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Hydrogen-induced nanohardness variations in a CoCrFeMnNi high-entropy alloy
Zhao, Yakai,Lee, Dong-Hyun,Lee, Jung-A,Kim, Woo-Jin,Han, Heung Nam,Ramamurty, Upadrasta,Suh, Jin-Yoo,Jang, Jae-il Elsevier 2017 INTERNATIONAL JOURNAL OF HYDROGEN ENERGY - Vol.42 No.16
<P><B>Abstract</B></P> <P>The influence of electrochemically charged hydrogen (H) on the hardness (<I>H</I> <SUB> <I>N</I> </SUB>) of a CoCrFeMnNi high-entropy alloy (HEA) was investigated with nanoindentation. Upon charging, <I>H</I> <SUB> <I>N</I> </SUB> of HEA increases by ∼60%, which decreases gradually during subsequent aging at room temperature, and on prolonged aging, the alloy softens to an extent that <I>H</I> <SUB> <I>N</I> </SUB> falls below that of the uncharged HEA. These H-induced mechanical property variations are rationalized in terms of the competition between solid solution hardening caused by H and excess vacancy creation due to deeply trapped H.</P> <P><B>Highlights</B></P> <P> <UL> <LI> Effects of hydrogen charging on hardness of high-entropy alloy were studied. </LI> <LI> Hardness changes dramatically during charging and subsequent aging. </LI> <LI> Competition of solution hardening and vacancy softening plays an important role. </LI> </UL> </P>
Hydrogen-induced softening in nanocrystalline Ni investigated by nanoindentation
Zhao, Yakai,Seok, Moo-Young,Lee, Dong-Hyun,Lee, Jung-A,Suh, Jin-Yoo,Jang, Jae-il Abingdon; Taylor & Francis Ltd 2016 PHILOSOPHICAL MAGAZINE Vol.96 No.32
<P>The influence of hydrogen on the plastic deformation of nanocrystalline nickel was analysed by recourse to nanoindentation on the uncharged and hydrogen-charged samples. It was revealed that, in nanocrystalline Ni, hydrogen significantly decreases hardness but does not alter the strain rate sensitivity. Through thermal desorption spectroscopy measurement, charged hydrogen was expected to reside in face-centred cubic lattice, grain boundaries (GBs) and vacancies rather than dislocations. The hydrogen-induced softening behaviour is discussed in terms of the possible roles of hydrogen in GB-assisted dislocation flow mechanism.</P>
The role of hydrogen in hardening/softening steel: Influence of the charging process
Zhao, Yakai,Seok, Moo-Young,Choi, In-Chul,Lee, Yun-Hee,Park, Seong-Jun,Ramamurty, Upadrasta,Suh, Jin-Yoo,Jang, Jae-il Elsevier 2015 Scripta materialia Vol.107 No.-
<P>Thermal desorption spectroscopy and nanoindentation techniques were employed to elucidate the key differences in the hydrogen (H) charging methods (electrochemical versus gaseous) and their consequences on the mechanical response of a low carbon steel. While electrochemical charging enhances the hardness, gaseous charging reduces it. This contrasting behavior is rationalized in terms of the dependency of the strength on the absorbed amount of H during charging and the H concentration gradient in the specimen.</P> <P><B>Graphical abstract</B></P> <P>[DISPLAY OMISSION]</P>
Resistance of CoCrFeMnNi high-entropy alloy to gaseous hydrogen embrittlement
Zhao, Yakai,Lee, Dong-Hyun,Seok, Moo-Young,Lee, Jung-A,Phaniraj, M.P.,Suh, Jin-Yoo,Ha, Heon-Young,Kim, Ju-Young,Ramamurty, Upadrasta,Jang, Jae-il Elsevier 2017 Scripta materialia Vol.135 No.-
<P><B>Abstract</B></P> <P>The influence of hydrogen on the mechanical behavior of the CoCrFeMnNi high-entropy alloy (HEA) was examined through tensile and nanoindentation experiments on specimens hydrogenated via gaseous and electrochemical methods. Results show that the HEA's resistance to gaseous hydrogen embrittlement is better than that of two representative austenitic stainless steels, in spite of the fact that it absorbs a larger amount of hydrogen than the two steels. Reasons for this were discussed in terms of hydrogen-enhanced localized plasticity mechanism and the critical amount of hydrogen required for it. These were further substantiated by additional experiments on electrochemically charged specimens.</P> <P><B>Graphical abstract</B></P> <P>[DISPLAY OMISSION]</P>
Zhao, Yakai,Park, Jeong-Min,Lee, Dong-Hyun,Song, Eun Ju,Suh, Jin-Yoo,Ramamurty, Upadrasta,Jang, Jae-il Elsevier 2019 Scripta materialia Vol.168 No.-
<P><B>Abstract</B></P> <P>The influence of charging method on hydrogen (H) distribution and the resultant nanomechanical behavior of CoCrFeMnNi high-entropy alloy was examined and compared with another face-centered cubic structured alloy, an austenitic stainless steel. Through thermal desorption spectroscopy measurement and theoretical analysis, it was revealed that electrochemical (E-) charging induces steep gradient of H concentration near the surface while H was homogenously distributed after gaseous (G-) charging. Nanoindentation results show significant hardening in E-charged alloys while the hardness of G-charged alloys remains invariant. These differences were rationalized in terms of the nature of H distributions induced by different charging methods.</P> <P><B>Graphical abstract</B></P> <P>[DISPLAY OMISSION]</P>
Influence of pre-strain on the gaseous hydrogen embrittlement resistance of a high-entropy alloy
Zhao, Yakai,Lee, Dong-Hyun,Kim, Woo-Jin,Seok, Moo-Young,Kim, Ju-Young,Han, Heung Nam,Suh, Jin-Yoo,Ramamurty, Upadrasta,Jang, Jae-il Elsevier 2018 Materials science & engineering. properties, micro Vol.718 No.-
<P><B>Abstract</B></P> <P>The effect of pre-strain on the resistance to gaseous hydrogen embrittlement of CoCrFeMnNi high-entropy alloy (HEA) was investigated through mechanical testing and thermal desorption analysis. The results reveal that prior plastic deformation does not affect either the hydrogen contents or the excellent hydrogen embrittlement resistance of the HEA.</P>
Influence of hydrogen on incipient plasticity in CoCrFeMnNi high-entropy alloy
Yang, Guanghui,Zhao, Yakai,Lee, Dong-Hyun,Park, Jeong-Min,Seok, Moo-Young,Suh, Jin-Yoo,Ramamurty, Upadrasta,Jang, Jae-il Elsevier 2019 Scripta materialia Vol.161 No.-
<P><B>Abstract</B></P> <P>The influence of hydrogen on the onset of plastic deformation in a CoCrFeMnNi high-entropy alloy (HEA) was examined through the analysis of the load at which first pop-in during spherical nanoindentation experiments occurs on hydrogen-charged and subsequently aged specimens. Results reveal that the dissolved hydrogen lowers the plastic flow resistance, indicated by the shear yield strength, <I>τ</I> <SUB>y</SUB>, by modifying defect formation energies. Aging, subsequent to charging, leads to recovery of <I>τ</I> <SUB>y</SUB>, but only partially. The results are discussed in terms of the vacancy-mediated dislocation nucleation, which is supported by the estimated activation volume for deformation.</P> <P><B>Graphical abstract</B></P> <P>[DISPLAY OMISSION]</P>
Lee, Dong-Hyun,Seok, Moo-Young,Zhao, Yakai,Choi, In-Chul,He, Junyang,Lu, Zhaoping,Suh, Jin-Yoo,Ramamurty, Upadrasta,Kawasaki, Megumi,Langdon, Terence G.,Jang, Jae-il Elsevier 2016 Acta materialia Vol.109 No.-
<P>Time-dependent plastic deformation behavior of nanocrystalline (nc) and coarse-grained (cg) CoCrFeMnNi high-entropy alloys (HEAs) was systematically explored through a series of spherical nano indentation creep experiments. High-pressure torsion (HPT) processing was performed for achieving nc microstructure in the HEA, leading to a reduction in grain size from similar to 46 pm for the as-cast state to similar to 33 nm at the edge of the HPT disk after 2 turns. Indentation creep tests revealed that creep deformation indeed occurs in both cg and nc HEAs even at room temperature and it is more pronounced with an increase in strain. The creep stress exponent, n, was estimated as similar to 3 for cg HEA and similar to 1 for nc HEA and the predominant creep mechanisms were investigated in terms of the values of n and the activation volumes. Through theoretical calculations and comparison of the creep strain rates for nc HEA and a conventional face-centered-cubic nc metal (Ni), the influence of sluggish diffusion on the creep resistance of nc HEA was analyzed. In addition, sharp indentation creep tests were performed for comparison purposes and the results confirmed that the use of a spherical indenter is clearly more appropriate for investigating the creep behavior of this HEA. (C) 2016 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.</P>
Lee, Dong-Hyun,Lee, Jung-A,Zhao, Yakai,Lu, Zhaoping,Suh, Jin-Yoo,Kim, Ju-Young,Ramamurty, Upadrasta,Kawasaki, Megumi,Langdon, Terence G.,Jang, Jae-il Elsevier 2017 ACTA MATERIALIA Vol.140 No.-
<P>The influence of annealing on the constitutive stress-strain response of nanocrystalline (nc) CoCrFeMnNi high-entropy alloy (HEA) was investigated through a series of nanoindentation experiments using five different three-sided pyramidal indenters. The nc HEA, produced by high-pressure torsion (HPT), was subjected to annealing at 450 degrees C for 1 and 10 h. Microstructural analysis using transmission electron microscopy (TEM) showed that three different nano-scale precipitates (NiMn-, FeCo-, and Co-rich phases) form in the primary single-phase matrix of nc HEA after annealing. The strain-dependent plastic flow response of nc HEA pre- and post-annealing was estimated using the indentation strain and constraint factor, revealing a significant strain softening in nc HEA, which becomes pronounced after annealing. TEM analysis of the deformed material underneath the indenter suggests that the plastic deformation aids in the dissolution of the annealing-induced intermetallic precipitates, which could be the mechanism for the pronounced softening. The dissolution mechanism was rationalized by the destabilization of precipitates during plastic deformation due to the increase in interface energy. (C) 2017 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.</P>