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The evolution of η phase and its effect on the tensile properties at 700 °C in 718Plus superalloy after three solution (at 940 °C for 1 h) and single aging (at 870 °C for 25 h, 50 h and 100 h) heat treatments, were investigated. The results showed that there were several various morphologies of η phase in 718Plus samples. As aging time increased, the volume fraction of needle-like/plate-like increased continuously from 5.32%, 7.78% to 29.52%, while the volume of granular/short rod-like η phase firstly increased from 0.46% to 0.68% then decreased to 0.56%. The high temperature tensile properties were affected by η phases with various morphologies. With aging time increasing, the ultimate tensile strength was obviously improved from 797.11 MPa to 860.57 MPa, which could be attributed to a certain number of plate-like/needle-like η phases at the grain boundaries which could hinder the dislocations motion near the grain boundaries. And the elongation weakly increased firstly from 16.84% to 18.76% and then decreased to 18%, which was closely related to granular/short-rod η phases. Enhancing tensile strength and acceptable ductility could be obtained by morphology control of η phase applying SA heat treatments in the study: promoting the simultaneous precipitation of an appropriate amount of plate-like/needle-like η phases and a small amount of granular/short rod-like η phases. • The morphologies of η phases found in studied 718Plus superalloy samples are plate-like/needle-like, granular/short rod-like. • With aging time extending, the volume fraction of needle-like/plate-like increased, resulting in improving tensile strength. • By applying SA aging heat treatments, an enhancing tensile strength could be obtained by morphology control of η phase [ABSTRACT FROM AUTHOR]

Allvac 718Plus alloy is a newly-developed Ni-based superalloy with excellent thermal stability and mechanical performance at elevated temperatures up to 704 °C. The precipitation behavior of the rapidly-solidified Allvac 718Plus superalloy during aging is systematically investigated in this work. The microstructure evolutions of the strengthening γ' phase and the γ'/γ″ co-precipitates during aging are examined. During aging, the "duplet" γ'/γ″ and "triplet" γ″/γ'/γ″ occur and then disappear due to the dissolution of γ″ phase, which is promoted by the δ precipitation. On the contrary, the "triplet" γ'/γ″/γ' forms and remains after aging, resulting from the low diffusion rate of solute atoms through the spheroidal γ' precipitates. The lamellar δ-Ni 3 Nb phase occurs at grain boundaries. Its size and content increase with the prolongation of aging time. After long-term thermal exposure, the hexagonal η phase forms at the δ-γ interface due to the variations of local chemical composition, and then the tetragonal Cr-rich σ phase nucleates adjacent to the δ and η particles because of the segregation of Cr atoms rejected from the δ and η particles. • The rapidly-solidified γ-matrix of 718Plus is obtained with a supersaturated solid solution and a high density of defects. • The dissolution of ellipsoidal γ″ results in the disappearance of the "duplet" γ'/γ″ and "triplet" γ″/γ'/γ″. • The "triplet" γ'/γ″/γ' is remained due to the low diffusion rate of solute atoms through the spheroidal γ' precipitates. • The lamellar δ forms first and the η and σ phases successively nucleate at the δ-γ interface after long-time thermal exposure. [ABSTRACT FROM AUTHOR]

Precipitation and growth behavior of mushroom-like Ni 3 Al phase along the dual phase (γ + γ′) and eutectic γ − γ′ area interfaces of as-cast JG4246A Ni 3 Al-based alloy were investigated by using short time cyclic heat treatment. Results showed that vacancies, small angle grain boundaries and dislocations formed along the dual phase (γ + γ′) and eutectic γ − γ′ area interfaces during the cyclic heat treatment, which promoted the precipitation of Ni 3 Al phase along this location. In addition, concentration gradient of Al elements between the eutectic γ − γ′ area and dual phase (γ + γ′) areas and the more rapid pipe diffusion provided by dislocations contributed to growth of directional mushroom-like Ni 3 Al phase. [ABSTRACT FROM AUTHOR]

The correlation of microstructure-properties of the Ni-based superalloy with coprecipitation of γ'/γ'' phases has been studied in details. To obtain the different γ'/γ'' microstructures, various heat treatments were attempted. The experimental results show that high amounts of the compact γ'/γ'' phases contribute to the tensile strength of Ni-based superalloy, which decreases with the coarsening of γ'/γ'' structures. The γ'/γ'' phases can restrain the motion of the dislocations. Further increasing the size of the γ' and γ'' phases, the deformation mechanism transforms from dislocation pairs shearing to Orowan bypassing, and then to dislocation shearing. Stacking faults (SFs) also appears in γ'/γ'' phases. Additionally, the moderate η/δ phases effectively pin the grain boundary, which enhances the alloy's ductility. [ABSTRACT FROM AUTHOR]

The precipitation of η phase during the solution treatments of Allvac 718Plus was investigated in this study. The results showed that there was free-γ′ zone appearing around η phase which was consistent with the hexagonal η-Ni 3 (Al/Ti) 0.5 Nb 0.5 after heat treatments. The granular η phase was found to be the transition of the plate-like η phase and have no pinning effect against the grain boundary, while the plate-like η phase has a pinning effect on the grain boundary and leads to the formation of serrated grain boundaries. With the solution temperature increasing, the size and fraction of η phase were observed to increase and then decrease with the morphology changing from granular to plate-like and then to granular. With the solution time prolonging, the fraction and size of η phase were increased, and the η phase formed in a cellular mode and then grow in a Widmanstätten morphology into the grain with its morphology changing from the dominant granular/plate-like to Widmanstättan structure. It was found that the increase in room temperature microhardness with the solution time changing from 6 h to 24 h was caused by the formation of Widmanstätten η phase. • The granular η phase is the transition of the plate-like η phase,having no pinning effect against the grain boundary. • The content and morphology of η phase changes with the solution temperature increasing. • The η phase forms in a cellular mode and then grow in a Widmanstätten morphology with the solution time prolonging. • The increase in microhardness with the solution time from 6 h to 24 h is closely related to the Widmanstätten η phase. [ABSTRACT FROM AUTHOR]