Novel CoNiCr-based superalloys. Microstructure, mechanical properties and oxidation resistance

Recent research on high and medium entropy alloys revealed that the CoNiCr-system provides interesting properties, however, the high temperature strength of such single-phase alloys is not sufficient. Therefore, new high temperature superalloys based on the CoNiCr-system and strengthened by γ${}^{\prime }$-(Ni,Co)${}_3$(Cr,Al,Ti) precipitates have been developed within this work and the effect of different refractory element additions (Nb, Mo, Ta, W) on microstructure, mechanical properties and oxidation behavior was investigated. The newly designed alloy compositions were determined based on literature research and guided by CALPHAD calculations. The elemental partitioning behavior and the lattice misfit between the γ and γ${}^{\prime }$ phases were determined by STEM-EDS and HEXRD. Ta and Nb strongly concentrate within the γ${}^{\prime }$ phase, whereas Mo weakly partitions to the γ phase. W distributes equally between the γ and γ${}^{\prime }$ 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 different temperatures was evaluated. Alloying with Nb or Ta significantly improves the high temperature mechanical properties. By modelling it was found that additions of Ta or Nb strengthen the γ${}^{\prime }$ phase more than Mo and W do. However, Mo has the highest solid solution strengthening effect in the γ phase, followed by W. Creep experiments were done under compression at 750 °C and 850 °C, respectively. Additions of W and Ta can efficiently improve the creep resistance of the alloys in the intermediate temperature range. The high temperature oxidation in novel CoNiCr-based superalloys was investigated. During high temperature oxidation, static recrystallization was triggered at the specimens' surface. Nb, W, Ta and Mo alloying cannot avoid the formation of recrystallization at the surface completely. However, Ta addition decreases the depth of recrystallization at the surface. Compared with some conventional Ni-based superalloys, the investigated new CoNiCr-based superalloys exhibited better mechanical properties at high temperature, which indicates that these alloys with their complex compositions are possible candidates for high temperature applications. However, there are some critical problems, such as discontinuous reactions at grain boundaries and Portevin-le-Chatelier phenomenon during compressive deformation, that still need to be solved in the future.