First-principles Study On The Properties Of δ And γ" Phases And The Recrystallization Behavior Of Cold Rolled Inconel 718 Super-alloy

The mechanical and electronic properties ofδphase andγ" phase in Inconel 718 super-alloy have been studied by using Materials Studio-CASTEP calculation module which was developed based on the density functional theory, then the recrystallization behavior of cold rolled Inconel 718 super-alloy pre-treated with different progresses was studied by metallographic observation and variation of hardness.The structures of both the phases are optimized and the calculated results are in good agreement with the experimental results. The formation energies ofδphase andγ" phase are calculated to be -1.48 eV per atom and -0.69 eV per atom, respectively. This suggests thatδphase is thermodynamically more stable thanγ" phase. The relatively high values of C11-C12 and B/G indicate that both the phases are hard and have good ductility. From the Mulliken population analysis and PDOS, it is found that a combination of covalent and ionic bonding exists between Ni-Nb atoms. The charge density distribution reveals that there exist a covalent bonding between Ni-Ni atoms in both the phases and a covalent bonding between Nb-Nb atoms inγ" phase. Therefore, the bonding characteristic in both phases is a unuaual combination of covalent, metallic and ionic bonding.In order to study the recrystallization behavior of cold rolled Inconel 718 super-alloy, three different pretreatments were adopted. The pre-treated samples were cold rolled to different reductions and then heated at diiferent temperatures for different time. The results reveal that the recrystallization temperature range of cold rolled samples pre-treated by A pretreatment is from 860oC to 910 oC. For B pre-treatment, the range for 25% and 50% cold rolled samples are 910 oC-960 oC and 860 oC-910 oC, respectively. For C pretreatment the range is from 810 oC to 860 oC. The small grains can be obtained under low heating temperature and large cold rolling reduction conditions in the case of C pretreatment. The existence ofδphase will prevent the grain growth and reduce the recrystallization grain size. The relationship between hardness and heating time was given. The recovery and recrystallizaton make the hardness decrease while the precipitation of theδ,γ′andγ" phases will promote the increase of the hardness. The variation of hardness reflects the comprehensive function of the recovery, recrstallization and the precipitated phases.