A Correlative In Situ and Ex Situ Analysis of Static Recrystallisation in a New Superalloy for 3D-Printing

By making particular use of high-temperature confocal laser scanning microscopy, static recrystallisation is studied correlatively both in situ and ex situ in an as-fabricated superalloy made by laser-powder bed fusion (L-PBF). In this way, insights are gained into important recrystallisation phenomena with direct observations made-for the first time in this class of material-of phenomena such as nucleation of recrystallisation, subsequent grain growth, jerky flow of boundaries due to pinning, and twin formation. The nucleation process-requiring strain-free lattice to be created by grain boundary migration-is visualised, and its role in limiting the kinetics of recrystallisation is elucidated. Moreover, it is demonstrated that boundary mobility is initially prevented by Smith-Zener pinning due to a fine dispersion of secondary phases but also with a role played by solute drag caused by cellular micro-segregation. With increasing annealing time, the retarding pressure reduces due to carbide coarsening and/or dissolution as well as matrix compositional homogenisation, eventually allowing recrystallisation to take place. Further work will allow rich quantitative datasets to be gained which will allow for the testing of recrystallisation models.