| High W content superalloy is one of the equiaxed superalloys with the highest temperature bearing capacity,which is widely applied in aero-engines and gas turbines.K416B is a typical high W content superalloy,which contains 16 wt.%tungsten.The mechanical property of K416B is directly affected by its microstructures.At present,the researches have been focused on the mechanical properties in the service temperature region,while few attentions have been paid on the morphology evolution behavior during casting and heat treatment.Therefore,this dissertation aims to systematically study the microstructure evolution of K416B at elevated temperatures,and the effect on the properties is also investigated.The main results are summarized as follows:The results show that the microstructure of K416B superalloy consists of γ phase,γ’ phase and MC carbide.In addition,a vast amount of γ/γ’ eutectic can be observed in K416B superalloy.The area fraction of γ/γ’ eutectic is about 18%,which directly affects mechanical properties of the alloy.In order to investigate the interdendritic solidification of K416B,water quenching experiments were carried out on the samples.The results showed that γ-γ’ clusters in eutectics nucleated at the front of secondary dendritic arm,which was controlled by the cooling rate.The proportion of γ phase progressively increased as the γ/γ’ eutectic growing,resulting in the concentration of γphase forming elements raising in the residual liquid.In this process,the solidification reaction transmuted from eutectic reaction into peritectic reaction.Meanwhile,islandbanding of γ phases perpendicular to the solid-liquid interface precipitated from the liquid,together with the remelting of secondary dendritic arm.Severe plastic deformation occured at the interface between the γ/γ’ eutectic and matrix,where cracks were prone to initiate during the deformation process.To study the microstructure evolution of K416B alloy at elevated temperatures,the samples were heated at 1000 ℃、1100℃ and 1200 ℃ for different hours.The Lifshitz-Slyozof-Wagner(LSW)model was applied to calculate the growth of γ’precipitation at 1000℃ and 1100℃.When the temperature increased to 1200℃,the average size of γ’ sharply increased,and the secondary γ’ and ultra-fine tertiary γ’precipitated at the center of γ channel during the cooling process.The forming temperature(1147℃)of secondary γ’ could be obtained from the cooling curve of DSC results.Additionally,the morphological evolution of γ/γ’ eutectic was obviously observed at high temperatures.The dissolution kinetics of γ phase were quantitatively assessed by Johnsom-Mehl-AvramiKolmogorov(JMAK)model.The γ phase completely transformed into γ’ at 1200℃.In addition,The morphology of γ phases were converted from lamellae to platelets in the dissolution process,and the spheroidization time could be predicted by static spheroidization kinetics.The properties of tensile and stress rupture were analyzed.The results showed that the mechanical properties of the high tungsten content superalloys are manifestly influenced by γ’ precipitation and γ/γ’ eutectic morphology.The crack propagation inγ/γ’ eutectic was impeded by γ plates during the deformation process,and thus the yield strength and creep life of K416B were improved.Moreover,dislocations cut through primary γ’ in the form of full dislocations and part of them decomposed into stacking faults,while bypassing secondary γ’ by Orowan mechanism during the tensile deformation process at room temperature.The secondary γ’ phase coarsened rapidly during the durable test,which promoted dislocations cutting into γ’ phase and basically damaged the durable life of K416B alloy.Additionally,nano W6C particles precipitated along the γ-γ’ interface to alleviate the strain concentration caused by high-density dislocations. |
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