Synergistic strengthening by bimodal co-precipitates: strengthen mechanisms in a new directly aged corrosion-resistance nickel-based alloy
Qubo He, Fusheng Pan, Fei Guo, Dongzhe Wang, Haiding Liu, Jinzhi Wang, Dadi Zhou, Wei Wu, Linjiang Chai, Qingqing Ding
Abstract
: This study investigates the microstructure and strengthening mechanisms of a modified 955 nickel-based alloy using direct aging after forging, contrasting it with conventional solution treatment and duplex aging. Direct aging resulted in an 11% yield strength increase (1354 MPa) over conventionally treated material. This enhanced strength is primarily due to residual stress retention and a unique bimodal precipitate distribution formed during forging and aging. Directly aged samples exhibited this bimodal distribution, comprising large, disc-shaped γ″ (∼140 nm) and nano-scale γ′/γ″ phases. γ′-γ″ duplets and triplets were identified as key strengthening precipitates in the directly aged condition. These co-precipitates, with their sandwich structure, offered superior resistance to dislocation motion compared to uniformly distributed precipitates in conventionally treated conditions. Direct aging also reduced defects in strengthening phases by recovering anti-phase boundaries and precipitate misalignment. This research elucidates strengthening mechanisms in direct aging, emphasizing the critical roles of residual stress, grain refinement, and effective bimodal co-precipitates in achieving high strength in this nickel alloy. Beyond residual stress, bimodal co-precipitates in directly aged alloys demonstrated superior strengthening behavior compared to uniform nano-precipitates. This direct aging approach offers a promising route to optimize heat treatments for high-strength nickel-based superalloys.