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SCRIPTA

In this study, a novel analytical system was developed to monitor the environmental hydrogen embrittlement of Al-Zn-Mg-based aluminum alloys dynamically and quantitatively under atmospheric air pressure. The system involves gas chromatography using a SnO₂-based semiconductor hydrogen sensor, a digital image correlation step, and the use of a slow strain rate testing machine. Use of this system revealed that hydrogen atoms are generated during the plastic deformation of Al-Zn-Mg alloys caused by the chemical reaction between the water vapor in air and the alloy surface without oxide films. Digital image correlation also clarified that the generated hydrogen atoms caused numerous localized grain boundary cracks on the specimen surface, resulting in a localized grain boundary fracture. The amount of hydrogen atoms evolved from the embrittled fracture surface was 2.7 times as high as that from the surface without embrittlement.

SCRIPTA

The traditional solution (1120 °C/4 h) and aging (845-1080 °C/24 h) processes are the keys for K417G superalloy to obtain the expected microstructure and ensure excellent mechanical properties. It is found here that the investigated microstructure can be achieved by electropulsing solution treatment at 1000 °C, and the processing time is 10 minutes. Meanwhile, the morphology of the harmful γ/γ′ eutectic phase can be quickly modified during the pulsed process, thereby the crack propagation near the eutectic phase changes from a direct pass to a zigzag pass, which is beneficial to performance improvement. The theoretical analysis and calculation results show that high current density area existed between the heterogeneous phases and the system thermodynamic barrier reduction are the fundamental reasons for the primary γ′ dissolution and the γ/γ′ eutectic morphology evolution.

SCRIPTA

The strength-ductility (or thermal conductivity) trade-off is unavoidable in most conventional alloys due to the limited composition space. Herein, we propose a mechanism-based design strategy for concurrent optimization of high temperature (HT) strength, room temperature (RT) ductility, and HT thermal conductivity by considering atomic size misfit for solid-solution hardening and lattice distortion, and valence electron concentration for shear instability. As a test case, we extend the composition space of binary W-Ta alloys to W-Ta-V-Ti-Cr refractory high entropy alloys (RHEAs), and the present RHEAs (e.g. WTaVTiCr) exhibit superior HT strength (1210±43 MPa at 1073 K) and RT ductility (23.4±5.7%). Further, we show that the thermal conductivities of the present RHEAs increase with increasing temperature, and theoretically can reach to ~40% of that of pure W at 2000 K. This work thus provides a general design rule of RHEAs that enables effective utilization of compositional complexity for potential applications.

SCRIPTA

Crystallographic twins are critical to the properties of numerous materials from magnesium alloys to piezoelectrics. Since the onset of the twin formation is highly sensitive to the triaxial mechanical boundary conditions, non-destructive bulk microscopy techniques are required. Elastic strains can be mapped via X-ray diffraction with a 100-200 nm resolution. However, the interplay of strains with nanotwins cannot be characterized. Here, a method based on dark-field X-ray microscopy to quantify the density of nanotwin variants with twin lamellae of sizes as small as several tens of nanometers in embedded subvolumes (70x200x600 nm³) in millimeter-sized samples is introduced. The methodology is corroborated by correlating the local density of twin variants to the long-ranging strain fields for a high-performance piezoelectric material. The method facilitates direct, in situ mapping and quantification of nanoscale structural changes together with their elastic driving fields, which is the key towards controlling and engineering material's performance at nanometric scales.

SCRIPTA

Eutectic highentropy alloys (EHEAs) have attracted wide attention in the metallic materials community because of its excellent castability, fine and controllable microstructure. However, the design method for eutectic EHEAs is still a challenge. In present work, a new infinite solid solution strategy was proposed to design EHEAs. Using this strategy, a series of EHEAs composed of ordered body centeredcubic (B2) and bodycentered cubic (BCC) phases were successfully developed with nano-structured precipitated phase. Some of the designed alloys in the current work have exhibited excellent compressive mechanical properties.

SCRIPTA

The variation in stacking fault energy (SFE) with the change in temperature has been evaluated experimentally for Fe40Mn40Co10Cr10 high entropy alloy. The distance between partial dislocations was measured using transmission electron microscopy (TEM) based weak-beam dark field (WBDF) technique. SFE of the system was found to be 37.7 (±7) mJ/m2 and 19.5 (±5) mJ/m2 at room temperature (RT) and -100°C, respectively. Owing to the decrease in SFE, a transition in the deformation behavior occurred from limited twin formation and slip dominance at RT to mixed-mode consisting of FCC→HCP transformation and twinning at -100°C. Simultaneous occurrence of twins and deformation-induced martensitic transformation led to superior strength-ductility combination in the specimen deformed at -100°C. SFE of the studied alloy at RT was 42% higher than that of the equiatomic FeMnCoCrNi alloy. This increase in SFE, despite removal of Ni can be understood by considering the effect of the alloy chemistry re-adjustment on ΔG^γ→ε.

Vol. 199, 1 July. 2021, 113893

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