Research on the Microstructure Evolution of A-USC Ni-based Superalloy and the Effect of Creep Deformation
Author:Li Hui Fang
At present,thermal power generation is still the main source of electricity in today’s society.The increasingly prominent environmental problems and the continuous increase of power demand facilitate power plants to improve the conversion efficiency by increasing the operating temperature and pressure.The improvement of working temperature requires the superalloy to have much higher creep resistance.Ni-based superalloys,which possess both excellent microstructure stability and high temperature creep resistance have been regarded as one type of the candidate materials used under the 700 ℃ level power generation technology,and the most representative two kinds of superalloys are Inconel 740H and Alloy 617B respectively.Under the guidance of the strategy of introduction,absorption and reproduction,domestic enterprises had developed nickel-based alloys by referring to the chemical composition of Inconel 740H and Alloy 617B,and they were named TG700A and TG700B respectively.However,the microstructure stability of domestic nickel-based alloys after long-term service needs to further study.From the previous study on microstructural evolution of Inconel 740H and Alloy 617B,it was found that high-temperature deformation could lead to the change of microstructure near grain boundaries.Therefore,the specific performance in the effect of high-temperature creep deformation on the microstructural evolution of domestic TG700A and TG700B alloys also needs to be further clarified.At the same time,the mechanism of the effect of high temperature deformation on the microstructural evolution needs to be further studied,so as to enrich the understanding of the microstructural evolution of similar alloys such as Inconel 740H and Alloy 617B.The specific research conclusions are as follows:(1)For the microstructural evolution study after long time creep tests:the size of γ’ within alloys could have a certain increase,and some Cr-rich carbides in TG700B alloy could transform into Mo-rich phase.Through microstructural comparative study between the grip section and the gauge section of the same creep sample,it could be found that the effect of creep deformation was mainly embodied in two aspects,as the rupture time increased from hundreds of hours to tens of thousands of hours in the process:First,Cr-rich carbides in TG700A grew from 0.3～0.9 pm to 1.2～2.4 μm;and the morphology of Mo-rich phase in TG700B alloy changed from granular to long strip(along the main boundary).But the Cr-rich carbides in TG700B alloy didn’t change obviously.Second,the local microstructure of grain boundaries would change to form CZ(containing rod γ’ phase)and PFZ(not containing γ’ phase).(2)For the study of the influencing factors on CZ and PFZ(creep deformation stage,stress and temperature):these two kinds of grain boundary area began to emerge from the early period of the tertiary creep stage;the number of these two is small in the area far from fracture and is large in the area near fracture.The higher creep temperature is,the larger size of CZ area is and the longer γ’ is.The shorter the creep stress is,the slender rod-shaped γ’ phase is,at the same time,the smaller the width of PFZ is.In the ruptured sample under high stress for TG700B alloy,there are CZ and PFZ with smaller size.In the ruptured sample under low stress,only PFZ with a little larger size is found.After comparison between TG700A and TG700B,the size and quantity of CZ and PFZ in TG700A are bigger than TG700B alloy,and the γ’ number of CZ in the former is more than the latter.(3)For the analysis of two kinds of microstructure:the precipitates in CZ were identified as rod-shaped γ’ phases finally,their growth direction was parallel to the<1 1 0>direction.According to the back-scattering information of FIB sample,it could be known that one edge of the CZ area is distributed with equiaxed Cr-rich carbides particles,and the other edge is the location of the grain boundary.The above phenomenon reflects that the CZ area is the region where the grain boundary has passed through.According to the EBSD technique,it can be found that the orientation of the CZ and PFZ is only consistant with the orientation of one grain nearby.Based on the statistical analysis of nearly 30 CZ areas,the angles of migrated grain boundaries(near CZ),are between 45° and 65°,and about 85%of these migrated grain boundaries are between 500° and 60°.(4)For the strain information and local misorientation on both sides of migrated grain boundary:on the both sides of migrated grain boundary,the value of KAM was lower in soft oriented grains(with high SF value),while the value of KAM was higher in hard oriented grains(with low SF value).The crystal orientation rotation from the grain boundary to the inner of soft oriented grain was small,but the crystal rotation from the grain boundary to the inner of hard oriented grain was large.These results reflect that the hard oriented grain contain higher GNDs density and higher strain gradient,which could lead partial grain boundary to migrate and decrease the local strain energy.The migration direction of grain boundary was from soft oriented grain to hard oriented grain.More slip bands,which were terminated at the moving grain boundary were found in soft oriented grains.Thus,it can be inferred that the dislocations in the soft oriented grains could not pass through the grain boundaries smoothly,and then continued to accumulate around boundaries during the creep deformation process,resulting in the continuous increase of the force on the hard oriented grain,which eventually led to the rotation of the hard oriented crystal and produced plenty of GNDs density.(5)For the distribution of local misorientation(KAM)and the rotation of crystal orientation:during the creep deformation,the peak value of KAM angle would increase from 0.15° to higher,and the angle range of local misorientation would become bigger.At the same,the significant increase of R0.5°-5° was consistent with the formation of CZ/PFZ.From the analysis of the ruptured sample,the higher the value of R0.5°～5° is,the greater the total number of CZ/PFZ is and the higher the strength of texture with specific crystal orientation is.