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In present work, a multiphase Ni 3 Al-based intermetallic alloy with complex components of γ'+γ dendrite comprising extremely high volume fraction (81 vol %) of γ′ phase and interdendritic β phase is investigated, the microstructure evolution and phase transformation behavior tailored by different solution cooling rates are studied by applying different cooling methods of water cooling (WC, 138 °C/s), air cooling (AC, 72 °C/s), and furnace cooling (FC, 0.05 °C/s) after solution treatment at 1200 °C for 10 h. Results show that the cubic degree, size and volume fraction of cuboidal primary γ′ as well as size of spherical secondary γ′ particles in the γ'+γ dendrite are improved by decreasing cooling rates, and the average sizes of the primary and secondary γ′ particles are found to meet logarithmic relationships with cooling rates. There are three kinds of precipitates in the interdendritic β phase resulted from different cooling methods, i.e., the rod-like Cr 3 C 2 carbides, the semi-spherical α-Cr particles, and the long acicular γ′ precipitates. The lower cooling rate decreases the number of rod-like Cr 3 C 2 carbides, increases the number and size of the semi-spherical α-Cr particles, and promotes the formation of long acicular γ′ precipitates in the interdendritic β-matrix. The semi-spherical α-Cr particles play the role of pinning dislocations in the β-matrix and have the orientation relationship of [‾110] α-Cr //[‾110] β-matrix. In addition, the precipitation of semi-spherical α-Cr particles in β-matrix is observed to accomplish in two sequential stages and grow in an onion-like ordered way. Stacking faults existed in partial of α-Cr particles due to the nanotwins in β-matrix. Microtwins are observed in the interdendritic β-matrix after water cooling, which have a bimodal configuration of nanotwins at the microtwinning boundaries and interior stacking faults. Besides, γ′-envelope formed between the γ'+γ dendrite and interdendritic β phase dependent on the solution cooling processes. Image 1 • Re-precipitation sizes of primary and secondary γ′ particles have the logarithmic relationship with solution cooling rates. • Microtwins in the β-matrix have a bimodal configuration of nanotwins at their boundary and interior stacking faults. • Precipitation pattern and growth mode of semi-spherical α-Cr particles in β-matrix phase after air cooling. • Precipitation behaviors influenced by solution cooling rates of precipitates in the interdendritic β. [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]

A polycrystalline Ni 3 Al-based superalloy was joined through transient liquid phase (TLP) bonding technique. A post bond heat treatment (PBHT) was successively conducted. Creep tests of the TLP and TLP + PBHT specimens were then carried out under 800 °C and 160 MPa. The creep deformation and fracture behaviors were further investigated through interrupted creep tests. During the PBHT procedure, microstructure evolution of the joint occurred with creep life prolonged from 64 h to 368 h. The superdislocations in TLP and TLP + PBHT specimens were determined to study the γ′ phase shearing modes. Both individual superdislocations and APB-coupled partial dislocation pairs are observed. The variation of dominate superdislocation configurations among different joint zones and the evolution of the superdislocations during the creep process are discovered. Besides, the formation of different γ′ rafting microstructures and the initiation of intergranular microcracks in the polycrystalline joint are both studied. [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]

A CoNi-based alloy (various cooling rates after solution heat treatment) was utilized to investigate the microstructure-dependent coarsening behavior of γ′ precipitates. The fast cooling rate leads to the formation of initial fine γ′ precipitates with high number density. The different initial microstructures of CoNi-based alloy lead to various coarsening behaviors of γ′ precipitates during subsequent aging treatment at 900 °C. The coarsening behavior of large γ′ precipitates with low density (formed in slow cooling rate, 0.1 °C/s) is in accordance with the Lifshitz-Slyozov-encounter modified (LSEM) model. Whereas for the fine γ′ precipitates with high density (formed in fast cooling rate, 60 °C/s or 240 °C/s), its coarsening behavior is consistent with the TIDC model at early stage and LSEM model at later stage, respectively. • The different initial microstructures of Cobalt-based alloy lead to various coarsening behaviors of γ′ precipitates. • Fast cooling rate leads to the formation of fine γ′ precipitates with high density. • Coarsening of fine γ′ precipitates is consistent with the TIDC model in the early stage and LSEM model in the later stage. • Coarsening of large γ′ precipitates is in accordance with the LSEM model. [ABSTRACT FROM AUTHOR]