First Principles Study on Effects of Ordering on Structural Stability and Brittleness in High Silicon Steel

Abstract

Many studies have shown that embrittlement of high silicon steel is closely related to the formation of B2 and D03 ordered phases. In most researches, main attention has been paid to the effect of heat treatment on the size and distribution of these ordered phases. However, the underlying mechanisms for the formation of B2 and D03 ordered phases and embrittlement of high silicon steel has not yet been understood. In this work, we explore the effects of ordering on structural stability and embrittlement of Fe-Si alloys by using first principles method based on density functional theory. To examine the structural stability of Fe-Si alloys, we calculate the total energies of Fe-Si alloys according to arrangements of Si atoms in Fe medium for various Si concentrations. It is found that the D03 like arrangement of Si atoms is the most stable among possible arrangements for each Si composition. This preference of D03 like arrangement implies that Si atoms prefer to cluster in some degree, rather than be distributed uniformly in Fe medium. Also, calculations from the coherent potential approximation (CPA) which describes randomly disordered state consistently show that both B2 and D03 ordering of Si atoms give rise to energetically more stable structures than disordered distribution of Si atoms. To understand the effects of ordered phases on embrittlement of Fe-Si alloys, we study surface energies and generalized stacking fault (GSF) energies of B2 ordered phases and pure iron. It is known that brittle or ductile response of materials to external loads is determined by two competing processes, the extension of the crack by creation of surfaces and the bluntness of the crack by emission of dislocations that absorbs the stress at the crack tip. Consequently, surface energy can be regarded as a barrier for brittle behavior and unstable stacking fault energy as a barrier for ductile barrier. The results of our calculations show that the (110) surface energy of B2 ordered phase is 61 percent lower than that of pure iron, whereas unstable stacking fault energy barrier of B2 ordered phase is only 9 percent lower than that of pure iron for the slip system. It is shown that the difference in unstable stacking fault energy between B2 ordered phase and pure iron is negligible compared to that in surface energy.It is concluded that very low surface energy of B2 ordered phases may contribute to the brittleness of Fe-Si alloys. This conclusion, however, is not fully guaranteed since the proposed mechanisms of brittle versus ductile behavior of materials based on the analysis of surface energy and unstable stacking fault energy are still far from complete. The development of more rigorous scheme to determine the brittle or ductile behavior is required for better understanding of embrittlement mechanism of high silicon steel.