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Laser-powder bed fusion (L-PBF) of alloys often leads to large, elongated grains, which are the cause for anisotropic mechanical properties and, sometimes, solidification cracking. While grain refinement has been used before in the design of Al alloys for AM in order to avoid these issues, it is much less common in steels. Here, we report that Ti(O,N) is an effective inoculant for ferritic steels, owing to the Baker-Nutting crystallographic orientation relationship and matching lattice constants. We show that this inoculating phase can be added to steel powder ex situ as TiN micro-powder, but that it also can be formed in sub-µm-size in the melt by reaction of the alloying element Ti with nitrogen from the process atmosphere. Thereby, the refining effect can be “switched” by the choice of process gas. By both, in-situ synthesis and ex-situ addition, ultrafine grains with an average diameter just larger than 1 µm and a completely random texture are achieved. We show that this refinement method is effective in several ferritic steels (Fe-Ti, Fe-Al, Fe-Cr) and discuss the required amount of Ti addition. The addition of 5wt.-%Ti to Fe-19wt.-%Cr leads to a doubling of the yield strength and UTS, which we argue is mostly due to the grain refinement by Ti(O,N).

MATERIALS FOR HUMANITY (MH 21)

Ultrafine-grained ferritic steels by L-PBF via ex-situ addition or in-situ synthesis of inoculants

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MH 21 is arranged around 6 themes that focus attention on pressing needs that should be addressed to enhance the well-being of communities and provide a forum for presenting breakthrough discoveries.

technical paper

Porosity in laser powder bed fusion is a concern due to the influence of such defects on mechanical properties of parts. Different mechanisms lead to different types and shapes of pores, in different quantities and it is important to be cognizant of these differences, for best approaches to minimize their formation and even remove them by different methods. Results are presented showing how this can be done for different types of pores in the laser powder bed fusion of metals.

Porosity in laser powder bed fusion of metals

We recently presented a new approach inspired by the strengthening mechanisms in crystalliine alloys to design a new class of damage-tolerant architected materials called meta-crystals [1, 2][1]. The mimicry of crystalline microstructure enables a uniquely new way to program meta-crystals to have a defined set of properties in response to the changes in surrounding environments. While crystal-inspired structures can be readily changed via computer-aided design, the intrinsic microstructure of materials can be tailored by changing the base material and processing parameters used to fabricate the meta-crystals. The combined opportunity offered by the crystal-inspired approach opens the new paradigm of engineering and programming materials with desired mechanical properties. In this talk, I will present our studies of the programming of meta-crystals such as store, communication and actuation of mechanical information.

Reference
[1] Pham M-S, Liu C, Todd I, Lertthanasarn J. Damage-tolerant architected materials inspired by crystal microstructure. Nature 2019;565:305.[1] Lertthanasarn J, et al. Synergistic effects of crystalline microstructure, architected mesostructure, and processing defects on the mechanical behaviour of Ti6Al4V meta-crystals. Materials Science and Engineering: A 2021;818:141436.
[2] Liu C, Lertthanasarn J., Pham M-S The origin of the boundary strengthening in meta-crystal architected materials. Nature Communications 2021 (accepted).

Mechanical programmable meta-crystals

Synthesis of magnetic nanoparticles for rapid quantification of mycotoxins and pesticide residue in foods

In IMRE, we developed a series of technologies for cascade utilization of SCG to achieve “zero waste”. The whole process includes solvent extraction, transesterification, compounding, moulding etc, and we are able to obtain various functional materials from SCG, such as coffee oil, surfactants, bioplastics. 

Cascade utilization of spent coffee grounds towards a zero-waste strategy

Our group has optimised and performed the pyrolysis of PE, PP and PS plastics alongside 2 commonly found waste plastic with good conversion rates and selectivity. The incineration ash and pyrolysis oil generated will be characterised and attempts will be made to functionalise them into specialty chemicals. 

Catalytic pyrolysis of plastic waste with incineration fly ash into fuels for upcycling

Spent coffee grounds (SCGs) are often disposed by conventional waste disposal methods which are environmentally unfriendly. As such, we aim to maximize the potential of SCGs for coffee oil and as a filler material for bio-based composites. Approximately 10 % of coffee oil was successfully extracted from the SCGs. Its chemical composition was evaluated and was proven to exhibit radical scavenging activity. Afterwards, the extracted SCGs (ESCGs) were added as fillers into epoxy resin to form ESCG composites. Although the composites displayed lower mechanical properties relative to epoxy, improvements were seen for ESCG composites in comparison to SCG composites. Composites filled with acetone ESCGs demonstrated the most improvement with tensile strength, flexural modulus and flexural strength up to 23.4 MPa, 3.02 GPa and 42.9 MPa respectively. This work suggests an alternative method to repurpose common food waste while moving towards a more sustainable future.

An environmental alternative to composites using waste coffee grounds

1) Traditional Thermosets:
Excellent mechanical strength,
Thermal stability,
Solvent resistance.
2) Vitrimers (recyclable thermosets)
Excellent mechanical strength,
Good recyclability
Self-healing and repairing

Catalyst-Free Epoxy Vitrimer for Recyclable and Self-Healing Solid Polymer Electrolytes

Super-absorbent fabric can provide a durable and comfortable protective microenvironment for the protective clothing users. In this work, we prepared the recirculated super-absorbent and strong water-retaining fabrics by depositing starch-based double network (St-PAA/SA) aerogel on fiber surface via graft-cross-linking copolymerization and freeze-drying strategy, with two biological macromolecules of starch and sodium alginate as raw materials, ammonium persulphate (APS) as the initiator, and N, N-methylene bisacrylamide (MBA) as a cross-linking agent. FTIR and XPS provided an evidence for graft copolymerization and cross-linking interaction among St-PAA/SA aerogel. SEM showed that St-PAA/SA aerogel presented typical double network macroporous characteristic, and the pore size distribution was 23~52 μm, which provided space for the absorption and storage of water molecule. Moreover, the strong interactions were excited between St-PAA/SA aerogel and fabric. When the mass ratio of starch to sodium alginate was 7/3, and the molar ratio of APS/MBA/AA was 0.8/0.3/1, the water absorption volume reached 420 g/g, which exceeded that of the super-absorbent fiber (120 g/g). Also, the super-absorbent fabric exhibited strong water storage capacity and excellent characteristics of recirculated water-absorbent and storage. The water absorption and softness of the fabric can be adjusted by controlling the aerogel deposition amount. This work provides a new way for the development of comfortable medical protective fabrics.

Recirculated Super-Absorbent and Strong Water-Retaining Fabric by Depositing Starch-based Double-Network Aerogels on Fiber Surface

Marine degradation polymers seems like a promising substitute to existing plastics issues. Beside the cost issue, the lower thermal stability also cause processing challenges. The CML aims to connect the different stakeholders and find the next generation of plastics materials.

Development of Marine Biodegradation Testing Capability and Materials

Pressure-driven membrane processes are widely-applied techniques in the field of water and wastewater treatment. Currently, polymeric membranes are of dominance in this market for their low fabrication cost, high scalability and high separation factor. On the other hand, ceramic membranes have been receiving global attention in this field as well, attributed to their apparent advantages including easy surface functionalization, high intrinsic permeability, high thermal and chemical stability and excellent mechanical strength, although the main hindrance for wide acceptance of ceramic membranes is their higher fabrication cost compared to that of polymeric membrane. However, the ceramic membrane offers novel applications, for instance, to impart a reactive functionality via introducing nanocatalysts onto/into the ceramic membrane, such that the membrane can work as a hybrid filtration and catalytic reactor. The present work interpreted the advantages of ceramic membranes over their counterparts of polymeric membranes, showed specific work to demonstrate excellent performance of ceramic membrane and gave the perspectives on ceramic membranes, which as key players to replace polymeric membranes in the water-treatment field.

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Ultrafine-grained ferritic steels by L-PBF via ex-situ addition or in-situ synthesis of inoculants

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