Thermodynamic Modelling on Nanoscale Growth of Magnesia Inclusion in Fe-O-Mg Melt

Abstract

Nano-magnesia is the intermediate product during the growth of magnesia inclusion in Mg-deoxidized steel. Understanding the thermodynamics on nano-magnesia is important to explore the relationship between magnesia product size and deoxidation reaction in molten steel. In this work, a thermodynamic modeling is developed to study the Mg-deoxidation reaction between nano-magnesia inclusions and liquid iron. The thermodynamic results based on the first principle method show that the Gibbs free energy change for the forming magnesia product decrease gradually with the increasing nano-magnesia size in liquid iron. The published experimental data about Mg-deoxidation equilibria in liquid iron are scattered across the region between the thermodynamic curves of 2 nm magnesia and bulk-magnesia. It is suggested that these scattered experimental data of Mg-deoxidized liquid iron are in different thermodynamic states. Some of these experiments are in equilibrium with bulk-magnesia, while most of these experiments do not reach the equilibrium state between bulk magnesia and liquid iron, but in quasi-equilibria between nano-magnesia and liquid iron. This is the reason that different researchers gave different equilibrium constants. Furthermore, the behavior of the metastable magnesia is one of the most important reasons for the supersaturation ratio or the excess oxygen for MgO formation in liquid iron.

Thermodynamic insight into the growth of nanoscale inclusion of Al-deoxidation in Fe–O–Al melt

“…In addition, Wang et al [32][33][34] suggested that the thermodynamic of Al-deoxidation in liquid iron is closely related with the size of alumina. Xiao et al [35][36][37] reported that the deoxidation thermodynamics of metal in liquid iron have depended on the structures and properties of reaction products. Hence, it can be concluded that there is a close relationship between the thermodynamics of Al-deoxidaiton reaction in liquid iron and metastable phase, such nano-Al 2 O 3 .…”

“…The initial structures of (Al 2 O 3 ) 2 cluster and α-Al 2 O 3 crystal were selected from the previous work 33 . The Gibbs free energy of (Al 2 O 3 ) 2 cluster and α-Al 2 O 3 crystal were calculated by the equations [33][34][35][36][37] where E is the total energy of alumina cluster or crystal at 0 K; H and S are the enthalpy and entropy of alumina cluster or crystal, respectively. The H and S were obtained by the analysis of atomic harmonic vibrational frequency of alumina cluster or crystal, which are the functions of temperature T. The calculation details of the relationship among the atomic harmonic vibrational frequency, the thermodynamic properties and temperature can be found in our previous study [33][34][35][36][37] .…”

“…However, most of the researchers are focused on the thermodynamic equilibrium between the bulk-alumina and iron melt, while less known about the thermodynamic properties of nano-alumina in liquid iron. Many previous work proved that the thermodynamic properties of the nano-alumina are different from that of the bulk-alumina, and the thermodynamic difference among the nanoscale inclusions is more obvious with the decreasing size of inclusions [31][32][33][34][35][36][37][38] . It was reported that the interfacial free energy between nano-alumina and liquid iron are decreased with the decreasing of the size of alumina, and the Gibbs free energy change of Al-deoxidation reaction has very close relation with the size change of alumina inclusion 31 .…”