Equilibrium segregation of phosphorus at grain boundaries of Fe–P, Fe–C–P, Fe–Cr–P, and Fe–Cr–C–P alloys

Abstract

The equilibrium grain-boundary segregation of phosphorus was investigated in Fe–P, Fe–C–P, Fe–Cr–P, and Fe–Cr–C–P alloys after annealing at temperatures in the range 400–800°C. The grain-boundary concentrations were determined by Auger electron spectroscopy on intergranular fracture surfaces. The phosphorus segregation in Fe–P alloys with 0·003–0·3%P can be described by the McLean equation, the free energy of segregation is ∆G° = −34 300–21·5T (J mol−1). The percentage of intergranular fracture is increased by the presence of phosphorus at grain boundaries. In Fe–C–P alloys grain-boundary segregation is affected by site competition. Increasing carbon content of the samples causes an increase of the carbon concentration and a decrease of the phosphorus concentration at the grain boundaries; the intergranular part of fracture is simultaneously lowered. The addition of chromium causes an increase of phosphorus segregation in Fe–C–P alloys but no change in Fe–P alloys. The behaviour in carbon-containing alloys is explained by the decrease of carbon activity by chromium which allows enhanced phosphorus segregation. As chromium does not affect phosphorus segregation in carbon-free alloys, there is no ‘synergistic cosegregation’ of chromium and phosphorus. The results lead to a new understanding of the influence of phosphorus on temper embrittlement of unalloyed and low-alloyed steels.