Materials Science Forum Vols. 706-709

Abstract: Soldering is a potential technique for joining metallic glasses. It can be performed at far below the crystallization temperature of various metallic glasses; thus, there is no possibility of crystallization. However, Cu-Zr-based metallic glass displays poor wettability to Pb-free solder, because a strong native oxide film prevents direct contact between the solder and the glass. To overcome this problem, Cu-Zr-based metallic glass clad with a thin film of Cu has been developed. This was produced by casting the melt of a Cu₃₆Zr₄₈Al₈Ag₈ pre-alloy into a Cu mold cavity, inside which a thin film of Cu with a thickness of 2 μm was placed. Cu₃₆Zr₄₈Al₈Ag₈ metallic glass was successfully formed and welded to the Cu thin film. From microstructure analysis, it was found that a reaction layer was formed at the interface between the Cu and the Cu₃₆Zr₄₈Al₈Ag₈ metallic glass. However, no oxide layer was observed in the Cu-clad layer. It was found that the Cu cladding played an important role in preventing the formation of the surface oxide film. Consequently, solderability to the Cu-Zr-based metallic glass was drastically improved.

Abstract: The crystal structure of the phase precipitated in a Zr-Cu-Al ternary alloy during quenching was investigated. The crystal structure is constructed by stacking several atomic layers periodically and is similar to that of the Laves phases. The reason why a Zr-Cu-Al alloy tends to crystallize into Laves-like periodic stacking order structure is discussed.

Abstract: The controversy about the start up of the abnormal growth as heterogeneously engendered in the microstructure or produced by peculiar continuous incubation process has still not been solved. In this work the statistical theory of grain growth has been applied to treat the case of grain growth in the presence of an homogeneously unstable Zener drag. By the simulations presented it will be shown as abnormal grain growth is a result out of a continuous and homogeneous process without requiring any heterogeneity in the microstructure presumed to give local advantages to some grains. The mechanism by which during an incubation period the preconditions for the unstable growth are built up in the microstructure is clarified and discussed. Moreover the peculiar shape of Grain Size Distribution (GSD) approaching the “structural instability” will be also analytically defined and compared with experimental results obtained in a grain oriented Silicon Iron just before the onset of abnormal grain growth.

Abstract: The flow curves of an austenitic stainless steel deformed at temperatures 700-1000°C with strain rates 10^-5-10^-2 s^-1 were modelled with the Voce equation. The parameters needed to draw the Voce equation, are the saturation stress σ_V that defines the height of the flow curve, the critical strain ε_C that defines the velocity to achieve σ_V, and the stress σ_o, namely the back-extrapolated flow stress to zero strain. A modified strain hardening analysis based on the one-parameter model was used to analyze the strain hardening rate dσ/dε vs. the flow stress σ in order to obtain σ_V and ε_C. The modified approach was based on the assumption that the dislocation multiplication component of strain hardening was temperature and strain rate dependent through the thermal activation term s of flow stress. A parameter s’ proportional to s was obtained from the strain hardening analysis and a relationship between s’ and temperature and strain rate was found. Relationships between σ_V, σ_o, ε_C and s’ were finally established and at this stage the Voce equation could reproduce the experimental flow curves at any imposed deformation conditions of temperature and strain rate.

Abstract: A cyclic phase transformation concept has been proposed to investigate the growthkinetics of the austenite (γ) to ferrite (α ) and vice versa in Fe-Mn-C and Fe-C alloys. In the caseof cyclic partial transformations in Fe-Mn-C alloys, two new and special stages are observed:a stagnant stage in which the degree of transformation does not vary while the temperaturechanges and an inverse phase transformation stage, during which the phase transformationproceeds in a direction contradictory to the temperature change. The local equilibrium (LE)and paraequilibrium (PE) are both applied to analyzing the new observations. The stagnantstage was found to be caused by the Mn partitioning, while the inverse phase transformationstage was due to equilibrium conditions not being reached at the transition temperatures.A mixed-mode model is applied to simulating the cyclic phase transformation in Fe-C alloy,and it is found that the cyclic phase transformation concept is a very promising method forinvestigating the interface mobility.

Abstract: Sandwich structures are widely used, especially in areas where the performance of conventional materials is simply not adequate. Sandwich components achieve the same structural performance as conventional materials with weight savings of up to 75 %. They are basically made from two thin skins (faces) and a lightweight thicker core. Their structural, physical, and mechanical characteristics can be tailored based on service requirements by selection of different materials and manufacturing processes. In this study, the geometry and property of each separate component is utilized to the structural advantage of the whole assembly. Although Lagrangian method has been widely applied in other engineering disciplines, it has received less attention for optimization of sandwich components. The Lagrangian method is therefore introduced and expanded to find solutions for multipurpose design problems. This new optimization approach will enable us to find analytical solutions for complicated design problems which were conventionally solved by utilizing graphical methods. This paper aims to present a generic optimization method which can be used in the variety of applications in this field.

Abstract: The numerical modelling technique is successfully used for simulation of liquid metal movement and behaviour of non-metallic inclusions in the continuous casting devices. The CFD (Computational Fluid Dynamic) method allows information on physical, chemical and hydrodynamic phenomenon in the metallurgical processes to be obtained. In the continuous steel casting process, where the tundish performs the function of a device batching steel to the mould, the monitoring especially of non-metallic inclusions is very essential because a quantity of non-metallic inclusions in the liquid steel limit the quality of final steel product. The paper presents the results of computing simulation of behaviour a liquid steel flow and non-metallic inclusions in the one-strand tundish with stopper rod system. The subject of simulation was a one-strand tundish of a nominal capacity of 30 tons. The tundish is used in the polish steel mill to process of continuous casting slabs. In the paper authors analysed two tundish i.e. tundish with a low dam and tundish with a subflux turbulence controller and a low dam. The Ansys-Fluent^® program was used for solved mathematical model of casting process. The computer simulations were performed for unsteady and nonisothermal conditions. The population balance model was used to description of non-metallic inclusions growth process. As a result of computations, fields of liquid steel flow, fields of secondary phase volume fraction, fields of volume fraction for particular bins and non-metallic inclusions growth curve were obtained.

Abstract: Developing the metallurgical purity of steel products requires, among other things, the understanding of the behavior of non-metallic inclusions (NMI) in the bulk of liquid steel in the mould zone within the forming skin of a concast billet. The identification of the mode of NMI distribution with different values of casting parameters influencing the state of the metal in the mould, including electromagnetic stirring intensity, may be of key importance to developing the metallurgical purity of concast billets being cast. The present article discusses the analysis of the results of simulation of NMI flowing out from the liquid steel volume in the mould zone of the steel continuous casting machine (CCM). As the investigation object, two different types of square cross-section mould were chosen, while for carrying out computations for two selected steel grades, a hydrodynamic module (HDM) being an extension of the FLUENT^® program was employed. The use of this module made it possible to take consideration of the influence of the EMS-M type electromagnetic stirrer on the conditions of NMI flotation and distribution in the metal volume within the mould for defined thermal – dynamical conditions.

Abstract: Cold cracks occur during the cooling down of welded joint at low temperatures or later at room temperature after the end of welding. It is associated with the formation of brittle microstructures as martensite in the presence of diffusible hydrogen as well as of tension stresses. By using an enhanced Simulation-und Testing Center Gleeble 3500, a procedure for physical simulation of cold cracking under laser beam welding conditions is suggested. The approach reproduces combinations of the cold crack main parameters, a brittle microstructure, tension stress and high local hydrogen concentration under welding conditions which induce a cold crack. A specimen geometry and technique were developed to enable the gaseous hydrogen charging from pure hydrogen atmosphere. The amount of charged hydrogen can be adjusted through varying the charging parameters like temperature, gas pressure and charging time. The hydrogen charging technique and the cold crack testing procedure were proven with high strength steel specimens.

Abstract: Energy-efficient production in today’s metal industry includes the usage of casting heat to allow hot deformation directly after solidification for saving time and cost of expensive reheating processes. Due to the different initial state this processing route also leads to differences in deformation behavior (strain, flow stress) and softening and precipitation kinetics compared to a reheated material. This is caused by the different microstructures in the material directly after solidification, showing a much more coarsed grain structure and a higher amount of dissolved microalloying elements. Therefore, a material directly deformed after solidification usually shows a retarded softening behavior accompanied by precipitation processes. The different behaviour of a reheated material and a material directly deformed after solidification is shown in this work. The different modes of action for micro-alloying elements in different initial states are compared at the example of steels with different chemical compositions. Differences in deformation behaviour were simulated with semi-empirical models including specific coefficients to consider the processing parameters strain, strain rate, temperature, and chemical composition. The models are capable of describing the retarded softening caused by a superimposed precipitation kinetic leading to a typical plateau. The results of these models are compared with established physical models.