Precipitation and dissolution processes in γ/γ′ single crystal nickel-based superalloys

An experimental analysis of the γ↔γ+γ′ phase transformations in two single-crystal Nickel-based superalloys is presented. The transformations are characterised macroscopically by differential thermal analysis and dilatometry measurements and microscopically by qualitative and quantitative metallography of samples quenched during various thermal cycles. This fully covers the large changes in precipitate morphology and volume fraction encountered in the upper temperature transformation domain (between 1100 and 1300°C). It is shown that dissolution and precipitation processes are strongly influenced by the local elastic distortions resulting from the lattice mismatch between the two phases. The complex microstructure evolution observed is explained by considering the successive equilibrium shapes of a coherent precipitate growing in more or less strong interaction with its neighbors.