钴基高温合金碳化物演变及相关性能研究

发布时间：2018-08-18 12:51

【摘要】：钴基高温合金因其具有较高的熔化温度、优异的抗热疲劳以及抗热腐蚀性能,广泛应用于航空发动机中导向叶片材料。热疲劳是导向叶片最主要的失效形式之一。热疲劳裂纹主要萌生于碳化物和晶界处,并且会沿着碳化物和晶界扩展。为了提高合金的热疲劳性能,需要探索改善碳化物和晶界形态的途径。热处理是影响碳化物类型、形貌、尺寸以及分布的重要手段。本文重点研究了热处理条件下DD640M和DD6509两种钴基高温合金碳化物组织演变规律及其对热疲劳行为的影响,并关注了相应条件下合金持久性能的变化规律。另外,为避免晶界对碳化物演变产生影响,合金应用单晶凝固技术制备。同时,晶界的消除也有利于合金热疲劳性能的提高。本研究为钴基高温合金的发展和应用提供理论基础。DD640M合金的铸态组织为粗大的富Cr网状M_7C_3和富Ta、Zr的骨架状MC的两种共晶碳化物。在1140～1260℃/4h热处理过程中,随着温度升高初生碳化物的数量和尺寸逐渐减小。另外,热处理过程中发生了 M_7C_3→M23C6反应以及MC碳化物的蜕化。本研究中首次揭示M_7C_3碳化物以原位方式向M23C6碳化物发生转变。M23C6碳化物在M_7C_3碳化物和基体界面处形核并朝着M_7C_3碳化物长大。初生MC碳化物在热处理过程中发生蜕化释放大量的W和Ti,温度较低时分解形成M6C碳化物,温度较高时则仅以固溶形式发生蜕化。DD6509合金的铸态组织为粗大的富Ta骨架状MC和富Cr不规则块状M23C6两种共晶碳化物。在1260～1330℃/4h热处理过程中,初生碳化物逐渐发生溶解。初生M23C6碳化物在1300℃/4h热处理时完全溶解到基体中,另外,高温下部分骨架状MC碳化物分解成颗粒状。固溶处理促进DD640M和DD6509合金在1000～1200℃时效条件下析出更均匀更细小的二次碳化物。DD640M合金基体中只析出二次M23C6碳化物;DD6509合金中析出二次MC和M23C6碳化物,其中二次MC碳化物分布在基体中而二次M23C6碳化物分布在碳化物周围。DD640M合金初生M_7C_3共晶碳化物在1280℃发生熔化,重凝组织为片层更细的M23C6共晶碳化物。M_7C_3共晶碳化物的熔化过程为M_7C_3碳化物先转变成M23C6碳化物再发生熔化。初生MC共晶碳化物在1320℃发生熔化,重凝后形成骨架更细的MC共晶碳化物。DD6509合金中初生M23C6共晶碳化物熔化发生在1335℃以及MC共晶碳化物熔化发生在1340℃。本研究首次关注了钴基高温合金中初生共晶碳化物的熔化现象,为优化钴基高温合金的化学成分以及微观组织提供借鉴。采用合适的热处理方法可有效改善DD640M和DD6509合金的热疲劳性能。其中,DD640M 和 DD6509 合金分别在 1260℃/24h 和 1330℃/24h+1100℃/100h热处理后热疲劳性能提高最为明显。热处理使得合金中碳化物更加弥散和细化,减缓热疲劳裂纹萌生与扩展,从而提高合金热疲劳性能。DD640M和DD6509合金高温固溶处理后持久寿命均有提高,其缘于热处理后获得良好的组织稳定性、细小MC碳化物以及过饱和固溶体。DD640M合金铸态样品在高温持久过程中发生了 M_7C_3→M23C6, M23C6→M6C和MC→M23C6转变。热处理使得钴基高温合金热疲劳性能和持久性能均得到显著提高,这改变了热处理对钴基高温合金性能影响有限的认识,钴基高温合金热处理应该得到应有的重视。两种合金的成分差异影响合金碳化物组织的组成和稳定性。DD6509合金的热疲劳性能和高温持久性能均优于DD640M合金,归因于DD6509合金更加稳定的碳化物组织、较高的碳化物含量和二次MC碳化物的析出。
[Abstract]:Cobalt-based superalloys are widely used in aero-engine guide vanes because of their high melting temperature, excellent thermal fatigue resistance and thermal corrosion resistance. Thermal fatigue is one of the most important failure modes of guide vanes. In order to improve the thermal fatigue properties of alloys, it is necessary to explore ways to improve the carbide and grain boundary morphology. Heat treatment is an important means to influence the type, morphology, size and distribution of carbides. In addition, in order to avoid the effect of grain boundary on carbide evolution, the alloy was prepared by single crystal solidification technique. At the same time, the elimination of grain boundary was beneficial to the improvement of thermal fatigue properties of the alloy. The as-cast microstructure of the alloy is two kinds of eutectic carbides with thick Cr-rich reticulate M_7C_3 and rich Ta, Zr skeleton MC. The amount and size of primary carbides decrease gradually with the increase of temperature during the heat treatment process from 114 to 1260 c/4h. In addition, the reaction of M_7C_3 M23C_6 and the decay of MC carbides occur during the heat treatment. M23C6 carbides nucleate at the interface between M 7C 3 carbides and matrix and grow towards M 7C 3 carbides. Primary MC carbides decay and release large amounts of W and Ti during heat treatment, and decompose to form M6C carbides at lower temperatures, but only solid solution at higher temperatures. The as-cast microstructure of DD6509 alloy is composed of coarse Ta-rich skeleton MC and Cr-rich irregular block M23C6 eutectic carbides. Primary carbides dissolve gradually during heat treatment at 1260-1330 c/4h. Primary M23C6 carbides dissolve completely into the matrix at 1300 c/4h, and some skeleton MCs at high temperature. Solid solution treatment promotes the precipitation of more homogeneous and finer secondary carbides in DD640M and DD6509 alloys at 1000-1200 C. Only secondary M23C6 carbides are precipitated in DD640M alloy matrix; secondary MC and M23C6 carbides are precipitated in DD6509 alloy matrix, and secondary MC carbides are distributed in matrix while secondary M23C6 carbides are precipitated in DDD6509 alloy matrix. The primary M_7C_3 eutectic carbide of DD640M alloy melts at 1280 C, and the recrystallized structure is M23C_6 eutectic carbide with finer lamellar structure. The melting process of M_7C_3 eutectic carbide is that M_7C_3 carbide first transforms into M23C_6 carbide and then melts. The primary MC eutectic carbide melts at 1320 C and then re-solidifies. MC eutectic carbides with finer skeleton. Melting of primary M23C6 eutectic carbides in DD6509 alloy occurred at 1335 C and melting of MC eutectic carbides occurred at 1340 C. The melting phenomenon of primary eutectic carbides in cobalt-based superalloys was studied for the first time in this study, which provided a reference for optimizing the chemical composition and microstructure of cobalt-based superalloys. Thermal fatigue properties of DD640M and DD6509 alloys can be effectively improved by proper heat treatment. Thermal fatigue properties of DD640M and DD6509 alloys are improved most obviously after heat treatment at 1260 c/24h and 1330 c/24h + 1100 c/100h, respectively. The rupture life of DD640M and DD6509 alloys after high temperature solid solution treatment is improved because of their good microstructure stability, fine MC carbides and supersaturated solid solution. As-cast samples of DD640M alloy occur M_C_3_M23C6, M23C6_M6;MC and MC_M23C during high temperature rupture. 6 transformation. The thermal fatigue and rupture properties of Co-based superalloys have been significantly improved by heat treatment, which changes the understanding that heat treatment has limited effect on the properties of Co-based superalloys. The heat treatment of Co-based superalloys should be given due attention. The composition difference between the two alloys affects the composition and stability of the carbides. The thermal fatigue properties and high temperature rupture properties of alloy DD6509 are superior to those of DD640M alloy, which is attributed to the more stable carbide structure, higher carbide content and secondary MC carbide precipitation.
【学位授予单位】：中国科学技术大学
【学位级别】：博士
【学位授予年份】：2017
【分类号】：TG132.3

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