Investigation of microstructure evolution and mechanical properties of multi-precipitation Ni–Co base superalloys

Abstract

The microstructure evolution and mechanical properties were investigated in a multi-precipitation Ni–Co base superalloy system with different chemical compositions (wt. %) of Ni30–Co33–C0.02 (1#) and Ni28.5-Co34.5-C0.01 (2#). The interaction of multi-precipitates and its influence on the creep behavior were studied in correlation with the evolution of deformed microstructure. The multi-precipitates mainly consist of γ′-Ni3Al phase, β-NiAl phase, carbides, and η phase. After a long term aging, both of the β and γ′ phase possess a strong thermal stability in the two superalloys, while the number density of η precipitates has increased a lot (volume fraction: 0.02%–0.21%) in the 2# Ni–Co alloys compared with that in the 1# Ni–Co alloys. The room temperature mechanical properties of both Ni–Co alloys almost remain stable such as ultimate tensile strength (1260–1280 MPa) and total elongation (22–24%) even after a long aging time of 1500 h. However, different from the stress-free aging process, the γ′ precipitate in the 2# Ni–Co alloy shows a higher rafting rate compared with that in the 1# Ni–Co alloy after the creep (903 K, 450 MPa) for 500 h. Thus, the 2# Ni–Co alloy exhibits a significantly higher creep strain rate and an advanced accelerating creep stage than the 1# Ni–Co alloy. The differences of the creep properties are mainly attributed to a change in deformation mechanism from dislocation gliding to dislocation climbing over the rafted γ′ phase during the steady-state stage. A high stacking fault energy (SFE) of the matrix results in strong stress concentration and dislocation network formation in the γ/γ′ interfaces, contributing to an accelerated creep strain rate of the 2# Ni–Co alloy. In contrast, sustainable precipitation hardening from spherical MC carbides can delay the steady-state stage to a very long life until 1500 h in the 1# Ni–Co alloy.