To improve the crack propagation resistance of disk alloys, a study of the influence of the different heat treament parameters, which control the grain boundary geometry, was performed on Astroloy fabricated by a powder metallurgy process. Grain boundary serrations, associated with the precipitation of adjacent coarse y ' precipitates, were obtained by heat treating above the solvus for the alloy followed by slow cooling. The size of these coarse y ' precipitates can be controlled by the heat treatment parameters which include: homogenizing temperature, cooling rate and cooling range.The presence of grain boundary precipitates creates serrations by discontinuous precipitation of the y ' phase in the grain boundary region. The precipitates responsible for serrations were uniformly distributed on both sides of the boundaries. The amplitude of the serrations increase with the misorientation between grains. Tensile tests at 750°C showed no real difference between the microstructures, with and without serrations; however the creep fatigue crack growth rate in the alloy with serrated boundaries was found to be significantly slower than that observed in the unserrated material.

The growth kinetics of particles producing serrations on grain boundaries were studied on Astroloy and an experimental low misfit alloy. The serrations developed after applying an isothermal heat treatment a few degrees below the solvus temperature of the y ' phase. In both materials, it was observed that arms which originated from dendritic particles pushed the grain boundaries on both sides and consequently produced serrations of a few microns in size (category A). In addition, in the alloy with low misfit, fans characteristic of discontinuous reaction which were approximatively a hundred microns in size were observed in regions of low particle density (category B). To identify the growth mechanisms of each category, the experimental kinetics were compared with those predicted by models of continuous and discontinuous precipitation. For the formation of large fans, it appeared that the model of discontinuous growth is in good agreement with the experimental results. However, for the second category of serrations, the continuous growth model provides a better description of the growth kinetics.