Precipitation Behavior Of δ Phase In Alloy K325 And Its Effect On Mechanical Properties

Advanced ultra-supercritical power plant(A-USC)is the main developing trend of coal-fired power plant.Its operating temperature and pressure are 700 ℃ and 35 MPa,respectively.Higher demand is necessary for the materials used in A-USC power plants.Nickel-based superalloys,which possess high strength,creep resistance,and weldability,are the major candidate materials.To address those issues related to large castings in A-USC power plants,cast alloy K325 was developed by Institute of Metal Research(IMR),Chinese Academy of Science.This paper investigates the precipitation of δ phase in K325 after aging and its effect on mechanical properties.This will impulse the application of alloy K325 A-USC power plants.The first part is about the precipitation behavior of δ phase by employing K325 aging at 700 ℃ and 750 ℃.It is found that the initial precipitation positions of the δ phase are along the grain boundaries and NbC carbides,and then extend to the interdendritic region.The precipitation of δ phase in K325 are concentrated in the interdendritic region.This can be attributed to the segregation of Nb.The precipitation of δ phase promotes the decomposition of NbC carbide.The transformation of M23C6 from NbC is found after aging at 750 ℃ for 3,000 h and 700 ℃ for 7,500 h.Formation process of δ phase is a solid phase transformation process from γ" phase.The variants of 8 phase with a particular orientation relationship is observed.The second part is on the factors that affect the precipitation of δ phase in alloy K325:aging temperature,aging time and compositions of the alloys.The volume fraction of δ phase precipitated in K325 alloy after aging at 750 ℃ was much higher than that of aging at 700 ℃,while little δ phase was observed after aging at 650 ℃ for up to 10,000 h.The precipitation of 8 phase is sensitive to aging temperature.The content of δ phase increases with the increasing aging temperature.As for aging at 750 ℃,it reached the steady state finally.As for compositions of the alloys,the precipitation of δ phase is strongly affected by Nb content while limitedly affected by Mo and Cr.The effect of Nb、Cr、Mo on the precipitation of δ phase are very different.This is closely related to their influence on the segregation degree of Nb.The third part is about the relationship between δ phase and mechanical properties.The results showed that both yield strength and ultimate tensile strength increased nearly linearly with increasing δ phase volume fraction,while the elongation was relatively limitedly affected by δ precipitation when its volume fraction was above 10%.It is found that the reinforcement of 8 phase is a little weaker than that of y" phase,but it’s basically the same.8 phases inhibit the dislocation motion and result in the strengthening effect.The effect of δ phase on stress-rupture properties was also studied.Comparied with that of the solid solution alloy,the rupture time of aged alloys with 8 phase increase.When the fraction of 8 phase increases from 18%to 35%,the stress-rupture life is approximately the same at high stress(400 MPa)and decreased obviously at low stress(375 MPa).