Effects of alloying elements on the microstructure of precipitation strengthened Co-based superalloys

The alloying effects on the microstructure, especially precipitation of harmful phases and solvus temperature of L1(2)-structured gamma' phase, etc., are essential for alloy design and development of precipitation strengthened Cobased superalloys. Based on the newly reported Co30Ni9Al5Mo10Cr2Ta0.03B alloy, alloying elements Mo, Cr, Al, Ta and Ni are systematically changed and alloys are fabricated to investigate their microstructures in two heat treatment statuses, namely homogenization and aging, respectively. Results show that Mo and Cr are gamma forming elements and have limited effect on gamma' solvus temperature, but high concentration of Mo and Cr facilitates the formation of harmful mu phase. Al, Ta, and Ni are all gamma' stabilizers and can effectively increase gamma' solvus temperature, but roles of Al, Ta and Ni are different. Increasing Al and Ta would result in other harmful phases, such as the mu phase in the 6Ta and 4Ta alloys, and mu + B2 phases in the 13Al and 11.5Al alloys. However, Ni addition significantly increases solvus temperature and area fraction of gamma' phase without occurrence of harmful phases. The current findings might provide guidelines for design and development of precipitation strengthened Cobased alloys.