A model to describe the effect of initial grain size distribution on abnormal grain growth with application to Rene 88DT

Critical grain growth (CGG) in powder metallurgy nickel base superalloys has been defined as the development of coarse grains in strained material that has been exposed to a heat treatment above the γ′ solvus. The large grain sizes that result from CGG can render the material unsuitable for further processing or use, resulting in substantial production losses. The goal of the present research project is to use modeling and experimental results to identify the dominant CGG mechanism, and to identify microstructure characteristics that may be used to prevent CGG in the nickel base superalloy René 88DT. Results from heating-rate experiments indicate that primary annealing processes occur well before the material reaches the reported gamma prime solvus temperature. Thus, CGG is a manifestation of abnormal grain growth, and not a direct result of recrystallizaton.; A model that describes the temporal evolution of a grain size distribution has been used to analyze the effect of initial grain size distribution (GSD) on abnormal grain growth. Based upon the work of Hillert and Hunderi and Ryum, this model uses a finite-difference technique to numerically solve the continuity equation. Two parameters describe synthetic initial GSDs used in the model: u_max is the ratio of the largest grain radius to the critical grain radius and F_o is the initial fraction of grains larger than twice the critical radius. These parameters describe the deviation of the initial GSD from steady state. The model then analyzes the evolution of initial GSDs that deviate from steady state.; In the absence of a pinning force from particles, a transient period of abnormal grain growth occurs as the GSD evolves toward steady state for all initially non-steady state GSDs. If stable particles are present, grain growth stagnation occurs. When the pinning force associated with particles is removed after stagnation, a transient period of abnormal grain growth occurs during the initial stage of evolution to a steady state GSD. By using a time-dependent particle pinning force, abnormal grain growth may be avoided. F_o and u_max allow approximate prediction of the duration and extent of abnormal grain growth.; The GSD evolution model was used in conjunction with experimental results to identify the importance of heating rate, particle volume fraction, particle coarsening and initial GSD on abnormal grain growth in René 88DT. The results of this effort imply that abnormal grain growth in this alloy could be due to a recrystallized GSD with large u_max or reduction in the pinning force from particles at high temperatures.