The Extension And Application Of Miedema’s Model In O And S Alloying Systems

In order to obtain high-quality metallurgical synthesis for metals and materials, the accurate controlling of O and S in alloying melts is one of the crucial issues. It is well-known that, during the metallurgical melting process, the impurity segregations of oxygen and sulfur usually occur, and even form some extremely stable fatal oxides and sulfides, appeared as typical inclusions, in particular, in various steels. Their segregations and the formation of oxides and sulfides may result in the severe damages on the important properties of materials, such as bendability, ductility, toughness and weldability, and so on. Therefore, as one of the most primary tasks in metallurgical process, the concentrations of oxygen and sulfur are usually kept down to a level as low as possible, as documented in the basic concept of many superpure steels, which are being developed in our contury. However, to achieve this aim, the prerequisite is to know that the fundamental thermodynamic behaviors of oxygen and sulfur and their interactions with other elements in alloys. However, the current reality is that the necessary thermodynamic data related with S and O is indeed not enought. Metallurgists mainly derive the necessary thermodynamic data through experimental methods although most of the experimental techniques have been perfectly improved, nowadays. But, quite often, it is noteable that the experimental data are even highly scattering by several different experimental groups. In addition, sometimes, it is also highly difficult to perform high-temperature experiments to measure necessary thermodyanic data, such as entropies and activities. Therefore, it is highly desirable and meaningful to develop valid theoretical methods to evaluate thermodynamic properties of O-and S-based alloying systemsAs early as1990s, by combining Miedema’s models with semi-empricial geometric models, Ding et al. successfully developed the models of evaluating the activity and interaction coefficients of binary and ternary high-tmperature melts. The models have been extensively applied to Fe-, Cu-and Co-based alloying melts. However, the problem arises for oxygen-and sulfur-alloying melts because Midema’s model did not specify the modeling parameters of oxygen and sulfur elements. At that time, the difficulties lie on the definition of the corresponding parameters of oxygten and sulfur because they are both gas. Accordingly, there is no way to find out their electronic densities which were defined on basis of solid condensed phases. However, this problem is now overcome in terms of the spirit of Chen’s method proposed in2006. By combining experimental data with Miedema’s nodel, the parameters of oxygen and sulfur are successfully derived in our current thesis as follows,Oxygen:electronegativity7.04, electronic density6.03, molar volume4.59, andSulfur:electronegativity5.8, electronic density3.24, molar volume6.97.In comparison with the results from literature, our results have been turned out to be highly reasonable to Miedem’s model. We even derived the activities and their interaction coefficients of Fe-based alloying melt in1873K and further compared the results with available experimental data. Calculated results are confirmed to be in good agreement with experimental data, except some special cases. Therefore, through our proposed method, the long-term problem related with the parameters of oxygen and sulfur for Miedema’s model has been resolved, successfully.In particular, we even found some special elements, such as Nb, Pt and Ag-their parameters are even in problem even that Miedema gave out their parameters in1970s. Here, it is suggested that the substantial improvements have been achieved by the similar method. By considering the experimental enthalpies of formation for the Nb-, Pt-and Ag-compounds obtained from Kleppa’s study, we revised the corresponding electronegativities of Nb(4.05), Pt(5.65)and Ag(4.35)to be4.31,5.57and4.17, respectively. Our results uncovered that the revised parameters are much more reasonable than the original parameters derived by Miedema et al.