Activation energy for growth in single size distribution and the dissolution features of γ′ precipitates in the superalloy IN738LC

Activation energy for the growth of γ′ precipitates in single size unimodal distribution has been determined by annealing the solution-treated and quenched alloy with fine 70 nm size cooling precipitates at 1,040°, 1,080°, and 1,100 °C for different periods of time up to 100 h. Results obtained using the LSW matrix diffusion model concur with the deductions of earlier work that the activation energy for growth of precipitates in single size distribution is not a constant, but increases with increasing size of precipitates. Also, the activation energy plotted against the corresponding precipitate sizes yields a straight line with a positive slope. During long-time annealing, precipitate particles line up to reduce interactive free energy and grow to fairly coarse sizes along the matrix grain boundaries. Some of the particles in isolated islands or those lined along the matrix grain boundaries attain critical maximum sizes and begin to dissolve into the matrix in four different ways, designated as Modes 1 through 4. The various modes of dissolution are described and the possibility of repeated growth and dissolution in cycles is envisaged.