Three-dimensional atom probe analysis of solute distribution in thermomechanically processed TRIP steels

Complex multiphase microstructures were obtained in transformation induced plasticity C–Mn–Si–(Nb–Al–Mo) steels by simulated controlled thermomechanical processing. These microstructures were characterized using transmission electron microscopy, X-ray diffraction and three-dimensional atom probe tomography (APT), which was used to determine the partitioning of elements between different phases and microconstituents. The measured carbon concentration (∼0.25 at%) in the ferrite of carbide-free bainite was higher than expected from para-equilibrium between the austenite and ferrite, while the concentrations of substitutional elements were the same as in the parent austenite suggesting that incomplete bainite transformation occurred. In contrast, the distribution of substitutional elements between the ferrite lath and austenite in carbide-containing bainite indicated a complete bainite reaction. The average carbon content in the retained austenite (3.2 ± 1.6 at%) was somewhat higher than the T0 limit. On the basis of the APT measured composition, the calculated Ms temperatures for retained austenite were above room temperature, indicating its low chemical stability.