Surendra Kumar Makineni

Thermophysical and Mechanical Properties of Advanced Single Crystalline Co-base Superalloys

A set of advanced single crystalline γ′ strengthened Co-base superalloys with at least nine alloying elements (Co, Ni, Al, W, Ti, Ta, Cr, Si, Hf, Re) has been developed and investigated. The objective was to generate multinary Co-base superalloys with significantly improved properties compared to the original Co-Al-W-based alloys. All alloys show the typical γ/γ′ two-phase microstructure. A γ′xa0solvus temperature up to 1174xa0°C and γ′ volume fractions between 40 and 60xa0pct at 1050xa0°C could be achieved, which is significantly higher compared to most other Co-Al-W-based superalloys. However, higher contents of Ti, Ta, and the addition of Re decrease the long-term stability. Atom probe tomography revealed that Re does not partition to the γ phase as strongly as in Ni-base superalloys. Compression creep properties were investigated at 1050xa0°C and 125xa0MPa in 〈001〉 direction. The creep resistance is close to that of first generation Ni-base superalloys. The creep mechanisms of the Re-containing alloy was further investigated and it was found that the deformation is located preferentially in the γ channels although some precipitates are sheared during early stages of creep. The addition of Re did not improve the mechanical properties and is therefore not considered as a crucial element in the design of future Co-base superalloys for high temperature applications. Thermodynamic calculations describe well how the alloying elements influence the transformation temperatures although there is still an offset in the actual values. Furthermore, a full set of elastic constants of one of the multinary alloys is presented, showing increased elastic stiffness leading to a higher Young’s modulus for the investigated alloy, compared to conventional Ni-base superalloys. The oxidation resistance is significantly improved compared to the ternary Co-Al-W compound. A complete thermal barrier coating system was applied successfully.

On the segregation of Re at dislocations in the γ' phase of Ni-based single crystal superalloys

Abstract We report evidence of Re and Mo segregation (up to 2.6 at.% and 1 at.%) along with Cr and Co to the dislocations inside of γ precipitates in a second generation Ni-based single crystal superalloy, after creep deformation at 750u202f°C under an applied stress of 800u202fMPa. The observed segregation effects can be rationalized through bridging the solute partitioning behavior across the γ/γ interface and the pipe diffusion mechanism along the core of the dislocation line. This understanding can provide new insights enabling improved alloy design.