Designing, Processing and Properties of Advanced Engineering Materials

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Authors: Jong Taek Yeom, J.H. Park, J.W. Lee, Nho Kwang Park
Abstract: Friction welding of dissimilar materials, Ni-base superalloy IN713LC and oil-quench plus tempered AISI 4140 steel, was investigated. Friction welding was carried out with various process variables such as friction pressure and time. The quality of welded joints was tested by applying bending stresses in an appropriate jig. Microstructures of the heat-affected zone (HAZ) were investigated along with micro-hardness tests over the friction weld joints. DEFORM-2D FE code was used to simulate the effect of welding variables in friction welding process on the distributions of the state variables such as strain, strain rate and temperature. The formation of the metal burr during the friction welding process was successfully simulated, and the temperature distribution in the heat-affected zone indicated a good agreement with the variation of the microstructures in the HAZ.
Authors: I.G. Lee, A.K. Ghosh
Abstract: In order to analyze high temperature deformation behavior of NiAl alloys, deformation maps were constructed for stoichiometric NiAl materials with grain sizes of 4 and 200 µm. Relevant constitute equations and calculation method will be described in this paper. These maps are particularly useful in identifying the location of testing domains, such as creep and tensile tests, in relation to the stress-temperature-strain rate domains experienced by NiAl.
Authors: Takao Kozakai, Tadaaki Shikama, Toshiyuki Koyama, Minoru Doi
Abstract: Precipitation of disordered bcc Co (A2) particles has been investigated in Co-rich B2 ordered Co-Al-Fe and Co-Al alloys by means of a macroscopic composition gradient method using a transmission electron microscope. When Co-Al-Fe ternary and Co-Al binary alloys were annealed at 823-1023K for a relatively short time, bcc disordered particles precipitated within B2 ordered matrix, followed by the formation of equilibrium fcc α-Co (A1) on the phase boundaries. The precipitation limit of metastable A2 phase was evaluated using the binary and ternary specimens with macroscopic compositional gradient. As a result, it was found that the limits are located within the equilibrium A1+B2 two-phase fields in both ternary and binary alloy systems, and that the limit curve on the isothermal section at 923K for Co-Al-Fe system successfully connects with the phase boundary of A2+B2 two-phase field at the Co-Fe binary side of the ternary system.
Authors: Jae Wan Song, Jeong Whan Han, Mok Soon Kim, Sun Keun Hwang
Abstract: In order to investigate the extrusion characteristics of magnesium alloy (AZ31), a computer simulation was attempted. Tensile properties of as-cast billet with different strain rates were incorporated into the simulation. The results showed a great change in distribution of stress and strain at near die region by the ram travel. It was found that the average gain size at longitudinal edge of the extruded sheet was smaller than that at center region, which might be attributed to recrystallization caused by severe plastic deformation.
Authors: Xiao Ying Yuan, Kunio Takahashi
Abstract: The modified embedded atom method (MEAM) can describe the physical properties of bulk systems for a wide range of advanced engineering materials. However, the MEAM is found to return negative surface energy for Li(100), Li(110) and Li(111), if the relaxation of atomic positions on the surface is taken into account. In order to solve this problem, a new scheme of MEAM for lithium has been developed, by modifying the expression of embedding function. In this work, the new scheme is also applied to the other alkali metals, and the parameter sets of MEAM have been determined by fitting to not only bulk properties but also some non-bulk properties. The new MEAM potentials for alkali metals have been applied to calculate the elastic stiffness of crystal, the vacancy formation energy, the surface energies for low index crystal faces and the bond length and the binding energy for dimer. The results have been compared with experimental values.
Authors: Yong-Seok Hwang, Jae Chang Kim
Abstract: In this work, we propose integral photography using bi-convex variable focusing lens arrays (hereafter will be abbreviated as bi-convex VFLA-IP) to obtain wider viewing angle and faster response time and the enhanced depth without decreasing resolution of integrated image. We designed a bi-convex lens structure to apply to 3-dimensional moving pictures. We fabricated twodimensional bi-convex liquid crystal lens arrays using lithography. The response time and the focal lengths of the lens array are measured. Applied voltage to the cell alters the effective refractive index of the liquid crystal layer and results in a change of the focal length. From the results of measuring electro-optic properties of the fabricated cell, we discovered that synchronized elemental image array for real or virtual mode can be integrated in front of or behind the lens array without decreasing resolution by adjusting the focal length.
Authors: Masahiko Morinaga, M. Yoshino, A. Shimode, K. Okabayashi, H. Nakamatsu, R. Sekine
Abstract: The electron density distributions in a series of metal oxides are calculated using the DV-Xα molecular orbital method. It is found that the logarithm of the electron density, logρ(r), decreases with the distance, r, from the oxygen nucleus, while keeping a constant slope relevant to oxygen atom. The magnitude of the slope is about 15.75 for O-1s electrons, and about 6.61 for O-2s, 2p electrons, being nearly close to the respective values of 16 and 8, expected from the radial distribution functions of hydrogen-like atom containing only one electron. The extent of the region for the O-2s, 2p electrons changes with metal species in the oxides, but the slope remains unchanged. Furthermore, it is shown that the nature of the chemical bonding is well represented in log (ρ minZ-3) vs. 2(Z/n) rminb plots, where ρmin is the minimum electron density, rmin is the distance r at ρmin, Z is the atomic number, and n is the principal quantum number.
Authors: Seong Jin Kim, Sang Jin Park, Rena H. Basch, James W. Fash, Ho Jang
Abstract: The role of transfer film on high temperature wear properties of a multiphase composite for a brake friction material was investigated using a pad-on-disk type tribometer. A novolac resin-bonded composite based on a simple formulation with 6 ingredients (aramid pulp, cashew dust, Cu fiber, graphite, potassium titanate, and zirconium silicate) was used in this study. Results showed that the wear properties of the composite were significantly affected by the temperature at the frictioninterface when the transfer film was present on the counter face during sliding. In particular, the transfer film on the disk surface was well developed at approximately 200°C, resulting in theimproved wear resistance. It suggested that the transfer film on the disk surface effectively prevented direct contacts of the composite onto the counterface. On the other hand, no apparent relationship between transfer film thickness and friction coefficient was found in this experiment.
Authors: Bong Ju Lee, Duck Young Yoon, Young Tae Cho
Abstract: In particle or short-fiber reinforced composites, cracking of reinforcements is a significant damage mode because the cracked reinforcements lose load carrying capacity. This paper deals with elastic stress distributions and load carrying capacity of intact and cracked ellipsoidal inhomogeneities. Axisymmetric finite element analysis has been carried out on intact and cracked ellipsoidal nhomogeneities in an infinite body under uniaxial tension. For the intact inhomogeneity, as well known as Eshelby’s solution, the stress distribution is uniform in the inhomogeneity and nonuniform in the surrounding matrix [1-3]. On the other hand, for the cracked inhomogeneity, the stress in the region near the crack surface is considerably released and the stress distribution becomes more complex. The average stress in the inhomogeneity represents its load carrying capacity, and the difference between the average stresses of the intact and cracked inhomogeneities indicates the loss of load carrying capacity due to cracking amage. The load carrying capacity of the cracked inhomogeneity is expressed in terms of the average stress of the intact inhomogeneity and some coefficients. It is found that a cracked inhomogeneity with high aspect ratio still maintains higher load carrying capacity than one with low aspect ratio.
Authors: Jang Hyuk Yoon, Seong Jin Kim, Ho Jang
Abstract: Molecular dynamics (MD) simulation was performed to study the stress induced grain boundary migration caused by the interaction of dislocations with a grain boundary. The simulation was carried out in a Ni block (295020 atoms) with a Σ = 5 (210) grain boundary and an embedded atom potential for Ni was used for the MD calculation. Stress was provided by indenting a diamond indenter and the interaction between Ni surface and diamond indenter was assumed to have a fully repulsive force to emulate a traction free surface. Results showed that the indentation nucleated perfect dislocations and the dislocations produced stacking faults in the form of a parallelepiped tube. The parallelepiped tube was comprised of four {111} slip planes and it contained two pairs of parallel dislocations with Shockley partials. The dislocations propagated along the parallelepiped slip planes and fully merged onto the Σ = 5 (210) grain boundary without emitting a dislocation on the other grain. The interaction of the dislocations with the grain boundary induced the migration of the grain boundary plane in the direction normal to the boundary plane and the migration continued as long as the successive dislocations merged onto the grain boundary plane. The detailed mechanism of the conservative motion of atoms at the grain boundary was associated with the geometric feature of the Σ = 5 (210) grain boundary.

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