Strengthening mechanisms in polycrystalline multimodal nickel-base superalloys

Polycrystalline γ-γ′ superalloys with varying grain sizes and unimodal, bimodal, or trimodal distributions of precipitates have been studied. To assess the contributions of specific features of the microstructure to the overall strength of the material, a model that considers solid-solution strengthening, Hall-Petch effects, precipitate shearing in the strong and weak pair-coupled modes, and dislocation bowing between precipitates has been developed and assessed. Cross-slip-induced hardening of the Ni3Al phase and precipitate size distributions in multimodal microstructures are also considered. New experimental observations on the contribution of precipitate shearing to the peak in flow stress at elevated temperatures are presented. Various alloys having comparable yield strengths were investigated and were found to derive their strength from different combinations of microconstituents (mechanisms). In all variants of the microstructure, there is a strong effect of antiphase boundary (APB) energy on strength. Materials subjected to heat treatments below the γ′ solvus temperature benefit from a strong Hall-Petch contribution, while supersolvus heat-treated materials gain the majority of their strength from their resistance to precipitate shearing. © The Minerals, Metals & Materials Society and ASM International 2009