Under specific conditions, secondary precipitate free zones (PFZ) can form in the vicinity of grain boundaries in polycrystalline Ni-base superalloys used for turbine disks. SEM, TEM and APT analysis have shown that these zones form and grow during close-to-γˈ-solvus heat treatments due to diffusion of γˈ-forming elements both into the matrix and grain boundaries. At these temperatures, intergranular γˈ formation and coarsening occurs simultaneously to intragranular precipitates coarsening, resulting in a loss of γˈ-forming elements from the bulk of grains to the grain boundaries.

In order to quantify the effect of such zones on mechanical properties, macroscopic and in-situ tensile tests have been conducted on specimens with different PFZ widths and different tertiary γˈ sizes into the PFZ. Deformation mechanisms have been analysed with SEM and TEM, post-mortem and on interrupted tests.

It has been shown that PFZ width could affect deformation mechanisms near to grain boundaries, at room temperature and at 700 °C, but tertiary precipitates size also. These modifications have a limited impact on macroscopic properties at room temperature, but they particularly affect yield strength, toughness and ductility at 700 °C.