Materials Science Forum Vols. 706-709

Abstract: In a wide range of materials, precipitation hardening is the key for optimizing properties such as strength or creep performance. In order to model strengthening effects with physically based concepts, precipitate kinetic simulations have to be linked to micromechanical models. Part of this link is the precipitate distance distribution in the glide planes of dislocations. Recently, a new model for the calculation of distance distributions has been introduced, which is specially designed for arbitrary size distributions and, thus, capable of handling more realistic microstructures when compared to classical approaches. Up to now, this model has been restricted to spherical precipitates. In this work, the model is advanced to account for all kinds of spheroids, that is, ellipsoids with rotational symmetry. Any prolate, oblate or globular precipitate shape can be represented by a specific shape factor, or aspect ratio, and an effective radius. The result is represented in the form of a multiplicative factor for particle distances depending on the aspect ratio only, and can be expressed as a single explicit formula. It is shown, that prolate shape is most effective for minimizing particle distances in glide planes, followed by oblate shape and finally spheres. Since numerous precipitate types feature needle-or platelike shapes, the present model offers a wide field of applications.

Abstract: In the present study, the effects of inclusions on the local ductility of DP steel are investigated using finite element analysis (FEA). In order to evaluate local ductility, a continuum damage mechanics (CDM) model has been incorporated into the Abaqus/Explicit® commercial finite element code. Furthermore, three-dimensional representative volume elements (RVEs) with ferrite, martensite, and inclusion phases have been used to evaluate the stress-strain response. Simulation results show that the volume fraction of the martensite as well as the difference in hardness between the ferrite and the martensite phases dominates the effect of inclusions on local ductility.

Abstract: Numerical modelling of the round bar rolling process, while considering the wear of the passes depending on their shape, was carried out within the present work. For the theoretical study of the rolling process, the Forge2008® was employed, which is finite element method-relying software that enables the thermomechanical simulation of rolling processes in a triaxial strain state. The wear model implemented in the Forge2008® permits no quantitative evaluation, but only comparative analysis of the wear of rolls. In order to use the results of simulation employing the simplified Archard model for the quantitative evaluation of roll wear, it is necessary to define the wear factor and hardness of the tool as a function of temperature. The paper present a methodology for the determination of the quantitative wear of rolls based on the results of computer simulations performed using the Forge2008® software for a selected round pass during rolling of round bars.

Abstract: We study the chemical bonds of small Al clusters (Al_n, n=2-8) and hydrogenated Al clusters (Al_nH_m , n=1-8 and m=1,2) using electronic stress tensor. We calculate the bond order based on energy density for these clusters. We also study the electronic structure under the presence of electronic current by the electronic stress tensor for AlH₃ molecule.

Abstract: The problem of whisker formation in tin (Sn) wiring in small electronic devices has become an important issue with the requirement of lead-free wiring, because doping of Pb to reduce whisker formation cannot be applied. It is therefore urged to better understand stress migration in tin, which is suspected to play a key role in whisker growth. We aim to study grain boundary diffusion in tin by atomistic simulation. After constructing an efficient interatomic potential suitable for diffusion of atoms using the genetic algorithm (GA), we perform molecular dynamics (MD) simulation of grain boundary diffusion in Sn under stress. We find that the magnitude of stress effect on diffusion depends on the boundary structure. Moreover, we examine the effect of impurities on vacancy migration by ab initio calculation to find atom doping that has potential to suppress diffusion.

Abstract: The paper explores possible fracture laws that can be used in meso-level lattice-typemodels for simulating the behaviour of cohesive frictional materials like concrete and rock. A newfailure criterion is proposed, which includes three possible mechanisms that appear to explain thefracture of these materials at the macro-level.

Abstract: Constantly increasing customers’ demands for the production of high and very high-quality steels, promote the intensive technological development of their production. Today, the dominating method of global steel production is continuous casting. A continuous casting plant includes the ladle turret, several steel ladles, a tundish, and a mold followed by a framework of rolls to support the strip. The tundish has become a very important metallurgical unit in the continuous casting process. Nowadays its role is not only to guaranty the link between the process of secondary metallurgy and the continuous casting process in the mould but it becomes an active metallurgical reactor. Therefore in recent years a lot of researches were done to establish a better understanding of the physical phenomena accompanying the steel flow through the tundish and non-metallic inclusion separation into the top slag cover. The article presents computational studies of the three-dimensional turbulent steel flow and non-metallic inclusions separation in a multi-strand tundish for steady-state casting conditions. Simulations of the steel flow in the tundish are performed with boundary conditions that are derived from the real casting process. The mathematical model used for simulations, was partly validated with experimental measurements. Numerical calculations are carried out by the finite-volume code Fluent using k-ε standard turbulence model. With the calculated flow field, micro-inclusions removal due to flotation to the coving slag is investigated numerically. For the particle separation at the interface a modified boundary condition is implemented.

Abstract: The ferrite transformation kinetics of severely hot-deformed austenite has been studiedby considering ferrite nucleation from dislocation cell blocks inside austenite grains. The size ofdislocation cell blocks and ferrite grain size just after phase transformation are acknowledged to beinversely proportional to the square root of dislocation density. It is found that the ferrite nucleationrate in this area can reach the saturated state at a high temperature just under Ae3, and the ferritetransformation finishes within a very short time. The kinetics of ferrite volume fraction and theferrite grain growth after phase transformation for plain carbon (0.1%C, 0.2%Si, 1.0%Mn) steelhave been studied using a THERMECMASTER hot-compression testing machine. These modelscan be applied to the hot and warm forming processes of plain carbon steel to predict the ferritetransformation from severely deformed austenite.

Abstract: The role of microstructure in susceptibility to hydrogen uptake and property degradation is being evaluated using a number of high strength pipeline steels. To do so, a cellular automaton (CA) model has been used to examine the effect of grain size, as a first step in assessing the influence of microstructure. The simulation results of hydrogen diffusion into microstructures with different grain sizes are presented.

Abstract: Based on the solute drag model, a practical model incorporating the segregation effect is proposed to calculate grain growth rates in carbon steels. The segregation effect is modeled using two factors: the difference in atomic diameter between a solvent and a substitutional element, and the solubility of a substitutional element. By including the segregation energy, the proposed model enables the simulated retardation of grain growth by the addition of microalloying elements. The calculated grain growth rate by the proposed model shows reasonable correspondence between grain growth rates for experimental and calculated results. The temperature dependence of the grain growth rate is also well simulated.