Mechanisms of Formation of Serrated Grain Boundaries in Nickel Base Superalloys

Abstract

The growth kinetics of particles producing serrations on grain boundaries were studied on Astroloy and an experimental low misfit alloy. The serrations developed after applying an isothermal heat treatment a few degrees below the solvus temperature of the y ' phase. In both materials, it was observed that arms which originated from dendritic particles pushed the grain boundaries on both sides and consequently produced serrations of a few microns in size (category A). In addition, in the alloy with low misfit, fans characteristic of discontinuous reaction which were approximatively a hundred microns in size were observed in regions of low particle density (category B). To identify the growth mechanisms of each category, the experimental kinetics were compared with those predicted by models of continuous and discontinuous precipitation. For the formation of large fans, it appeared that the model of discontinuous growth is in good agreement with the experimental results. However, for the second category of serrations, the continuous growth model provides a better description of the growth kinetics.