| Because of its excellent high-temperature comprehensive properties,nickel-based single crystal superalloy is widely used in the manufacture of heat resistant parts such as aerospace engine.With the rapid development of the aerospace industry,the performance requirements of turbine blades are constantly improving.However,in our country,the research of nickel based single crystal superalloy started relatively late;on the other hand,the research on heat treatment and properties of single crystal alloy is not perfect enough.Hence,it is of great scientific and practical value to study and optimize the heat treatment process and fatigue fracture properties of high-performance nickel based single crystal alloy.By studying the heat treatment process of a second generation low-rhenium nickel base single crystal alloy,the effect of heat treatment process on hardness of single crystal alloy was investigated.At last,the fracture behavior of the single crystal alloy under high cycle fatigue is discussed.The results of the study are as follows:Through differential thermal analysis,the DSC of the alloy was got and the heat treatment window(solution temperature range)for single crystal alloy was determined.Moreover,the parameters of solution treatment are designed for single crystal alloy and through analyzing the changes of microstructure and the properties(hardness)of single crystal alloy before and after solution treatment thus the optimum heat treatment scheme after optimization was determined.After homogenization treatment and solution treatment: they can almost completely eliminated the segregation of alloy composition,make the composition uniform,adjust the size of the dendritic arm.On the other hand,the temperature of the high temperature solution treatment influence the size,quantity and cubic morphology of ?' phase structure.In addition,with the increase of temperature,the sizes of the precipitated ?' phase decreases(at 1320?,the size is the best and it is 355~370nm)and the morphology changes from a mellow round to cubic.It is found that,the temperature and time of high temperature aging affect the morphology,size and quantity of ?' phase: with the increase of high temperature aging temperature(1200?),the size of the ?' phase increases gradually and the shape change from the irregular sphere to regular cubic shape,on the other hand,with the extension of the aging time,the cubic degree of the ?' phase presents a tendency to change first and then become worse.When the temperature is too high,the angle of the ?' phase is passivated and part of the ?' phase is interconnected.Besides the higher cubic degree of ?' phase is obtained after 4 hours.The two times temperature affects the uniformity of the cubic degree of ?' phase.When temperature increases,the cubic degree of ?' phase is regular and the size is more uniform,when the time of aging treatment at 870? and 900 ?.In this experiment,the best heat treatment scheme obtained is 1270?/2h+1280?/2h+1320?/4h,AC+1100?/4h,AC+870?/24 h,AC.After the best heat treatment,the hardness of single crystal alloy increased from 357 MPa to 537 MPa.In the short time aging process,with the increase of short-time aging temperature and the prolongation of time,the hardness increases first and then decreases,moreover,due to the temperature is different the time to reach the peak is different.Through the high cycle fatigue test,the fatigue limit of the single crystal alloy at 900? is about 400 MPa,the fatigue limit of the notched specimen is about 315 MPa and the notch sensitivity of the material is about 0.135.It shows that the single crystal alloy has little sensitivity to the notch at 900 ?,which is related to the higher high-temperature plasticity and good oxidation resistance of the alloy.The fatigue fracture of smooth and notched specimens of this single crystal alloy are both divided into crack initiation area,crack expansion area and instantaneous fracture zone.The fracture surface is a mixed fracture of ductile fracture and brittle fracture. |
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