Effects of Al content on microstructures and high-temperature tensile properties of two newly designed CoNi-base superalloys

Abstract

Abstract CoNi-base superalloys have recently attracted intensive research interests due to their wider γ/γ′ domains and higher γ′ solvus temperatures compared to Ni-free Co-base superalloys. In this work, two CoNi-base superalloys with Al content of 9 at.% or 13 at.% were designed with the aid of CALPHAD approach. The 9Al alloy exhibits a γ/γ′ dual-phase microstructure in its as-solidified and aged states, but the β-(Co,Ni)Al phase precipitates on the grain boundaries in the 13Al alloy. The measured Tγ′-solvus of the as-solidified 9Al and 13Al is 1238 °C and 1220 °C, respectively, and the fraction of γ′ in the aged 9Al and 13Al is 64% and 80%, respectively. Tension tests at 900 °C reveal that the Al content plays a vital role on high-temperature tensile properties of the current CoNi-base superalloys. The lower yield strength but a larger tensile strain of the aged 13Al alloy, as compared with the aged 9Al alloy, are attributed to irregular shape of the softer γ′ precipitates and the easy formation of the local tiny cracks around the β particles. This implies that the existence of β particles can effectively relieve the grain-boundary brittleness of superalloys at high temperature.