| GH742 alloy is a high alloying nickel-base superalloy with excellent performance at high temperature, serving up to 750～800℃as turbine disc materials. However, the hot workability becomes quite poor due to the higher alloying level, which limits the further development of the alloy. In this dissertation, the solidification and segregation of as-cast GH742 and GH742y alloys were firstly analyzed, and then the hot deformation behavior and nucleation mechanism of dynamic recrystallization both in as-cast GH742 and as-rolled GH742M alloys were investigated, and finally the microstructure after standard heat treatment and mechanical properties of as-forged and as-rolled GH742M alloys were discussed. The main results are summarized as following:The as-cast GH742 alloy exhibits severe dendritic segregation with Cr, Co, Al segregated to the dendrite cores and Nb, Ti, Mo segregated to the interdendritic regions. The morphology ofγ’ particles is varied at different regions, which are spherical and small in the dendrite cores, while cubical and coarse in the interdendritic regions. The precipitates such as MC type carbide, (γ+γ’) eutectic, Laves phase andδphase are precipitated in the interdendritic regions because of the intensive segregation of Nb and Ti. High content of Mo as well as its segregation is a significant reason for the precipitation ofσphase. Two phases containing rare earth elements, Ni5Ce phase and a RE-O-S phase, are precipitated in the interdendritic regions due to the enrichment of La and Ce. The solidification temperature of GH742 alloy ranges between 1346℃and 1190℃, and the solidification sequence isγmatrix, MC carbide, (γ+γ’) eutectic, Laves phase, Ni5Ce phase. Theδphase,σphase andγ’ phase are precipitated after solidification. Additions of La and Ce decrease the contents of O and S, modify the distribution and dimension of MC carbide, and refine the grain size. However, they aggravate the segregation of Nb and Ti, lower the solidus temperature, and cause the precipitation of many detrimental phases.The incipient-melting temperature of RE-O-S phase in as-cast GH742 alloy is 1120℃, while the absolutely soluble temperature of coarseγ’ phase is over 1120℃, thus a two-step homogenization treatment via low temperature pretreatment at 1100℃followed by high temperature diffusion at 1160℃is established to eliminate the dendritic segregation and obtain uniform austenitic microstructure without incipient-melting. Increasing the homogenization temperature can remarkably increase the rate of elemental diffusion, which is more effective than increasing the homogenization time. The diffusion coefficient of Nb is much smaller than that of Ti, requiring more time to be absolutely homogenized. The two-step homogenization treatment by 1100℃x 30h+1160℃x 40h for the ingot improves the hot deformation plasticity, decreases the flow stress, and favors the dynamic recrystallization process.The flow stress of as-cast GH742 and as-rolled GH742M alloys during hot compression deformation decreases with the increasing of temperature and the decreasing of strain rate. Dynamic recrystallization is easier to take place at higher temperature and higher or lower strain rate, and the corresponding recrystallized grain size is larger. Increasing strain can increase the volume fraction of the recrystallized grains, but the grains become coarser at low strain rate. The existence of y’particles decreases the hot deformation plasticity, and significantly increases the flow stress. Theγ’particles can pin the grain boundaries, which restrain the migration of grain boundaries and the nucleation of dynamic recrystallization.Both the stress exponent and apparent activation energy of as-cast GH742 and as-rolled GH742M alloys decrease as the temperature increases during hot compression deformation. The strain rate sensitivity is less than 0.3, indicating that the GH742 alloy is very difficult to achieve superplastic forming. The strain has an obvious influence on the hot processing map, especially for the as-rolled GH742M alloy, which reflects the microstructure evolution during deformation. The adiabatic shear bands occur at lower temperature and higher strain rate, while the wedge type cracks are prone to produce at higher temperature and lower strain rate. The constitutive equation of as-rolled GH742M alloy during hot deformation is as follows:ε= 8.89x1017σp5.33 exp(-809.48 x 103/RT)The dislocation morphology and density develop dynamically, relating with the deformation conditions such as temperature, stain rate and strain, which are the integrative effect of work hardening, dynamic recovery and dynamic recrystallization. Nucleation of new recrystallized grains starts preferentially at the initial grain boundaries to form a necklace structure. The starting nucleation mechanism of dynamic recrystallization in as-cast GH742 and as-rolled GH742M alloys is discontinuous dynamic recrystallization, though continuous dynamic recrystallization simultaneously takes place as the strain increases in the as-cast GH742 alloy. Twinning plays an important role in the nucleation and growth of dynamically recrystallizaed grains. The MC carbide in the alloy can accelerate the nucleation of new grains. The strain rate does not affect the nucleation mechanism of dynamic recrystallization, but can affect the rate of nucleation and growth of recrystallized grains.The elongation to failure as well as the tensile strength of as-rolled GH742M alloy increases with the increasing initial strain rate ranging from 10-4 to Is-1 when deformed at high temperatures. The tensile rupture surface at lower strain rate is a typical intergranular fracture, while it exhibits some characteristic of transgranular fracture at higher strain rate.The grain size of as-rolled GH742M alloy does not change obviously when the temperature of solid solution treatment is below 1080℃, and it begins to grow intensively when the temperature is over 1080℃. Three dimensions ofγ’ phase and lots of strengthening phases along the grain boundaries by 1020℃x 8h, AC+780℃x 16h, AC treatment exist in the alloy, which can not only improve the deformation plasticity below 1080℃, but also refine the recrystallized grains.The microstructure of as-rolled GH742M alloy after standard heat treatment consists of y matrix, y’phase, MC and M6C carbides, M5B4 boride. The tensile property at room temperature of as-rolled GH742M alloy is better than that of as-forged GH742M alloy, but its stress rupture life is less than that of as-forged GH742M alloy. The co-addition of 0.10%La and 0.01% Ce does not influence the tensile property at room temperature of as-forged GH742M alloy.GH742y is a derivative of GH742 alloy, which has a higher level of Al, Ti, Nb, and W, V in addition. The higher alloying level of GH742y alloy makes the dendritic segregation more serious and the precipitates more complicated. The primary and secondaryγ’ phases, (γ+γ’) eutectic, MC and M6C carbide, Laves phase,8 phase, a phase,μphase and Ni5Ce phase are precipitated in the interdendritic regions. The solidification temperature of GH742y alloy ranges between 1348℃and 1167℃, and the solidification sequence is y matrix, MC carbide, primary y’phase, (γ+γ’) eutectic, Laves phase, Ni5Ce phase and M6C carbide. The phase calculation reveals that the electron vacancy number of severe segregation regions is beyond 2.30, causing the precipitation of TCP phases such asσphase andμphase. The incipient-melting temperature of Ni5Ce phase is 1120℃, and the absolutely soluble temperature of coarseγ’phase is over 1140℃. A three-step homogenization treatment via 1100℃+1160℃+1180℃is established to obtain uniform austenitic microstructure. |
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