Influence of competitive C-P segregation on austenite grain growth in iron alloys

The influence of C on the P-induced solute drag effect (SDE) retarding austenite grain growth is studied using in situ high-temperature laser scanning confocal microscopy (HT-LSCM) and corroborated by ab initio density functional theory (DFT) calculations. Three Fe–C–P alloys, each containing 0.20 wt.-% C and varying P concentrations, are subjected to isothermal annealing between 1050 and 1350 °C. HT-LSCM enabled real-time observation of grain size evolution, supported by ex situ digital microscopy (DM) to improve statistical validity. Critical comparisons with austenite grain growth studies in binary Fe–P and Fe–C alloys revealed that phosphorus, along with trace amounts of sulfur, significantly reduces grain boundary (GB) migration rates. Carbon, however, enhances grain growth rates, because of suppressing the segregation of slower diffusing retarding elements like sulfur and phosphorus. For the Fe–C–P alloys investigated, the addition of 0.20 wt.-% C is found to mitigate phosphorus GB segregation at elevated temperatures, thereby weakening its SDE. DFT simulations further confirm that C and P compete for GB sites, providing additional atomistic insight into the experimentally observed behavior. The consistency between in situ HT-LSCM results and ab initio calculations reinforces the proposed methodology and align with literature on C and P GB segregation in austenite.