Effects of long-term thermal exposure on the microstructure and properties of a cast Ni-base superalloy

Microstructural stability and microstructure-property relationship during long-term thermal exposure in K452 alloy (a new Ni-base cast superalloy with ∼21 pct Cr, 11 pct Co, 3.5 pct W, 2.5 pct Al, 3.5 pct Ti, and others) are investigated. It is found that exposure temperature and time have significant effects on the microstructure and properties of the alloy. During exposure, the microstructure is degraded by γ′ coarsening, MC carbide (M mainly represents Ti, W, and Nb) degeneration, precipitation and evolution of grain interior (GI) M₂₃C₆ carbide, evolution of grain boundary (GB) microstructure, and precipitation of η phase. Among them, the γ′ coarsening is the leading reason for the decrease of strength of the alloy. The GI M₂₃C₆ and the η phase have negligible influence on the properties due to their relatively small populations. Blocky, closely spaced GB M₂₃ C₆ particles engulfed in γ′ increase the stress-rupture life, whereas the formation of a continuous GB M₂₃C₆ chain has an opposite effect. A life peak occurs when the M₂₃C₆/γ′ structure at the GBs is in an optimal form. The degenerated MC is the preferred initiation site of microcracks. Its presence at the GBs promotes the onset of intergranular fracture, and leads to the decrease in mechanical properties.