The growth pattern of $\gamma^\prime$ -precipitates in the grains and at the grain boundaries has been investigated in two model Ni base superalloys ; Ni-24Co-4Al-4Ti-5Cr-5Mo (wt%) alloy (designated as Alloy M) of very small lattice misfit between the precipitate and the matrix phases and Ni-23.4Co-4Al-4. 3Ti-4.7Cr alloy (designated as Alloy W) of quite large lattice misfit. When aged at temperatures lower than the solvus temperature by more than 30 ℃ after direct cooling from the solution treatment temperature, the nucleation density is high. In this condition the supersaturation is quickly removed because of the overlapping diffusion fields, and the precipitates undergo Ostwald ripening from the early stage. The precipitates then have an equilibrium shape of spheres for Alloy M and cubes for Alloy W with nearly straight grain boundaries. The precipitates at the grain boundaries are coherent with one of the grains and their number density is not much larger than that in the grains, apparently because of a large contact angle (about 150°) with the grain boundary. Quenching the alloy after the solution treatment and aging at any temperature also produce high precipitate number density and equilibrium shape. When aged at temperatures just below $\gamma^\prime$ solvus the nucleation density is low, the precipitates grow dendritically in the grains, and the grain boundaries become serrated. The observed dendritic growth characteristics do not quantitatively agree with the predictions of Mullins and Sekerka theory, but the discrepancy may be due to the uncertainties in both the observed and calculated quantities. By deeply etching the matrix, it is shown that the grain boundary serration is produced by the precipitates growing preferentially in the direction of the incoherent boundary because of the rapid solute diffusion along the grain boundary. The dendritic growth and grain boundary serration can be obtained also by slowly cooling through the temperature range just below $T_s$. In Alloy W, both the equilibrium shape and the growth shape show some directionality due to large lattice misfit of this alloy, but the underlying principles of solid state dendrite and grain boundary serration are not changed. $\gamma^\prime$ precipitates with their equilibrium shape can develop morphological instability if supersaturation in matrix are abruptly increased. When undergoes dendritic growth, $\gamma^\prime$ can meet twin boundaries and then develop complex pate-based morphology.
