Key Engineering Materials Vols. 345-346

Abstract: The ideal strength of a nano-component, which is the maximum stress of the structure, provides an insight into the mechanical behavior of minute material. We conducted tensile simulations for cylindrical-shaped Cu nano-wires composed of an atomic chain as a core wrapped around by shell(s) with the structure of (111) layers in an fcc crystal. The results are compared with Cu atomic chain and sheet which are components of the nanowire. Young’s moduli and the ideal strengths of the wires are less than a single atomic chain and a sheet. The mechanical strength of the wire is weakened by the following three factors: (A) Change in electron arrangement caused by combining core and shell; (B) Larger interatomic distance (inherent tensile strain) of the outer shell introduced by the mismatch of atomic layers due to the curvature difference; (C) Mismatch between shells due to curvature difference. Factor (A) reduces the bonding strength in the shell(s) that occupy a greater part of the wire. 5-1 wire, which consists of a core and a shell, is weaker than the single atomic chain and the single sheet due to (A) and (B). 10-5-1 wire, consisting of a core and two shells, has less strength than 5-1 wire due to (C) in addition to (A) and (B).

Abstract: Nanoindentation is an interesting technique used to probe the local mechanical properties of a material. Although this test has been widely used and developed over the world during the past few years, it remains a lot of uncertainties regarding the interpretation of nanoindentation data. In this study, we propose to simulate the nanoindentation test of FCC single crystals like Cu or Ni using three numerical models. At the lowest scale, molecular dynamics simulations give details of the nucleation of the first dislocations induced by the indentation. At an intermediate scale, discrete dislocation dynamics simulations are performed to study the evolution of the dislocation microstructure during the loading. Finally, at the upper scale, 3D finite element modelling using crystal plasticity constitutive equations give a continuum description of the indentation induced plasticity. It is shown how the different models are interconnected together.

Abstract: For the pressure-assisted master sintering surface (PMSS), the sintered density during hot pressing is a unique function of the integral of a temperature function over time at a fixed pressure, irrespective of the heating path. The present research was undertaken to develop the pressure-assisted master sintering surface for silicon nitride known as potential material for high-temperature structural applications due to its excellent mechanical and thermal properties. High-purity α-Si3N4 powder (E-10 grade, Ube Industries) were mixed with 6.25% Y2O3 by alumina ball milling in ethanol for 3 hours. From the weight loss of ball mill after mixing, 1% Al2O3 was added. Consequently, SiN, 6.25% Y2O3 and 1% Al2O3 of powder mixture were prepared. After drying the resultant slurry at 100°C, the mixed powder were cold pressed at 300Mpa and made as disk of 25.4mm diameter and 2mm thickness. Then, densifications of silicon nitride were continuously recorded during heating at two different ramping rates of 5°C/min and 10°C/min up to 1800°C at fixed pressures from 7 to 34 MPa. The pressure-assisted master sintering surface of silicon nitride was successfully constructed. Using this surface, the final density can be predicted to about 1% accuracy for a fixed pressure and an arbitrary temperature-time path.

Abstract: The integrated simulation model for microstructural design of Fe-C alloy using the phase-field method and the homogenization method is proposed. First, the phase-field simulation is performed to simulate the morphological change of the grain boundary ferrite to Widmanstätten ferrite. Then, in order to clarify the effects of the morphology of the ferrite phase on the micro- and macroscopic mechanical properties, the finite element analysis based on the homogenization method is conducted with the representative volume element obtained from the phase-field simulation. This numerical approach provides a powerful tool to investigate systematically the micro and macroscopic mechanical behavior with the morphological change of the ferrite phase in the Fe-C alloy.

Abstract: The establishment of the coupled numerical model which enable to simulate the spherulite formation and its mechanical behavior continuously is our final goal. In this paper, we have developed Phase-field model for spherulte growth of polymer by generalizing the model proposed by Granasy et. al.. The numerical simulations for single spherulite and multi-sperulites have been performed with isotropic interface energy.

Abstract: Convenient structure adjustment and thereby the achievement of suitable material and technological properties is one of the very important areas of technological as well as material research. In general, this issue includes a great number of parameters and variables. To find suitable technological conditions, it is possible to use various kinds of modeling processes. One of them is the utilization of thermomechanical simulators, which allow simulating the conditions of the real processes to be simulated with sufficient accuracy. It is then possible to perform the optimization on smaller specimens, while monitoring the real conditions with higher accuracy. This method was used for the optimization of unconventional technological processes for selected alloying strategies of low-alloyed multiphase steels. These strategies are designed to be applied to technologies, which combine anisothermal forming and thermomechanical treatment of quasimassive components using intensive plastic deformation. Incremental deformations allow a high amount of deformation to be reached. It is also possible to obtain very fine grained structures by a suitable choice of temperature. By a suitable choice of temperature it is also possible to obtain structures with very fine grain. At the same time, the morphology of the structure and thus also its final mechanical properties can be significantly influenced this way.

Abstract: The interactions of a dislocation with commonly observed irradiation induced defects such as a stacking fault tetrahedron (SFT) and a void are studied using molecular dynamics (MD) simulation methods. The simulation of an SFT interacting with a dislocation in face centered cubic (FCC) copper (Cu) reveals that an SFT is a strong obstacle against a dislocation motion, with dislocation detachment often involving an Orowan like mechanism. The resulting SFT generally involves a shear step, although partial absorption is also observed in some specific interaction geometries. Dislocation interaction with a void has been studied in body centered cubic (BCC) molybdenum (Mo). The dislocation locally annihilates upon contact with the void and then re-nucleates on the void surface as the dislocation glides past the void. The interaction results in the simple shear of the void by one Burger’s vector. The obstacle strength of the void is measured using conjugate gradient molecular statics (MS) method as a function of void size. A large increase in the obstacle strength is observed for a void size greater than 3 nm in diameter.

Abstract: By using molecular dynamics simulation, misfit dislocation networks are made on semi-coherent interfaces in a laminate structure of Ni and Ni3Al single crystals. The core structure of the networks is discussed in detail, focusing on the different atomic configuration at the interfaces; e.g. with or without Al atoms on the Ni3Al side. It is revealed that the networks can be a source of partial dislocation loops under the external loading; however, the loops tend to form immobile wedge-like stacking faults, analogous to the stacking fault tetrahedron (SFT), near the interface with Al atoms. On the other hand, the loops propagate into both Ni and Ni3Al phases, from the network dislocations on the interface without Al atoms.

Abstract: Strategy and function of a new developed FEM code COSMAP(COmputer Simulation of MAterial Process) for surface hardening during of thermo-mechanical processing, including heat treatment, carbonizing and nitriding, is briefly introduced in this paper. The simulation code is developed based on the metallo-thermo-mechanical theory considering the coupled equations of diffusion, heat conduction, inelastic stresses and kinetics of phase transformation. Some examples of simulation and the experimental verification for carbonized quenching, carbonizednitrided- quenching process of a cylinder and ring as well as a gear are illustrated, and comparison of the simulated values of distortion, residual stresses and profile of induced phases with the experimental data is made with some discussions.

Abstract: Using the first-principles calculation, the elastic constant C44 of Ag/Al multilayers with different modulation periods from 0.43 nm to 2.27 nm has been evaluated in order to examine the effect of atomic and electronic structures on it. With increasing modulation period, C44 decreases and becomes close to that obtained by the conventional mixing rule, however, the difference of 8 % still remains at the modulation period of 2.27 nm. As C44 correlates with the average interplanar spacing, the decrease of C44 can be explained by the decrease of the charge density in the stacking direction due to the increase of the average interplanar spacing. The difference in the electronic structure is included in the effect of atomic structure.