[en] Highlights: • Solidification of Al-5wt%Fe alloys was monitored via high-speed synchrotron tomography. • Formation dynamics of Al₁₃Fe₄ intermetallics were revealed. • Hole-like defects filled with aluminium melt were observed within the intermetallics. • Oriented particle attachment as a potential growth hypothesis of the Al₁₃Fe₄ intermetallics was proposed. -- Abstract: High-speed synchrotron tomography was used to investigate the nucleation and growth dynamics of Al₁₃Fe₄ intermetallic during solidification of an Al-5wt%Fe alloy, providing new insights into its formation process. The majority of Al₁₃Fe₄ intermetallics nucleated near the surface oxide of the specimen and a few nucleated at Al₁₃Fe₄ phase. Al₁₃Fe₄ crystals grew into a variety of shapes, including plate-like, hexagonal tabular, stair-like and V-shaped, which can be attributed to the crystal structure of this compound and its susceptibility to twinning. Hole-like defects filled with aluminium melt were observed within the intermetallics. Oriented particle attachment mechanism was proposed to explain the formation of the Al₁₃Fe₄ intermetallic, which needs further experiments and simulation to confirm.

[en] Constructing 0D-2D mixed-dimensional van der Waals (MvdW) heterostructures is an effective strategy to improve photovoltaic properties of photovoltaic devices. However, addressing the underlying mechanism on the role of array periodicity of 0D and thickness of 2D components on the photovoltaic properties at the atomic-level still remains challenging. Herein, the interfacial charge transfer and photovoltaic conversion mechanism in C${}_{60}$/MoS${}_2$ 0D-2D MvdWs based on the atomic-bond-relaxation consideration, Marcus theory, modified-detailed balance principle, and first-principles calculations are investigated. Compared with the case of monolayer MoS${}_2$, it is found that C${}_{60}$/MoS${}_2$ MvdW heterostructures have lower interface reflectivity and type II band alignment, leading to obvious improvement of power conversion efficiency (PCE) from 0.27% to 1.40%. Moreover, the photovoltaic properties can be tuned by changing the array periodicity of the C${}_{60}$ and the thickness of the MoS${}_2$, and the optimized PCE can be up to 5.35%. The results reveal that the modification of 0D C${}_{60}$ is an effective strategy to enhance the photovoltaic properties of MoS${}_2$-based solar cells. (© 2021 Wiley‐VCH GmbH)

[en] A trace (0.2 wt.%) addition of Ge significantly increased both the strength and elongation of Al–3.5Cu–0.4 Mg alloy. An aged Al–3.5Cu–0.4 Mg–0.2Ge (wt.%) alloy revealed good tensile properties after being exposed at 150 °C. Transmission electron microscopy observations showed that these properties are closely correlated to the high-density microstructure of nanoscale precipitates and the good coarsening resistance of those precipitates; Ge stimulated the formation of nanoscale precipitates and suppressed the formation of S′ precipitates in aged Al–3.5Cu–0.4 Mg–0.2Ge (wt.%) alloy.

[en] Controlling the final grain size in a uniform manner in powder metallurgy nickel-based superalloys is important since a number of mechanical properties are closely related to it. However, it has been widely documented that powder metallurgy superalloys are prone to suffer from growth of abnormally large grains (ALGs) during supersolvus heat treatment, which is harmful to in-service mechanical performance. The underlying mechanisms behind the formation of ALGs are not yet fully understood. In this research, ALGs were intentionally created using spherical indentation applied to a polycrystalline nickel-based superalloy at room temperature, establishing a deformation gradient underneath the indentation impression, which was quantitatively determined using finite element modeling and synchrotron diffraction. Subsequent supersolvus heat treatment leads to the formation of ALGs in a narrow strain range, which also coincides with the contour of residual plastic strain in a range of about 2%–10%. The formation mechanisms can be attributed to: (1) nucleation sites available for recrystallization are limited, (2) gradient distribution of stored energy across grain boundary. The proposed mechanisms were validated by the phase-field simulation. This research provides a deeper insight in understanding the formation of ALGs in polycrystalline nickel-based superalloy components during heat treatment, when subsurface plastic deformation caused by (mis)handling occurs or small residual strain has been retained from hot/cold working before supersolvus heat treatment.

[en] The deformation responses at 77 and 293 K of a FeCoNiCr high-entropy alloy, produced by a powder metallurgy route, are investigated using in situ neutron diffraction and correlative transmission electron microscopy. The strength and ductility of the alloy are significant improved at cryogenic temperatures. The true ultimate tensile strength and total elongation increased from 980 MPa to 45% at 293 K to 1725 MPa and 55% at 77 K, respectively. The evolutions of lattice strain, stacking fault probability, and dislocation density were determined via quantifying the in situ neutron diffraction measurements. The results demonstrate that the alloy has a much higher tendency to form stacking faults and mechanical twins as the deformation temperature drops, which is due to the decrease of stacking fault energy (estimated to be 32.5 mJ/m² and 13 mJ/m² at 293 and 77 K, respectively). The increased volume faction of nano-twins and twin-twin intersections, formed during cryogenic temperature deformation, has been confirmed by transmission electron microscopy analysis. The enhanced strength and ductility at cryogenic temperatures can be attributed to the increased density of dislocations and nano-twins. The findings provide a fundamental understanding of underlying governing mechanistic mechanisms for the twinning induced plasticity in high entropy alloys, paving the way for the development of new alloys with superb resistance to cryogenic environments.

[en] A FeCoCrNiMo_0_._2_3 high entropy alloy was processed by powder metallurgy with two conditions: hot extruded and annealed. In situ neutron diffraction, together with electron microscopy, was used to study the deformation mechanisms and concomitant microstructural evolution for both conditions. The as-extruded alloy has a single face-centered-cubic structure with a calculated stacking fault energy of ∼19 mJ/m"2. When the alloy is tensile deformed, nano-twins and microbands are induced, resulting in an excellent combination of strength and ductility (784 MPa ultimate tensile strength and over 50% elongation). Annealing at 800 °C for 72 h increases the strength of the alloy but decreases its ductility. This is due to the decomposition of the alloy after annealing, causing the formation of Mo-rich intermetallic particles and a decrease of the stacking fault probability. These results highlight that combined mechanisms (i.e. solute strengthening and twin/microband induced plasticity) can effectively improve both the strength and ductility of high entropy alloys.

[en] The scale of solidification microstructures directly impacts micro-segregation, grain size, and other factors which control strength. Using in situ high speed synchrotron X-ray tomography we have directly quantified the evolution of dendritic microstructure length scales during the coarsening of Mg-Zn hcp alloys in three spatial dimensions plus time (4D). The influence of two key parameters, solute composition and cooling rate, was investigated. Key responses, including specific surface area, dendrite mean and Gauss curvatures, were quantified as a function of time and compared to existing analytic models. The 3D observations suggest that the coarsening of these hcp dendrites is dominated by both the re-melting of small branches and the coalescence of the neighbouring branches. The results show that solute concentration has a great impact on the resulting microstructural morphologies, leading to both dendritic and seaweed-type grains. It was found that the specific solid/liquid surface and its evolution can be reasonably scaled to time with a relationship of ∼ t"−"1"/"3. This term is path independent for the Mg-25 wt%Zn; that is, the initial cooling rate during solidification does not greatly influence the coarsening rate. However, path independence was not observed for the Mg-38 wt%Zn samples because of the seaweed microstructure. This led to large differences in the specific surface area (S_s) and its evolution both between the two alloy compositions and within the Mg-38 wt%Zn for the different cooling rates. These findings allow for microstructure models to be informed and validated to improve predictions of solidification microstructural length scales and hence strength.

[en] Engineered porous materials are proposed as random absorption masks for X-ray phase-contrast imaging in high-energy regions (i.e. over 50 keV). X-ray phase-contrast imaging can substantially enhance image contrast for weakly absorbing samples. The fabrication of dedicated optics remains a major barrier, especially in high-energy regions (i.e. over 50 keV). Here, the authors perform X-ray phase-contrast imaging by using engineered porous materials as random absorption masks, which provides an alternative solution to extend X-ray phase-contrast imaging into previously challenging higher energy regions. The authors have measured various samples to demonstrate the feasibility of the proposed engineering materials. This technique could potentially be useful for studying samples across a wide range of applications and disciplines.