Language：Japanese   Publisher：Acta Materialia  

The hydrogen trapping and micromechanical behaviors of additively manufactured CoCrFeNi high-entropy alloy (HEA) using the laser powder bed fusion (L-PBF) technique in the as-built and pre-strained states were explored through nanoindentation and micro-tensile experiments combined with thermal desorption analysis. To analyze the influence of pre-straining, both global pre-strains, imposed using the interrupted tensile tests, and local strain levels, estimated using the digital image correlation measurements, were employed. It was revealed that pre-straining (which increases the dislocation density in the alloy) does not enhance the hydrogen effects on the micromechanical performance of the L-PBF HEA. To understand this, rather unexpected, result, we investigated the trapping behavior of diffusional hydrogen in detail, through thermal desorption analysis combined with the Ag decoration technique. The results are discussed in terms of the hydrogen contents and trapping sites in the L-PBF HEA.

DOI： 10.1016/j.actamat.2024.119886

Scopus

Language：Japanese   Publisher：Scripta Materialia  

Nanocrystalline (NC) dual-phase Al0.7CoCrFeNi HEAs containing face-centered cubic (FCC) and body-centered cubic (BCC) microstructural phases were fabricated by high-pressure torsion (HPT). The influences of hydrogen on the thermal desorption and nanoindentation responses of NC HEA were compared with the coarse-grained alloy. The plastic zone size and indentation size effects were carefully considered to identify the distinct contributions of the constituent phases to the hardness and its variation with hydrogen charging. Results show that the FCC phase is susceptible to a larger degree of hydrogen-induced hardening than the BCC phase. Such difference is negated in the NC samples. These results are discussed in terms of the distinct responses of FCC and BCC HEA phases to hydrogen and the governing deformation mechanisms in coarse grained and NC samples.

DOI： 10.1016/j.scriptamat.2021.114472

Scopus

Language：Japanese   Publisher：The Iron and Steel Institute of Japan  

<p>In order to investigate the adaptability of small punch (SP) creep testing technique to the remaining-life assessment, the SP creep test was carried out with 2.25Cr-1Mo steel hot rehear steam piping, which had been actually used in a fossil power plant for long periods of time. The SP load (<i>F</i>) was converted to the stress (<i>σ</i>) by three different equations, which were derived based on the displacement to maximum load in the SP test (<i>u</i>_m) and the deflection to minimum deflection rate in the SP creep test (<i>u</i>_min) for correlating the SP creep rupture data with the uniaxial ones. The experimental results showed that the SP creep rupture time of specimen removed at around outer surface of piping tended to be slightly shorter than those taken at around inner surface and center. It was also found that the SP creep rupture data were relatively in good agreement with the uniaxial ones by converting <i>F</i> to <i>σ</i> with the equations, and the creep remaining-life was well predicted by the extrapolation of short-term SP creep rupture data. The highest prediction accuracy was obtained by using the equation derived from the SP test result, that is, <i>u</i>_m. Consequently, it was confirmed that the SP testing technique could be a strong tool for creep remaining-life assessment of boiler piping.</p>

DOI： 10.2355/tetsutohagane.tetsu-2021-077

Scopus

Language：Japanese   Publisher：Scripta Materialia  

The effect of marked change in grain size from coarse-grained to nanocrystalline can affect the hydrogen absorption and plastic deformation behavior in two face-centered cubic high-entropy alloys (HEAs), viz. equiatomic CoCrFeNi and CoCrFeMnNi. Thermal desorption analysis of the hydrogen-charged samples proved that grain boundaries act as hydrogen traps and thus largely increase the hydrogen contents in the nanocrystalline samples. A direct comparison in the hydrogen absorption between two HEAs confirms that both chemical composition and grain size are crucial factors contributing to the hydrogen solubility of the HEAs. The parameters for the thermally activated deformation from nanoindentation rate-jump tests suggest enhanced lattice friction by hydrogen, leading to a reduction in activation volume and thus modification of the plastic deformation processes. The results are discussed in two aspects, viz. the effect of grain size and chemical composition on the hydrogen-affected plastic deformation.

DOI： 10.1016/j.scriptamat.2021.114069

Scopus