Role of volume fraction in the coarsening of Ni 3Si precipitates in binary NiSi alloys

Abstract

The coarsening behavior of coherent γ′ (Ni 3Si) precipitates in binary NiSi alloys containing 5.50–5.93 wt.%Si was investigated at an aging temperature of 650°C. Over this range of concentrations the volume fractions ƒ γ′ vary from approximately 0.015 to 0.081. The kinetics of coarsening and the distributions of particle sizes appear to be independent of volume fraction for ƒ γ′ = 0.050, 0.058 and 0.081 , but semiquantitative evidence was found for anomalously rapid coarsening kinetics when ƒ γ′ = 0.018 . The larger precipitates in the more dilute alloys exhibit concave-cuboidal morphologies and in some instances were observed in the process of splitting into groups of eight new particles. These morphologies were also attained under even smaller supersaturations than those experienced during normal isothermal aging, i.e. in a microstructure produced by a double-aging experiment. 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 elastic energy in affecting morphology and coarsening kinetics may be more important than previously suspected, since the Ni 3Si-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 literature, that the coarsening kinetics might increase with increasing ƒ γ′ at volume fractions exceeding 0.10; this is more in keeping with the kind of behavior expected theoretically.