Thermodynamics of phosphorus grain boundary segregation in low alloy steels

Experimentally determined values of grain boundary and bulk concentrations of P, Cr, V, Mo, and Fe for Cr, Cr-V, and Cr-V-Mo steels aged at 773 and 823 K for 5000 h were used to calculate thermodynamic parameters (DeltaH(P)(0), DeltaS(P)(0), alpha'(PCr), alpha'(PMo), and alpha' (PV)) of phosphorus segregation. The enrichment of segregating species at grain boundaries were measured on freshly fractured surfaces by means of Auger electron spectroscopy. Altogether 30 equations reflecting the site competition effect, Fowler-type binary interaction, and P-Cr, P-V, and P-Mo ternary interactions in all possible combinations were proposed for systems corresponding to the investigated steels. With respect to the recently published results, experimental data were fitted by 15 equations free of Fowler term. There was confirmed a linear relationship between enthalpy and entropy of phosphorus segregation. The consideration of P-Mo interaction in the calculations led to the less negative values of segregation enthalpy. There was recorded a high scatter of alpha' (PCr), alpha' (PV), and alpha' (PMo) values calculated according to the different segregation equations. It was shown that the higher a number of ternary interactions considered the higher a scatter of values of interaction coefficients.