Microstructural Evolution Of Nickel-based Alloy TG700A In Service Condition

To achieve high thermal efficiency of coal-fired power plants and reduce carbon dioxides?CO₂?emissions,it is imperative to develop 700°C advanced ultra-supercritical?A-USC?power plants.The material of high temperature components of A-USC steam boilers is a key to operating safely and improving efficiency.Ni-based superalloys become a major class of candidate materials for the A-USC boilers,due to their high creep strength and corrosion resistance.Microstructural stability is important to keep their excellent mechanical properties and security during long-term service at elevated temperature.It is of great significance to study microstructural stability of Ni-based superalloys in long-term service condition.Alloy TG700A,as a new 740H-type nickel-based superalloy with a base composition of Ni-25Cr-20Co,is a promising candidate materials designed for applications in A-USC power plants.In this paper,microstructural evolution of alloy TG700A during long-term aging and creep at 770?were investigated,by means of stress rupture test and microstructure observation.Then,effects of stress on microstructural evolution were explored.Also,effects of aging treatment on microstructural evolution and mechanical properties were studied.In addition,creep rupture mechanism was discussed.The results showed the coarsening of??precipitates in the grain interior had occurred during thermal exposure in the presence and absence of stress.The coarsening behavior follows LSW growth kinetics model diffusion-controlled,and the coarsening rate of??precipitates under standard heat treatment condition is faster than as-received during thermal exposure.The microstructure around grain boundary was altered remarkably under stress.The precipitate-free zones?PFZs?commonly existed near the grain boundary after long-term creep.Coarsened and elongated particles in the PFZs were identified as??-Ni3?Ti,Al?precipitates by chemical composition analysis and diffraction pattern.And,the primary mechanism of PFZ formation was discussed.Furthermore,PFZs may influence distribution and size of the adjacent M₂₃C₆ carbides at the grain boundary.Compared to stress free,relative frequency of low angle grain boundaries increased distinctly under stress.The hardness of the alloy increased first and then continuously decreased with increasing aging time.And the room temperature impact toughness decreased first and then continuously maintained constant with increasing aging time,due to precipitation and coarsening of M₂₃C₆carbides at the grain boundary.Alloy TG700A exhibited good creep resistance,as a result of precipitation strengthening,but its activation energy increased first and then decreased with increasing aging time.Furthermore,intergranular fracture is the dominant failure mode during creep.High-density dislocations accumulated at the grain boundary through planar slip.And large local plastic deformation mostly occurred due to carbides inhibiting movement of dislocations and low strength of PFZs,which resulted in localized stress concentration.So,creep cavities initiated at the interface between grain boundary carbides and the PFZs.