| 其他摘要 | In this dissertation, microstructure of a nickel base Re-containing superalloy for conventional casting at different conditions was investigated.
Microstructure of the as-cast alloy is mainly composed of γ matrix，γ΄ precipitation，(γ + γ΄ ) eutectic, MC carbides and borides. The results show that the solidification and precipitation sequence of this superalloy is as follows：
γ΄ precipitations at dendrite core are bigger than those at interdendritic region. There are two kinds of MC carbides in the alloy, one is graininess MC carbide; the other is Chinese script MC carbide. They are mainly composed of Ti、Ta and Nb containing carbide. Severe dendritic segregation leads to the formation of a lot of (γ + γ΄ ) eutectics. Ti, Nb, Ta and Al are rich in interdendritic region, while Co and W are rich in dendrite core. No strong segregation profile of Cr, Mo and Re are observed.
During solution treatment, γ΄precipitations and (γ + γ΄ ) eutectics are gradual solutionised as the temperature of solution treatment rises. During cooling process,γ΄reprecipitates, but in a much smaller size. The faster specimen cools, the smaller γ΄ becomes. The morphology of γ΄ depends on both the temperature and time of high temperature ageing treatment. The higher the temperature is, the bigger the γ΄ becomes; the longer the time is, the bigger the γ΄ becomes. During high temperature ageing treatment, M6C with γ΄ cladding precipitates along grain boundary; graininess TCP precipitates in dendrite core. Relation between the temperature of ageing and hardness is: hardness rises as the temperature rises to a peak value, and then hardness falls as the temperature rises.
During creep process, γ΄ raftes in parts of grain, orientation of γ΄ rafting is [001]. The stress causes γ΄ precipitating in the grain and along grain boundary, without dislocation. The more sufficient degree of solution becomes, the less γ΄ precipitates in the dendrite core. SEM observation on the fracture surfaces and longitudinal sections showed that the cracks initiate at the sample surface or the microporocities and the coarse MC carbide particles on the inner grain boundaries, and propagate along grain boundaries.
An investigation on the microstructure evolution of the experimental alloy during long-term aging shows that the γ΄ precipitate coarsens gradually during the long-term aging at both 900℃. After long-term aging at 900℃ , M6C, M23C6 and γ΄ precipitated along grain boundary. After long-term aging at 900℃ for 1000h, no TCP precipitates; for 2000, much acicular TCP precipitate.
Re causes an increase in (γ + γ΄) eutectic temperature, MC carbide formation temperature and γ΄ precipitating temperature. Re causes an increase in eutectic reaction segregation as well as solidification segregation. There is a eutectic reaction segregation region around eutectic caused by eutectic forming, which is rich in Cr and Mo, and devoid of Al and Ta. In the region, with γ΄ ladding, rich in Cr, W and Mo, form from residual liquid; the pseudo-dendritic γ΄ precipitates form, which result from latent heat of eutectic reaction and impact of Re increasing in precipitating temperature together. |
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