Impact of microstructural evolutions during thermal aging of Alloy 625 on its monotonic mechanical properties

Alloy 625 is widely used for petrochemical, marine and aerospace applications owing to its outstanding corrosion and mechanical properties at high temperatures. However, this alloy is prone to complex microstructure evolutions above 500  ∘ C that may impact its mechanical properties. In this study, the impact of its microstructure evolutions occurring upon thermal aging on the monotonic mechanical properties has been studied. Thermal exposures of up to ∼2000 hours in the 550  ∘ C – 900  ∘ C temperature range have been investigated. TTT diagrams of the δ and γ′′ phases were established based on high resolution scanning electron microscopy observations. The evolutions of secondary carbides distributions were also followed. It has been observed a steep increase of the room temperature micro-hardness after overagings performed at 650  ∘ C and 700  ∘ C due to the precipitation of the γ′′ phase. Moreover, it is clearly demonstrated a strengthening effect of the δ phase observed after long term thermal exposures at temperatures in excess of 700  ∘ C. Finally, the impact of a thermal aging in the γ′′ precipitation domain on the tensile properties was evaluated from room temperature up to 800  ∘ C. It is shown that the loss of high temperature ductility is not correlated to the precipitation of grain boundary secondary carbides.