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

Abstract

The alloying effects on the microstructure, especially precipitation of harmful phases and solvus temperature of L12-structured γʹ phase, etc., are essential for alloy design and development of precipitation strengthened Co-based 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 γ forming elements and have limited effect on γʹ solvus temperature, but high concentration of Mo and Cr facilitates the formation of harmful μ phase. Al, Ta, and Ni are all γʹ stabilizers and can effectively increase γʹ solvus temperature, but roles of Al, Ta and Ni are different. Increasing Al and Ta would result in other harmful phases, such as the μ phase in the 6Ta and 4Ta alloys, and μ + B2 phases in the 13Al and 11.5Al alloys. However, Ni addition significantly increases solvus temperature and area fraction of γʹ phase without occurrence of harmful phases. The current findings might provide guidelines for design and development of precipitation strengthened Co-based alloys.