China Science

Finite element simulations of indentations on elastic and elastic–plastic materials have been performed to systematically study the effects of indenter geometry and mechanical properties, namely Poisson’s ratio and the E/?y ratio, on the correction factor for Sneddon’s equation used for analysis of nanoindentation data. Two indenter geometries, namely conical and Berkovich indenters, have been considered. It has been shown that the first-order correction for conical indentation on elastic materials previously developed by Hay et al. [Hay JC, Bolshakov A, Pharr GM. J Mater Res 1999;14:2296–305] can be applied only to conical indentation of elastic deformation-dominated materials but not to Berkovich indentation. A new general relationship for the estimation of the correction factor for Berkovich indentation is proposed based on the finite element simulation results. This relationship gives a better estimation of correction factor for Berkovich indentations on both elastic and elastic–plastic materials.

Zhi-Hui Xua?Xiaodong LiaEmail:lixiao@engr.sc.edu
[a]Department of Mechanical Engineering, University of South Carolina, 300 Main Street, Columbia, SC 29208, USA

Nano and microcrystalline, Mn/CeO2 solid solutions (5 mol.% Mn) have been prepared by solution combustion synthesis using urea, glycine or polyethylene glycol (PEG) as fuel. The nature of the fuel and its concentration (fuel to metal mole ratio, F/M) have a strong influence on the physical and chemical characteristics of the resulting Mn/CeO2 solid solutions. The variations in the physicochemical properties are attributed to differences in (i) the adiabatic/real flame temperature realized with these fuels at different F/M ratios; (ii) the sustenance of the temperature or the quenching effect of the fuel at higher F/M ratios; (iii) combustion or decomposition of the precursors as the main course of the reaction; and (iv) the generation of gaseous products during combustion. Since the addition of the fuel to the initial precursor solution does not change the pH of the medium, the differences in the type of Mn species formed are mainly attributed to the combustion process.

B. Murugana?A.V. Ramaswamya Email:avram@iitm.ac.in?D. Srinivasb?C.S. Gopinathb?Veda Ramaswamyb
[a]Department of Chemistry, University of Pune, Pune 411 007, India;[b]Catalysis Division, National Chemical Laboratory, Pune 411 008, India

Duplex grain structure consisting of coarse-cell and fine-cell dendritic grains is frequently found in the central portion of direct-chill cast billets and ingots. Coarse-cell grains are usually considered as free-floating crystals settled to the bottom of the billet sump. These grains are assumed to be solute-lean and contribute to the negative centerline segregation. In this paper the contribution of coarse-cell and fine-cell grains to macrosegregation is for the first time studied experimentally by direct measurements of their composition. It is shown that the coarse-cell, floating grains are depleted of solute and the areas of their accumulation contribute to the negative macrosegregation. The areas of fine-cell grains can be either enriched in solute or be close to the nominal composition. It is argued that their composition results from the interplay between thermo-solutal and shrinkage-induced flows. The roles of casting speed and grain refining are also under scrutiny in this paper.

D.G. EskinaEmail:d.g.eskin@tudelft.nl?R. Nadellaa?L. Katgermanb
[a]Netherlands Institute for Metals Research, Mekelweg 2, 2628CD Delft, The Netherlands;[b]Delft University of Technology, Department of Materials Science; Engineering, Mekelweg 2, 2628CD Delft, The Netherlands

Sn and Sn-based compounds have attracted great interest as candidates for anode materials in lithium-ion batteries. Despite the great deal of attention focused on the effects of the volume change of the Sn anode during the lithiation/delithiation process on the cyclic property of the batteries, its influence on the electrode potential is still not well understood. In this study, by constructing a simple Sn–Li battery system, we have investigated the effects of the volume change associated with the formation of Li–Sn compounds on the electrode potential from the viewpoint of the Gibbs free energy and associated elastic-strain energy. Our experimental results show that (i) -Sn, which is a low-temperature phase and in thermodynamic non-equilibrium at around 298 K (our experimental temperature), is also formed together with usual ?-Sn after several cycles of the lithiation and delithiation processes and (ii) when a Sn plate-shape electrode is lithiated, the experimental electrode potential underruns the value expected thermodynamically. These experimental results can be consistently explained by considering the contribution of the elastic-strain energy to the chemical free energy of formation.

K. Hiraia?T. Ichitsubo aEmail:tichi@mtl.kyoto-u.ac.jp?T. Udaa?A. Miyazaki1?S. Yagia?E. Matsubaraa
[a]Department of Materials Science; Engineering, Kyoto University, Kyoto 606-8501, Japan

Although important, ductility remains difficult to predict and there is a tremendous need for more precise modelling. Progress in this field is hampered by a lack of quantitative experimental results to assess the validity of these models due to the stochastic nature of ductile fracture. In this paper, tensile tests have been carried out in a scanning electron microscope on model materials made of thin metallic sheets containing laser drilled holes. Depending on the material and hole configuration, different failure modes and strains are observed. The results show the importance of void spacing and orientation, constraining effects, materials yield stress and work hardening rate, and the competition between ductile fracture and shear localization. Finally, it is shown that the Thomason model for void coalescence is not appropriate for predicting fracture of the model material. However, the McClintock model for void growth, and the Brown and Embury and the McClintock models for void coalescence provide relatively good predictions.

A. Weck aEmail:weckag@mcmaster.ca?D.S. Wilkinsona
[a]Department of Materials Science; Engineering, McMaster University, Hamilton, Ontario, Canada L8S 4L7

Recent experiments on face-centered cubic (fcc) and hexagonal close packed (hcp) nanocrystalline metals reported an increase of more than 10-fold in strain-rate sensitivity in contrast to their conventional coarse-grained counterparts. To improve our understanding of this issue, we consider a mesoscopic continuum model of a two-dimensional polycrystal with deformation mechanisms including grain interior plasticity, grain-boundary diffusion and grain-boundary sliding. The model captures the transition from sliding- and diffusion-dominated creep in nanocrystals with relatively small grain sizes at low strain rates to plasticity-dominated flow in nanocrystals with larger grain sizes at higher strain rates. The strain-rate sensitivity obtained from our calculations matches well with the experimental data for nanocrystalline Cu. Based on this analysis, an analytical model incorporating the competition between grain interior plasticity and grain-boundary deformation mechanisms is proposed to provide an intuitive understanding of the transition in strain-rate sensitivity in nanostructured metals.

Yujie Weia?Allan F. Bowera?Huajian Gao aEmail:huajian_gao@brown.edu
[a]Division of Engineering, Brown University, Providence, RI 02912, USA

The anisotropy of fracture toughness in AA2139 (Al–Cu–Mg) alloy sheet has been investigated via synchrotron radiation computed tomography of arrested cracks in Kahn tear test pieces for different loading cases. The three-dimensional distribution and morphology of pores and defects in the as-received state are seen to be anisotropic, with chains of voids and void elongation in the L (longitudinal) direction. For toughness testing in L–T orientation (T is long transverse), voids ahead of the crack grow and link in the L direction. In T–L tests, voids ahead of the crack tip also grow in the loading direction, although a high degree of alignment is retained in the L direction. The present work provides quantitative microstructural data that can be used as input for and validation of recent idealized model formulations, and it is shown that the measured void dimensions and evolution are consistent with measured toughness anisotropy.

T.F. MorgeneyeraEmail:tm504@soton.ac.uk?M.J. Starinka?I. Sinclaira
[a]Materials Research Group, School of Engineering Sciences, University of Southampton, Southampton SO17 1BJ, UK;[b]Alcan Centre de Recherches de Voreppe, BP 27, 38341 Voreppe Cedex, France

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