Out of the multitude of researched processing routes for sustainable ironmaking, hydrogen-based direct reduction and hydrogen plasma smelting reduction (HyPSR) are currently the most promising ...candidates for a successful industrial application. Both processes operate under gaseous atmospheres, which turn the partial and absolute pressure of hydrogen into a relevant process parameter. Here, we present first insights into the influence of total pressure and concentration of hydrogen on the reduction of hematite, focusing on the more pressure-sensitive route (HyPSR). The effect of pressure on the dissociation of H
2
molecules into metastable H atoms or H
+
ions,- and the overall hydrogen utilization is discussed using a thermodynamic approach. Validation experiments were conducted to testify the practical feasibility of adjusting these parameters. A decrease in the total pressure of the system from 900 mbar to 450 mbar resulted in an improved hydrogen utilization, thus suggesting that a larger population of H atoms can exist in the plasma arcs ignited under 450 mbar. An increase in the hydrogen concentration to 20 vol.% lead to undesired evaporation, likely because of a parallel increase in plasma temperature. Possibilities and challenges for exploiting these pressure-related phenomena for the industrial production of green steel are outlined and discussed.
The enormous magnitude of 2 billion tons of alloys produced per year demands a change in design philosophy to make materials environmentally, economically, and socially more sustainable. This ...disqualifies the use of critical elements that are rare or have questionable origin. Amongst the major alloy strengthening mechanisms, a high-dispersion of second-phase precipitates with sizes in the nanometre range is particularly effective for achieving ultra-high strength. Here, we propose an alternative segregation-based strategy for sustainable steels, free of critical elements, which are rendered ultrastrong by second-phase nano-precipitation. We increase the Mn-content in a supersaturated, metastable Fe-Mn solid solution to trigger compositional fluctuations and nano-segregation in the bulk. These fluctuations act as precursors for the nucleation of an unexpected α-Mn phase, which impedes dislocation motion, thus enabling precipitation strengthening. Our steel outperforms most common commercial alloys, yet it is free of critical elements, making it a new platform for sustainable alloy design.
The formation of strain-induced martensite in AISI 201 austenitic stainless steel was followed by in situ synchrotron X-ray diffraction during tensile testing. Real-time information allowed tracking ...microstructural changes related to the decomposition of metastable austenite (γ) into hcp-ε-martensite and bcc-α′-martensite. Approximately 78% of α′-martensite was found after an equivalent strain of 0.39 and a corresponding elongation of 0.70 in the analyzed region. Microstructural characterization was carried out using electron backscatter diffraction and scanning electron microscopy. The work hardening behavior of each phase was evaluated and a strain partitioning between martensite and austenite was observed. Microstrain in austenite increased with strain whereas it remained nearly unchanged in α′-martensite during the experiment. At the end of the test, these two phases showed similar values of microstrain. The formation of martensite and the accumulation of defects in austenite are responsible for the overall work hardening behavior. Kernel average misorientation analyses showed very similar values of stored elastic energy in austenite and α’-martensite. Orientation relationships among the phases close to {111}γ//{0002}ε//{011}α′ and γ// ε// α′ were found.
There is a need to find new paths for van der Waals 2D-systems detachment and transfer or to control their adhesion state in different environments. We have observed that supported multilayer ...graphene immersed in a fluid can be detached from a substrate through pressure application. The process is based on the development of wrinkles originated by the difference of in-plane-compressibility between the graphene stacks and the substrate. Graphene stacks comprised between 9 and 110 layers and immersed in various fluids allowed to investigate the growth and evolution of wrinkles with increasing pressure. The detachment from the substrate stops at the pressure-induced fluid solidification. Methanol, ethanol or their mixtures favor the pressure-induced wrinkle formation in SiO2/Si substrates. In these cases, the pressure evolution of the delamination process follows a universal behavior independently of the number of graphene layers with a complete delamination at ∼4GPa. The quantitative analysis of the wrinkle geometry evolution can be consistently interpreted as due to a pressure-induced increase of the bending stiffness of the graphene stacks, or a reduction of the adhesion forces between the sample and the substrate, or both. These results should also be of practical use in high-pressure experiments of van der Waals systems.
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Alloys processed by laser powder-bed fusion show distinct microstructures composed of dislocation cells, dispersed nanoparticles, and columnar grains. Upon post-build annealing, such alloys show ...sluggish recrystallization kinetics compared to the conventionally processed counterpart. To understand this behavior, AISI 316L stainless steel samples were constructed using the island scan strategy. Rhodonite-like (MnSiO
3
) nanoparticles and dislocation cells are found within weakly-textured grains in the as-built condition. Upon isothermal annealing at 1150 °C (up to 2880 min), the nucleation of recrystallization occurs along the center of the melt pool, where nuclei sites, high stored elastic energy, and local large misorientation are found in the as-built condition. The low value of the Avrami coefficient (
n
= 1.16) can be explained based on the non-random distribution of nucleation sites. The local interaction of the recrystallization front with nanoparticles speeds up their coarsening causing the decrease of the Zener-Smith pinning force. This allows the progression of recrystallization in LPBF alloys, although sluggish. These results allow us to understand the progress of recrystallization in LPBF 316L stainless steel, shedding light on the nucleation mechanisms and on the competition between driving and dragging pressures in non-conventional microstructures. They also help to understand the most relevant microstructural aspects applicable for tuning microstructures and designing new LPBF alloys.
Graphical abstract
Strain partitioning and texture evolution of AISI 201 austenitic stainless steel were investigated upon cold rolling up to a true strain of ε = 0.92. ε-martensite formation is the main work hardening ...mechanism at low strains (ε = 0.11). With increasing strain, the volume fraction of α’-martensite increases with a sigmoidal-like behavior. Remaining untransformed austenite is intensely fragmented by mechanical microtwins. The in-grain misorientation increases for all phases up ε = 0.51 and then levels off for further strain. Strain partitions evenly between austenite and α’-martensite during cold rolling. X-ray texture measurements revealed that austenite develops Goss, Brass and S texture components up to the largest investigated strain. The presence of Brass component at the highest deformation seems to be assisted by mechanical twinning. The texture components of α’-martensite belong to the α- and γ- fibers. Texture evolution of ε-martensite was followed by electron backscatter diffraction data and results show that texture evolves up to ε = 0.51 and remains nearly unchanged at larger strains, similarly as observed for austenite and α’-martensite.
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•Austenite reversion starts at 500 °C.•Austenite reversion, recrystallization, and ferrite decomposition occurs at 800 °C.•Precipitation reactions take place at 700 and 800 °C in ...recrystallized austenite.•The coercivity of deformed and annealed materials is mainly influenced by ferrite.
Strain-induced α′-martensite and austenite reversion in a cold rolled UNS S32304 lean duplex steel were tracked by means of magnetic measurements, with emphasis on both Ms (saturation magnetization) and Hc (coercive field) parameters. Grain-averaged quality metrics derived from EBSD (electron backscatter diffraction) analysis were also used to distinguish the phases during austenite reversion. The material was cold rolled to a true strain (ε) of 1.61 and subjected to isothermal and continuous annealing, the latter conducted in the presence of an external magnetic field. The evolution of the α′-martensite fraction upon straining and after isothermal annealing was monitored by coupling the Ms values and thermodynamic simulations, as well as from EBSD analysis. For the isothermally annealed material (ε = 1.61), the overall behavior of Ms and hardness displayed similar trends with a strong decrease for temperatures higher than 500 °C, suggesting austenite reversion. Results confirmed the occurrence of austenite reversion for the temperature interval investigated here. At 800 °C, austenite reversion is complete, and the steel is fully recrystallized. Besides, from the EBSD analysis, evidence of ferrite transformation into austenite was rather noticeable, in accordance with thermodynamic simulations and magnetic probing. Complementary electron channeling contrast imaging (ECCI) revealed that precipitation reactions mainly occur in the recrystallized austenite at 700 and 800 °C. The Hc behavior of both, the strained and annealed conditions was inferred to be mostly driven by microstructural changes in ferrite.
The formation of submicron structural defects within austenite (γ), ε- and α′-martensite during cold rolling was followed in a 17.6 wt.% Mn steel. Several probes, including XRD, EBSD, and ...ECCI-imaging, were used to reveal the complex superposition of the strain hardening mechanisms of these phases. The maximum amount of ε-martensite is observed at a strain of ε = 0.11. At larger strains, the amount of ε decreases suggesting that it precedes the α′-formation (γ → ε → α′). Stacking faults and twins are the main planar defects noticed in ε-martensite. The remaining γ is finely subdivided by stacking faults and twins up to ε = 0.22. From ε = 0.51 on, twinning and multiplication of dislocations are the principal strain hardening mechanisms in austenite. Deformation is accommodated in α′ by the rearrangement of dislocation tangles into dislocation cells plus shear banding at ε = 1.56. During cold rolling, austenite develops a Brass-type texture component, which can be associated to mechanical twinning. ε-martensite presents its basal planes tilted ∼24° from the normal direction towards the rolling direction. The α′-martensite develops and strengthens both, the bcc α- and γ-texture fibers during cold rolling.
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•Coercivity of the α′-martensite is characterized by strong magnetic shape anisotropy.•α′ → γ transformation is split into two stages during continuous annealing.•Magnetic properties ...present an annealing time-dependence between 500 and 600 °C.•Formation of γ-nanograins in the early stages of reversion induces strong magnetic shape anisotropy.•Athermal α′-formation within the prior athermal ε-phase is observed for temperatures lower than 100 °C.
Strain-induced martensite (SIM) formation was evaluated upon cold-rolling of a 17.6 wt.%Mn-TRIP steel by means of magnetic measurements, X-ray diffraction, and high-resolution electron backscatter diffraction (EBSD). α′-martensite formation was observed to be dependent on the presence of prior ε-martensite. Upon deformation, the coercivity of the ferromagnetic α′-martensite is characterized by strong magnetic shape anisotropy. Austenite (γ) reversion was evaluated by means of in situ magnetic measurements during continuous annealing. The experimental results were compared to thermodynamic simulations. It turned out that γ-reversion was not completed in the regime where a γ-single phase field is expected, which suggests the splitting of α′ → γ transformation into two stages. The Curie temperature of remaining α′-martensite was determined as being ∼620 °C. Magnetic properties presented an annealing time-dependence within the temperature range of 500–600 °C, suggesting long-range diffusional α′ → γ transformation. With the aid of electron channeling contrast image technique (ECCI), we noticed that the formation of γ-nanograins in the early stages of reversion is sufficient to induce strong magnetic shape anisotropy in this steel. After full austenitization at 800 °C, further in situ magnetic measurements were also used to track the magnetic response of the material upon controlled cooling. Athermal formation of α′-martensite within the prior athermal ε-phase was clearly observed for temperatures lower than 100 °C. Using thermodynamic modeling we also calculated the start temperature for ε-formation (Msε). Results showed that ε-martensite is indeed expected to form before α′, which was confirmed in all cases by means of EBSD.
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