Precipitation of η-Ni₃(Ti, Al) Phase at Grain Boundaries in an Austenitic Precipitation-Strengthened Stainless Steel

The precipitation of the η-Ni₃(Ti, Al) phase at grain boundaries in austenitic stainless steels significantly affects grain-boundary-related properties of the bulk material. In this study, the precipitation behavior of the η-Ni₃(Ti, Al) phase in a thermally aged austenitic precipitation-strengthened stainless steel was evaluated during aging at 750 °C for 0.5 to 64 hours using scanning electron microscopy, transmission electron microscopy, and atom probe tomography. A higher number of plate-like η-Ni₃(Ti, Al) phase particles precipitated at the grain boundaries with increasing aging time, and grew into the grain interior until the entire grain was occupied by this phase. The η phase had a {0001}_η//{111}_γ, \( \langle 11\bar{2}0\rangle_{{{\upeta }}} \)//〈110〉_γ semi-coherent orientation relationship with the matrix, and contained many stacking faults. Ni- and Ti-rich layers alternated periodically between adjacent stacking faults in the η phase. Al segregated at stacking faults and the interface between the η phase and matrix. The partition coefficients of Ni, Al, and Si between the η precipitates and matrix decreased with increased aging time; however, the partition coefficient of Ti did not change significantly. The partition coefficients of the other elements increased with increasing aging time. The Ti:Al ratio of the η phase significantly increased with increasing aging time. Based on the experimental results, the precipitation and growth behaviors of η-Ni₃(Ti, Al) phase precipitated at grain boundaries were discussed, which are expected to contribute to the design of austenitic stainless steel compositions with improved mechanical properties.