex connected to the implant, and (3) first failure away from the screw with cracks propagating toward the screw-bone interface. Mechanical properties of in vivo integrated bone-metal screws rely on a combination of multiple parameters that are difficult to identify and separate one from the other.

Characterization of high-pressure die-cast hypereutectic Al-Si alloys based on microstructural distribution and fracture morphology

The fracture behavior of high-pressure die-cast hypereutectic (HPDC) Al-Si alloys was investigated using a high-resolution laboratory CT and synchrotron X-ray tomography with a particular focus on the influence of HPDC microstructure. Results showed that microstructure of the alloy was mainly comprised of primary silicon particles (PSPs), Al dendrites, Cu-rich phases and pores. Most of the coarse PSPs, Cu-rich phases and pores were located in the center of the specimen. The rapid solidification of HPDC led to a heterogeneous microstructural feature. Elemental Cu was enriched in the frontiers of solid-liquid interface, causing the formation of large size dendritic arms. The pores were formed in the interdendrites which endured high stress intensity under high applied stress. Microcracks were originated from pores and further connected Cu-rich phases causing intergranular fracture. PSPs worked as obstacles causing piling-up dislocations in the phase interface. In the regions where large size of PSPs enriched in, PSPs ruptured rather than debonded from matrix, indicating transgranular fractures of PSPs. Microcracks originated around pores and PSPs tended to converge on the main cracks to decrease the energy required for crack propagation.

3D characterization of the propagation of small fatigue cracks in steels with different forging conditions

The influence of forging conditions on the propagation of physically small fatigue cracks has been studied for two high strength steels. Two surface conditions were produced after the forging process. The subsurface microstructure of the materials has been characterized by EBSD. Small samples extracted from the original specimens were used to perform in situ fatigue tests monitored by high resolution synchrotron X-ray tomography. Fatigue cracks were initiated from an artificial defect (100 μm × 50 μm deep) introduced in the forging skin by laser machining. 3D images of the initiation and growth of those physically small fatigue cracks have been obtained. It was found that the presence of a shot-blasted skin containing a hardness and microstructure gradient influences the 3D crack shape during propagation in comparison with the materials without material properties gradient. The 3D crack shapes are rationalized in terms of crack closure effects induced by the forging processes, close to the surface.

Influence of forging conditions on the fatigue mechanisms of low alloy steels: a 3D study

The influence of forging conditions on the propagation of physically small fatigue cracks has been studied for two high strength steels. Two surface conditions were produced after the forging process. The subsurface microstructure of the materials has been characterized by EBSD. Small samples extracted from the original specimens were used to perform in situ fatigue tests monitored by high resolution synchrotron X-ray tomography. Fatigue cracks were initiated from an artificial defect (100 ?m wide x 50 ?m deep) introduced in the forging skin by laser machining. 3D images of the initiation and growth of those physically small fatigue cracks have been obtained. It was found that the presence of a shot-blasted skin containing a hardness and microstructure gradient influences the 3D crack shape during propagation in comparison with the materials without material properties gradient. The 3D crack shapes are rationalized in terms of crack closure effects induced by the forging processes, close to the surface.

Refinement and growth enhancement of Al2Cu phase during magnetic field assisting directional solidification of hypereutectic Al-Cu alloy.

Understanding how the magnetic fields affect the formation of reinforced phase during solidification is crucial to tailor the structure and therefor the performance of metal matrix in situ composites. In this study, a hypereutectic Al-40 wt.%Cu alloy has been directionally solidified under various axial magnetic fields and the morphology of Al2Cu phase was quantified in 3D by means of high resolution synchrotron X-ray tomography. With rising magnetic fields, both increase of Al2Cu phase’s total volume and decrease of each column’s transverse section area were found. These results respectively indicate the growth enhancement and refinement of the primary Al2Cu phase in the magnetic field assisting directional solidification. The thermoelectric magnetic forces (TEMF) causing torque and dislocation multiplication in the faceted primary phases were thought dedicate to respectively the refinement and growth enhancement. To verify this, a real structure based 3D simulation of TEMF in Al2Cu column was carried out, and the dislocations in the Al2Cu phase obtained without and with a 10T high magnetic field were analysed by the transmission electron microscope.

Synchrotron 3D characterization of arrested fatigue cracks initiated from small tilted notches in steel

High resolution synchrotron X-ray tomography has been used to obtain 3D images of arrested cracks initiated at small artificial defects located on the surface of cylindrical steel specimens subjected to mode I fatigue loading. These defects consist in small semi-circular slits tilted at 0¢X, 30¢X or 60¢X with respect to the plane normal to the loading axis; all of them had the same defect size, area = 188 £gm, where the area denotes the area of the domain defined by projecting the defect on a plane normal to the loading axis. Arrested cracks initiated from the notch were observed for all tilt angles at the surface of samples cycled at the fatigue limit (stress amplitude at which the specimen does not fail after 1¡Ñ107 cycles). High resolution synchrotron X-ray tomography has been used to obtain 3D images of those small defects and non-propagating cracks (NPC). Despite the fact that steel is a highly attenuating material for X rays, high resolution 3D images of the cracks and of the initiating defects were obtained (0.65 ƒÝm voxel size). The values of surface crack length measured by tomography are the same as those obtained by optical microscope measurements. The area values present the same tendency as the surface length of NPC, i.e. larger non-propagating cracks areas were observed in the softer steel. In the extreme case of 60o tilted defect, the crack fronts appear much more discontinuous with several cracks propagating in mode I until arrest.

Studying SiC/SiC Composites by X-Ray Tomography

Continuous SiC fiber reinforced SiC matrix composites (SiC/SiC) have been studied and developed for high temperature applications and nuclear applications. In this study, SiC/SiC composites were fabricated via polymer impregnation and pyrolysis (PIP) process and studied by X-ray tomography. The SiC/SiC composites were first scanned using a Metris X-tek 320 kV source at the Henry Moseley X-ray Imaging Facility at the University of Manchester, the closed porosities were investigated after three dimensional (3D) imaging of the samples. Furthermore, high-resolution synchrotron X-ray tomography was applied to the SiC/SiC composite at Diamond Light Source. Digital volume correlation was employed for Hertzian indentation testing of the SiC/SiC composite, quantifying damage by measurement of the displacement fields within the material. A Cellular Automata integrated with Finite Elements (CAFE) method was developed to account for the effect of microstructure on the fracture behavior of the SiC/SiC composite. Graded microstructures, textures and multiple phases were simulated and a mesh-free framework was developed to compute the damage development through the microstructure. The results indicated that we could study the development of discontinuous cracking and damage coalescence, and its sensitivity to microstructure with this method.

A 4D Synchrotron X-Ray-Tomography Study of the Formation of Hydrocarbon- Migration Pathways in Heated Organic-Rich Shale

Summary Recovery of oil from oil shales and the natural primary migration of hydrocarbons are closely related processes that have received renewed interest in recent years because of the ever tightening supply of conventional hydrocarbons and the growing production of hydrocarbons from low-permeability tight rocks. Quantitative models for conversion of kerogen into oil and gas and the timing of hydrocarbon generation have been well documented. However, lack of consensus about the kinetics of hydrocarbon formation in source rocks, expulsion timing, and how the resulting hydrocarbons escape from or are retained in the source rocks motivates further investigation. In particular, many mechanisms have been proposed for the transport of hydrocarbons from the rocks in which they are generated into adjacent rocks with higher permeabilities and smaller capillary entry pressures, and a better understanding of this complex process (primary migration) is needed. To characterize these processes, it is imperative to use the latest technological advances. In this study, it is shown how insights into hydrocarbon migration in source rocks can be obtained by using sequential high-resolution synchrotron X-ray tomography. Three-dimensional images of several immature “shale” samples were constructed at resolutions close to 5 μm. This is sufficient to resolve the source-rock structure down to the grain level, but very-fine-grained silt particles, clay particles, and colloids cannot be resolved. Samples used in this investigation came from the R-8 unit in the upper part of the Green River shale, which is organic rich, varved, lacustrine marl formed in Eocene Lake Uinta, USA. One Green River shale sample was heated in situ up to 400°C as X-ray-tomography images were recorded. The other samples were scanned before and after heating at 400°C. During the heating phase, the organic matter was decomposed, and gas was released. Gas expulsion from the low-permeability shales was coupled with formation of microcracks. The main technical difficulty was numerical extraction of microcracks that have apertures in the 5- to 30-μm range (with 5 μm being the resolution limit) from a large 3D volume of X-ray attenuation data. The main goal of the work presented here is to develop a methodology to process these 3D data and image the cracks. This methodology is based on several levels of spatial filtering and automatic recognition of connected domains. Supportive petrographic and thermogravimetric data were an important complement to this study. An investigation of the strain field using 2D image correlation analyses was also performed. As one application of the 4D (space + time) microtomography and the developed workflow, we show that fluid generation was accompanied by crack formation. Under different conditions, in the subsurface, this might provide paths for primary migration. Key words in this work include 4D microtomography, 3D image processing, shale, strain field analysis, kerogen, petroleum generation, primary migration, petrography, and thermogravimetry.

Cooperative material transport during the early stage of sintering

The complex transport processes contributing to sintering are not yet fully understood, partially because in-situ observations of sintering in three dimensions (3D) are very difficult. Here we report a novel experiment in which monocrystalline copper spheres are first marked with microscopic boreholes drilled using a focused ion beam, after which high-resolution synchrotron X-ray tomography is carried out to measure translational, rolling and intrinsic rotation movements of some hundred spheres during sintering. Unlike in 1D and 2D systems, we show that, in 3D, intrinsic rotations are more pronounced than angular rolling rearrangements of the particle centres and become the dominant mechanism of particle movement. We conclude that in addition to the well-known neck growth and centre approach mechanisms, grain boundary sliding caused by the different crystallographic orientations of the individual spheres occurs.

Creep cavitation can establish a dynamic granular fluid pump in ductile shear zones

The feedback between fluid migration and rock deformation in mid-crustal shear zones is acknowledged as being critical for earthquake nucleation, the initiation of subduction zones and the formation of mineral deposits. The importance of this poorly understood feedback is further highlighted by evidence for shear-zone-controlled advective flow of fluids in the ductile lower crust and the recognition that deformation-induced grain-scale porosity is a key to large-scale geodynamics. Fluid migration in the middle crust cannot be explained in terms of classical concepts. The environment is considered too hot for a dynamic fracture-sustained permeability as in the upper crust, and fluid pathways are generally too deformed to be controlled by equilibrium wetting angles that apply to hotter, deeper environments. Here we present evidence that mechanical and chemical potentials control a syndeformational porosity generation in mid-crustal shear zones. High-resolution synchrotron X-ray tomography and scanning electron microscopy observations allow us to formulate a model for fluid migration in shear zones where a permeable porosity is dynamically created by viscous grain-boundary sliding, creep cavitation, dissolution and precipitation. We propose that syndeformational fluid migration in our 'granular fluid pump' model is a self-sustained process controlled by the explicit role of the rate of entropy production of the underlying irreversible mechanical and chemical microprocesses. The model explains fluid transfer through the middle crust, where strain localization in the creep regime is required for plate tectonics, the formation of giant ore deposits, mantle degassing and earthquake nucleation. Our findings provide a key component for the understanding of creep instabilities in the middle crust.

Three-dimensional shape of the early stages of fatigue cracks nucleated in nodular cast iron

High resolution synchrotron X-ray tomography has been used to obtain three-dimensional (3D) images of the early stages of fatigue crack nucleation in a nodular cast iron. Microcracks were observed to initiate at casting defects (microshrinkage) and graphite nodules. The 3D observations have shown that the microcracks form in the material ligament comprised between the specimen surface and the defect. The probability of a defect to initiate a crack was correlated with the size of the defect and its position with respect to the surface. This correlation has been explained on the basis of local stress concentrations in the vicinity of the defect. The 3D observations of samples submitted to different fatigue cycles revealed that a large part of the fatigue life consisted in the progressive fracture of the ligament. The majority of the observed cracks stopped after this fracture process and, therefore, their size did not exceed the initiating defect size even if the crack seemed larger than the defect on the optical surface observations. Only few microcracks, nucleated on the largest defects, continued to grow with short-crack behaviour.

Three-dimensional study of a fretting crack using synchrotron X-ray micro-tomography

High resolution synchrotron X-ray tomography has been used to obtain three-dimensional images of a crack initiated under fretting wear loading in a 2024 Al alloy. Quantitative information about the 3D crack shape is extracted from the images. It is found that the local microstructure may play a key role in the fretting crack initiation stage. In addition, the crack extension process seems to be quite independent of the microstructure encountered by the crack front, at least in the first 300 μm for the investigated experimental conditions. This depth is likely to correspond to the zone where the propagation is completely dominated by the mechanical loadings imposed by the contact conditions. Further growth beyond this length seems again more influenced by the local microstructure. This last point can account for the scatter observed in classical fretting experiments where fretting crack length is measured.