| Superalloys are critical materials in aviation, aerospace engines and various industrial gas turbine engines. As a developed nickel-based superalloy by ourself, the formation of equiaxed grain during the casting process of K424 superalloy is advantageous of excellent overall performance and is widely used in the engine and the turbine rotor, the auxiliary power unit by entire casting. Rapid solidification technology is treated as an important means for developing new high-performance structural materials, which can be achieved by significant grain refinement, forming microcrystalline, amorphous and nanocrystalline materials. As a result, the performance of conventional metallic materials can be improved largely. In addition,the employing of non-equilibrium effects by using rapid solidification process can be applied to fabricate supersaturated solid solution. By controlling the subsequent precipitation of solid phase transformation process, fine dispersed phase structure obtained as well. Therefore, optimization the preparation of high-temperature alloys,so as to improve its performance has obvious application value, due to the importance for the precipitation strengthening phase in nickel-based superalloy and further understanding the strong, toughening mechanisms and performance characteristics of high-temperature alloys.Applying vacuum induction melting technology combined with stepped mold,detail study of the microstructure formation of K424 superalloy at different cooling rates was performed in present work. On this basis, different isothermal heat treatment process was adopted to analyze the precipitation behavior of rapid solidified supersaturated solid solution. Combined with the hardness results, the relationship between fabrication method, microstructure characteristic and property measurement was established.Rapid solidification results show that the increase of cooling rate can effectively refine the K424 alloy γ matrix structure, where the secondary dendrite arm spacing of is reduced to about 5μm, which is nearly 1/16 of that under near-equilibrium solidification. Dendrite structure refinement and solute entrapping occurs effectivelyso as to reduce the segregation of alloys. As a result, the formation of γ+γ’ eutectic phase and MC carbides size is suppressed. Moreover, the precipitation of γ’ is inhibited effectively after rapid solidification due to the reduction of the average size.Heat treatment results indicate that dendritic morphology is not obvious anymore and the dendrite pattern becomes unclear after different cooling of solution treatment.Under water cooling condition, the morphology of γ’ phase shows irregular angular,while it changes into long strips, and relatively blunt corners under air cooling and furnace cooling conditions. After aging the rapid solidified K424 alloy at 800 ℃,900℃ and 1000℃ under 240 min, the size of γ’ phase size increases continuously and its value changes from 0.09μm to 0.35μm, where the morphology of γ’ phase is changed from spherical to rectangle.After solution treatment through the furnace cooling, the hardness of alloy is422 HV, and the maximum value is obtained after air cooling, reaching 470 HV.Hardness of the alloy after water cooling is the lowest, which is only 367 HV. As for the aging temperature in the range of 800℃~1000℃, the hardness of K424 alloy increases from 477 HV to 489 HV firstly and then decreases to 440 HV when the temperature rising to 1000℃. As for the aging time of 30 min, the hardness is 477 HV and it increases to a maximum value of 493 HV after aging 60 min. When the aging time was increased to 120 min, the hardness of alloys decreased to 488 HV and continuous reduction occurs to 484 HV as for the aging time of 240 min. |
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