Thermal metallurgical analysis of GMA welded AH36 steel using CFD-FEM framework

Cheon, J.,Kiran, D.V.,Na, S.J. Elsevier Ltd 2016 Materials & Design Vol.91 No.-

<P>A temporal combination of CFD mass and heat transfer, and FEM conductive heat transfer analysis was conducted using a proper temperature history implantation scheme. The phase distribution in an AH36 steel weldment was predicted and compared with experimental results. The numerical phase fraction estimation was performed using the critical austenite temperature model in the heating process as a function of heating rate. The CCT information based transformation starting and finishing temperature, and the maximum phase fraction models were utilized with instant cooling rate in the cooling process. The thermal analysis result agreed well with the FZ shape and measured temperature history. The calculated hardness slightly overestimated the measured hardness. The steep reduction of hardness in the HAZ and the tempered zone was much more affected by the change in austenite critical temperature than the cooling rate. Based on the potential results of this work, predicting weldment deformation by considering phase transformation will be extended. (C) 2015 Elsevier Ltd. All rights reserved.</P>

CoSn(OH)₆ hybridized with anionic and cationic graphenes as a new high-capacity anode for lithium ion batteries

Richard Prabakar, S.J.,Han, S.C.,Jeong, J.,Sohn, K.S.,Pyo, M. Elsevier Ltd 2017 Materials & Design Vol.118 No.-

<P>Herein is the first use of a CoSn(OH)(6)/graphene composites as a high-performance anode in lithium ion batteries (LIBs). CoSn(OH)(6) alone is found to be electrochemically active and to possess electrochemical properties that are superior to those displayed in its dehydrated form (CoSnO3). In contrast to the formation of 3Li(2)O in CoSnO3 during the first discharge (CoSnO3 + 6Lit(+) 6e(-) -> Co + Sn + 3Li(2)O), the production of 6 mol of LiOH in CoSn(OH)(6) (CoSn(OH)(6) + 6Li(+) + 6e(-) -> Co + Sn + 6LiOH) seems to provide nanometric Co/Sn particles with more efficient reversibility for the subsequent Li+-insertion/de-insertion via conversion/alloy reactions. The electrochemical performance of CoSn(OH)(6) is further improved when composited with graphenes. This is accomplished by electrostatically combining negatively charged Co2+/Sn2+-anchored graphene oxide (GO) with positively charged amine-functionalized graphene (GN) in a solution. The subsequent hydrothermal reaction produces CoSn(OH)(6) nanocubes that are tightly held by graphene sheets (GO/CS/GN). The GO/CS/GN delivers unprecedentedly high capacity and excellent cyclability (discharge capacities of 1475 mAh.g(-1) for the 100th charge/discharge (C/D) cycles at a 0.1 A.g(-1)). In contrast to the comparison materials (CoSnO3/graphene composites) the rate performance is also remarkable, delivering a capacity of 650 mAh.g(-1) at 2.0 at A.g(-1). (C) 2017 Published by Elsevier Ltd.</P>

XFEM investigation on Knoop indentation cracking: Fracture toughness and aspect-ratio of radial-median cracks

Rickhey, F.,Lee, J.H.,Lee, H. Elsevier Ltd 2016 Materials & Design Vol.107 No.-

<P>In sharp indentation of brittle materials, cracks form below the impression or at its corners and propagate further during unloading. The dimensions of radial-median cracks can be exploited to derive the fracture toughness. Knoop indentation has the merit that only one large crack is produced. This and the shallow plastic zone make it the preferred method for crack growth experiments, but which require information on the initial crack shape. For investigation of the complex indentation stress field, which is moreover perturbed by cracking, the extended finite element method (XFEM) is an ideal tool. Results show that the point load assumption holds for sufficient loading, which means that Knoop indentation can be used for fracture toughness evaluation. During loading the plastic zone evolves linearly with depth h, while crack depth c(z) is found to evolve according to h proportional to c(z)(3/4). For well-developed cracks, the crack aspect-ratio rho equivalent to c(z)/c (c is the length of the crack on the surface) is load independent. The XFE model is validated through comparison with experimental results,from the literature. Based on parameter studies, we establish functions that allow determination of fracture toughness and crack aspect-ratio. It is demonstrated that the mapping functions work well. (C) 2016 Elsevier Ltd. All rights reserved.</P>

Surface energy and wettability control in bio-inspired PEG like thin films

Javid, A.,Kumar, M.,Wen, L.,Yoon, S.,Jin, S.B.,Lee, J.H.,Han, J.G. Elsevier Ltd 2016 Materials & Design Vol.92 No.-

<P>Tailoring of chemical functionalities in polymer films can induce interesting biocompatibility, however the sequential process of polymerization followed by functionalization imposes surface-interface complexities and inhomogeneity of functional groups across the thickness. Here, a single-step plasma process, enabling the simultaneous polymerization-functionalization, is demonstrated to control the surface energy and wettability of polyethylene glycol-like thin films. Chemical studies, carried out by Fourier transform infra-red spectroscopy and X-ray photoelectron spectroscopy, confirm the evolution and enhancement in amide functionalities, owing to the increase in the electronic transitions related to nitrogen based ions/radicals (independently confirmed by optical emission spectroscopy). In present case, the evolution and control over amide functionalities lead to the enhancement in wettability and surface energy tailoring in 60.5-67.5 mJ/m(2) range. Excellent growth of L-929 fibroblast cells is obtained by the synergic contribution of plasma power and N-2 flow rate via enriching the amide functionalities in these films. (C) 2015 Elsevier Ltd. All rights reserved.</P>

Mechanical behavior of ultrafine-grained high-Mn steels containing nanoscale oxides produced by powder technology

Jeon, J.,Nam, S.,Kang, S.,Shin, J.,Choi, H. Elsevier Ltd 2016 Materials & Design Vol.92 No.-

<P>A new class of ultrafine-grained high-Mn steels containing nanoscale oxides has been developed by spark plasma sintering of ball-milled powders. During spark-plasma sintering, nanoscale manganese oxides were generated in Fe-15Mn steel, while nanoscale aluminum oxides were produced in Fe-15Mn-3A1-3Si steel because of the high affinity of aluminum for oxygen. Ultrafine-grained high-Mn steels that contain nanoscale oxides exhibited superior strength without significant loss of toughness, owing to the combined effects of grain refinement, twinning-induced plasticity (TWIP), and dispersion of nanoscale oxides. These new materials have potential for application in powder metallurgy components used in the automotive industry, such as gear sets, connecting rods, and bearing caps, which require high surface hardness as well as good core toughness. (C) 2015 Elsevier Ltd. All rights reserved.</P>

Molecular simulation for thermoelectric properties of c-axis oriented hexagonal GeSbTe model clusters

Vora-ud, A.,Rittiruam, M.,Kumar, M.,Han, J.G.,Seetawan, T. Elsevier Ltd 2016 Materials & Design Vol.89 No.-

<P>Using a combination of molecular orbital and molecular dynamics simulations, electronic and thermoelectric properties of GeSbTe model clusters are presented. The unit cells of Ge13Sb20Te52, Ge7Sb12Te40 and Ge14Sb6Te26 model clusters are designed corresponding to GeSb2Te4, GeSb4Te7 and Ge2Sb2Te5 compositions in hexagonal phase, oriented in the c-axis direction. The electronic structures of clusters have been simulated by discrete-variational molecular orbital calculation using Hartree-Fock-Slater approximation to determine the electrical conductivity and Seebeck coefficients in Mott expression. For thermal properties, molecular dynamics simulations have been employed on clusters in amorphous, cubic and hexagonal phases using Verlet's algorithm and subsequently using Green-Kubo relation for lattice thermal conductivity. We assumed inter-atomic interaction, defined by the Morse-type potential function added to Busing-Ida potential function, which considers partial electronic charges on the ions, bond length of the cation-anion pair, and depth and shape of the potential. Based on our simulations, detailed variation of electrical conductivity, carrier thermal conductivity, lattice thermal conductivity, Seebeck coefficients, power factor and figure of merit, are presented as a function of temperature in 300-700 K range. Thermoelectric parameters obtained in present study were compared and explained with those of experimentally results of Ge2Sb2Te5 composition in hexagonal phase. (C) 2015 Elsevier Ltd. All rights reserved.</P>

Robust fault-tolerant control for power systems against mixed actuator failures

Kaviarasan, B.,Sakthivel, R.,Kwon, O.M. Elsevier Ltd 2016 NONLINEAR ANALYSIS HYBRID SYSTEMS Vol.22 No.-

<P>This paper employs linear matrix inequality (LMI) based optimization algorithm to develop a method for designing fault-tolerant state feedback controller with mixed actuator failures for power systems subject to random changes. Meanwhile, the random abrupt changes are determined by a finite set Markov chain so the considered system is equivalently represented as a discrete-time Markov jump linear system (MJLS). Further, due to the variations of loading conditions in power system, an uncertainty term is incorporated to MJLS. For the proposed system, we construct a novel actuator fault model containing both linear and nonlinear terms which is more general than the conventional actuator fault models. The main purpose of this paper is to design the robust fault-tolerant controller such that for all possible actuator failures, time-varying delays and admissible parameter uncertainties, the closed-loop uncertain discrete-time MJLS is robustly stochastically stable. Based on free-weighting matrix approach and linear matrix inequality theory, anew set of sufficient conditions that guaranteeing the robust stochastic stability is presented by choosing an appropriate Lyapunov-Krasovskii functional candidate. In addition, a single-machine infinite-bus (SMIB) power system is considered as an application example and its simulation results demonstrate the effectiveness of the proposed design techniques. (C) 2016 Elsevier Ltd. All rights reserved.</P>

Growth and residual stresses in the bonded compliant seal of planar solid oxide fuel cell: Thickness design of window frame

Jiang, W.,Zhang, Y.C.,Zhang, W.Y.,Luo, Y.,Woo, W.,Tu, S.T. Elsevier Ltd 2016 Materials & Design Vol.93 No.-

<P>Bonded compliant seal (BCS) is a new sealing method for planar solid oxide fuel cell. The BCS design uses a thin foil to bond the cell and window frame, which generates a multilayer structure. However, the high temperature bonding generates large residual stresses that greatly affect the fracture. This paper presents a numerical method and neutron diffraction measurement to study the residual stress, and effect of window frame thickness has been discussed. A grain boundary diffusion model incorporated with a power-law creep constitutive model is developed to calculate the growth stress in the oxide film. Then, the thermal elasto-plastic finite element method is applied to calculate the thermal stress. A neutron diffraction experiment is performed to measure the through thickness stresses. A good agreement is found between the calculation results and the neutron diffraction measurements. Compressive stress is generated in the oxide scale because of the substrate constraint. Furthermore, a competition exists between the generation of growth stress and the creep relaxation in the oxide layer. The residual stresses in the oxide layer decrease with the decrease in the substrate thickness. The thicknesses of the window frame and foil are designed to be 500 and 50 mu m, respectively. (C) 2015 Elsevier Ltd. All rights reserved.</P>

Understanding the relationship between microstructure and mechanical properties of Al-Cu-Si ultrafine eutectic composites

Kim, J.T.,Lee, S.W.,Hong, S.H.,Park, H.J.,Park, J.Y.,Lee, N.,Seo, Y.,Wang, W.M.,Park, J.M.,Kim, K.B. Elsevier Ltd 2016 Materials & Design Vol.92 No.-

<P>Systematic investigations for the influence of the microstructural change derived from compositional tuning in Al-rich corner of Al-Cu-Si system on mechanical properties demonstrate that mechanical characteristic of ultra fine eutectic composites strongly depend on the crystallinity, length scale and volume fraction of constituent phases. Ultrafine eutectic composites can be divided into two categories: 1) whether primary phases exist or not and 2) the types of matrix phases, i.e., single eutectic or bimodal eutectic. The features of primary phases play a principal role on the macroscopic property. Ductile alpha-Al phase is very effective to improve the plasticity while brittle Al2Cu intermetallic phase deteriorates obviously mechanical performance. In addition, bimodal eutectic matrix alloys composed of eutectics with different length scale and morphology show superior mechanical properties than single eutectic matrix alloys. Based on the microstructural studies, it is believed that high strength is originated from ultrafine scale microstructure and enhanced plasticity is supported by strain gradient plasticity. These results reveal that mechanical properties of the ultrafine eutectic composites can be properly optimized via control of chemical and topological governing factors. (C) 2015 Elsevier Ltd. All rights reserved.</P>

Joining of metal-ceramic using reactive air brazing for oxygen transport membrane applications

Raju, K.,Muksin,Kim, S.,Song, K.s.,Yu, J.H.,Yoon, D.H. Elsevier Ltd 2016 Materials & Design Vol.109 No.-

<P>This study examined the joining of dense Ce0.9Gd0.1O2 (-) (delta)-La0.6Sr0.4Co0.2Fe0.8O3 (-) (delta) (GDC-LSCF) ceramics to high temperature metal alloys for the fabrication of multilayered oxygen transport membrane (OTM) stacks. Reactive air brazing using a silver-based paste was performed at 1050 degrees C for 30 min in air to join GDC-LSCF/high temperature alloys, such as AISI 310S, Inconel 600 and Crofer 22 APU. The effects of the various filler materials, including CuO, GDC, LSCF, and GDC-LSCF mixture, in the Ag paste were also examined. The Ag-10 wt% CuO braze filler ensured in a reliable and compact joining without the formation of cracks and voids at the joining interface, while the addition of other ceramic fillers resulted in incomplete joining. Although none of the GDC-LSCF/metal alloy joints showed gas leakage at room temperature, the GDC-LSCF/Crofer joint only maintained the gas-tightness up to 800 degrees C under pressurized air up to 7 bars, which was explained by the microstructural rigidness of the oxide layer formed on the filler/alloy interface at high temperatures. This was supported by the minimal decrease in shear strength of the GDC-LSCF/Crofer joint, which was 91.1 and 88.3 MPa for the as-brazed and isothermally aged joint at 800 degrees C for 24 h in air, respectively. (C) 2016 Published by Elsevier Ltd.</P>