Optimization of the high-temperature strong plasticity of the new precipitation-enhanced nickel-iron-based superalloy GH4650T

with the increasing global greenhouse effect and the shortage of fossil fuels such as coal, how to further improve the efficiency of coal-fired power generation and reduce CO emissions has become an urgent problem to be solved. In this paper, a new type of precipitation-reinforced ni-fe Base Superalloy GH4650T is studied, and its heat treatment process is optimized by isothermal aging test and mechanical properties test, the quantitative relationship between microstructure evolution and macroscopic properties in the service temperature range is revealed. The results provide a theoretical basis for the application of GH4650T alloy in industry.

the master alloy ingots were obtained by a dual process of Vacuum induction melting and vacuum consumable melting, and then homogenized, finally, die casting ingot, forging billet and hot extrusion were carried out to produce pipes with a diameter of about 44.5 mm and a wall thickness of 10 mm. The round bar blanks with diameter of 8mm and length of 51mm are cut from the pipe. Alloy samples were treated by two-step solid solution heat treatment in a heat treatment furnace. After two-step treatment, alloy samples with almost the same grain size were obtained. The GH4650T alloy was treated by isothermal aging at 750℃. After heat exposure, the samples were removed and the oxide scale was removed. Then, more than # 280, # 400, # 600, # 800 and # 1200 sandpaper were used to polish the samples. After mechanical polishing, the samples were immersed in corrosive solution. The microstructure of the alloy was observed by ZEISS SIGMA HD field emission scanning electron microscopy. Tensile tests were carried out at 700℃using MTS E45.105 universal electronic material testing machine with a strain rate of 2.5.10-4s. The samples were placed at the temperature and kept for 10 minutes after the temperature was stabilized. The samples were quickly water-cooled to retain the deformed tissue. Three parallel samples were tested and discussed using average results. The fracture morphology was observed by Zeiss Sigma HD field emission scanning electron microscopy. WEDM was used to cut the 0.5 mm from the creep sample in a direction perpendicular to the tensile axis. After mechanical grinding to 50μm, the thin films were prepared by EPMA-1 electrolytic double-jet thinning apparatus. The corrosion agent used in electrolysis process is 40ml perchloric acid + 360ml ethanol. The microstructure of the alloy was characterized and analyzed by using the jeol-2100 plus transmission electron microscope.

a lot of MC carbides were found in the grain boundary and twinning of the alloy by scanning electron microscopy. It is found that there is no acicular phase or flake TCP phase precipitated in the grain after 5h-500 h aging time, which indicates that the microstructure of the alloy is relatively stable at 750℃. At the same time, after aging, many γ′ phase particles are precipitated in the grain of the alloy, which is consistent with the results calculated by the thermodynamic calculation software JMat-Pro V 13.2. By calculating the particle size of γ′ phase during isothermal aging at 750℃, we find that the coarsening growth kinetics follows the Lifshitz-Slyozov-Wagner maturation law. After the tensile test, we found that the tensile strength of the alloy increased at first and then decreased with the increase of aging time. The tensile yield strength of GH4650 alloy at 700℃ is only 362.0 ± 3.0 MPA. After aging at 750℃ for 5 H, the yield strength of the alloy increases to 533.3 ± 5.3 MPa. With the increase of aging time, the yield strength of the alloy increases slowly, then decreases slowly, and at about 48H, the alloy has peak strength. Contrary to the trend of strength changing with aging time, the tensile plasticity of the alloy shows the opposite trend. The relationship between tensile properties and γ′ phase particle size was also obtained. It is found that the tensile strength of the alloy reaches its maximum value when the average diameter of γ′ phase particles is 25.2 nm. At about 12.2 nm, the tensile ductility of the alloy reaches the minimum.After studying the dislocation configurations in different heat-treated alloys after about 1.0% plastic strain at 700℃, we find that for GH4650T alloy, the dislocation configurations in the alloy are similar to those of GH4650T alloy, the deformation mechanism changed from weak coupling dislocations to strong coupling dislocations during 700℃ tensile deformation and then to Orowan circumambulation with aging time. By means of Sem, we found that the tensile fracture morphology of the alloys in different heat-treated states at 700℃was mainly in the form of plastic transgranular fracture, and after 5 h short-time aging, the tensile fracture morphology of the alloys in the solution state was mainly in the form of plastic transgranular fracture, the fracture of the alloy is mainly along the brittle grain boundary and the port plane is relatively smooth. However, the fracture of the alloy is mainly along the grain boundary and through the grain boundary with the further aging time, the more obvious the transgranular fracture is, the smoother the port plane is.Conclusion: at 750℃, the main precipitates in the new ni-fe based superalloy are MC type carbide, MC type carbide and γ′ phase, and the first two precipitates are mainly distributed at grain boundary and twin boundary, the coarsening of γ′ phase follows the ripening law of Lifshitz-Slyozov-Wagner. With the increase of aging time, the strength of the alloy increases first and then decreases, and the peak tensile strength is about 25.2 nm. This is due to the deformation mechanism from weak coupling dislocation to cutting particles to strong coupling dislocation to cutting particles and then to Orowan bypassing particles. With the increase of aging time, the tensile ductility of the alloy decreases first and then increases, and the minimum value of tensile ductility is about 12.2 nm. This is because the fracture mechanism changes from plastic transgranular fracture to brittle intergranular fracture and then to transgranular and intergranular mixed fracture.