Structure and formation probability of contact interface between adjacent M23C6 in aged Fe-15Mn-3Al-0.7C TWIP steel

• Three possible interface structures may form when two carbides contact each other. • Different interface structures have different formation probabilities. • A twin structure can be formed by growing-contacting of carbide. M 23 C 6 carbides are characterized by a complex unit cell structure and high stacking fault (SF) energies, theoretically impeding the formation of SFs within the carbides. However, SFs can form upon contact and coarsening. Given their significance in understanding the deformation mechanism of nano-crystalline structures, their formation mechanisms and probabilities should be clarified. The paper deploys optical microscopy (OM), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) to explore the contact interface structures of adjacent grain boundary M 23 C 6 in aged Fe-15Mn-3Al-0.7C high-manganese TWIP steel. Results show the internal lattice of grain boundary M 23 C 6 features a prominent 0.8165-nm-wide SF structure parallel to the close-packed plane. This width is 33.5 % greater than that of the close-packed plane and {1 1 1}γ is found within it. It is proved that the SF is generated by the growth and contact beween adjacent M 23 C 6 carbides. Based on these findings, a new model is established to predict the possible contact interface structures and formation probabilities: a SF structure where remaining austenite planes exist in between (probability: 66.7 %), a coherent interface with SFs when the distance between the nuclei of the two carbides conforms to S = nd/sinθ (probability: 33.3 %), or a coherent interface without SFs (probability: 11.1 %).