| As an important means to influence the microstructure of superalloys,heat treatment plays an important role in the research of superalloys.Understanding the microstructural evolution of alloys is helpful to optimize the composition,microstructures ancd processing technology of alloys,thus providing theoretical guidance for the development and design of a new generation of high performance superalloys.In this paper,the equilibrium phase diagrams and solidification paths of alloys with different boron content are calculated by JMatPro thermodynamic simulation software.The phase transformation temperature of the alloy was determined by differential scanning calorimetry.Six heat treatment processes were developed according to the solidification characteristic temperature of the alloy.The heat treatment of the alloy was carried out in different processes.The as-cast structure and the structure after heat treatment were researched.The effects of heat treatment on dendritic structure,?/?'eutectic structure,morphology and size of ?' phase,carbon/boride,micropore and mismatch degree of ?/?' phase lattice of the alloy were studied.(1)The precipitation phases of the alloy are L phase,? phase,?' phase,carbide phase,M3B2 boride phase and TCP equal between 600 and 1500?.With the increase of B element content,the precipitation amount and precipitation temperature of TCP phase decrease,which indicates that B element can inhibit the harmful TCP phase in the superalloy.The characteristic temperature of the alloy decreases gradually with the increase of B element content,and the thermodynamic calculation results of the alloy show that B element can inhibit the harmful TCP phase in the superalloy.It is close to the DSC experimental results and shows the accuracy of the simulation results.The incubation period of a and ? phases in B3 alloy is longer,but the nose tip temperature in TTT curve is lower.TCP harmful phases will gradually precipitate in the alloy during the incubation period up to 30.9 h at 1070 ? or 7.3 h at 1140?.Therefore,the ageing treatment system of the alloy can be set at 1080?/6 h+870?/24 h.(2)The dendrite patterns of BO and B1 alloys were eliminated completely under the heat treatment systems of T6(1310?)and T5(1305?)respectively.Mapping analysis of elements in B1 alloy after T5(130?)heat treatment shows that the homogenization effect of elements in B1 alloy is remarkable.The dendrite pattern of B2 and B3 alloys is eliminated obviously after heat treatment of T1(1285?)and T2(1290?).With the increase of solution temperature,the dendrite structure of B2 and B3 alloys is basically eliminated and the homogenization effect is more remarkable under the treatment of T4(1300?)and T3(1295 ?)respectively.(3)After heat treatment,the morphology of the carbide in the alloy changes from the as-cast Chinese character to the granular shape and the thin rod shape,and the carbide size decreases with the increase of the solution treatment temperature.The EDS elemental energy spectrum analysis of the carbides showed that the carbide phase in the alloy is mainly MC-type carbides rich in Ta and Hf,indicating that the carbide type in the alloy did not change.(4)After heat treatment,the morphology of ?' phase begins to change into regular cubic morphology;the size of ?' phase of B0 and B1 alloys increases gradually with the increase of solution temperature,and the maximum size of ?' phase of B0 and B1 alloys reaches 0.355 and 0.351?m respectively.The ?' phase size of B2 and B3 alloys increased and then decreased with the increase of solid solution temperature.The two exhibited the largest size at T2(1290?),reaching a maximum of 0.438?m and 0.398?m respectively.The ?' phase morphology precipitated during the subsequent cooling process is represented by a regular cubic morphology. |
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