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Helium accumulation in advanced nuclear systems would accelerate the failure of structural materials. Nanostructured materials have the potential to mitigate helium effects by providing more nucleation sites for He bubbles. La-doped nanocrystalline 304 austenitic stainless steel (NC304-La) possesses ultra-fine and stable nano-grains ~45 nm in diameter and high-density fine nano-precipitates. In this work, we implanted NC304-La with helium ions, then annealed at different temperatures after implantation. We found that combining the key features of nanocrystalline steels and oxide dispersion-strengthened steels, NC304-La showed excellent bubble swelling resistance. We characterized the interaction between He bubbles and the two features in NC304-La: grain boundaries and nano-precipitates. Through detailed study for the heat-treatment related behaviors of He bubbles in this multi-sink system, the mechanisms of He bubble coarsening resistance in NC304-La were proposed.

SCRIPTA

We examine the effect of transformation sequence on the microstructure evolution and mechanical properties of complex phase steel consisting of bainite and martensite. Formation of martensite prior to bainite transformation provides uniform lath-type mixture of constituent phases, but preceding isothermal bainite transformation produces carbon-enriched blocky martensite embedded in the bainite. Even though the presence of carbon-enriched blocky martensite increases the strength of the alloy more effectively, the impact toughness is significantly deteriorated with a small fraction of martensite. On the other hand, uniform lath-type mixture of martensite and bainite provides moderate decreased or conserved impact toughness at martensite fraction up to 70%, which be considered as an exceedingly beneficial option to enhance the strength of complex phase steel without compromising impact toughness.

SCRIPTA

Interface engineering is a useful method to reduce the accumulation of irradiation defects in metals. Here, we study the effect of interface on the formation of dislocation loops in helium and krypton ions irradiated nanolayered Zr-2.5Nb. Nanolaminated α/β-Zr duplex-phase structures remain stable after irradiation at 400 °C up to 20 dpa. Dislocation denuded zones with width of 30 to 40 nm are formed at interfaces. The number density and size of both a-loops and c-loops are smaller in nanolayered sample because of interface accelerating point defects recombination. These observations indicate that α/β-Zr interfaces effectively reduce dislocation loops formation.

SCRIPTA

Microstructure development of 18Cr-9Ni-3Cu-Nb-N steel during creep was discussed by observing the grip portion of the crept specimen that ruptured after 50214.3 h at 973 K. Focused ion beam-scanning electron microscopy (FIB-SEM) serial sectioning and image processing with machine learning were combined for the three-dimensional visualization of the precipitation behavior of the steel. It was revealed that coarsening of σ phase induce the depletion of M₂₃C₆ phase on the grain boundary and aggregation of Cu phase at the interface between the σ phase and the matrix. These microstructural changes related to the σ phase formation were suggested to cause premature failure in the longer-term creep due to the reduced contribution of grain boundary precipitation strengthening by M₂₃C₆ phase and precipitation strengthening by Cu phase around the σ phase.

SCRIPTA

The tensile deformation behaviour of a crack-free additively manufactured (AM) Ni base superalloy and its wrought (W) counterpart has been investigated by in-situ synchrotron X-ray diffraction. In order to understand the evolution of dislocation character, dislocation density (ρ), coherent scattering domain size (D) and dislocation arrangement parameter (M), the diffraction data corresponding to the first six{hkl} reflections have been analyzed using modified Williamson-Hall and modified Warren-Averbach methodologies. The ρ was found to be relatively higher in AM superalloy as compared to W superalloy during both the regimes of elastic and plastic deformation. In the plastic deformation regime, while the fraction of edge and screw dislocations was found to be nearly equal in proportion in W superalloy, the fraction of screw dislocations remained higher in AM superalloy. In view of the relatively high initial ρ and inefficacy of D in AM superalloy, heat treatment in as-built parts is suggested.

SCRIPTA

Slip properties of TiZrNbHfTa were evaluated to obtain ψ–χ relationships, where χ is defined as the angle between the maximum shear stress plane and (-101). ψ is defined as the angle between the apparent slip plane and (-101). Bending tests were conducted with micrometer-sized cantilevers fabricated from a single grain with a focused ion beam. Slip bands appear to be straight on a micrometer length-scale, as for Fe–Si alloys, which suggests a persistent slip. However, the ψ–χ relationship indicated that ψ is nearly the same as χ, suggesting that there is no persistent slip. Detailed observations with an atomic force microscope confirm that cross-slipping is so frequent at a sub-micrometer length-scale that the slip bands appear to be straight and the maximum shear stress plane is observed on a micrometer length-scale. Here, TiZrNbHfTa shows abnormally frequent cross-slipping, which is a novel characteristic of high-entropy body-centered cubic alloys.

Vol. 200, 15 July. 2021, 113899

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