COARSENING KINETICS OF COHERENT PRECIPITATES IN NI-AL-MO AND FE-NI-ALALLOYS

The late stage coarsening kinetics of coherent gamma' and beta' have b een investigated in single crystalline specimens of Ni-14.5 at.% Al-5. 9 at.% Mo and Fe-10 at.% Ni-15 at.% Al after aging at. 1453 K for the Ni alloy and 1233 K for the Fe alloy. Scanning electron microscopy (SE M) shows that early in the coarsening regime there is particle splitti ng in the Ni-base alloy. Continued aging changes the spatial distribut ion of particles giving rise to the formation of groups of many partic les. In the Fe-base alloy, formation of particle groups is seen earlie r. Longer aging treatments produce large particles that split forming plate like precipitates. Particle migration is evident in both alloys. The coarsening kinetics of particles in the Ni-base alloy show deviat ions from the usual linear behavior between the cube of the average pa rticle radius and the aging time. There is a reduction of the coarseni ng rate and in addition broader particle size distributions are obtain ed. In the Fe-base alloy, direct measurements of particle areas on SEM images show a linear dependence of the cube of the average particle r adius with the aging time. However, after correcting for particle shap e an increase of the coarsening rate is suggested. (C) 1997 Elsevier S cience S.A.