Multimodal Precipitation in the Superalloy IN738LC

IN738LC is a polycrystalline, nickel-base superalloy, which is used in aggressive environments at high temperatures. The required strength is provided by precipitate strengthening. Both unimodal and multimodal precipitate distributions are observed in IN738LC. After reaching a critical size, a unimodal precipitate microstructure transforms to a bimodal one. This transformation is controlled by the precipitate-matrix interface, which is under compression in IN738LC. As the unimodal precipitates grow, the strained interface, due to differential lattice parameter of the matrix and the precipitate phase, stops solute diffusion into the growing precipitates. Hence, the solute atoms, entrapped in the matrix, saturate the matrix and form new, fine precipitates. Dissolution of some large precipitates also supplies solute to supersaturate the matrix. On the other hand, a multimodal precipitate distribution tends to become unimodal at low aging temperatures and bimodal at high aging temperatures. Interestingly, the activation energy is calculated for the coarsening of large precipitates in multimodal distribution and is found to vary with aging time.