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Recent research on Co-based and CoNi-based alloys revealed that the Co–Al–W-system provides interesting properties, however, the high content of W addition triggers high mass density of alloys which limits its industrial application. Therefore, new high temperature superalloys based on the Co–Ni–Al–Ti-system with high content Cr and strengthened by γ′-(Ni,Co)3(Cr,Al,Ti) precipitates, have been developed, and the effect of different refractory element additions was investigated. STEM-EDS and HEXRD were employed to determine the elemental partitioning behavior and the lattice misfit between the γ and γ′ phases. Ta and Nb strongly concentrate within the γ′ phase, whereas Mo weakly partitions to the γ phase. W distributes equally between the γ and γ′ phases. These new superalloys have an unexpectedly high positive misfit compared with some conventional Ni-based superalloys and Co-based superalloys. Nb and Ta additions increase the lattice misfit further, while Mo and W decrease the lattice misfit. The effect of refractory elements alloying on the yield stress at room temperature was evaluated by analyzing the contributions of different strengthening mechanisms. Alloying with Nb or Ta significantly improves precipitation strengthening by increasing the antiphase boundary energy. Mo has the highest solid solution strengthening effect in the γ phase, followed by W. Compared with some conventional Ni-based superalloys, the investigated new CoNi-based superalloys exhibited better mechanical properties at high temperature, which indicates that these compositionally complex alloys are possible candidates for high temperature applications.
Usually, Co-based superalloys contain a high fraction of W and/or Mo. The refractory elements stabilize the precipitate phase, but cause a high density. In this work, new L12-phase hardened, low-density CoNiCr-based superalloys were developed with the assistance of CALPHAD calculations. Several alloys were studied experimentally and their microstructures, elemental distributions, lattice parameters and the lattice misfit between the γ and γ′ phases were evaluated using scanning electron microscopy, transmission electron microscopy, atom probe tomography and high energy X-ray diffraction. The alloys exhibit a very high misfit, good phase stability and excellent mechanical strength. It was found by hardness tests that a two-step heat treatment improves the strength of the new alloys even further. In comparison with conventional Co-based superalloys, Co–Al–W based superalloys and Co–Ti based superalloys, as well as Ni-based superalloys, i.e. Udimet 720Li, they have a low mass density and high yield stress. They also overcome the problem of a generally low lattice misfit of previous L12-phase hardened CoNiCr-based superalloys with high Ni and Cr contents. As a result, they show cuboidal-shaped precipitates similar to Co–Al–W and Co–Al–Mo based superalloys.
The alloying elements Nb, Ta and Zr improve the creep properties of fully lamellar γ/α2 titanium aluminides significantly. Since high temperature deformation mainly occurs in the γ-phase of γ/α2 titanium aluminides, the diffusivity and the solid solution hardening effect of these three elements in the γ-phase is studied by analyzing the concentration gradients of the alloying elements and the resulting hardness across the interdiffusion zone of diffusion couples by energy dispersive X-ray diffraction and nanoindentation. The results reveal that Zr has the highest interdiffusion coefficient but also the largest solid solution hardening coefficient. The mechanical properties of single γ-phase Ti-54Al-5X alloys were investigated by strain rate jump tests. The addition of 5 at.% Nb or Ta lead to an increased strength compared to a binary γ-Ti-54Al alloy. The Zr-containing γ-TiAl alloy reveals the highest strength at 750°C and 900°C, which is discussed to be due to the strong solid solution hardening effect of Zr. However, in comparison to the other alloys, Ti-54Al-5Zr shows quite brittle behavior up to 900°C. The lower diffusivity of Ta compared to Nb leads to a higher strength of the Ta-modified alloy at 900◦C.