y-y' Lattice misfit and its influence on the microstru-cture of UDIMET 520 during the rejevenation heat treatment : an X-ray diffraction study

The excellent strength of UDIMET 520 is due to the Y' precipitates (L12 structure) embedded in the y (fcc) matrix. These precipitates are generally of spherical shapes with a diameter - 90nm. This increases with service exposure of the material. The high misfit stress due to the difference in the lattice parameters matrix and precipitates along with applied stress is believed to be the decisive factor for the mechanical properties of this alloy, during elevated temperature exposure. At room temperatures, in most Ni-base superalloys, the lattice misfit is small but positive.