Dendrite fragmentation induced by massive-like δ-γ transformation in Fe-C alloys

Highly anisotropic Fe3C microflakes constructed by solid-state phase transformation for efficient microwave absorption

Soft magnetic materials with flake geometry can provide shape anisotropy for breaking the Snoek limit, which is promising for achieving high-frequency ferromagnetic resonances and microwave absorption properties. Here, two-dimensional (2D) Fe3C microflakes with crystal orientation are obtained by solid-state phase transformation assisted by electrochemical dealloying. The shape anisotropy can be further regulated by manipulating the thickness of 2D Fe3C microflakes under different isothermally quenching temperatures. Thus, the resonant frequency is adjusted effectively from 9.47 and 11.56 GHz under isothermal quenching from 700 °C to 550 °C. The imaginary part of the complex permeability can reach 0.9 at 11.56 GHz, and the minimum reflection loss (RLmin) is -52.09 dB (15.85 GHz, 2.90 mm) with an effective absorption bandwidth (EAB≤-10 dB) of 2.55 GHz. This study provides insight into the preparation of high-frequency magnetic loss materials for obtaining high-performance microwave absorbers and achieves the preparation of functional materials from traditional structural materials.

Dendritic patterns from shear-enhanced anisotropy in nematic liquid crystals

Controlling the growth morphology of fluid instabilities is challenging because of their self-amplified and nonlinear growth. The viscous fingering instability, which arises when a less viscous fluid displaces a more viscous one, transitions from exhibiting dense-branching growth characterized by repeated tip splitting of the growing fingers to dendritic growth characterized by stable tips in the presence of anisotropy. We controllably induce such a morphology transition by shear-enhancing the anisotropy of nematic liquid crystal solutions. For fast enough flow induced by the finger growth, the intrinsic tumbling behavior of lyotropic chromonic liquid crystals can be suppressed, which results in a flow alignment of the material. This microscopic change in the director field occurs as the viscous torque from the shear flow becomes dominant over the elastic torque from the nematic potential and macroscopically enhances the liquid crystal anisotropy to induce the transition to dendritic growth.

In Situ Observation of the Movement of Magnetic Particles in Polyurethane Elastomer Densely Packed Magnetic Particles Using Synchrotron Radiation X-ray Computed Tomography

In situ observation of the migration and structure formation of magnetic particles in polyurethane elastomers was carried out by X-ray computed tomography using synchrotron radiation. The mean diameter of the magnetic particles was 7.0 μm, and the volume fraction was ϕ= 0.24 at its maximum. The exposure time was 100 ms/frame, and the pixel size was 0.458 μm/pixel. The orientation angle and the volume fraction of the maximum aggregate were analyzed using commercial software for image analysis. The orientation angle for magnetic elastomers with ϕ = 0.24 was approximately 55° at 0 mT and decreased remarkably with the magnetic field. At magnetic fields above 150 mT, the orientation angle gradually decreased with the field and showed a constant value of 38° at 300 mT, suggesting that magnetic particles move and form a chain-like structure although the chains do not align perfectly in the direction of the magnetic field. On the other hand, the volume fraction of the maximum aggregate was constant at magnetic fields below 100 mT, and it significantly increased with the field, indicating that magnetic particles were connected to each other and developed into a macroscopic structure with anisotropy. Dynamic viscoelastic measurements revealed that the storage modulus of the magnetic elastomers cannot be simply scaled by the orientation angle. It was also found that the volume fraction of the maximum aggregate is a good parameter for explaining the huge increase in the storage modulus. The dynamic movement of magnetic particles when a magnetic field of 300 mT was switched on and off was also successfully observed. When the field was switched on, magnetic particles connected instantly and their aggregates were rapidly elongated in the direction of the magnetic field. When the field was switched off, some of the connections between aggregates were broken; however, most of the aggregates did not return to the original position even 5 min after being switched off.

Determining Alloy Nucleation Core Origin and Grain Refinement Strategy Based on the Dependence Degree of Content Difference

What is nucleation core origin during alloy solidification, especially for equiaxed grains? Different dependence degrees of the magnitude or occurrence of element content variation could shed light on this long-standing issue in actual large ingots. Here, based on etched surface height and grayscale, element content distributions within the solid fraction in continuous casting billets and additive manufacturing samples are first obtained by only a two-dimensional surface. Then, combined with the phylogenetic trees, the rank correlation is applied to measure the dependence of content differences during initial solidification. Assessments of external dependence degrees are helpful to determine nucleation core origin and low internal dependence degree facilitates grain refinement. Moreover, in continuous casting, some nucleation cores in the central equiaxed grain zone are confirmed to originate from the edge-chilled zone and high equiaxed grain area ratio under a low superheat, which is attributed to the low ratio of temperature gradient to growth rate rather than remelting fewer cores originating from the chilled zone. In addition, the floating behavior of separated grains originating from the chilled zone can be affected by gravity force, but these grains should be more active when increasing the casting superheat that may weaken the influence of gravity to a certain extent.

Progress on In Situ and Operando X-ray Imaging of Solidification Processes

In this review, we present an overview of significant developments in the field of in situ and operando (ISO) X-ray imaging of solidification processes. The objective of this review is to emphasize the key challenges in developing and performing in situ X-ray imaging of solidification processes, as well as to highlight important contributions that have significantly advanced the understanding of various mechanisms pertaining to microstructural evolution, defects, and semi-solid deformation of metallic alloy systems. Likewise, some of the process modifications such as electromagnetic and ultra-sound melt treatments have also been described. Finally, a discussion on the recent breakthroughs in the emerging technology of additive manufacturing, and the challenges thereof, are presented.

Dynamics of grain boundary premelting

The mechanical strength of a polycrystalline material can be drastically weakened by a phenomenon known as grain boundary (GB) premelting that takes place, owing to the so-called disjoining potential, when the dry GB free energy [Formula: see text] exceeds twice the free energy of the solid-liquid interface [Formula: see text]. While previous studies of GB premelting are all limited to equilibrium conditions, we use a multi-phase field model to analyze premelting dynamics by simulating the steady-state growth of a liquid layer along a dry GB in an insulated channel and the evolution of a pre-melted polycrystalline microstructure. In both cases, our results reveal the crucial influence of the disjoining potential. A dry GB transforms into a pre-melted state for a grain-size-dependent temperature interval around [Formula: see text], such that a critical overheating of the dry GBs over [Formula: see text] should be exceeded for the classical melting process to take place, the liquid layer to achieve a macroscopic width, and the disjoining potential to vanish. Our simulations suggest a steady-state velocity for this transformation proportional to [Formula: see text]. Concerning the poly-crystalline evolution, we find unusual grain morphologies and dynamics, deriving from the existence of a pre-melted polycrystalline equilibrium that we evidence. We are then able to identify the regime in which, due to the separation of the involved length scales, the dynamics corresponds to the same curvature-driven dynamics as for dry GBs, but with enhanced mobility.

A study on bone tissue engineering: Injectable chitosan-g-stearic acid putty

BACKGROUND

Bioengineering products can help bone tissue regeneration.

OBJECTIVE

There is an ongoing research for more effective biomaterials in bone regeneration. Chitosan (Ch) grafted stearic acid (Ch-g-Sa) polymer was synthesized and its usability as a putty was evaluated in this study.

METHODS

The chemical structure of Ch-g-Sa polymer was investigated using Proton nuclear magnetic resonance (H-NMR) and Fourier-transformed infrared spectroscopy-attenuated total reflectance (FTIR-ATR). Thermal properties of Ch-g-Sa polymer were determined by thermal gravimetric analysis (TGA). Putties containing nano-hydroxyapatite were prepared and in-vitro degradation properties and viscosity of the putties were determined.

RESULTS

The cytotoxicity, oxidation effect and osteogenic potential of the putties were investigated on MC3T3 cells while the inflammatory effect of the putties was studied on THP-1 cells. For the determination of the osteogenic effect of the putties, ALP and RUNX2 gene expression of MC3T3 cells were studied.

CONCLUSION

Ch-g-Sa/HA putties are promising biomaterials for bone tissue regeneration.