Effect of creep stress on microstructure of a Ni-Cr-W-Al-Ti superalloy

The tensile stress resulted in the α-W and γ′ precipitate free zones near the grain boundaries, or within the grains in the Ni-Cr-W-Al-Ti superalloy during creep. These PFZs did not appear at the secondary stage, but appeared at the tertiary stage or ruptured specimens. For a less deformed region of the creep-ruptured or the interrupted specimens at the tertiary stage, PFZs were primarily formed near the grain boundaries inclined to the tensile axis and folds were formed within some grains. The direction of PFZs and folds were generally inclined to the tensile axis. In some areas, their directions coincided with the direction of maximum shear stress. However, PFZs were rarely formed in the region where the amount of plastic deformation was very small. In the creep-ruptured specimens, the area fraction of PFZs increased with the increase in reduction of area. It is concluded that the PFZs were not formed by diffusional flow such as in Nabarro-Herring or Coble creep, but formed by the inhomogeneous plastic deformation due to GBS and its accommodation process.