Improved High-Temperature Microstructural Stability and Creep Property of Novel Co-Base Single-Crystal Alloys Containing Ta and Ti

The influence of Ta and Ti additions on microstructural stability and creep behavior in novel Co-Al-W base single-crystal alloys has been investigated. Compared to the ternary alloy, the gamma' solvus temperature and gamma' volume fraction were raised by individual additions of Ta and Ti, and increased further in the quinary alloy containing both alloying additions. In contrast to ternary and quaternary alloys, an improved microstructural stability with the stable gamma-gamma' two-phase microstructure and more than 60% gamma' volume fraction existed in the quinary alloy after prolonged aging treatment at 1050 degrees C for 1000 h. The creep behavior at 900 degrees C revealed lower creep rates and longer rupture lives in the quaternary alloys compared to the ternary alloy, whereas the quinary alloy exhibited even better creep resistance. When the creep temperature was elevated to about 1000 degrees C, the creep resistance of the quinary alloy exceeded the previously reported Co-Al-W-base alloys and first-generation Ni-base single-crystal superalloys. The improved creep resistance at approximately 1000 degrees C was considered to be associated with high gamma' volume fraction, gamma' directional coarsening, and dislocation substructure, which included gamma-gamma' interfacial dislocation networks and the sheared gamma' precipitates containing stacking faults and anti-phase boundaries.