Synchrotron Radiation X-ray Computed Tomography Research Articles

Effect of Na3AlF6 contents in refining flux on 3D characteristics of pore and mechanical properties of recycled Al-Mg-Si alloy

Fluoride is an essential and effective component in solid refining flux due to its ability to dissolve Al2O3. In this paper, the effects of Na3AlF6 content in refining flux on the pores and mechanical properties of recycled Al-Mg-Si alloy was investigated by tensile testing, scanning electron microscope (SEM) and Synchrotron radiation X-ray computed tomography (SRXCT). The results show that addition of Na3AlF6 transformed the shrinkage pores with large-sized irregular-shaped to highly spherical hydrogen pores, resulting in a significant reduction in pore volume fraction. When the refining flux with 5% Na3AlF6 is added to the melt, the volume fraction and average equivalent diameter of pores in the recycled alloy decreased by 84% and 23.2%, respectively, compared to those flux of without Na3AlF6. Furthermore, adding Na3AlF6 improves pore distribution uniformity while reducing the equivalent diameter of the largest pore by 83.7%. Although increasing Na3AlF6 content slightly increases pore equivalent diameter, it significantly reduces both larger pores (Equivalent diameter ≥20 μm) and their proportion. Moreover, the elongation of recycled Al gradually increases with higher Na3AlF6 content and reaches up to 21%, which is 40% higher than that without Na3AlF6. This improvement can be attributed primarily to reduced size and sphericity of shrinkage pores.

Effects of processing parameters on pore defects in blue laser directed energy deposition of aluminum by in and ex situ observation

A single track, as the basic unit of laser directed energy deposition (L-DED) process, plays a significant role in the dimensional accuracy and mechanical performances of the ultimate products. However, there is almost no systematic investigation on the formation process and three-dimensional characteristics of the internal pore defects. Here, we used a high-speed camera, laser scanning confocal microscope (LSCM), and synchrotron radiation X-ray computed tomography (SR-CT) to study single tracks of AlSi10Mg alloy fabricated by blue laser directed energy deposition (BL-DED). A comprehensive investigation is conducted on the impact of processing parameters on the sizes, shapes, and formation mechanism of pore defects. Three types of pore defects are examined in single tracks: Type I lack of fusion, Type II spherical gas pores and Type III large irregular pores. Besides, large irregular pores are the transition between other two types. In particular, the results of SR-CT show that porosity decreases gradually with the increment of laser power and scanning speed. Therefore, high laser power accompanying with fast scanning speed will reduce the porosity. The lowest porosity of 0.074% is achieved under the power at 1600 W with scanning speed at 1080 mm/min, which has an obvious improvement over the current infrared L-DED. In addition, the mapping relationship among laser power, scanning speed and pore defects is established, which will provide a fundamental understanding of the origin of the defect and strategies for controlling the defect in L-DED towards high-quality printing.

Characterization of internal fatigue crack initiation in Ti‐6Al‐4V alloy via synchrotron radiation X‐ray computed tomography

AbstractThe initiation of internal fatigue cracks in very high cycle fatigue of Ti‐6Al‐4V alloy was investigated using synchrotron radiation X‐ray computed tomography (SR‐CT). Micro‐CT detected 28 cracks that were distributed across the examined volume of Ф1.8 × L 2.5 mm. No apparent correlation was observed between the spatial distribution, initial lengths, and initiation lives of cracks. The crack growth rate of the facet‐sized crack varied widely; some cracks propagated rapidly, whereas no crack growth was observed for other cracks over a specific period of time after detection. Using nano‐CT, the several grain‐sized internal cracks and their microstructures were clearly and nondestructively visualized. In the field of view, many primary α phases were detected; however, no other cracks were observed. The multiple facet initiation site, which is commonly observed for titanium alloys, might not be due to the concurrently initiated facets but may be caused by the small crack growth accompanied by facet formation.

Ultrasound-assisted dispersion of TiB2 nanoparticles in 7075 matrix hybrid composites

Refinement and dispersion of rigid ceramic particles enhance the mechanical properties of particulate reinforced aluminum matrix composites (PRAMCs). However, nanoparticles are intrinsically clustered or agglomerated together in melts and thereby reduce their strengthening efficacy. Ultrasound cavitation and acoustic streaming can effectively improve the distribution of nanoparticles in melts. In this work, we use synchrotron radiation X-ray computed tomography (SR-CT) to unveil the particle dispersion mechanism by ultrasound vibration treatment (UVT) from three-dimensional perspective. The SR-CT results indicated that the mesoscale agglomerates in a high-strength 7075 aluminum alloys can be eliminated completely upon UVT. Two types of TiB2 particles have been identified, termed as micro-size TiB2 particles (MTPs) and nano-size TiB2 particles (NTPs), which were observed to be aggregated along the grain boundaries and dispersed uniformly within the α-Al grains, respectively. Tensile tests reveal significant strengthening of the composites in the as-cast state, suggesting effective Orowan strengthening. This strength enhancement is attributed to the dispersed NTPs that have been retained after solidification. It is also inspiring to see the concurrent increase in the ductility of the composites after UVT, thanks to the improvement in particle distribution.

The novel effect mechanism of Al2O3 nano-powder in the pack cementation process to prepare SiC coating on C/C composites

As an additive, Al2O3 nano-powder can significantly improve the preparation of SiC coating by the pack cementation process. The three-dimensional morphology of the SiC coating and the interface between coating and substrate were analyzed by synchrotron radiation X-ray computed tomography (SR-CT). Combined SR-CT and other analysis results, an unpredicted effect mechanism of Al2O3 was analyzed in this work, which is obviously different from previous studies. The holes left by the chemical reaction between the Al2O3 and SiC can be the penetrating channels for the liquid Si into the C/C composites, which could obviously increase the thickness of SiC coating. At the same time, the density of SiC coating is also improved by adding the Al2O3 nano-powder.

Characterization of a 3D Auxetic Warp-knitted Spacer Fabric by Synchrotron Radiation X-ray Computed Tomography

Auxetic fabrics have many potential applications in technical textiles. Auxetic 3D warp-knitted spacer fabric can be produced by adoption of some special geometrical configuration. Characterization of the geometrical configurations is of vital significance to understand their auxetic property. In this paper, a typical 3D auxetic warp-knitted spacer fabric was studied by synchrotron radiation x-ray computed tomography (SR-CT). Fiber distribution in the fabric was clearly obtained and different types of yarn were extracted separatively. 3D image reconstructed by synchrotron radiation X-ray computed tomography was consistent with the designed geometrical configuration. Besides, 3D model reconstructed lays the foundation for further numerical analysis by FEA.

Effects of aging treatment and peripheral coarse grain on the exfoliation corrosion behaviour of 2024 aluminium alloy using SR-CT

The exfoliation corrosion behaviour of 2024 aluminium alloy was investigated through exfoliation corrosion test, together with synchrotron radiation X-ray computed tomography (SR-CT) to achieve the 3D segmentation of precipitated phases, cracks and corrosion products. The results demonstrate that aging treatment has a significant effect on the distribution of the precipitated phases in the 2024-T4 aluminium alloy, and the precipitated phases continuously distributed along the grain boundary would lead to intergranular corrosion (IGC) which is easily evolved into exfoliation corrosion. In the 2024-T2 aluminium alloy, the longest crack was found to locate at the boundary between the inner grains structure and outer peripheral coarse grain (PCG) structure, which indicates that the corrosion is attributed to electrochemical property difference of two structures.

Real internal microstructure based key mechanism analysis on the micro-damage process of short fibre-reinforced composites.

In this work, the underlying micro-damage mechanisms of randomly oriented short fibre-reinforced composites were revealed based on real internal microstructural characteristics obtained by high-resolution (0.7 μm/pixel) synchrotron radiation X-ray computed tomography (SR-CT). The special ‘pore dominant micro-damage processes’ were directly observed through SR-CT three-dimensional reconstructed images, which were different from the well-known ‘fibre breakage dominant failure mode’. The mechanisms of pore formation and pore evolution were further investigated on the basis of the microstructural parameters extracted from the SR-CT results. On one hand, the pore formation mechanism caused by shear stress concentration was proposed by combining the shear-lag model with the microstructural parameters obtained from the experiment, including the fibre length and orientation angle. On the other hand, the ‘fibre-end aggregation-induced pore connection’ mode of crack initiation was proposed through a composites model, which considered the parameters of real internal microstructure, including the critical value of the distance between neighbouring fibre ends and the number of neighbouring fibre ends. The study indicated that the shear stress concentration was significant in the region with a large number of neighbouring fibre ends, thus causing pore connection and crack initiation.

Combined SRCT & FXCT – The next steps

One of the goals in developing synchrotron radiation x-ray computed tomography (SRCT) for biomedical specimens, is allowing particular tissues and cell types to be marked in the images. This is equivalent to the staining in histology, which enables researchers to visualise and measure tissue structure and biochemical processes within the specimen. Some progress in this direction for SRCT is being made, using a variety of contrast agents that alter the natural x-ray attenuation of the marked tissue [1]. However there are limits to the usefulness of these attenuation altering techniques. Often high concentrations of potentially disruptive chemicals are required with reduced compatibility for in-vivo studies. Another image highlighting technique which might prove more sensitive is x-ray fluorescence imaging. In this case usually endogenous elemental markers are visualised. We would like to develop a lower resolution, but wider field of view means of three-dimensional (3-D) fluorescence imaging compatible with SRCT. We have previously proposed a technique in which x-ray fluorescence CT (FXCT) and SRCT data can be collected simultaneously [2]. This work resulted in proof of concept modelling, and a simple experiment test system. We show data here which demonstrate a two-dimensional (2-D) reconstruction of an iodine fluorescence map from a phantom. Measurements were performed with a fixed beam modulating mask using the Imaging and Medical beam line (IMBL) at the Australian Synchrotron. Fluorescence data was obtained during a CT scan using a single point detector, while transmission data was simultaneously collected using an area detector. A maximum likelihood expectation maximisation (MLEM) iterative algorithm was used to reconstruct the fluorescence map. We report on technique development and now believe compressive sensing (CS) imaging techniques suit SRCT and may overcome the issues encountered so far in combining SRCT and FXCT.

Stimuli dependent impedance of conductive magnetorheological elastomers

The structure dependent impedance of conductive magnetorheological elastomers (MREs) under different loads and magnetic fields has been studied in this work. By increasing the weight fraction of iron particles, the conductivity of the MREs increased. Dynamic mechanical measurements and synchrotron radiation x-ray computed tomography (SR-CT) were used and they provided reasons for the electrical properties changing significantly under pressure and magnetic field stimulation. The high sensitivity of MREs to external stimuli renders them suitable for application in force or magnetic field sensors. The equivalent circuit model was proposed to analyze the impedance response of MREs and it fits the experimental results very well. Each circuit component reflected the change of the inner interface under different conditions, thus relative changes in the microstructure could be distinguished. This method could be used not only to detect the structural changes in the MRE but also to provide a great deal of valuable information for the further understanding of the MR mechanism.

TiB2 reinforced aluminum based in situ composites fabricated by stir casting

In this study, a new technique involving mechanical stirring at the salts/aluminum interface was developed to fabricate TiB2 particulate reinforced aluminum based in situ composites with improved particle distribution. Processing parameters in terms of stirring intensity, stirring duration and stirring start time were optimized according to the microstructure and mechanical properties evaluation. The results show that, the first and last 15min of the entire 60min holding are of prime importance to the particle distribution of the final composites. When applying 180rpm (revolutions per minute) stirring at the salts/aluminum interface in these two intervals, a more uniform microstructure can be achieved and the Al-4wt% TiB2 composite thus produced exhibits superior mechanical performance. Synchrotron radiation X-ray computed tomography (SR-CT) was used to give a full-scale imaging of the particle distribution. From the SR-CT results, the in situ Al–xTiB2 composites (x=1, 4 and 7, all in wt%) fabricated by the present technique are characterized by fine and clean TiB2 particles distributed uniformly throughout the Al matrix. These composites not only have higher yield strength (σ0.2) and ultimate tensile strength (UTS), but also exhibit superior ductility, with respect to the Al–TiB2 composites fabricated by the conventional process. The σ0.2 and UTS of the Al–7TiB2 composite in the present work, are 260% and 180% higher than those of the matrix. A combined mechanism was also presented to interpret the improvements in yield strength of the composites as influenced by their microstructures and processing history. The predicted values are in good agreement with the experimental results, strongly supporting the strengthening mechanism we proposed. Fractography reveals that the composites thus fabricated, follow ductile fracture mechanism in spite of the presence of stiff reinforcements.

Application of synchrotron radiation X-ray computed tomography to investigate the agglomerating behavior of TiB2 particles in aluminum

Agglomeration of reinforcing particles has a number of deleterious effects on the properties of in situ metal matrix composites (MMCs). In order to better understand this phenomenon, the agglomerating behavior of TiB2 particles in aluminum based in situ MMCs was investigated using synchrotron radiation X-ray computed tomography (SR-CT) and field emission scanning electron microscope (FESEM). SR-CT was shown to be a powerful tool for visualizing and quantifying the three-dimensional (3D) features within the composites. Based on the SR-CT and FESEM results, a formation mechanism of the flaky agglomerates, flocculent agglomerates and clusters of coarse TiB2 particles, which are most frequently presented in the in situ Al/TiB2 composite, has been proposed. The mechanism shows that the formation of these three kinds of agglomerates can be attributed to three parallel processes, i.e. diffusing titanium atoms reacting with AlB2, aluminum melt directly reacting with emulsified salt, diffusing boron atoms reacting with TiAl3, respectively. Moreover, the mechanism may shed some light on how to design better processing techniques for obtaining homogenous particle distribution in in situ Al/TiB2 composites in the future.

In situ observations of fractures in short carbon fiber/epoxy composites

In contrast with traditional methods of observation, synchrotron radiation X-ray computed tomography (SR-CT) is an advanced technique that allows direct three-dimensional (3D) and non-destructive observation of microstructures in materials. High-resolution in situ observations (0.7μm/pixel) of fractures in short carbon fiber/epoxy composites are achieved using the SR-CT technique, and the mechanical load response of short carbon fibers treated with oxidation and those untreated are compared. By the quantitative extraction and analysis of microstructure parameters in high-resolution 3D images, the failure mechanisms of the two materials were studied. The proportion of broken fibers to other types of fiber damage in the sample with oxidation-treated fibers increases by about 6%. Also, the oxidation treatment is able to reduce the ineffective length of the fibers by about 20%, thereby improving the mechanical properties of these composites. The results show that computed tomography can promote characterization of the internal microstructures in carbon fiber-reinforced polymer composites, which will facilitate further theoretical research on the failure mechanisms of these composites.

Role of Second Phase Powders on Microstructural Evolution During Sintering

The influence of second phase fine powders on solid state sintering was investigated in situ by synchrotron radiation X-ray computed tomography (SR-CT). The evolutions of microstructure during sintering of pure SiO2 powders and SiO2 powders mixed with Si3N4 powders were observed from reconstructed SR-CT images. Typical sintering parameters, which determine mechanical properties, were extracted from the experimental data, and the effects of the second phase powders were quantitatively analyzed. The experimental results showed that with second phase powders, 1) the specific surface area decreases more efficiently; 2) pore shape evolution is accelerated; 3) sintering necks grow faster and 4) densification process is notably accelerated. It was indicated that the Si3N4 fine powders enhanced the sintering of the original SiO2 powders rather than hindering the process. A qualitative explanation based on the experimental results and existing powder sintering theory is provided.

Three-dimensional observation of carbonaceous chondrites by synchrotron radiation X-ray CT – Quantitative analysis and developments for the future sample return missions

Carbonaceous chondrites were observed by synchrotron radiation X-ray computed tomography (SR-CT) three-dimensionally. We analyzed the chemical composition of the components of the carbonaceous chondrites, such as chondrules, matrix and iron inclusions, from the linear attenuation coefficient (LAC) of X-rays. Histograms of the LAC were used for the analysis of chemical composition of the internal material, and we could find a clear relation of the compositions between the chondrules and matrix. We also analyzed the size distribution of low-Fe inclusions (chondrules) using image processing of CT images. Cumulative size distribution of chondrules shows power-law distribution, and slopes of the distribution changed for different chondrite classes. Results of these analyses show that the chondrule and matrix would have formed from the same precursor, and were incorporated into the parent bodies without large scale mixing in the solar nebula.The method will also provide important information for the analysis of future sample return missions. Our examinations demonstrate great potential of the quantitative determination of chondrite class nondestructively, by SR-CT.

In situ investigation of metal's microwave sintering

In this paper, we present the in-situ investigation on microstructure evolution of metal powders during microwave sintering by using synchrotron radiation X-ray computed tomography (SR-CT) technique. Through the experimental reconstructed images, sequential 2-D and 3-D microstructure evolution phenomena of the same grains set were clearly observed, such as the rearrangement and migration of particles, the formation and growth of sintering neck, and the removal of large initial differences in surface curvature. Moreover, anomalous sintering neck growth kinetics mechanisms (log( x/a) − log ( t)) during microwave sintering process were revealed and compared with the conventional sintering experiment, the reasons for these phenomena may be the “non-thermal effects”.

In situ investigation of SiC powder’s microwave sintering by SR-CT technique

Microwave sintering is being developed as a novel technique for the preparation of dense structural ceramics, but the mature theory has not been established due to the technical difficulties. The synchrotron radiation X-ray computed tomography (SR-CT) technique was introduced for the first time into the study of microwave sintering to in-situ observe the microstructure evolution of silicon carbide (SiC) material in this paper. By applying the SR-CT technique, the reconstructed 2D and 3D images of the specimen were obtained and the double logarithm curve of mean neck size and time (Ln(x)-Ln(t)) were obtained from these reconstructed images. Various sintering phenomena including sintering neck growth during microwave treatment were observed from the reconstructed images. Furthermore, the differences in microstructure evolution and sintering kinetics between microwave and conventional sintering were analyzed based on the reconstructed images and the Ln(x)-Ln(t) curve. 1) The sharp surface of grains near the contact region distinctly grew blunt and the sintering neck growth between these grains were obviously observed at the early stage. Besides, the larger particles grew faster than smaller ones. The main reason for these phenomena may be the micro-focusing effect of electric fields. 2) During each of the three sintering stages, the sintering kinetics curve of double logarithm relationship between mean neck size and time shows a good linear relationship, but at the middle stage the slope of the curve increases dramatically, which is quite larger than conventional sintering. The preliminary interpretation for these extraordinary phenomena has been discussed in details.

The development of a novel SR-CT technique-originated equipment for microwave sintering

Microwave sintering has been paid a considerable amount of attention by researches as a novel technology for the preparation of dense structural ceramics recently. However, the mature theory has not been established due to the technical difficulties. Synchrotron Radiation X-ray Computed Tomography (SR-CT) technique is a latest nondestructive detection technology. Applying the SR-CT technology on the research of microwave sintering can realize the observation of the evolution of microstructure under microwave and high-temperature field in a nondestructive, 3D and real-time way, provide more accurate experiment data for revealing the kinetics mechanism, and offer direct foundation for establishing the theory of microwave sintering. But due to the high requirements of experiment skills, it is difficult to apply the SR-CT technique into the in-situ observation of the microstructure evolution process during microwave sintering. Especially, duo to the restriction of SR-CT experiment platform, to design a microwave sintering equipment is quite a big problem. In this paper, according to the analysis of the requirements of SR-CT technique and the restriction of SR-CT experiment platform, the corresponding solutions for these difficulties such as separated structure arrangement, two-ply heat insulation and high precise rotation device were put forward, and a microwave sintering furnace exclusively to SR-CT experiment was designed. The testing on the heat insulation structure and the precision calibration of the rotation device was conducted, the results show that the heat insulation effect is good and the rotation precise achieves the required standard. Besides, depending on this equipment, the first observation of the microstructure evolution of SiC during microwave sintering was carried out, and a few sintering phenomena during three sintering stages were clearly observed. The experiment results indicate that the all the specifications of the equipment meet the design requirements.

SR-CT filter-back-projection algorithm and application in detection of microstructure evolution

Noise has been spotted in Synchrotron Radiation X-ray Computed Tomography (SR-CT) experiment; however, traditional filter functions (such as S-L filter function and R-L filter function) do not take the presence of noise into account. Therefore, the quality of reconstructed image is affected by noise seriously. In this paper, actual situation of SR-CT experiment was considered and a new filter function was applied. Simulation experiment was performed in order to verify the advantages of new filter function. The images reconstructed by different filter functions were compared by using image-evaluated parameters. The reconstructed images from ceramic sintering experiment projection were also obtained. The result shows that the new filter function is more suitable than the traditional filter functions in actual situation. To be close to the actual situation, the experiment noise in projections was analyzed. A new method to apply new filter function in reconstruction was presented by which image quality can be improved and the problem that image quality is affected by noise can be relieved. Therefore, this method could be better used in the real-time microstructure detection.

In situ observation of grain evolution in ceramic sintering by SR-CT technique

Grain evolution of boron carbide ceramic powder during isothermal sintering process was in situ investigated by synchrotron radiation X-ray computed tomography (SR-CT) technique. The process of grain growth and material migration during three sintering stages was clearly distinguished from the 2-D and 3-D reconstructed images. The results show that from room temperature to 1 200 °C (0–270 min), grains gradually approach each other and form the sintering neck but grain growth does not start, which is indicated as the initial sintering stage. While the sintering time is between 270–390 min (temperature is 1 200 °C), material migration between grains starts, while grains and sintering neck grow up, which is defined as the middle sintering stage. As the sintering time exceeds 390 min (temperature is 1 200 °C), pores become isolated and spheroidized, which shows the final sintering stage. The double logarithm curve of mean grain radius and time logarithm during middle stage of isothermal sintering process is obtained from reconstructed images and the grain growth exponent is 0.364 03, falling in the predicted range of the traditional sintering theory. The experiment results are in accordance with those of the traditional sintering theory and provide effective experimental data for further analysis of the sintering process and the mechanical characteristics of ceramics.