| 其他摘要 | The influence of heat treatment and long term aging on microstructure and mechanical properties of cast nickel-base superalloy M951 are investigated in this dissertation.
Five heat treatments are carried out on the as-cast alloy, including three direct aging treatments and two solution and aging treatments. The direct aging treatments are 870℃/ 24h, 1050℃/4h and 1100℃/4h respectively. The results show that γ'size increases obviously with the increase of aging temperature. Fine secondary γ'phase precipitates in the alloy aged for 4h at 1050℃ or 1100℃. The MC carbide transforms during solution and aging treatment (1220℃/4h+870℃/24h), granular and acicular M23C6 carbide precipitates in interior grain and at grain boundary respectively. The corresponding γ' precipitates have a smaller size, their squareness reduces, and are aligned irregularly. After a solution and two stage aging treatments (1220℃/4h+1050℃/4h+870℃/24h), no acicular carbide is found, the size and squareness of γ'precipitates increase, and they are aligned more regularly. High temperature aging treatment has obvious influence on the morphology, size and alignment of γ'precipitates.
Room and high temperature tensile tests are carried out on the alloy after different heat treatments. The results show that the tensile properties of the direct aged alloy are almost the same as that of the as-cast. After aged for 4h at 1100℃, the tensile strength of the alloy increases slightly, while its ductility has a little decrease. The tensile strength increases notably, while its ductility decrease obviously after solution and low temperature aging treatment. Stress rupture tests are also carried out on different heat treated samples at 1100℃/35MPa. The results show that alloy treated by 1050℃/4h has the best stress rupture property, and that treated by 870℃/24h has the worst. The longitudinal microstructure observation of the stress ruptured alloys shows that cracks are mainly initiated at grain boundaries. The standard heat treatment of M951 alloy is 1100℃/4h, bimodal γ'distribution are obtained through standard heat treatment, which make the alloy has a good combination of tensile and stress rupture properties.
Tensile tests at different temperatures are carried out on alloys treated by standard heat treatment. The result shows that different deformation mechanisms are operated at different temperature regimes. At low temperatures (<600℃), the dominant deformation mechanism is γ'shearing by dislocation and slip bands. At high temperatures (>850℃), the deformation is dominated by γ'by-pass. At intermediate temperatures (600-850℃) there is a transition from γ'shearing to by-pass.
The alloys are long term aged at 900℃ and 1100℃ after standard heat treatment. It shows that during aging treatment at 900℃, the MC decomposes into M23C6. While the morphology of MC carbide changes from script to blocks during aging treatment at 1100℃. The growth kinetics of γ'at both temperatures obeys the LSW coarsening theory. There is no occurrence of obvious raft phenomenon during aging at 900℃, γ'is stable at this temperature. During early aging at 1100℃, there are a few γ'precipitates coalescing into raft-like shape, during the later stage,γ'precipitates coarsen and raft more obviously. The thermal exposure temperature has significant influence on the morphology and size of γ'.
During aging treatment at 900℃, the yield strength-temperature behavior is similar to that of standard alloy. As aging times increases, the yield strength of the alloy decreases, while its ductility increases compared to that of standard alloy. The main reason that the yield strength decreases is γ'coarsening. The stress rupture tests show that as the aging time extends, the stress rupture life of the alloy decreases during aging treatment at 900℃, it decreases as a whole during aging at 1100℃, at both temperatures the elongation increases. The reason that the stress rupture life decreases after long term aging relates to the γ'coarsening and formation of γ'rafts. As aging time extends, the dislocation networks at the interface of γ'/γ are destroyed gradually, and there are more dislocations cutting into γ'phase, resulting the decrease of creep rupture strength.
Key words: nickel-base superalloy M951, heat treatment, long term aging, tensile, stress rupture |
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