The effect of elastic misfit strain on the morphological evolution of γ’-precipitates in a model Ni-base superalloy

The morphological evolution of coherent γ’-precipitates in a Ni-23.4Co-4.7Cr-4Al-4.3Ti (wt%) alloy has been observed under systematically varying heat-treatment conditions. By deeply etching the matrix and observing under a scanning electron microscope, the precipitate morphologies are determined accurately. By quenching after solution treatment and aging isothermally, high initial precipitate density is obtained, and the cuboidal precipitates cluster and align to form regularly spaced arrays. The dimensionality of the aligned structure increases with increasing precipitate volume fraction as the aging temperature decreases. By directly cooling to an aging temperature after the solution treatment intermediate precipitate density is obtained. During the isothermal aging, the cuboidal precipitates split into octets and clusters of several pieces which disintegrate subsequently into aligned cuboids. Upon further aging the coherency is broken. When directly cooled to aging temperatures very close to the solvus temperature, the dispersed precipitates grow dendritically along the 〈111〉 directions. Various thermodynamic and kinetic theories for the elastic strain effect stemming from the lattice misfit agree quantitatively with the observed clustering, alignment, and splitting, but presently there is no complete theory which accurately describes the observed morphologies.