Grain boundary crystallography in two Fe-C-P alloys

Misorientation, grain growth and brittle fracture were investigated in two iron - carbon alloys containing 0.06 wt-% phosphorus (0.06P) and 0.12 wt-% phosphorus (0.12P) after selected heat treatment schedules. A ‘fracture surface serial sectioning’ technique was devised and combined with misorientation measurements to reconstruct specimens after fracture. Anomalous grain growth occurred in the 0.06P specimen only, after 1000°C annealing. This was attributed to the inhomogeneous distribution of phosphorus at the interfaces. No evidence was found for the direct influence of misorientation angle distributions or coincidence site lattice distributions on anomalous grain growth. The proportion of Σ3s increased greatly after annealing at 1000°C, attributed to the twinning that developed in the austenite range. There was strong evidence that Σ3s were in general more resistant to brittle fracture than were random boundaries. It is suggested that alloys of this type could be ‘grain boundary engineered’ to improve fracture resistance.