DIRECTIONAL COARSENING IN NICKEL-BASE SINGLE-CRYSTALS WITH HIGH-VOLUME FRACTIONS OF COHERENT PRECIPITATES

Directional coarsening has been investigated experimentally in two nic kel-base single crystal alloys with high volume fractions of precipita tes. The initial elastic stresses due to the positive or negative misf it of the coherent precipitates biased the orientation of the precipit ates developed during coarsening. However, directional coarsening of t he precipitates did not occur until some limited amount of creep defor mation was initiated. With the application of an external stress, disl ocations penetrated preferentially into the most highly stressed matri x channels and directional coarsening occurred by coalescence of the g amma' precipitates in the plane of the initially less highly stressed channels, where the misfit stresses were not altered by the presence o f misfit relieving dislocations. The diffusional processes responsible for rafting apparently operate on the local scale of the precipitates , with preferential local dissolution of the gamma' and diffusional fl ow of alloying elements around the periphery of the precipitates.