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Gamma Prime Precipitation Modeling and Strength Response in Powder Metallurgy Superalloys Jian Mao Precipitation-hardened nickel-based superalloys have been widely used as high temperature structural materials in gas turbine engine applications for more than 50 years. Powder metallurgy (P/M) technology was introduced as an innovative manufacturing process to overcome severe segregation and poor workability of alloys with high alloying contents. The excellent mechanical properties of P/M superalloys also depend upon the characteristic microstructures, including grain size and size distribution of γ′ precipitates. Heat treatment is the most critical processing step that has ultimate influences on the microstructure, and hence, on the mechanical properties of the materials. The main objective of this research was to study the γ′ precipitation kinetics in various cooling circumstances and also study the strength response to the cooling history in two model alloys, Rne88DT and U720LI. The research is summarized below: 1. An experimental method was developed to allow accurate simulation and control of any desired cooling profile. Two novel cooling methods were introduced: continuous cooling and interrupt cooling. Isothermal aging was also carried out. 2. The growth and coarsening kinetics of the cooling γ′ precipitates were experimentally studied under different cooling and aging conditions, and the empirical equations were established. It was found that the cooling γ′ precipitate versus the cooling rate follows a power law. The γ′ precipitate size versus aging time obeys the LSW cube law for coarsening. 3. The strengthening of the material responses to the cooling rate and the decreasing temperature during cooling was investigated in both alloys. The tensile strength increases with the cooling rate. In addition, the non-monotonic response of strength versus interrupt temperature is of great interest. 4. An energy-driven model integrated with the classic growth and coarsen theories was successfully embedded in a computer program developed to simulate the cooling γ′ precipitation based on the first principle of thermodynamics. The combination of the thermodynamic and the kinetic approaches provided a more practical method to determine the critical nucleation energy. 5. The simulation results proved the γ′ burst theory and the existence of the multistage burst of γ′ precipitates, which shows good agreement with the experimental data in a variety of aspects.

Gamma prime precipitation modeling and strength responses in powder metallurgy superalloys