Communication Thermodynamic Self-interaction of Si in Ferritic Fe-Si Alloys SUJIN LEE and YOUNGJO KANG In order to clarify the thermodynamic properties of Si in ferritic Fe-Si alloy, equilibrium experiments were carried out with Fe-Si foils and Ag-Si alloys. The activity coefficient of Si at infinite dilute solution and the self- interaction parameters of Si were investigated from the distribution equilibrium of Si between the Fe and the Ag alloys. The self-interaction parameters of Si in the ferritic Fe-Si alloy were determined from 1323 K to 1423 K as follows: e Si Si ¼ 44065:8 T þ 3:17 https://doi.org/10.1007/s11661-019-05392-w Ó The Minerals, Metals & Materials Society and ASM International 2019 In order to appropriately control alloying elements and inclusions, knowledge of thermodynamic proper- ties, such as the interaction parameters of alloying elements, even in solid Fe is required. In fact, there are many studies on the thermodynamic interaction param- eters between alloying elements in liquid Fe. [1] In solid Fe, however, the interaction parameters of solutes have been rarely reported. Even the few reported values of interaction parameter in solid phase are mostly related to carbon and nitrogen, rather than metallic alloying elements. Wada et al. determined the thermodynamic properties of carbon in Fe-(Ni, Mn, Si)-C alloys by equilibration with controlled CH 4 -H 2 atmosphere. [2,3] As for the interaction parameters between other ele- mental solutes besides carbon, there is an example about the interaction parameter of Si and Ti in both austenitic Fe and ferritic Fe at 1273 K based on equilibrium experiments between solid Fe and liquid Ag by one of the present authors. [4] Si has been used as a deoxidizing agent in steelmaking of many typical steel grades to reduce bubbles of oxygen from molten steel. For semi or fully killed steels, addition of Si should be achieved, which is in molten steel Si must be a role as essential alloying element. Not only in molten steel but also in solid steel, Si has significant impact on the properties and process of steel products. Si helps to improve mechanical properties and protect an oxidizing layer formed on high-strength steel grades, e.g., transformation-induced plasticity steel dur- ing annealing or rolling process. [5,6] In particular, electrical steel grades, which contain more than 3 wt pct Si for minimizing eddy current loss, are widely used in many industries. These steels are specifically utilized in transformers and electric motors due to their excellent magnetic properties, such as low core loss and high permeability. For improvement of their magnetic prop- erties, the control of grain size during heat treatment has significant importance. In addition, various and small precipitations of inclusions such as AlN and MnS are utilized as inhibitors in secondary recrystallization. The interaction between solute elements in the solid phase has an impact on the grain growth of Si steels and thus the thermodynamic driving force to develop secondary inclusions and particles which may result in undesired nucleation and unexpected growth of smaller grains can be controlled. [7] Therefore, information on the interac- tions between solutes in solid phase is important to clearly understand formation conditions of the precip- itates. The influence of Si on the solute elements, particularly in solid Fe containing considerable content of Si, should be studied. In order to precisely investigate the interaction between Si and other solutes, knowledge of the thermo- dynamic behavior of Si in solid Fe is one of the most important prerequisites. However, few data on the thermodynamic properties of Si in the ferrite phase have been reported. [8,9] Sakao et al. determined the thermodynamic properties of Si in solid Fe from the equilibration of molten steel with a gas mixture of SiO, H 2 , and H 2 O at the vicinity of liquidus temperature. [8] Similarly, the same authors investigated the thermody- namic properties of Si in solid Fe-Si alloys using an electrochemical cell. [9] However, it may be necessary to verify their rather large temperature dependence of the activity coefficients of Si in ferritic solid Fe. In the present study, therefore, the aim was to obtain reliable information about the thermodynamic proper- ties of Si in solid steel in ferritic Fe approximately with 3 wt pct Si. The interaction parameter regarding Si can be determined by equilibrium between solid Fe and liquid Ag in which the activity of Si is known. [4,10] The self- interaction parameters of Si in the ferritic Fe-Si alloy were measured at 1323 K to 1423 K on the basis of the distribution equilibrium of Si between ferritic Fe-Si and liquid Ag-Si phases. SUJIN LEE and YOUNGJO KANG are with the Department of Materials Science and Engineering, Dong-A University, Busan 49315, Republic of Korea. Contact e-mail: youngjok@dau.ac.kr Manuscript submitted March 28, 2019. METALLURGICAL AND MATERIALS TRANSACTIONS A