ROLE OF VOLUME FRACTION IN THE COARSENING OF NI3SI PRECIPITATES IN BINARY NI-SI ALLOYS

The coarsening behavior of coherent gamma' (Ni3Si) precipitates in bin ary Ni-Si alloys containing 5.50-5.93 wt.%Si was investigated at an ag ing temperature of 650-degrees-C. Over this range of concentrations th e volume fractions f(gamma') vary from approximately 0.015 to 0.081. T he kinetics of coarsening and the distributions of particle sizes appe ar to be independent of volume fraction for f(gamma') = 0.050, 0.058 a nd 0.081, but semiquantitative evidence was found for anomalously rapi d coarsening kinetics when f(gamma')= 0.018. The larger precipitates i n the more dilute alloys exhibit concave-cuboidal morphologies and in some instances were observed in the process of splitting into groups o f eight new particles. These morphologies were also attained under eve n smaller supersaturations than those experienced during normal isothe rmal aging, i.e. in a microstructure produced by a double-aging experi ment. This suggests to us that the morphologies in question are not a product of dendritic growth. These rather unexpected results indicate that the role of clastic energy in affecting morphology and coarsening kinetics may be more important than previously suspected, since the N i3Si-matrix lattice misfit is smaller than in any other binary Ni-base alloy. There is also evidence, based on our results and data in the l iterature, that the coarsening kinetics might increase with increasing f(gamma') at volume fractions exceeding 0.10; this is more in keeping with the kind of behavior expected theoretically.