Effects Of High Density Pulse Current On Microstructure And Mechanical Property Of Nickel-base Superalloys

Since 1861, the phenomenon of electromigration or mass transportation in the metallic materials was discovered, the effects of electric current on metallic materials have been paied attention. The new phenomena caused by high density pulse current on the fabrication, working processing and heat treatment of the materials was focused with many researches.Superalloy as one kind of the important metallic materials, it is said to be the level symbol of industry for a country. In order to adapt the increasing requirements, more much alloying elements have to be added into the superalloys, it leads the poor deformability, especially, the decrease of toughness and plasticity after long-term aging. In the present study, a high density pulse current was used to improve the mechanical property of Nickle-base supperalloys, and the mechanisms of microstructure evolution and mechanical property were also discussed.A microcomputer controlled electropulsing instrument was used for the electropulsing treatment, with a current capisity of 4000A/mm2, a pulse frequency of 2Hz-50Hz and a pulse duration of 25μs-105μs. The temperature variation during electropulsing treatment is measured by a thermal-couple attached on the specimen.The effects of high density pulse current on the aging process and recrystallization behavior of GH4199 alloy and GH3625 alloy were studied, comparing with that treated by normal aging or annealing treatment.Based on the experimental results, it can be found that the pulse current accelerated the aging process of GH4199 alloy, both theγ' phase and carbides on the grain boundary of GH4199 alloy are enlarged by the electropulsing. Theory calculation indicated that the diffusion activation energy of GH4199 alloy was decreased by the pulse current, the dislocation density was decreased and the twin and fault of GH3625 alloy formed easy by the electropulsing.It is clear that the recrystallization process of both GH4199 and GH3625 alloys can be accelerated by the pulse current. After electropulsing treatment for 20min, the recrystallization occurred, at the same time, M6C carbides with two different size can be observed in GH4199 alloy. While, the recrystallization behavior of GH4199 alloy can not be found by the normal annealing treatment at 800℃for 40min. After electropulsing treatment for 40min, the recrystallization of cold-worked GH3625 alloy can be completed, while, the recrystallization can not be found by the normal annealing treatment at 600℃for 40 min, so that, only can be found by the normal annealing treated at 800℃for more than 40min. However, if GH3625 alloy is electropulsing treated for 40 min, the dislocation density will be decreased and the fault will form.Based on the results of micro-tensile testing, it is notice that both the yielding strength and tensile strength of GH4199 alloy at room temperature can be improved by the pulse current, even if the fracture elongation somehow little decreased or kept at the same level as untreated one. With the examination by both TEM and SEM, it is found that the crack initial easy occurred round with the M6C carbides in larger-size, which there are strong stress concentration at the interface between the M6C carbides and matrix. Since both the M6C carbides and matrix can be easy controlled by the electropulsing treatment, the mechanical properties can be improved with optimum treating parameters.In summary, new effects of the pulse current on microstructure evolution and behaviors of deformation of Nickel-base superalloys have been found in the present research. Based on the present research, a new mothed for improving the microstructure and mechanical properties of Nickel-base superalloys can be considered.