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Microstructure evolutions of Ni-Ti-Nb-Al alloys with different Al addition subjected to casting and solution-treatment have been investigated using scanning electron microscope (SEM) and X-ray diffraction (XRD). The alloys were prepared by vacuum arc melting, and then solution-treated at 1273 K for 20 h. The alloys with 3 at.% Al addition and Al-free show a microstructure consisting by B2-NiTi matrix phase and soft β-Nb precipitate. With increasing Al addition, Al 3 Ti phase firstly forms, and Ni 3 Al only can be found in the 7 at.% Al alloy. The addition of Al promotes the diffusion of Nb during casting and solution-treatment, resulting in the refinement of β-Nb precipitate. For the as-cast alloys, the macro-hardness values increase with the increase in Al content and slightly increase after solution-treatment. [ABSTRACT FROM AUTHOR]

The discovery of ordered γ' phase in Co-Al-W based superalloys opens up a pathway for the development of novel Cobalt-based superalloys. γ' phase from binary compounds was calculated by first-principles methods, and the third alloying elements were also considered to stabilize and strengthen γ' phase, the ordered γ' phase with higher stability and more excellent mechanical properties was confirmed herein in novel Cobalt-based superalloys. The formation energies and lattice parameters of Co-X (X = Al, Mo, Nb, Ta, Ti, V, W) and Co-Ti-Z (Z = Al, V, Cr, Mo, Ta, W) were calculated, the results show that X = Ti and Z = Mo are better choices. The doping of Mo can stabilize the ordered L1 2 -γ' phase and improve the mechanical properties of the γ' phase when Mo occupies Ti sites. The occupying preference of alloying elements in Co 3 Ti lattice is analyzed, Al and Cr prefer to occupy Co sites in L1 2 -Co 3 Ti lattice, while V, Mo, Ta and W prefer to occupy Ti sites. The as-cast microstructure of the five superalloys is typical dendrite with γ/γ' two-phase region in the inter-dendrite. Besides, segregation phenomenon can be observed in the as-cast microstructure of the superalloys, and Ti prefers to enriched in the inter-dendrite, while other elements tend to be distributed in the dendritic arms. • The formation energy of different structure in Co-X binary system was calculated, and Co 3 Ti is the most strengthening γ' phase. • Al and Cr prefer to occupy the Co sites, while V, Mo, Ta and W tend to occupy the Ti sites in L1 2 -Co 3 Ti lattice. • Except for Al, other alloying elements in current work tend to occupy the Ti sites in D0 19 -Co 3 Ti lattice. • Co-Ti-Mo based superalloys have typical dendrite structure in as-cast state with occurrence of elements segregation. [ABSTRACT FROM AUTHOR]