Carbide phases in cobalt base superalloy: role of nucleation entropy in refinement

Abstract

The role of nucleation entropy in the formation of carbide phases in cobalt base superalloy HS 21 is investigated. X-ray and electron diffraction analyses show that M₂₃C₆ is the primary carbide, the composition approximating (Cr_0·77Co_0·15Mo_0·08)₂₃C₆. The composition of HS 21 was varied by remelting the as-received alloy in a vacuum induction furnace and adding small amounts of Ta and Nb. The addition of ∼1 at.-%Ta or 1·5 at.-%Nb resulted in the coarser M₂₃C₆ carbide being entirely replaced by a considerably finer MC carbide (TaC or NbC). It is shown that the carbide refinement is in agreement with the increased nucleation entropy and corresponding nucleation rate of the MC v. M₂₃C₆ carbide.