【作者】 赵坦；

【导师】 陈光； 张健；

【作者基本信息】 南京理工大学 ， 材料科学与工程， 2009， 博士

【摘要】 为了充分发挥定向柱晶在生产效率、制造成本方面的优势,避免单晶高温合金难以解决的问题,满足我国航空事业发展、特别是先进航空发动机对涡轮叶片承温能力的要求,本文在借鉴国外先进经验的基础上,以研制开发一种新型低成本高性能第二代定向柱晶高温合金为目标,系统研究了合金成分、工艺、服役条件等因素对组织、性能的影响以及相形成与演化规律,得到以下主要结论:1)通过成分设计与工艺优化,成功研制的第二代定向柱晶高温合金DZ59达到了预期目标要求,其密度为8.53g/cm³,室温性能σ_b＝1369MPa,σ_0.2＝1069MPa,δ＝10.8%,ψ＝11.4%,与基础合金DSжc6y相比,强度和塑性分别提高30%和20%;975℃/230MPa高温蠕变断裂寿命达370hr.,比DSжc6y提高290%;760℃瞬时拉伸强度σ_b＝1301MPa,比相同Re含量的第二代单晶高温合金DD6高18%;900℃瞬时拉伸强度σ_b＝947MPa,与第一代单晶高温合金DD3相当,较第一代定向柱晶DZ125合金高11%;由L-M综合蠕变指数表征的高温蠕变断裂性能全面超过第一代单晶高温合金,达到国际上典型的第二代单晶高温合金CMSX-4水平;综合制造成本比第一代单晶高温合金可降低25%,比类似成分的第二代单晶高温合金可降低35%。2)发现DZ59合金中的μ相具有枝晶间分布的特性,颗粒化的μ相可以有效的阻碍位错运动以及晶界滑移,而成为主要的次生沉淀强化相。DZ59合金枝晶间颗粒化μ相的形成机制为:时效过程中,μ相的形成元素（Re、Mo、Cr等）向界面偏聚以及γ’的形成和成分起伏促进了μ相的形成。3)在DZ59合金时效过程中晶界发生多次反应,形成由FGRZ、次生μ相、含Re的M₂₃C₆以及多相晶团所组成的二次晶界反应区（SGRZ）,其中含Re的M₂₃C₆型碳化物在以往的定向柱晶高温合金中从未被发现过。SGRZ中胞状晶团的产生受温度、时间、应力影响,在静态时效中,该晶团的产生同γ’的粗化、搭接合并以及μ相的析出等过程相关;在应力时效中,该组织受FGRZ边界位错密度的变化、Re等难溶元素沿位错等溶质高速扩散通道扩散以及γ’的搭接合并过程等综合因素的影响。尺寸较大的胞状晶团增加横向晶界受力界面,高负荷下可能成为蠕变空洞萌生和扩展的起源。4)Ta有助于提高DZ59合金凝固过程中各相的转变温度,促进骨架状MC形成,并加重晶界M₆C型碳化物的链状倾向。Ta大量进入γ’相,增大γ’-γ两相的错配度,有助于γ’相形貌的立方化,显著提高室温瞬时拉伸性能、比强度以及高温蠕变断裂性能。5)Re降低MC的形成温度,有效抑制M₆C的形成和脆性MC的生长,促进μ相的生成和MC分解。Re原子的弥散分布及难扩散性质还有助于改善晶界沉淀相的形貌和分布,大幅增加基体的强化效果,对高温蠕变断裂性能作用尤为明显。1%（wt）Re的作用同2%（wt）W的强化效果相当。6)DZ59合金在900℃/600hr.以内的长期热曝露过程中可以保持较高的高温拉伸性能,但是热曝露超过1000hr.后,性能大幅下降,其中断面收缩率下降尤为明显,此时晶界区域产生较高含量的胞状晶团增加了晶界横向受力面积是合金塑性下降的主要原因。在975℃/255MPa条件下,随着热曝露时间的延长,蠕变断裂强度单调下降,热曝露1000hr.时,合金塑性急剧下降。该合金长期热曝露以及蠕变断裂过程产生0.2-0.4%（体积分数）的μ相,但无论该μ相是短针状还是颗粒状对性能均影响不大。弱化的基体、大尺寸的MC碳化物以及γ/γ’共晶是DZ59合金服役过程主要裂纹萌生源。更多还原

【Abstract】 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.更多还原