The grain boundary wetting by the second solid phase in the iron-carbon alloys
摘要
The iron-carbon system is the basis of all binary, as well as many low- and high-alloyed steels. In this paper, the phenomenon of the so-called wetting of grain boundaries (GBs) by the second solid phase is studied for the first time in the Fe–C system. This phenomenon was observed in the regions of the Fe–C phase diagram where two different solid phases are in equilibrium. In the case of complete GB wetting by the second solid phase, its continuous layers completely separate the grains of the matrix. If the GB wetting is incomplete (or partial), then the second phase forms a chain of lenticular inclusions along the GB. With a change in temperature, a transition from complete wetting to incomplete wetting and vice versa can occur. The morphology of precipitates of the second solid phase in grain boundaries depends on the ratio of the GB energy σGB and the energy of interphase boundaries 2σαβ between two solid phases α and β. If σGB > 2σαβ, the second phase forms continuous layers between the matrix grains. In this case, one speaks about GB complete wetting by the second solid phase. If 2σαβ > σGB, the second phase forms chains of lenticular particles in the grain boundary, and the incomplete wetting is observed. Consider first the two-phase area of the Fe–C phase diagram where austenite (carbon solid solution in the fcc γFe) and cementite Fe3C are in equilibrium. Above 905 °C all γFe/γFe GBs are completely wetted by Fe3C layers. Below 850 °C all γFe/γFe GBs are partially wetted by Fe3C. In the αFe + γFe area, all γFe/γFe GBs are completely wetted by the αFe layers above 782 °C. Below 750 °C all γFe/γFe GBs are partially wetted by αFe. In the αFe + Fe3C area of the Fe–C phase diagram no complete GB wetting was observed.