Segregation sequence of phosphorus and its effect on impact toughness of GH984G alloy during thermal exposure

The segregation sequence of phosphorus and the influence on impact toughness of GH984G alloy, for the application of advanced ultra-super critical (A-USC) power plant, were studied during long-term thermal exposure at 700 °C. The results show that the segregation of phosphorus is associated with the phosphorus content and exposure time. Phosphorus atoms become rich around the MC carbides firstly, then segregates to the grain boundaries and twin boundaries in the form of solute P atoms with increasing phosphorus content and prolonging thermal exposure. Phosphorus can also exist around the MC carbides and at grain boundaries in the form of needle-like phosphide when the alloys with high P content are subjected to long-term thermal exposure. The impact toughness degradation of phosphorus-containing alloys is related to the phosphorus redistribution during thermal exposure. The segregation degree of phosphorus at grain boundaries rises with the increase of P content during thermal exposure. Excess phosphorus segregation at grain boundaries decreases the binding force of grain boundaries and results in the grain boundary embrittlement. Also, the needle-like phosphides around the MC carbides and at grain boundaries in high-phosphorus alloy induce the cracks initiation during impact. Hence, the impact toughness decreases with increasing phosphorus content after thermal exposure at 700 °C.