Microstructure Evolution And Thermal Fatigue Of DD5 Nickel-based Single Crystal Superalloy

Along with the development of the domestic aeroengine industry,nickel-based single crystal superalloy is in the period of fast development.At this stage,the research on preparation process of single crystal has been quite mature.Therefore,it is more practical to study the microstructure and properties of the single crystal superalloy during the heat treatment process and high temperature service,which is of great significance to improve the alloy performance and understand the high temperature stability and the life prediction of the alloy.In addition,with the application of film cooling technology in the domestic single crystal blade,the turbine engine temperature capacity has been further increased,while the thermal fatigue cycle cracks initiation and propagation of the area around the air film hole is lack of research and exploration.In this paper,the main studies are the improvement of microstructure and properties of the second generation nickel-based single crystal superalloy DD5 by heat treatment process,microstructure evolution during the high temperature service and the cracks initiation and propagation around the circular hole under thermal fatigue test.In this paper,the solution treatment temperature range of DD5 single crystal superalloy was determined by JMatPro and DSC curves.The researches of the solution treatment and aging treatment were found: The homogenization treatment and high temperature solution treatment could reduce the alloy structure of the dendritic segregation.The cooling mode of the solution treatment affects the ?' morphology.The size of ?' phase is below 100 nm by water coiling;the size of ?' phase is extremely large and over 600 nm after furnace cooling;and the size of ?' particles is homogeneous and orderly after air cooling.The Vickers hardness of the alloy is about 465 after solution treatment by air cooling,which is higher than that of the as-cast microstructure.And the microhardness of the water cooling sample is the highest.The size and morphology of ?' phase in the superalloy can be further adjusted by aging treatment.It is found that ?' particles appear circular shape and random arrangmeat,when the temperature of aging treatment is too low.With the temperature rising,the circular ?' particles gradually become cubic shape and ordered arrangement.When the temperature continues to rise,the standard cubic ?' phase particles become rough.The optimum heat treatment process of DD5 single crystal superalloy is 1270?,2h +1280?,2h +1290?,2h + 1300?,2h +1310?,2h +1320?,6h AC +1080?/4h AC +780?/24 h ACThe study of DD5 single crystal superalloy in the long-term aging test shows that as time goes on,the ?' phase becomes larger and finally appears raft phenomenon;with the long-term aging temperature rising,the ?' phase coarsens obviously.During the long-term aging process,the microhardness of the alloy generally decreases as time goes by.The coarsening coefficient of 1050? long-term aging is greater than that of 900?,which indicates the coarsening rate of ?' phase of 1050? long-term aging coefficient is higher than that of 900?.During thermal fatigue experiments,it was found that the angle between crack propagation direction of DD5 alloy and the direction of dendrites growth is 45°under the second orientation both of [110] and [100].And the cracks of the second orientation [110] sample are more than second orientation [100] samples.The internal defects of the single crystal superalloy have great influence on the crack propagation.