Effect Of Long-term Aging On Microstructure And Mechanical Properties Of A Single Crystal Superalloy DD407

With the continuous development of aero-engines,single crystal superalloys have been widely used in advanced aero-engines due to their advantages such as high operating temperature,good oxidation resistance and good structural stability.DD407 is a nickel-based precipitation-hardened single crystal superalloy with excellent comprehensive mechanical properties such as long-term durability,creep strength,and fatigue resistance.However,the high alloying makes the microstructure stability of the alloy become one of the focuses.For this reason,this article aims at long-term aging at high temperature for DD407 alloy,and studies the microstructure evolution and its influence on tensile mechanical properties and deformation behavior and mechanism under different temperature and time conditions.The results showed that the homogeneity and cubicness of the y' phase in the alloy after standard heat treatment increased,showing a small square with an average size of 0.32?m.During long-term aging at 900? and 1000?,the ?' phase continued to grow.After aging for 2000h,the average size of ?' phase reached 0.79?m and 1.58?m,respectively.During the long-term aging at 900?,the volume fraction of ?' phase continued to increase.During aging at 1000?,the volume fraction of ?' phase fluctuated around 70%,and after aging for 2000 hours,it was 64.08%.After aging at 900? and 1000? for 2000h,the size,composition and number of carbides did not change significantly.However,after aging at 1000? for 2000h,there was a phenomenon that the carbides were wrapped by the ?' phase.After aging at 900? for 2000h,the tensile strength of the alloy at 600? was similar to that of the standard heat treated state,but the yield strength was reduced by 30.7%,and the elongation at break was increased by 61.5%;the yield strength and tensile strength of the alloy at 750? were the same as those of the standard heat treated state,and the elongation at break increased by 43.1%;the yield strength and tensile strength of the alloy at 900? were significantly reduced compared to the standard heat treated state,but the elongation at break did not change significantly.After aging at 1000? for 2000h,the tensile properties of the alloy at 600? showed a difference from the aging at 900? for 2000h,that was,the yield strength and elongation at break were not significantly different from those before aging,but the tensile strength increased;At 750? and 900?,the tensile properties of the alloy showed the same tendency as after aging at 900? for 2000h that the yield strength and tensile strength both decreased.The analysis found that at a high deformation temperature(900?),the proportion of dislocation creep mechanism increased;after aging,the speeds of rafting between the dendrites and the dendritic stem structure of the alloy in the tensile procedure were different,which accelerated the tensile fracture of the alloy.After aging at 900? and 1000? for 2000h,the fracture modes of the alloy at the three deformation temperatures were all mixed fractures,but with the increase of the deformation temperature,the fracture toughness characteristics increased significantly.At 900?,the ductile fracture characteristics of the alloy dominated.At the same aging time and tensile deformation temperature,compared with 900? aging,long-term aging at 1000? had a greater impact on the high-temperature tensile properties of the alloy at 750?.After long-term aging at 900?,the alloy can still maintain good structural and mechanical properties stability to a certain extent;after 1000h aging at 1000?,the stability of the structure decreased and the short-term tensile mechanical properties at high temperatures were significantly degraded.