Influence of Grain Size and Volume Fraction of η/δ Precipitates on the Dwell Fatigue Crack Propagation Rate and Creep Resistance of the Nickel-Base Superalloy ATI 718Plus

Recently, it was found that the orientation of the η/δ plates in the Ni-base superalloys ATI 718Plus has a huge impact on the fatigue crack propagation behavior (Huenert et al. in Superalloys pp.781–792, 2016). However, the targeted alignment of η/δ precipitates in components with complex geometry can be challenging in certain applications. Therefore, the effect of other microstructural features of ATI 718Plus, which are easier to manipulate, is tested with respect to their fatigue crack propagation behavior and creep strength. The grain size, the γ′ precipitate size, as well as the γ′ and η/δ volume fraction are varied by applying different heat treatments. An increase in the η/δ volume fraction as well as the γ′ volume fraction and precipitate size enhanced the crack growth resistance, but worsened the creep properties. On the contrary, increasing the grain size leads to better creep properties and better fatigue crack growth resistance. The impact of the different microstructures investigated in this study on the mechanical properties is smaller compared to microstructures with strongly aligned η/δ plates. These results enable to design the desired microstructures that meet the particular demands of the given application regarding crack propagation and creep resistance.