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Electron Backscatter Diffraction (EBSD) in Scanning Electron Microscope (SEM) has been one of the most popular tools for characterizing microstructures of materials and in particular alloys made by additive manufacturing (AM) in recent years. This characterization technique can provide abundant microstructural data, including phase distribution, grain size, grain boundary character, crystallographic orientation, and texture. However, it has recently been noticed that there are several challenges in the characterization of AM alloys by EBSD, causing the misunderstanding of material microstructures. This presentation discusses two challenges associated with the grain size determination by EBSD.
Grain size is an important parameter used in understanding the relationship between the mechanical property and microstructure of a 3D-printed alloy, and is usually measured by optical microscope (OM) and EBSD. However, a common issue is that the grain size determined by EBSD sometimes is inconsistent with that from OM. Here We measured the grain size from the same areas in a Ti64 sample by OM and EBSD, respectively, and compared the results, in order to correlate two techniques. The result shows that when the grain size is greater than 10um, the data from OM and EBSD can completely match. When the grain size is in the range of 4~10um, there is a significant deviation between the size distributions. For fine grains, e.g. &lt;4um, the EBSD measurement is more reliable. This indicates that OM and EBSD are equivalent for analyzing grains which sizes are bigger than 10um, but EBSD is more suitable for measuring fine grains due to the higher spatial resolution than OM. Further analysis reveals that the accuracy of grain size measurement especially for the fine grains is strongly related to the threshold angle of grain boundary used in the EBSD data process.
Step size is another important parameter to determine the grain size. Another usual challenge in the EBSD analysis is how to choose the step size to obtain a high-quality EBSD map with high spatial resolution and high hit rate [1]. As the typical AM alloy has two different microstructural features in terms of grain size, melt pools with large columnar grains and fine-grained regions between the melt pools, researchers like to choose a small step size to map the sample. This is a conventional EBSD strategy used in the characterization of heterogeneous microstructures in materials. However, the recent characterization in Al printed alloys shows that such a strategy doesn’t work well at low magnification even a very small step size is used. Normally, grains in melt pools are visualized and measured well, but the fine grains at the melt pool boundaries are invisible at the low magnification due to the low hit rate. To improve EBSD map quality and grain size measurement at the fine-grained regions, higher magnification with a smaller step size has been used, but this condition limits the size of the mapping area. Compromising the magnification and grain size determination, a new EBSD strategy that includes two steps of mapping AM samples is recommended, firstly scanning a big area to show the domain microstructures of melt pools, and secondly mapping with higher magnification to show the typical fine-grained region in detail.


MATERIALS FOR HUMANITY (MH 21)

Challenges in EBSD characterization of heterogeneous microstructure in additively manufactured alloys

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Electron Backscatter Diffraction (EBSD) in Scanning Electron Microscope (SEM) has been one of the most popular tools for characterizing microstructures of materials and in particular alloys made by additive manufacturing (AM) in recent years. This characterization technique can provide abundant microstructural data, including phase distribution, grain size, grain boundary character, crystallographic orientation, and texture. However, it has recently been noticed that there are several challenges in the characterization of AM alloys by EBSD, causing the misunderstanding of material microstructures. This presentation discusses two challenges associated with the grain size determination by EBSD.
Grain size is an important parameter used in understanding the relationship between the mechanical property and microstructure of a 3D-printed alloy, and is usually measured by optical microscope (OM) and EBSD. However, a common issue is that the grain size determined by EBSD sometimes is inconsistent with that from OM. Here We measured the grain size from the same areas in a Ti64 sample by OM and EBSD, respectively, and compared the results, in order to correlate two techniques. The result shows that when the grain size is greater than 10um, the data from OM and EBSD can completely match. When the grain size is in the range of 4~10um, there is a significant deviation between the size distributions. For fine grains, e.g. <4um, the EBSD measurement is more reliable. This indicates that OM and EBSD are equivalent for analyzing grains which sizes are bigger than 10um, but EBSD is more suitable for measuring fine grains due to the higher spatial resolution than OM. Further analysis reveals that the accuracy of grain size measurement especially for the fine grains is strongly related to the threshold angle of grain boundary used in the EBSD data process.
Step size is another important parameter to determine the grain size. Another usual challenge in the EBSD analysis is how to choose the step size to obtain a high-quality EBSD map with high spatial resolution and high hit rate [1]. As the typical AM alloy has two different microstructural features in terms of grain size, melt pools with large columnar grains and fine-grained regions between the melt pools, researchers like to choose a small step size to map the sample. This is a conventional EBSD strategy used in the characterization of heterogeneous microstructures in materials. However, the recent characterization in Al printed alloys shows that such a strategy doesn’t work well at low magnification even a very small step size is used. Normally, grains in melt pools are visualized and measured well, but the fine grains at the melt pool boundaries are invisible at the low magnification due to the low hit rate. To improve EBSD map quality and grain size measurement at the fine-grained regions, higher magnification with a smaller step size has been used, but this condition limits the size of the mapping area. Compromising the magnification and grain size determination, a new EBSD strategy that includes two steps of mapping AM samples is recommended, firstly scanning a big area to show the domain microstructures of melt pools, and secondly mapping with higher magnification to show the typical fine-grained region in detail.

technical paper

In the powder bed fusion process, exposure to oxygen and its incorporation into the melt pool is inevitable. Although oxygen exposure is conventionally recognized as a source of problems, recent experiments show otherwise, where the deliberate oxidation or introduction of oxygen actually improves the process and part quality. Hence, the influence of oxygen on the powder bed fusion process warrants further investigation. Yet, in-situ observation of the oxygen incorporation and its influence on melt pool dynamics is experimentally inexpedient, while oxygen-related effects in numerical models remain lacking. This work aims to shed light on the effect of oxygen content on melt pool dynamics through the use of thermal-fluid flow simulations. Our simulations have captured the oxygen absorption mechanism during melt pool formation and reveal its influence on the Marangoni effect and the subsequent oxygen distribution in the melt pool. The qualitative results obtained agree with experimental observations.

Influence of Oxygen Content on Melt Pool Dynamics

With flexible robots' development, the requirements for highly stretchable sensors are getting higher and higher. At present, flexible sensors have been widely used in wearable electronic devices to obtain human physical parameters, such as wearable electronic devices, motion recognition, and healthcare. Though the measuring ability of sensitivity and range has been largely improved compared to the traditional ones, the hysteresis in the loading and unloading state has always limited these sensors' application, which negatively influences the sensors' reliability and accuracy. In this paper, through the combination of highly stretchable dielectric elastomer cross-linked with polar hydrophobic ionic liquid and polymerized under UV light, a highly sensitive and highly stretched sensor is prepared, reaching up to 10 times stretchability at most and maintaining a low hysteresis (2.5%). The investigated sensor showed a very short response time (100ms), adjustable conductivity. Combined with some of our structural innovations, we have used smart gloves, touch sensors, and other applications to prove this material's broad prospects in soft robots, motion detection, wearable devices, and medical care.

Near-Hysteresis Free Ionic Conductive Elastomers with Long-Term Stability for Artificial Skin Applications

DEM-CFD simulation on spattering and denudation of laser powder bed fusion additive manufacturing

In this modern society, plagued by climate change and uncertain energy supply, clean water and energy are precious resources. These resources conjoin in a term called water-energy nexus. The Water and Energy crisis grows increasingly dire, especially in developing countries. Highly efficient solar energy desalination is a green solution that produces potable water using solar energy without further processing. A novel fully ceramic structure of integrated solar absorber/thermal insulator/water transport based on the two-layered structure of modified graphene on silica (MG@Silica) and porous silica structure was proposed. The proposed highly efficient ceramic solar desalination device addresses the issues of reusability and sustainability. By utilizing 3D printing, nanopores and micropores can be tailored to effectively maximize capillary effects to increase the water uptake. As a result, an advanced complex structure can be realized for a high water evaporation rate. Altering the ceramic’s surface with modified graphene can allow excellent solar absorption, good resistance to photodegradation, and the ability to remove ions and bacteria. The resultant desalinated water from the device meets the World Health Organization (WHO) drinking water standards.

Fabrication of 3D-Printed Ceramic Structures for Portable Solar Desalination Devices

Electrospray of plant based hydrogel microbeads for lab-based meat culture

Polluted sea water is a common problem in coastal cities, but its decontamination remains a big challenge due to the existence of salt ions, which may invalidate many prevailing treatment methods originally developed for waste fresh water. This work presents the first attempt to decontaminate the polluted sea water by synergizing two separate processes: photocatalysis and ozonation (see Figure 1). For low cost and human safety, the ozone concentration is kept to be < 60 ppm in the pumping gas (0.026 ppm in solution). Mechanism studies and experimental comparisons show that the photocatalysis, the ozonation and the photocatalytic ozonation (PCO) all have lower efficiencies in sea water than that in fresh water, and the PCO is always more efficient than only the photocatalysis or only the ozonation. More specifically, the PCO has a reaction rate constant about 23 times higher than only the ozonation. In addition, the sea water shows a positive synergistic effect of photocatalysis and ozonation and reaches the maximum when the pumping gas has an ozone concentration of 50 ppm in gas. In contrast, the fresh water shows a negative synergistic effect. Mechanism study using experiments infers that the dissolved ozone can enhance and stabilize the photocurrent by rapidly scavenging the photoelectrons. This work may pave the way to practical applications of sea water decontamination with high efficiency and low cost.

This work is supported by Research Grants Council (RGC) of Hong Kong (152184/15E, 152127/17E, 152126/18E, 152219/19E and 152156/20E), the National Natural Science Foundation of China (Project No. N_PolyU511/20) and The Hong Kong Polytechnic University (1-ZE14 and 1-ZVGH).

Decontamination of polluted seawater using photocatalytic ozonation

Development of highly-efficient and cost-effective water treatment techniques is critical to tackle global water crisis and membrane processes are promising among state-of-the-art water treatment technologies. We recently developed a few novel carbon-based electro-active filter technologies that could not only physically adsorb, but also electrochemically oxidize various refractory chemical contaminants. The efficacy and efficiency towards organic compounds electrooxidation were examined by several commonly detected organic pollutants (e.g. antibiotic tetracycline, refractory dyes and phenols). The filter materials we are currently working on include graphene, carbon nanotubes and other conductive nanofibers. The energy consumption per volume for our electroc-active filters were calculated to be only <0.1 kW·hr/m3, which is better than other recently developed electrochemical systems, as well as other advance oxidation processes for water purification. 
In this presentation, we will share some of our recent experimental results that exemplified the advantages of contaminants removal using filters constructed with various nanomaterials integrated with a novel electrochemical approach in a flow-through system. The overall findings demonstrate that this integrated strategy is attractive and useful for both waste water treatment and drinking water purification.

Engineering Functional Electro-Active Filter toward Environmental Remediation

This presentation will cover Cr(VI) and methyl orange dye (MO) removal from wastewater by adsorption on biochar-derived porous carbon adsorbents. The first work presents a strategy for modifying carbon surfaces by doping both N and P functional groups for enhanced Cr(VI) adsorption from aqueous solutions. This talk will focus on Layered double hydroxides (LDHs) decorated porous carbons for simultaneous Cr(VI) and MO removal. Layered double hydroxides (LDHs) are currently attracting intense research interests as pollutant removing adsorbents due to their low cost, non-toxic, structure amenability, and anion exchange capability. Herein, 3-dimensional flower-like Ni/Al LDH was prepared to decorate H3PO4 activated biomass-derived porous carbons (PAB) via a facile and green hydrothermal method. The as-synthesized nanocomposites (Ni/Al@PAB) displayed favorable removal performance towards both Cr(VI) and MO from aqueous solution due to the inherited excellent Cr(VI) removal ability from PAB and MO removal ability from Ni/Al LDH. In a single-component removal system, the maximum adsorption capacity of 271.5 and 412.8 mg/g for Cr(VI) and MO were achieved on Ni/Al@PAB, respectively. Remarkably, the removal capacity of Cr(VI) and MO both increased in the Cr(VI)single bond MO binary system due to the synergistic effect including, (i) additional adsorption sites of N-containing groups from MO, (ii) electrostatic interactions by the reduced Cr(III), and (iii) mutual thermal compensation in the binary system. This work paved the way for designing novel LDHs-decorated porous carbon adsorbents and revealed that the acicular Ni/Al LDH decorated PAB could be applied as an efficient and simultaneous adsorbent for coexisting toxic pollutants situations.

Simultaneous and efficient removal of Cr(VI) and methyl orange on layered double hydroxides decorated porous carbons

Energy and Materials Research to Find Solutions for the Dual Challenge

Membranes used in water reuse and pollution control applications are often exposed to feed solution species (e.g., proteins, polysaccharides, humic materials) that adsorb or otherwise accumulate during the filtration run, and thus can cause fouling. This has many consequences, including increased energy usage, a need for more frequent cleaning, and shortened membrane life. Here we examine novel and feed-specific hydrophilic coatings to mitigate the effects of fouling with the goal of reducing fouling resistance and energy consumption.
The coatings examined here are based on dopamine (DA), a biomimetic self-polymerizing hydrophilic material. They include DA, dopamine blended with 2-(dimethylamino)ethyl methacrylate (DAEMA) and DAEMA polymer (PDAEMA), and a novel coating using the dopamine analog norepinephrine.
The coatings containing DAEMA or PDAEMA offered anti-microbial activity, whereas the norepinephrine coatings provided very smooth hydrophilic surfaces that resisted fouling at low coating amounts, thus reducing coating resistance. These coatings were applied to commercially available 30 kDa poly(ether sulfone) membranes using a batch dip-coating method for a range of reaction times up to 3 h. 
Membranes were tested in bench-scale filtration tests using a dead-end system with mixing. Measurements included mass of permeate as a function of time, temperature, and solute concentration. From these we calculated solution flux, resistance, and energy consumption. Membranes were characterized using contact angle, atomic force microscopy (AFM), attenuated total reflectance Fourier transform infrared spectroscopy (FTIR), x-ray photoelectron spectroscopy (XPS), and scanning electron microscopy (SEM). The coatings enhanced anti-fouling properties against model foulants (BSA, SA, and SRNOM), and two treated municipal wastewaters. The modified membranes exhibited less energy consumption and lower fouling resistance as compared to as-received membranes.

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Challenges in EBSD characterization of heterogeneous microstructure in additively manufactured alloys

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