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

The development of crystallographic texture during directional solidificati on has been quantitatively analyzed in columnar castings of the Ni-base sup eralloys, CMSX4 and CM186LC, produced with a range of cooling rates and liq uidus front curvatures. It is proposed that the more diffuse crystallograph ic texture developed in CMSX4 relative to CM186LC results from a combinatio n of the differing local orientation stability condition and the alloys' so lidification characteristics. The implications of these additional factors on the evolution of the axial grain texture, the grain orientations produce d in single-crystal processing, and the stability of spurious grains in pro cessing CMSX4 are discussed. An experimental method is presented to quantit atively 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 s tability of spuriously nucleated edge grains in a single-crystal matrix.