| IN718 superalloy demonstrates not only good tensile strength, yield strength, rupture strength and plasticity but also good anti-corrosion, anti-radiation, thermal processing and welding performance in the range of -253? 650?. Hence, IN718 has become a fundamental material in the filed of aviation, aerospace, nuclear and oil applications. However, the high content of Nb in alloy compositions results in complex solidification process and severe elements segregation, which makes the subsequent thermal processing and heat treatment considerably difficult. In this work, confocal laser scanning microscope is used to observe the solidification process of IN718 alloy in situ, with particular emphasis on the influence of cooling rate and melting treatment on solidification process and solidified microstructures. Homogenization treatment is a key technique to remove micro-segregation and obtain uniform composition. The microstructure evolution and element diffusion behavior in two-stage homogenization treatment are fully characterized, which helps manufacturers have complete control of the homogenization process.Moreover, it is proved experimentally that the service temperature of IN718 can be raised markedly through P and B additions, offering a possibility to develop P and B strengthened IN718 alloy in industrial scale. Therefore, it is greatly important to investigate the effect of P and B additions on solidification process, segregation behavior and homogenization treatment. For this purpose, three heats with different contents of P and B were prepared by means of vacuum induction melting. The in-situ observation results show that three stages can be defined for L??transformation of IN718 alloy: initial stage, stable stage and last stage. The relationship between liquid fraction (fL) and temperature (T) is in accordance with Avrami equation. The precipitation process of MC carbide is observed in the intermediate stage. At the meantime, some remaining liquid still exists in the final solidification stage. In reference with the Scheil equation, a great amount of Nb and Mo are enriched in the remaining liquid, leading to the sluggishness of solidification process.With the increase of cooling rate, the secondary dendrite arm spacing appears to be smaller. At the slow cooling rate, the morphology of carbide appears to be Chinese-script, connecting like a string. With increasing cooling rate, morphology change is observed from Chinese-script to blocky and the correspondent size decreases obviously. While, no similar monotone decreasing pattern is detected concerning the size and morphology of Laves phase with increasing cooling rate. Blocky Laves phase is observed in the samples at the cooling rates of 5?/min, 100?/min, and 200?/min. In contrast, eutectic Laves phase that contains less Nb and Mo is found instead in the 30?/min sample. By calculation, the Nb content in liquid of 30?/min sample is lower than that of 5?/min and 100?/min samples, which may be the possible reason for observing eutectic Laves phase instead of blocky Laves phase in the 30?/min sample.The in-situ observation results also show that secondary dendrite arm spacing is significantly reduced by means of melting treatment. The morphology of MC carbide becomes fine after melting treatment. In particular, the super-cooling degree is increased markedly after 1600?melting treatment. The increase of super-cooling degree may be one of the reasons for explaining fine dendrite structures.The incipient melting temperature of?406 mm INCONEL718 ingot is situated between 1170?and 1180?. Two-stage homogenization treatment (1140?/60 h + 1190?/30 h) was employed to relieve the segregation The Laves phase elimination model is proposed according to the study results and the remaining Laves phase fraction (R) can be derived as a function of homogenization temperature (T,?) and time (t, h) as below. R(Laves)=exp[(-2.48×10-19)/exp(-0.036T)]×100%In the characterization of the second-stage homogenization,?phase (Nb-enriched) was used to trace the diffusion of Nb. As a consequence, Nb concentration tends to be homogeneous eventually, judging from three aspects: average composition of?phase area, quantity of?phase precipitates and size of?phase precipitates, respectively.Combined additions of P and B significantly increased the size of Laves phase, promoting Laves phase from eutectic to blocky. Nb and Ti are much more segregated in the inter-dendritic regions, which therefore be contributed to the precipitation of?phase and?" phase. According to EPMA mapping results, P and B have very low solubility in the?matrix and gradually enriched in the residual liquid during solidification process. The combination of microstructure characterization and DSC data strongly propose that the solidification sequence experienced by P and B doped IN718 superalloy can be expressed as follows: L?L +??L +?+ MC?L +?+ MC + Laves??+ MC + Laves + M3B2.The incipient melting temperature ranges obtained by metallographic method are in good agreement with those by DSC test. The additions of P and B lead to the reduction of incipient melting temperature by 50?. Hence, the homogenization temperature of first-stage should be chosen as 1120?. Also, the calculation results suggest that P and B strongly reduce the diffusion coefficients of Nb. Accordingly, the homogenization time should be extended for P and B strengthened IN718 alloy.For the direct aging sample, particle?phase is detected at the grain boundary. While long rod, short rod and long needle-like?phase exists in the solution plus aging samples. According to the above 4 kinds of?phase, the model of long rod?phase precipitate at the grain boundary is available ( )exp[( 2.48 10 19) / exp( 0.036 )] 100%LavesR = ?×?t ? T× as well as the model of?phase growth from grain boundary to grain inner. Under same aging conditions, the micro-hardness value of solution & aging state is less than that of direct aging state. Upon same aging time, the micro-hardness value of aging at 720?is significantly greater than that of aging at 620?. A large number of?" phase precipitation happen in the 720?aging process while much?' phase and a little?" phase precipitate in the 620?aging process. |
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