Non-equilibrium solidification and ferrite in δ-TRIP steel

Microscopy and microanalysis experiments on two cast alloys, designed on the basis of equilibrium to contain substantial amounts of delta-ferrite, reveal zero or much reduced fractions of this phase in the solidified condition. It appears that the solid state transformation of delta-ferrite into austenite occurs without the required partitioning of solutes and that this is responsible for the development of non-equilibrium microstructures. This conclusion is supported by microanalytical data and through calculations of limiting phase diagrams based on paraequilibrium rather than equilibrium. Kinetic simulations confirm that this interpretation is consistent with the majority of austenite growing in the solid state without the partitioning of the substitutional solutes.