镍基单晶高温合金共晶溶解动力学研究

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	镍基单晶高温合金共晶溶解动力学研究
	陈柏凡
学位类型	硕士
导师	张健
	2012
学位授予单位	中国科学院金属研究所
学位授予地点	北京
学位专业	材料学
关键词	镍基单晶高温合金固溶热处理共晶相变动力学 Ni-base Single Crystal Superalloy Solution Heat Treatment Eutectic Kinetics
摘要	"镍基单晶高温合金是制造航空发动机涡轮叶片的关键材料，其使用温度是影响发动机性能水平和效率的关键因素之一。随着镍基单晶高温合金的发展，难熔元素含量不断增加，合金的铸态枝晶偏析也越来越严重，固溶热处理也变得越来越困难。固溶热处理的关键在于枝晶间共晶的溶解，因此深入研究共晶溶解过程，对优化固溶热处理制度，降低叶片制造成本等具有重要意义。本文以难熔元素含量达到20%（质量分数）的第二代镍基单晶高温合金PWA1484为研究对象，采用差热分析法、金相法、电子探针成分分析等手段，系统地研究了共晶溶解等温动力学和连续升温动力学，并通过共晶溶解过程中的显微组织与成分演化规律探讨了共晶溶解的特点。实验发现PWA1484单晶合金的初熔温度约1315°C。对单晶合金在1290°C、1300°C、1310°C、1320°C的共晶溶解等温动力学研究表明，固溶处理温度越高，共晶溶解的速率越快，达到平衡时共晶的溶解量也越大。在某一温度保温时，初始阶段共晶溶解速率较快，之后共晶溶解速率逐渐变慢。共晶溶解等温动力学符合等温转变的JMAK相变动力学模型。对PWA1484单晶合金在1290°C到1310°C之间的连续升温动力学研究表明，升温速率过快，固溶时间不充分，共晶溶解量较小；升温速率降低，共晶溶解量增加；进一步降低升温速率对共晶溶解的影响不大。在温度升至1310°C后保温4h，不同升温速率的样品中残留共晶量基本相同。说明固溶处理温度是影响共晶溶解的关键因素。对PWA1484单晶合金在1310°C共晶溶解过程中的显微组织与成分演化的研究表明，在共晶溶解过程中，Ta元素含量的变化很大，其他元素含量的变化较小，Ta元素含量的变化强烈地影响共晶溶解过程。1310°C保温过程中，枝晶轴和枝晶间中除Re、W两种元素外，其他元素很快达到均匀化。分析认为，γ/γ΄共晶中Ta元素的扩散主要有两个途径：（1）Ta越过γ/γ΄相界，扩散至枝晶间；（2）Ta越过γ/γ΄相界扩散至共晶中的γ相通道，再通过γ通道扩散至枝晶间。因此只有直接与枝晶间接触的大块γ΄相和邻近γ通道的大块γ΄相才能迅速溶解。细小γ/γ΄共晶溶解较慢，这主要是由于细小γ/γ΄共晶中的Ta元素需要通过很长的扩散通道才能到达枝晶间，而且由于细小γ/γ΄共晶中Ta元素含量与枝晶间Ta元素的成分差异相对较小，Ta的扩散驱动力较小。根据共晶溶解动力学设计了PWA1484合金的固溶热处理制度，热处理后，共晶基本消除，获得了立方度好、大小和体积分数合适的γ΄相，合金的持久性能优异。"
其他摘要	Ni-base single crystal superalloy is the critical material for aircraft engine turbine blades. The temperature capacity of the alloys is one of the major concern determining the performance and efficiency of engines. With the development of Ni-base single crystal superalloy, the amount of refractory elements increases. The stronger dendritic segregation leads to the increasing difficulties during solution heat treatment of the advanced single crystal alloys. One of the major targets of solution heat treatment is to dissolve the eutectics. Therefore, it is very important to understand the dissolution behavior of eutectics in order to optimize the heat treatment procedure and lower the manufacturing cost of the blade. In this dissertation, a second-generation Ni-base single crystal superalloy containing 20% (mass fraction) refractory elements is used. Differential scanning calorimeter, metallographic method and electronic probe microanalysis, etc, were used in this study. The isothermal kinetics and continuous heating kinetics of eutectic dissolution were monitored in detail. The microstructure evolution of eutectics during dissolution was discussed. The incipient melting temperature of as-cast alloy is determined as 1315°C. The results show that dissolution of γ΄ phase is the main process of eutectic dissolution during isothermal exposure at 1290°C, 1300°C, 1310°C and 1320°C. The higher the heating temperature, the faster and the more eutectic dissolved. At a certain temperature, dissolution of eutectics gradually slowed down as increasing the isothermal time. The isothermal kinetics fits the JMAK model well. When heating from 1290°C to 1310°C at different heating rate, 5oC/h，10oC/h，and 20oC/h，respectively, the fraction of residual eutectics increases with increasing the heating rate. The residual eutectics in sample heated at 5oC/h and 10oC/h are much lower than that observed in sample heated at 20oC/h. After 4h at 1310°C, similar amount residual eutectics was found in all samples subjected to different heating rates, which indicates that the dissolution of the eutectics is strongly temperature dependent. Evolution of microstructure and composition of eutectics at 1310°C were studied in detail. The results show that most of the elements except W and Re homogenized very quickly. Ta content changed significantly and showed strong influence on the volume fraction of the eutectics. There are two ways for Ta to diffuse during the dissolution of eutectics: (1) diffusion into the surrounding interdendritic area directly, or (2) diffusion into γ channels in the eutectics and migrating through these γ channels into the surrounding interdendritic area. Therefore, only areas adjacent to interdendritic region or near the γ channel dissolve first. The dissolution of fine γ/γ΄ eutectics was much slower comparing to the dissolution of large γ΄ blocks. The reason is attributed to the much longer diffusion paths of Ta and the lower diffusion driving force. Finally, solution heat treatment with maximum temperature of 1315°C was designed based on the observation of the eutectic dissolution behaviors, and typical creep properties of the alloy were examined."
文献类型	学位论文
条目标识符	http://ir.imr.ac.cn/handle/321006/64512
专题	中国科学院金属研究所
推荐引用方式 GB/T 7714	陈柏凡. 镍基单晶高温合金共晶溶解动力学研究[D]. 北京. 中国科学院金属研究所,2012.