The Effects Of Phosphorus Microalloying On Creep Behavior Of GH984G Alloy At Medium Temperature And Low Stress

GH984G is considered as a candidate material for 700℃ advanced ultrasupercritical(A-USC)power plant because of its ultra-long-time thermal stability,good creep strength,corrosion resistance and low cost.Previous investigations found that phosphorus(P)can significantly affect superalloy creep behavior,but mostly focused on medium temperature,high stress and short-life creep deformation.A-USC power plant service is characterized by medium temperature and low stress,therefore,the mechanism of P action under this creep condition needs to be clarified.In this parper,we studied the creep behavior of P microalloying GH984G alloy in medium temperature and low stress region,analyzed the influence of P on microstructure,creep deformation and damage behavior,enriched the phosphorus microalloying theory and provided the theoretical foundation for engineering application of high-performance Ni-Fe-based GH984G alloy.The creep behavior of P-free GH984G alloy was investigated under medium temperature and low stress.The results show that the true stress exponent is 5 and the true creep activation energy is 280 kJ/mol.Creep mechanism is mainly power-law creep and controlled by dislocation climbing.Fracture mode are mainly intergranular fracture,the main fracture features are wedge cracks at 700-750℃/300-400 MPa and 775℃/200-350 MPa,while the number of grain boundary cavities increases and the microcracks formed by interconnection cavities become the main fracture feature at 700-750℃/150-250 MPa.Creep damage tolerance λ were calculated to be in 5-10,indicating that the creep damage was mainly attributed to precipitated phases coarsening,subgrain formation and dislocation density reduction.The extrapolation creep strength of P-free alloy at 700℃/105 h is about 137 MPa,which meets the requirements for the use in A-USC power plants.The effect of P microalloying on microstructure of GH984G alloy after standard heat treatment was investigated.The results show that P segregation at grain boundaries promotes the transformation of M23C6 from films and short rods to granularity,the degree of granularity increases with increasing P content.P microalloying has no effect on grain orientation and size,γ’ morphology and size.The effect of P microalloying(0.009 wt%-0.041 wt%)on creep behavior of GH984G alloy at medium temperature and low stress was investigated.The results show that P microalloying significantly improve the creep strength(creep life,minimum creep rate)of GH984G alloy and the improvement increases with increasing P content.For the same P content(0.018 wt%),the improvement in creep strength decreases with decreasing stress.The main creep mechanism of P-doped alloys is power-law creep,but the role of diffusion creep on creep deformation increases with increasing P content.The calculated λ values for P-doped alloys at 700-750℃/200-250 MPa are in 2.5-5,indicating that the creep damage is mainly due to creep cavity growth controlled by both pow-law creep and diffusion creep.While at 700℃/400 MPa,the λvalue is close to 1.5,creep cavities growth controlled by diffusion creep has an increasing effect on creep damage.The fracture features of P-doped alloys are creep cavities and the fracture mode are mixed fracture.P microalloying has no effect on γ’coarsening(LSW)and the main mode of dislocation passage through γ’(climbing and Orowan),but P microalloying promotes the frequency of dislocation shearing and the number of stacking faults increases in P-doped alloys.The effect of P microalloying(0.018 wt%)on the microstructure evolution of GH984G alloy during creep at 700℃/250 MPa was investigated.In P-doped alloy,M23C6 has smaller length and thickness,less lattice rotation and less dislocation accumulation,fewer sub-structured grain boundaries,stable ∑3 twin boundaries proportion,and no creep damage is generated under the same interruption time compared with P-free alloy.It was found that the enhancements of P(0.018 wt%)on creep life decreased with decreasing stress at 700-750℃/150-400 MPa.When the stress decreased from 750℃/300 MPa to 150 MPa,the number of icing-like grains,grain boundary microcracks and stacking faults of P-doped alloy increased,conversely,the localorientation misorientations at grain boundaries decreased.A comprehensive analysis shows that the P microalloying improves the creep strength of GH984G alloy mainly due to P segregation at grain boundaries to promote the granulation of M23C6 and improve the grain boundary bonding force,thus making it difficult to extend grain boundary damage.Meanwhile,P microalloying inhibits microstructure degradation and impede dislocations slip,promote the influence of diffusion creep on creep deformation and creep damage,reduces the number of wedge cracks and microcracks formed by creep cavities interconnection,increases the number of creep cavities caused by the combined effect of power-law creep and diffusion creep.