Retention of δ-ferrite in aluminium-alloyed TRIP-assisted steels

There have been significant difficulties in the theoretical justification for the optimization of δ-TRIP steel, since the original concept was invented. In particular, the prediction of phase fractions has been notoriously unreliable using standard thermodynamic methods. New thermodynamic databases have become available and seem to explain the results pertaining to both published and new experiments conducted to probe the data. Solidification experiments are reported in order to test the non-equilibrium retention of excess δ-ferrite in the microstructure observed at an ambient temperature. These provided valuable information for comparison against kinetic simulations that prove that the excess ferrite cannot be attributed to growth phenomena. Instead, evidence is offered to show that it is the difficulty in nucleating austenite that prevents the thermodynamically required transformation of δ-ferrite.