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 SCRIPTA Vol. 188, 1 Nov. 2020, P179-182

Effects of macro segregation and homogenizing heat treatment on creep strength of 9Cr-1Mo-V-Nb (Grade T91) steel have been investigated and it has been concluded that texture is useful feature to evaluate creep strength of the steel as a straightforward index of macro segregation.

SCRIPTA Vol. 188, 1 Nov. 2020, P184-189

Using in situ nanomechanical testing under high-resolution transmission electron microscope, we reveal that the grain boundary at the twin-twin interaction region (referred to as GB_T) can either migrate via a shuffling process to contribute to the dislocation transmission or facilitate the dislocation cross-slip between the two intersected twins. Stress condition, energy barrier and geometric structure of the GB_T can influence these two modes of dislocation activities at the intersection region. Besides, the dislocations inside the incoming twin can traverse the GB_T by dislocation-GB_T reactions. These findings should have general implications to the understanding of the deformation of nanotwinned materials.

SCRIPTA Vol. 188, 1 Nov. 2020, P195-199

To understand the strengthening mechanism of precipitate against twin-mediated plasticity in magnesium–aluminum alloys, the measurements of the increase in the critical resolved shear stress due to impediment on twin thickening are accomplished based on atomistic simulation, specifically at a significantly low strain rate of 1.7 × 10⁶ s^-1. Results show conspicuous coupled effect of temperature and strain-rate on the precipitation strengthening. The strengthening effect vanishes if coherent twin boundary migration is purely stress-driven without thermal activations, due to the back-stress dominating over Orowan stress. At a finite temperature the strengthening effect becomes observable, and is distinctly amplified by high strain rates.

SCRIPTA Vol. 188, 1 Nov. 2020, P200-205

The static recrystallization, nuclei orientations, and texture evolution of cold-rolled Mg-Zn-Gd alloys during annealing were tracked by quasi-in-situ electron backscatter diffraction method. An unusual texture transition from “rolling direction (RD)-split” to “transverse direction (TD)-split” occurred and oriented nucleation was observed. Nuclei originating from boundaries of the parent grain with “RD” orientation mainly exhibited “TD” orientations, and showed preferential misorientation relationship around 62.5° <uvt0> (including basal axes <11–20>, <10-10>, <41-50> and <52-70>) with respect to the parent grain, which may be related to basal <a> dislocations and low boundary energy.

SCRIPTA Vol. 188, 1 Nov. 2020, P216-221

The CrCoNi medium-entropy alloy is not fully utilized at room temperature circumstance due to its low strength, despite excellent cryogenic mechanical properties. Here we report that, a superior room temperature strength (1.6 GPa and 407.6 Hv) has been obtained in the single phase CoCrNi medium-entropy alloy, by carefully tuning the formation of multiple lattice defects. Electron microscopy characterizations show that such high-strength CoCrNi alloy contains favorable heterostructures of coarsening grains (~30 μm) and ultrafine grains (~1 μm), together with the high density of dislocation cells, small stacking faults, ultrafine nanotwins (~9 nm in thickness) and Lomer–Cottrell locks.

SCRIPTA Vol. 188, 1 Nov. 2020, P238-243

Uniaxial tensile experiments on hydrogen-charged and non-charged polycrystalline nickel deformed to a true plastic strain of ~0.1 at room temperature and 77 K reveal that hydrogen has no measurable effect on texture evolution. Texture measurements were completed on samples of each condition, as well as an as-received reference specimen, using electron backscatter diffraction data collected from >20,000 individual grains. Crystal plasticity simulations support the conclusion that the similarity is attributable to the insensitivity of small-strain deformation texture evolution to potential hydrogen-induced modifications in deformation mechanisms.

SCRIPTA Vol. 188, 1 Nov. 2020, P259-263

Phase stability of a nanocrystalline CrCoNi alloy is investigated using the combinatorial processing platform approach, which enables synthesis, processing and direct atomic-scale characterizations of alloys by atom probe tomography and transmission electron microscopy. Phase decomposition with formation of CoNi-rich phase occurs faster in the smaller (10 nm) grain-sized region than the larger one (20 nm), both being present in the same sample. Chemical analyses indicate that diffusion of Co and Cr plays an important role in phase decomposition. Comparison of phase stability between CrMnFeCoNi and CrCoNi implies that elemental segregation may promote phase decomposition by providing an additional chemical driving force for it.

SCRIPTA Vol. 188, 1 Nov. 2020, P264-268

Tracer diffusion of the matrix elements in CoCrFeNiMn-based high-entropy alloys with 0.2, 0.5 and 0.8 at.% carbon is measured at 1373 K. The diffusion coefficients are found to depend non-monotonously on the carbon content. The diffusion retardation in low-C alloys is discussed in terms of the impact of interstitially-dissolved carbon on the vacancy concentration and the migration barriers in the alloy. In high-C alloys, formation of carbides alternates the observed trends, most probably by increasing the total vacancy concentration.

SCRIPTA Vol. 188, 1 Nov. 2020, P269-273

High-strength Al-Zn-Mg-Cu alloys are highly susceptible to stress corrosion cracking (SCC) which severely limits their lifetime. Reversion and re-aging (RRA) temper provides a higher SCC resistance at no loss in strength, yet the microstructural origins of these enhanced properties remain elusive. In an Al-Zn-Mg-Cu alloy, we show that the fine precipitate dispersion in the grain interiors is similar in the peak aged and RRA tempers. However, upon RRA, precipitates inside the grains are enriched in Cu, lowering the Cu matrix content, and reducing the relative difference in the Cu precipitate composition between bulk and grain boundaries. This study enriches the current understanding on the critical role of Cu related to SCC resistance in Al-Zn-Mg-Cu alloys.

SCRIPTA Vol. 188, 1 Nov. 2020, P280-284

We report an interesting phenomenon in a new zirconium alloy where the yield drop gradually disappeared and re-appeared by modulating the pre-strain history. The initial yield drop is caused by the lack of mobile dislocations in crystal, and increased density of mobile dislocations introduced by pre-strain (< 1%) lead to the disappearance of yield drop. The latter one can be explained by the recombination of partial dislocations. That is, the climb-dissociated partials introduced by pre-strain deformation (> 9%) recombine onto the same prism plane to regain its moving ability, resulting in the re-appearance of yield drop.

SCRIPTA Vol. 188, 1 Nov. 2020, P285-289

Supersaturated solid solutions subjected to severe plastic deformation often exhibit decomposition of the solid solution, leading to a peculiar grain boundary phenomenon, the formation of few nanometer thin layer enriched in solute atoms along grain boundaries. Based on localized grain boundary sliding, this phenomenon can be explained by the formation of hydrostatic stress field, which induces up-hill diffusion in the solute atmosphere, resulting in solute atom accumulation along the stressed grain boundaries.

SCRIPTA Vol. 188, 1 Nov. 2020, P296-301

Understanding microstructural and strain evolutions induced by noble gas production in the nuclear fuel matrix or plasma-facing materials is crucial for designing next generation nuclear reactors, as they are responsible for volumetric swelling and catastrophic failure. We describe a multimodal approach combining synchrotron-based nanoscale X-ray imaging techniques with atomic-scale electron microscopy techniques for mapping chemical composition, morphology and lattice distortion in a single crystal W induced by Kr irradiation. We report that Kr-irradiated single crystal W undergoes surface deformation, forming Kr containing cavities. Furthermore, positive strain fields are observed in Kr-irradiated regions, which lead to compression of underlying W matrix.

SCRIPTA Vol. 188, 1 Nov. 2020, P302-306

We show the giant magnetocaloric effect of HoNi₂ compound can be tuned around hydrogen liquefaction temperature by substituting Ho with Gd and Ni with Co in Ho_1-xGd_xNi_2-yCo_y. While the Curie temperature of Ho_1-xGd_xNi_2-yCo_y can be tuned from 15 to 32 K originating from the lattice expansion of the cubic structure, giant entropy change of 22.0 J/kgK is retained near the hydrogen liquefaction temperature. A table-like entropy change observed in a broad temperature range of 20 to 32 K makes them promising as magnetocaloric materials for cryogenic magnetic refrigeration utilizing an Ericsson cycle.