A superalloy is a multi-metal alloy system that exhibits a good combination of high-temperature properties, such as resistance to thermal creep deformation, oxidation, and corrosion and has a service temperature often above 0.7 times the absolute melting temperature. Alloy ATI 718Plus is a relatively new Ni-based superalloy developed to improve upon the properties of Inconel 718. It shows improvement in service temperature up to 704 oC (55 oC more than IN718) and formability similar to IN 718 and better than that of Waspaloy because of its chemical composition, microstructure and major strengthening phase, gamma prime (γ'). The microstructure plays a vital role in controlling the mechanism of particle-dislocations interaction during deformation as the mechanism changes with particle size. The basic understanding of the active deformation mechanism with a unimodal distribution with average γ' particle size is well studied in the literature, but consideration of smaller and larger precipitates together, called bimodal microstructure, is still lacking. This study seeks to understand the development of a bimodal γ' precipitate size distribution in ATI 718Plus and its stability under various thermal and tensile stress conditions. In this study, initial optimization of the solutionizing temperature was conducted for subsequent aging treatment. The as-processed sample underwent heat treatment at 1000°C for 1 hr, followed by water quenching (WQ). The aging process encompassed single-step and two-step methods with varied parameters, including time, temperature, and cooling rate. For the two-step treatment, the sample was heated to 900°C for 2 hr, then WQ to room temperature before being heated to 720°C for 10 hr and WQ again. The resultant microstructure displayed a uniform bimodal distribution of γ' precipitates, with sizes of 11 nm and 55 nm for smaller and larger precipitates, respectively. Samples were prepared for characterization using scanning electron microscopy (SEM), transmission electron microscopy (TEM), and Atom Probe Tomography (APT). The developed microstructures underwent tensile testing to failure to assess their yield strengths (YS), ultimate tensile strengths (UTS), and elongations-to-failure (εf). Some of the tensile samples, intentionally unloaded after achieving 2-4% engineering strain, were evaluated using TEM to investigate the γ' precipitate-dislocation interactions. In the case of unimodal samples, weak-pair shearing was observed to be the dominant mechanism for smaller γ' precipitates (~14 nm), while both strong-pair shearing and dislocation loops were observed for the microstructures containing larger γ' precipitates (~48 nm). The microstructure containing a bimodal distribution of γ' precipitates exhibited shearing as a dominant mechanism and also resulted in the highest strength values. The combined influence of temperature and elastic tensile stress on γ' precipitate stability was examined. Under simultaneous application of temperature and stress (creep), γ' precipitate growth accelerated in contrast to unstressed samples. The amount of growth varied with crystal orientations in the creep-deformed sample.
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