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Choi, I.C.,Kim, Y.J.,Wang, Y.M.,Ramamurty, U.,Jang, J.i. Elsevier Science 2013 ACTA MATERIALIA Vol.61 No.19
The influence of strain on the mechanical properties and deformation kinetic parameters of nanotwinned (nt) copper is investigated by a series of nanoindentation experiments, which were performed by employing sharp indenters with five varying centerline-to-face angles (ψ). Comparison experiments were also conducted on (110) single crystalline Cu. Experimental results indicate that, unlike coarse-grained materials, nt-Cu is prone to plastic flow softening with large material pile-up around the indentation impression at high levels of strains. Localized detwinning becomes more significant with decreasing ψ, concomitant with reduced strain-rate sensitivity (m) and enhanced activation volume (V<SUP>*</SUP>). The m of nt-Cu is found to depend sensitively on ψ with a variation of more than a factor of 3, whereas V<SUP>*</SUP> exhibits a much less sensitive trend. This paper discusses the validation of the experimental techniques and the implications of various deformation kinetic parameters on the underlying deformation mechanisms of nt-Cu.
Seok, Moo-Young,Lee, Jung-A,Lee, Dong-Hyun,Ramamurty, Upadrasta,Nambu, Shoichi,Koseki, Toshihiko,Jang, Jae-il Elsevier 2016 Acta materialia Vol.121 No.-
<P>Multi-layered steel (MLS) consisting of alternating soft/ductile austenitic and hard/brittle martensitic stainless steel layers is a new class of hybrid material for structural application as it offers excellent combinations of strength and ductility. In this study, the contributions of each of the constituent layers to the overall strength and ductility of an MLS (having tensile strength > 1.4 GPa and ductility > 20%) were examined by recourse to nanoindentation experiments on each of them. By adapting two different indenter tip radii for the spherical nanoindentation experiments, constituent layers' stress-strain responses within the plastic regime were obtained and then compared with the macroscopic flow curve of the MLS that was obtained through tensile tests, to show that the strength contributions of the constituent steels to the global strength of MIS is as per the rule of mixtures. In order to examine the sources of tensile ductility of the MLS, sharp tip nanoindentation experiments were conducted on specimens extracted from tensile coupons that were subjected to predetermined plastic strains a priori. Results of these experiments show that the tensile failure occurs at a strain at which hardness of the austenitic layer, which is found to be dependent on the prior-plastic strain, is almost equal to the strain independent hardness of the martensitic layer. The results are discussed in terms of martensitic transformation within austenitic layer and the role of the mechanical environment change imposed by the neighboring martensite layers on it. (C) 2016 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.</P>
Lee, Dong-Hyun,Choi, In-Chul,Yang, Guanghui,Lu, Zhaoping,Kawasaki, Megumi,Ramamurty, Upadrasta,Schwaiger, Ruth,Jang, Jae-il Elsevier 2018 Scripta materialia Vol.156 No.-
<P><B>Abstract</B></P> <P>Nanoindentation experiments in the temperature (<I>T</I>) range of 298 and 573 K were performed to determine the activation energy (<I>Q</I>) for the plastic flow in a nanocrystalline CoCrFeMnNi high-entropy alloy, which was synthesized using high-pressure torsion. A marked increase in <I>Q</I> from ~0.5 to ~1.8 eV was observed when <I>T</I> is increased from 473 to 523 K, which correspond to ~0.3 and ~0.34 <I>T</I> <SUB>m</SUB> (<I>T</I> <SUB>m</SUB>: melting temperature), respectively. Detailed analysis reveals that this transition is associated with the additional activation of the grain boundary diffusion mechanism in enhancing plasticity.</P> <P><B>Graphical abstract</B></P> <P>[DISPLAY OMISSION]</P>
Role of free volume in strain softening of as-cast and annealed bulk metallic glass
Yoo, Byung-Gil,Park, Kyoung-Won,Lee, Jae-Chul,Ramamurty, U.,Jang, Jae-il Cambridge University Press (Materials Research Soc 2009 Journal of materials research Vol.24 No.4
<P>Plasticity in amorphous alloys is associated with strain softening, induced by the creation of additional free volume during deformation. In this paper, the role of free volume, which was a priori in the material, on work softening was investigated. For this, an as-cast Zr-based bulk metallic glass (BMG) was systematically annealed below its glass transition temperature, so as to reduce the free volume content. The bonded-interface indentation technique is used to generate extensively deformed and well defined plastic zones. Nanoindentation was utilized to estimate the hardness of the deformed as well as undeformed regions. The results show that the structural relaxation annealing enhances the hardness and that both the subsurface shear band number density and the plastic zone size decrease with annealing time. The serrations in the nanoindentation load-displacement curves become smoother with structural relaxation. Regardless of the annealing condition, the nanohardness of the deformed regions is ∼12-15% lower, implying that the prior free volume only changes the yield stress (or hardness) but not the relative flow stress (or the extent of strain softening). Statistical distributions of the nanohardness obtained from deformed and undeformed regions have no overlap, suggesting that shear band number density has no influence on the plastic characteristics of the deformed region.</P>
Yoo, B.G.,Choi, I.C.,Kim, Y.J.,Ramamurty, U.,Jang, J.i. Elsevier Sequoia 2013 Materials science & engineering. properties, micro Vol.577 No.-
Anelastic and viscoplastic characteristics of Cu<SUB>50</SUB>Zr<SUB>50</SUB> and Cu<SUB>65</SUB>Zr<SUB>35</SUB> binary bulk metallic glasses at room temperature were examined through nanoindentation creep experiments. Results show that both the deformations are relatively more pronounced in Cu<SUB>50</SUB>Zr<SUB>50</SUB> than in Cu<SUB>65</SUB>Zr<SUB>35</SUB>, and their amount increases with the loading rate. The results are analyzed in terms of the influences of structural defects and loading rate on the room temperature indentation creep.
Alaneme, K.K.,Hong, S.M.,Sen, I.,Fleury, E.,Ramamurty, U. Elsevier Sequoia 2010 Materials science & engineering. properties, micro Vol.527 No.18
Small additions of Cu to the SUS 304H, a high temperature austenitic stainless steel, enhance its high temperature strength and creep resistance. As Cu is known to cause embrittlement, the effect of Cu on room temperature mechanical properties that include fracture toughness and fatigue crack threshold of as-solutionized SUS 304H steel were investigated in this work. Experimental results show a linear reduction in yield and ultimate strengths with Cu addition of up to 5wt.% while ductility drops markedly for 5wt.% Cu alloy. However, the fracture toughness and the threshold stress intensity factor range for fatigue crack initiation were found to be nearly invariant with Cu addition. This is because the fracture in this alloy is controlled by the debonding from the matrix of chromium carbide precipitates, as evident from fractography. Cu, on the other hand, remains either in solution or as nano-precipitates and hence does not influence the fracture characteristics. It is concluded that small additions of Cu to 304H will not have adverse effects on its fracture and fatigue behavior.
Yoo, B.G.,Choi, I.C.,Kim, Y.J.,Suh, J.Y.,Ramamurty, U.,Jang, J.i. Elsevier Sequoia 2012 Materials science & engineering. properties, micro Vol.545 No.-
Room temperature nanoindentation experiments, employing two different pyramidal (Berkovich and cube-corner) indenters, were performed on a Zr-based bulk metallic glass (BMG) to critically examine the possibility of indentation-induced nanocrystallization in BMGs. Cross-sectional transmission electron microscopy images obtained from high angle annular dark field (HAADF) and high resolution (HR) modes clearly indicate to the occurrence of nanocrystallization. Pronounced nanocrystallite formation in the case of sharper cube-corner indenter suggests that the structural transformation is favored by the high strains introduced during nanoindentation.
Hydrogen-induced hardening and softening of Ni-Nb-Zr amorphous alloys: Dependence on the Zr content
Zhao, Y.,Choi, I.C.,Seok, M.Y.,Ramamurty, U.,Suh, J.Y.,Jang, J.i. Pergamon 2014 Scripta materialia Vol.93 No.-
The influence of absorbed hydrogen on the mechanical behavior of a series of Ni-Nb-Zr amorphous metallic ribbons was investigated through nanoindentation experiments. It was revealed that the influence is significantly dependent on Zr content, that is, hydrogen induced softening in relatively low-Zr alloys, whereas hydrogen induced hardening in high-Zr alloys. The results are discussed in terms of the different roles of mobile and immobile hydrogen in the plastic deformation.
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>
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>