Effect Of Ta And Ti On Microstructural Stability And Creep Behavior Of Novel γ’strengthened Co-base Single Crystal Superalloys

The recent report of y’-strengthened Co-Al-W-base superalloys provides a pathway for the development of new class of Co-base superalloys, which has been a hot research topic in the field of superalloys. The expanded γ-γ’ two-phase region and higher γ’ solvus temperature with the corresponding improved high temperature microstructural stability are the most challenging requirements to develop this class of novel superalloys. The understanding of the deformation mechanism during creep is also required to further promote their strength, especially at higher temperature. As Co-Al-W-base superalloys are in their early stages, the related research and thermodynamic databases are very limited, leaving a great challenge to solve these problems.On the basis of previous work, the influence of alloying addition Ta, Ti, Nb, Mo and V on the γ’ solvus temperature, γ-γ’ two-phase microstructure and precipitation of secondary phase have been investigated in this study. According to the alloying effects on the γ’ solvus temperature and microstructural stability, the individual and synergetic effects of Ta and Ti on the elemental partitioning behavior, lattice misfit, long-term microstructural evolution and high temperature creep behavior have been studied to clarify the deformation mechanism of Co-base single crystal superalloys.The investigation of alloying effects on the microstructure suggests that the y’ solvus temperature was raised by individual Ta, Ti and Nb additions to different extents, while Ta and Ti showed more significant influence. The y’volume fractions in quaternary alloys were at very high level in the range of74%to83%, similar to that in the Co-9Al-10W base alloy. However, Ta, Nb and Mo obviously promoted the precipitation of μ and χ phase, which increased in volume fraction with the increasing content of alloying addition.Based on the understanding of alloying effects on γ’ solvus temperature and microstructural stability, Co-7Al-8W-base alloys containing Ta and Ti have been designed and single crystal alloys have been successfully produced to investigate the microstructural stability and creep behavior. The experimental results indicate that the segregation behavior during directional solidification was limited and Co-7Al-8W-base alloys were not prone to form solidification defects, which suggested that preparation of large single crystals may be possible for this type of alloys. The y’solvus temperature and volume fraction were raised to a higher level with4at.%Ti compared to1at.%Ta, whereas further increase was observed in the quinary alloy containing both alloying additions. With combination of Ta and Ti additions, alloying elements with large atom radius, e.g. W, Ta and Ti, preferentially partitioned to the γ’ phase, and resulted in relatively high lattice constant in γ’ phase and positive γ-γ’ lattice misfit. Moreover, improved microstructural stability with the stable y/y’two-phase microstructure and more than60%γ’ volume fraction existed in the quinary alloy after prolonged aging treatment at1050℃for1000h, obviously higher than that in previously reported Co-base alloys.The creep behavior of quinary alloy at about1000℃indicates that it consisted of transient creep stage, steady-state creep stage and accelerating creep stage. The creep resistance of the quinary alloy exceeded the previously reported Co-Al-W-base alloys at1000℃/137MPa and fell between1st and2nd generation Ni-base single-crystal superalloys at982℃/248MPa. With the superposition of coherency stress and external tensile stress, directional coarsening of γ’ precipitates occurred and γ’ rafts parallel to the applied stress axis were formed during creep. Additional to the γ’ rafts, the general defects were γ-γ’ interfacial dislocation networks and stacking faults and antiphase boundaries in γ’ precipitates during stead-state creep stage. It is thus indicated that the γ’ rafts with continuous γ-γ’ interface, shearing of γ’ precipitates with stacking faults and antiphase boundaries, and dislocation intersection in the γ’ precipitates impeded motion, leading to the improved creep resistance.