Impurity effects on the intergranular liquid bismuth penetration in polycrystalline nickel

Abstract

The effects of impurities on the intergranular penetration of Bi based liquids in polycrystalline Ni at 700 °C were systematically investigated. In comparison with a nominally pure Ni (99.9945%), the presence of a total amount of <0.5 at.% impurities of Mn, Fe and Si in the Ni increased the penetration length by six times when a near-equilibrium Bi–Ni liquid was applied; when an initially pure Bi liquid was applied, this increment further enlarged to ∼20 times in the initial penetration stage. In a second set of controlled experiments, the addition of Mn, Sn and Fe to the liquid Bi–Ni all enhanced the intergranular penetration, but produced different kinetics and morphologies. We extended a concept that was initially proposed in the Rice–Wang model for grain boundary embrittlement to explain our observations of the impurity-enhanced intergranular penetration based on a theory that segregation of an impurity could reduce the grain boundary energy more rapidly than the solid–liquid interfacial energy. Correspondingly, a new analytical model for the effect of adding a third impurity on changing the equilibrium dihedral angle and the associated intergranular penetration kinetics has been derived for the dilute solution limit. Furthermore, we demonstrated that the interplay of bulk phase equilibria, interfacial segregation, transport (dissolution, precipitation and diffusion) processes and stress generation could effectively explain a variety of different intergranular penetration behaviors and morphologies that have been observed in the experiments conducted using 10 different combinations of the solid and liquid metals. The framework for understanding the impurity effects on intergranular liquid penetration developed in this study can be applied to other materials systems. This study has practically importance for understanding and controlling liquid metal corrosion and embrittlement.