Three-dimensional analysis of the delta-ferrite to austenite phase transformation in an additively manufactured duplex stainless steel

A fundamental understanding of the δ-ferrite to austenite phase transformation and characteristics of the interfaces formed is currently lacking due to challenges in achieving fully ferritic starting microstructure during conventional processing. Here, a 2205 duplex stainless steel manufactured by laser powder bed fusion (LPBF) is used as a model system to reveal the fundamentals of the δ-ferrite to austenite phase transformation with the aid of three-dimensional electron backscattered diffraction (EBSD). A predominantly δ-ferritic non-equilibrium microstructure is obtained through the high cooling rate during LPBF. During a short thermal treatment of this starting microstructure, four distinct types of austenite (intergranular, instability-induced, sympathetic, and intragranular) are formed. The sympathetic and intragranular austenite present significantly higher area fractions of interfaces following the Kurdjumov-Sachs (K[sbnd]S) or Nishiyama-Wassermann (N[sbnd]W) orientation relationships (ORs) compared to intergranular austenite, owing to their different nucleation and growth mechanisms. The habit plane distributions of various interfaces reveal that ferrite and austenite terminate on (110) and (111) planes, respectively. Interestingly, the plane and curvature distributions do not always exhibit an inverse correlation in the sympathetic and intragranular transformation paths, while the non-K-S/N-W interfaces exhibit lower grain boundary curvatures compared to the K-S/N-W ones. This could be because the total energy minimization associated with phase transformation involves contributions from both the surface energy at grain boundaries and the elastic bulk energy. These new insights into the δ-ferrite to austenite transformation enable duplex microstructure design via additive manufacturing and subsequent post-processing to achieve superior properties.