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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]

In present work, the unique envelope structure (about 1.1 μm in width, and ~3 vol%) in an as-cast multiphase Ni 3 Al-based intermetallic alloy was characterized in detail, its formation mechanism during solidification and widening behavior during long-term aging process were analyzed, and relevant effects on creep behaviors of this Ni 3 Al-based intermetallic alloy under 800 oC/220 MPa were evaluated. Results show that the envelope structure located between the γ'+γ dendrite and interdendritic β phase is γ′-Ni 3 Al phase with ordered face-centered L1 2 structure, which plays a vital role in adjoining the heteroid γ'+γ dendrite/interdendritic β phases. The γ′-envelope possesses a coherent orientation relationship of [101] γ'-envelope //[01‾1] dendritic-γ with the dendritic-γ phase (face-centered A1 structure) due to their similar face-centered lattice type, while there exists a semi-coherent transition region between the γ′-envelope (face-centered L1 2 structure) and interdendritic β phase (body-centered B2 structure) to accommodate their relative lattice mismatching. The formation of γ′-envelope during solidification process is induced by nickel enrichment in the residual liquidoid after interdendritic β phase, and its widening during long-term aging at 800 oC is mainly controlled by the diffusion of Ni, Al, Hf and Mo atoms at the interfaces between γ'+γ dendrite/γ′-envelope/interdendritic β phase under thermal activations, involving the relevant phase transformation of β (NiAl) + γ (Ni) → γ' (Ni 3 Al). The widening of γ′-envelope resulted in a decrease of interdendritic β phase and an increase of γ′ phase in volume fraction, benefited in the improved creep rupture life and reduced steady-state creep rate. However, the γ′-envelope widening also plays a positive role in crack initiation during creep deformation, since besides the GBs and the semi-coherent interfaces of the γ′-envelope/interdendritic β, cracks are also found to initiate near the Cr 23 C 6 carbides surrounded by the widened γ′-envelope. [ABSTRACT FROM AUTHOR]

Abstract The decomposition of metastable β phase and microstructural evolution of a forged Ti-22Al-25Nb orthorhombic alloys were investigated by the electron microscopes and phase identification. For the near-Ti 2 AlNb alloy, the processing temperatures were super-transus (over the transition temperature of the β phase). β phase was ordered before its decomposition and cooling rate could strongly influence the precipitation of the explored Ti-22Al-25Nb alloy. Water quenching could prevent the transformation related to diffusion and air cooling hindered the occurrence of fine lenticular O phase in the matrix. Comparison with previous researches and selected area diffraction patterns revealed that there did exist a displacive precursor that provided nucleation sites for the α 2 phase during B2 → α 2 transition. Due to the transient structure and diffusive gradient of Nb component during the transformation, a "cellular structure" was assembled and quantities of equiaxed α 2 precipitates were acquired. Simultaneously allotriomorph α 2 precipitates inclined to grow into coarse plates on the boundaries, which would have a detrimental effect on the mechanical properties of the alloy. Widmanstätten structure was formed during the (B2+α 2 +O) field and massive transformation of lenticular O phase occurred in the (B2+O) field. Highlights • Effect of cooling rates on the decomposition of B2 phase in the Ti 2 AlNb-based alloy is presented. • Precipitation and microstructural evolution during slow-cooling process are discussed in detail. • The mechanism of the formation of cellular structure was put forward and verified. [ABSTRACT FROM AUTHOR]

Based on its excellent high-temperature performances, polycrystalline Ni 3 Al-based alloys have attracted more and more attention. In this work, different thermal cycle aging treatments (TCAT) were applied to investigate microstructure evolution behavior of Ni 3 Al (γ′) phase in eutectic γ-γ′ area of JG4246A alloy. SEM equipped with energy dispersive X-ray spectrometer (EDS) and high-resolution transmission electron microscopy (HRTEM) were used for microstructure observation and mechanism analysis. The results showed that plate-like Ni 3 Al (γ′) phase and twinning Ni 3 Al (γ′) formed in eutectic γ-γ′ area due to its initial state of the alloy and thermal cyclic aging treatment conditions. Meanwhile, blocky Ni 3 Al (γ′) phase precipitated along eutectic γ-γ′ area and dual phase (γ+γ′) area ((γ-γ′)/(γ+γ′)) interfaces. Vacancies and dislocations generated during short-term TCAT were responsible for the formation of plate-like Ni 3 Al (γ′) phases in eutectic γ-γ′ area of as-cast alloy. While, for the solution-treated alloy, dislocations and internal stress generated during long-term TCAT led to the formation of twinning Ni 3 Al (γ′) in eutectic γ-γ′ area and blocky Ni 3 Al (γ′) phase along (γ-γ′)/(γ+γ′) interface. [ABSTRACT FROM AUTHOR]