| GH4738 alloy is a precipitation strengthened nickel base superalloy,originated in the 1950 s in the United States.It has high strength,good thermal corrosion performance and excellent creep resistance and fatigue resistance.It is widely used in the manufacture of turbine disk,turbine blade and fastener of aeroengine.The microstructure evolution of cold drawn bar for GH4738 Alloy Fastener was studied in detail through the high temperature diffusion annealing test,hot deformation and cold deformation test.In the high temperature diffusion annealing test,the dendrite segregation behavior in as cast structure of the alloy was studied,and the diffusion annealing process was determined.The dendrite segregation is essentially the uneven distribution of elements.The main segregation elements in GH4738 alloy are Ti,Mo,CO and Cr,and the segregation ratios are 1.820,1.136,0.896 and 0.905,respectively.The segregation degree is Ti>Mo>Co>Cr,in which the elements Ti and Mo segregate between dendrites,while the elements Co and Cr segregate CR segregates in dendrites.By using different diffusion annealing processes,it is found that with the increase of heating temperature and holding time,the inhomogeneity of elements gradually improves.Through theoretical calculation and experimental verification,the best high temperature diffusion annealing process of GH4738 alloy is 1200 ?×50 h?In the hot deformation test,the as cast and as forged alloys were hot deformed by MTS thermal simulator under the same parameters.The results show that the stress-strain curves of the two kinds of alloy have typical dynamic recrystallization characteristics,and there are three stages: work hardening,flow softening and steady-state flow.The difference of peak stress between the two kinds of microstructure is kept within 14 MPa.Within the error range,it can be considered that the change rule of flow stress of the two kinds of microstructure is basically the same during hot deformation.The hot deformation activation energy of as cast GH4738 alloy is Q=575.89 k J/mol,and that of as forged GH4738 alloy is Q=588.04 k J/mol.In the cold deformation test,with the increase of deformation,the grains will deform,and the stress will be transferred from the grain boundary to the grain.When the deformation is large,the original grain boundary will change from continuous to discontinuous,and it is difficult to identify.In addition to the change of grain size,the grain orientation also changes during deformation.The static recrystallization of GH4738 alloy can be completed after 5 min solution at 1040 ? regardless of cold deformation from 10% to 65%.Therefore,the static recrystallization process of GH4738 alloy is very rapid.With the increase of solution temperature and holding time,the trend of grain growth is obvious after static recrystallization.The phenomenon of grain coalescence and growth is observed in detail by means of in-situ heating.The orientation difference of adjacent grains is 30°?60°.It's easy to swallow.During cold deformation,the grains with arbitrary orientation in the original structure will gradually adjust their orientation and tend to be consistent with each other,so that the grains have the preferred orientation,resulting in two kinds of deformation textures{100}<110>,{111}<112>,but the grain growth is not affected by the texture environment in the subsequent solid solution process. |
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