Interfacial boron segregation in a high-Mn and high-Al multiphase lightweight steel

Interface segregation affects the microstructure evolution and mechanical properties of alloys, including strength, ductility and damage tolerance. This is particularly true for multiphase high-strength steels containing multiple types of interfaces whose characteristics are key factors influencing the steels’ mechanical performance. The different tendencies of solute segregation to different types of interfaces can lead to complex segregation behavior, which needs to be understood. Here, we focus on the segregation behavior of B in a high-Mn, high-Al lightweight steel with a two-phase austenite-ferrite microstructure. We find distinct B segregation at both austenite and ferrite grain boundaries as well as at austenite-ferrite phase boundaries after high temperature annealing (1100°C) and fast quenching. The segregation process is governed by local equilibrium between bulk and interfaces as discussed in terms of thermodynamic and ab initio calculations. Our findings reveal a dependence of B segregation on the interface structure regardless of the adjacent phases, which can be explained in terms of respective interfacial energy in accord with the Gibbs adsorption isotherm. In addition, co-segregation of B and C is observed at both high-angle and low-angle ferrite grain boundaries due to the attractive interaction between the two solutes in the bulk ferrite phase. In contrast, for austenite grain boundaries, C depletion is observed owing to its site competition effect and repulsive interaction with B in austenite. These observations help to guide interface segregation engineering in complex multiphase lightweight steels to improve their mechanical performance.