Papers by Author: András Roósz

Abstract: The austenitization of steels can occur in a wide variety of initial microstructures. In this study we addressed the transformation of banded pearlite steels. Banded pearlite initial structures similar to the real ones were created. In these structures the entire transformation process was simulated whose part processes are nucleation and grain growth. The nucleation is described by a free energy based model, and the Fick II. diffusion equation by using Finite Difference Method describes the grain growth. These models have been coupled in cellular automata simulations.

Abstract: The so-called phase diagram is one of the oldest but at the same time one of the most useful method for the material science. The role of algorithms and software calculating the phase diagram have an extraordinary importance in the industrial and research-development applications because remarkable advantages can be obtained by installing these software or the detailed and exact data base developed by them into either the simulation programs or into the everyday production (process-control, checking). In our present paper, the investigation of data-requirement of calculation method of ESTPHAD phase diagram is described by processing binary-and ternary systems.

Abstract: This paper includes the binary and ternary liquidus temperature calculations of Sn-Bi-Cd system. The calculation was performed in cases of the surfaces of Sn, Bi and Cd phases too. First of all the liquidus curves were calculated in the binary systems (Bi phase in Bi-Cd and Bi-Sn systems, Sn phase in Sn-Cd and Sn-Bi systems, Cd phase in Cd-Sn and Cd-Bi systems). By using the calculated coefficients of the binary phase diagrams and the data from the digitalized ternary phase diagram, the liquidus temperature of Sn, Bi and the Cd phases were calculated. Finally the eutectic point of the binary liquidus curves and the eutectic valley of the Sn and the Bi surfaces were calculated by means of an iteration method.

Abstract: In this paper, the thermodynamic basis of ESTPHAD method is briefly shown and the development of the whole ternary calculation can be followed: the first step is the calculation of liquidus and solidus temperatures of the binary phase diagrams (Al-Cu and Al-Fe), the second step is the estimation of the liquidus and solidus surfaces (Al-Cu-Fe system), and the last step is the calculation of liquidus and solidus slopes by using the derivative function of the calculated ESTPHAD equations.

Abstract: The peritectic alloys, such as some types of steel, Ni-Al, Fe-Ni, Ti-Al, Cu-Sn, are commercially important. In contrast to other types of alloys, many unique structures (e.g. banded or island ones) can form when peritectic alloys are directionally solidified under various solidification conditions. It can be observed in the course of the directional solidification experiments performed in a rotating magnetic field (RMF) that the melt flow has a significant effect on the solidified structure of Sn-Cd alloys. This effect was investigated experimentally for the case of Sn1.6 wt% Cd peritectic alloy. For this purpose, a Bridgman-type gradient furnace was equipped with an inductor, which generates a rotating magnetic field in order to induce a flow in the melt. As a result, the forced melt flow substantially changes the solidified cellular microstructure. The cell size and the volume fraction of the primary tin phase were measured by an image analyzer on the longitudinal polished sections along the entire length of the samples. The microstructure was investigated by scanning electron microscope (SEM) equipped with an energy dispersive spectrometer (EDS).

Abstract: Cylindrical Pb-Sn alloy samples (diameter: 8 mm, length: 120 mm) of different compositions (30, 40 and 50 wt.% of Sn) were prepared from high pure (4N) components. The unidirectional solidification experiments have been performed according to the upward vertical Bridgman-method by using a rotating magnetic field (RMF) with a magnetic induction of 150 mT and with a frequency of 50 Hz. The sample-movement velocity was constant (0.05 mm/s) and the temperature gradient changed from 7 to 3 K/mm during the solidification process. The first half of samples was solidified without using the magnetic field and the second half was solidified by using the magnetic field. Under the influence of this strong flow induced by the magnetic field, the columnar microstructure of the first part decomposed and a characteristic "Christmas tree"- like macrosegregated structure with equiaxed Pb-dendrites was developed. The secondary dendrite arm spacing (SDAS) and the volume percent of primary Pb-phase (dendrite) were measured by an automatic image analyser on the longitudinal polished sections along the whole length of the samples. The effect of the forced melt flow on the micro-and macrostructure was studied in case of the different sample compositions.

Abstract: A comparison of the results of RANS k-ε and LES turbulence models was done via the simulation of the electromagnetic stirring of liquid 75,5%Ga-24,5%In alloy (in a 10 mm diameter & 30 mm high crucible) using Ansys Fluent. Each velocity component, the distribution of eddies inside the melt and other flow parameters were compared respectively. The accuracy was checked with measured angular velocity data of A. Rónaföldi. The turbulent energy spectra were also produced to see the validity of the LES models.

Abstract: Recently several experiments on directional solidification of Al-6.5wt.Si-0.93wt.%Fe (AlSi7Fe1) alloy were performed under terrestrial conditions and onboard the International Space Station (ISS) in the Materials Science Lab (MSL) with use of electromagnetic stirring and without it. Analysis of the samples showed that stirring with a rotating magnetic field lead to the accumulation of iron-rich intermetallics in the center of the sample and influenced the primary dendrite spacing while the secondary dendrite arm spacing were not affected. In the present paper the accumulation of the intermetallics b-Al₅SiFe in the center of the samples due to RMF stirring is demonstrated numerically and the evolution of primary and secondary dendrite arm spacing is discussed.

Abstract: The austenitization is a solid phase transformation process accompanied by nucleation and nucleus growth controlled by long-range carbon diffusion. In the course of our work, a method was developed by which spheroidite model structures were constructed such a way that their different parameters (the size of ferrite grains, the average value of carbon concentration, the size of cementite spheroids) could be changed optionally. In addition, a nucleation model of free enthalpy base was created by which the difference between the two different places of nucleation can be distinguished on the basis of their free enthalpy. The effects of structure parameters, interface free enthalpies and temperature on the nucleation rate of austenite were investigated by cellular automaton simulations.

Abstract: Al–7wt.-% Si–1wt.-% Fe alloy was solidified unidirectionally in the Crystallizer with High Rotating Magnetic Field (CHRMF). The diameter of sample was 8 mm and its length was 120 mm. The parameters of solidification were as follows: solid/liquid interface velocity ~0.082 mm/s, temperature gradient 7+/-1 K/mm, magnetic induction 0 and 150 mT, frequency of magnetic field 0 and 50 Hz. The structure solidified without rotating magnetic field (RMF) showed a homogeneous, columnar dendritic one. The structure solidified by using magnetic stirring showed a dual periodicity. On the one hand, the branches of the “Christmas tree”-like structure known from the earlier experiments contained Al+Si binary eutectic. On the other hand, bands with higher Fe- and Si-content formed in the sample, which were at a larger distance from each other than the branches of the “Christmas tree” structure. The developed microstructure was analyzed by SEM with EDS. The average Si- and Fe-concentrations were measured on the longitudinal section at given places along the length of sample. Furthermore the Si- and the Fe-concentrations close to the bands and among the bands as well as the composition of the compound phases were determined.