On the Precipitation-Strengthening Contribution of the Ta-Containing Co3(Al,W)-Phase to the Creep Properties of γ/γ′ Cobalt-Base Superalloys

The creep strength of single-crystalline Co-based superalloys was found to be comparable to first-generation Ni-base superalloys. However, considerable shearing of the γ′ precipitates was observed in the early creep stages. To determine the strengthening contribution of the Ta-containing γ′-Co3(Al,W) precipitates, the creep strength of several single-crystalline Co-Al-W-Ta superalloys was determined as a function of the γ′ volume fraction at 1223 K (950 °C) and stress levels between 25 and 600 MPa. Employing a Lagneborg–Bergman–Reppich (LBR) approach, it is found that the strengthening contribution of the γ′ precipitates increases significantly with increasing γ′ volume fraction. In a Co-base superalloy that exhibits a precipitate volume fraction of about 70 pct, the γ′-strengthening contribution calculated with the LBR approach ranges between the ones observed in first-generation Ni-base superalloy CMSX-6 and second-generation Ni-base superalloy CMSX-4.