FORMATION OF SERRATED GRAIN BOUNDARIES AND THEIR EFFECT ON THE MECHANICAL PROPERTIES IN A P/M NICKEL BASE SUPERALLOY

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.