Research On A Second Generation Directionally Solidified Ni Base Superalloy DZ59

In order to meet the demands of aviation career in China,especially turbine blade materials with high temperature bearing capacity in advanced aeroplane engine,on the conceptions of taking advantage of the production efficiency and cost in directionally solidification process and avoiding the blank walls at present in single crystal solidification, the thesis aims to develop a new low-cost high-performance 2^nd generation directionally solidified superalloy according to the experience abroad and systematically investigates the influence of composition,process,service environment and other factors on the microstructure,properties,phase transformation.The main conclusions are obtained as follows:1.By optimizing the composition and processing,high-performance 2^nd generation directionally solidified superalloy DZ59 has been prepared with anticipative requirement: density 9＝8.53 g/cm³,room-temperature tensile strengthσ_b＝1369MPa,yield strengthσ_0.2＝1069MPa,elongation ratioδ＝10.8%,section contract ratioΨ＝11.4%.The strength and plasticity is increased by 30%and 20%respectively compared with DSжc6y. High-tempeature creep rupture life at 975℃/230MPa is up to 370hr.,increased by 290% than that of DSжc6y.Tensile strength at 760℃is 1301MPa,18%higher than 2^nd single-crystal superalloy with same Re content.Tensile strengthσ_b＝947MPa at 900℃is comparable with 1^st generation single-crystal superalloy DD3,11%larger than the 1^st generation single-crystal superalloy.The high-temperature creep rupture properties characterized by L-M comprehensive creep parameters are superior to those of the 1^st generation single-crystal superalloy in all sides and comparable with the 2^nd generation international typical superalloy CMSX-4.And the total expense is cut down 25%lower than that of the 1^st single-crystal superalloy and 35%lower than the 2^nd single-crystal superalloy with similar content.2.It is found thatμphase in DZ59 alloy is distributed in the interdendritic region. Granularμphase can effectively inhibit the dislocation movement and grain boundary slide and become the main secondary precipitate strengthening phase.The formation ofμphase in the interdendritic region is due to the boundary segregation ofμphase content element. Meanwhile,the composition repel and vibration ofγ' phase also promote the formation ofμphase.3.During the aging heat treatment of DZ59 alloy,secondary grain boundary reaction zone(SGRZ) is formed by the first grain boundary reaction zone(FGRZ),secondaryμphase,Re-contained M₂₃C₆ and multiphase colonies,among which Re-contained M₂₃C₆ is seldom reported in DS superalloys.The formation of SGRZ cellular colonies is influence by temperature,time and strain.During strain-free aging heat treatment,the formation of the colonies is related with the coarsen and merge ofγ' phase and precipitation ofμphase. During the aging heat treatment,such microstructure is controlled by the evolution of dislocation density in the FGRZ boundary,diffusion of Re atoms along dislocations,and bridge and merge process ofγ' phase.The transverse interfaces are increased by the larger-size of cellular clonies,which become the possible origin of the creep voids initiation and propagation under high load.4.Ta is beneficial to increase the transformation temperature of each phase during the solidification process of DZ59 alloy,promote the formation Of script MC and chain M₆C, and dissolve intoγ' phase,which increaseγ-γ' misfit and cubic degree,at last tensile properties,specific strength and creep properties.5.Re decreases the formation temperature of MC phase,effectively inhibits the formation and brittle of M₆C phase and the formation of MC phase,and promotes the formation ofμPhase and decomposition of MC phase.The dispersion and difficulty of difussion of Re are also helpful to improve the morphology and distribution of boundary precipitate phases and increase the strength greatly.Besides,Re addition is significantly beneficial to the high-temperature creep and brittleness,which is as twofold as the same amount addition of W. 6.DZ59 alloy can be exposed at 900℃for more than 600hr.without too much loss of high-temeprature tensile properties.However,the properties,especially the contraction ratio of section and plasticity would significantly decrease if that is longer than 1000hr.. The creep rapture strength would decrease monotonously with longer heat exposure time at 975℃under 255MPa.Either needle-shaped or granularμphase,which is about 0.2-0.4 %in the heat exposure process or creep rupture process,does not influence the performance.The cracks initiate from the weakened matrix,large size MC carbides orγ/γ' coherence interfaces.Large amount of SGRZ cellular colonies would be formed at the grain boundary region in DZ59 alloys after exposing at 950℃for 1000hr..Decreased plasticity is mainly due to the increased transverse area of grain boundaries.