Title

Grain growth phenomena in P/M Ni-base superalloys

Date of Completion

January 2005

Keywords

Engineering, Materials Science

Degree

Ph.D.

Abstract

It is well recognized that the presence of second phase particles strongly influences grain boundary migration in polycrystalline materials. In this work experiments have been performed to assess the effects of the various different precipitates upon the grain growth behavior in two powder metallurgy (P/M) Ni-base superalloys, IN100 and a related "Matrix" alloy. Extensive microstructural characterization has been performed on the γ phase, γ' precipitates and inert precipitates in these alloys. The interactions between the precipitates and the γ grain boundaries have been investigated in detail. The investigation has been performed on alloys heat-treated at temperatures around the primary γ' solvus, because the grain structures of the alloys underwent the most significant changes in this range. Extensive grain size and size distribution data have been acquired from both alloys. ^ For the Matrix alloy, the grain size data indicate that normal grain growth (NGG) occurs and that this stagnates at a limiting grain size. These data have been related to the characteristics of the precipitates (such as chemistry, size, volume fraction and spatial distribution) on the basis of Zener pinning theory. The two main approaches for modeling NGG were applied independently to the alloy, and both were found to be able to model the grain size variation well. The relationship between these apparently different approaches has been established successfully. For IN100, the grain growth behavior is more complex. Either NGG or abnormal grain growth (AGG) may occur, depending on the heat-treatment conditions. The occurrence of AGG leads to remarkably larger grain sizes and inhomogeneous grain structures, which could have important consequences for the properties of P/M alloys processed under such conditions. A new model has been developed to explain the origins of this abnormal grain growth phenomenon. ^