Directional and single-crystal solidification of Ni-base superalloys: Part I. The role of curved isotherms on grain selection

The development of crystallographic texture during directional solidification has been quantitatively analyzed in columnar castings of the Ni-base superalloys, CMSX4 and CM186LC, produced with a range of cooling rates and liquidus front curvatures. It is proposed that the more diffuse crystallographic texture developed in CMSX4 relative to CM186LC results from a combination of the differing local orientation stability condition and the alloys' solidification characteristics. The implications of these additional factors on the evolution of the axial grain texture, the grain orientations produced in single-crystal processing, and the stability of spurious grains in processing CMSX4 are discussed. An experimental method is presented to quantitatively analyze the grain selection process in the case of curved liquidus isotherms by retaining the stereology of the primary [001] dendrite growth direction and the local thermal gradient vector. This can account for the stability of spuriously nucleated edge grains in a single-crystal matrix.