IMR OpenIR
液态金属冷却定向凝固工艺对定向合金组织与性能的研究
李凯文
Subtype硕士
Thesis Advisor张健
2012
Degree Grantor中国科学院金属研究所
Place of Conferral北京
Degree Discipline材料工程
Keyword定向高温合金 热裂 热处理 持久 拉伸 Directional Solidification Superalloy Hot Tearing Heat Treatment Stress Rupture Tensile Property
Abstract"本论文采用金相显微镜(OM)、扫描电镜(SEM)、电子探针(EPMA)、能谱分析(EDS)等手段,利用传统的快速凝固(HRS)定向凝固技术和先进的高温度梯度液态金属冷却(LMC)定向凝固技术对一种定向凝固高温合金铸态/热处理态组织、热裂倾向、拉伸以及持久性能和变形组织进行了系统研究。主要结论包括以下几个方面:
研究发现,HRS及LMC工艺下合金铸态组织一次枝晶间距分别为327±5μm,135±3μm,共晶百分含量分别为4.2±0.04%,3.2±0.08%。合金经过1220℃、1240℃、1250℃、1260℃进行固溶处理,发现在相同温度下,HRS工艺的样品共晶含量均高于LMC工艺下的共晶含量,且HRS工艺下的样品在1260℃时,出现初熔现象,而LMC工艺下的样品共晶完全溶解,未观察到初熔。
实验合金经过760℃/840MPa低温高应力以及980℃/235MPa高温低应力持久变形后,合金在低温高应力时持久断口呈阶梯状,合金在高温低应力持久变形后,持久断口呈韧窝状。不同状态(铸态、直接时效及固溶处理再经过时效处理)样品持久裂纹的萌生位置不同,当样品中存在残留共晶时,其裂纹主要的萌生处在共晶,当样品中无残留共晶时(固溶温度为1260℃的持久样品),其裂纹主要萌生于碳化物/基体界面。同一热处理制度下,HRS及LMC样品980℃/235MPa的持久寿命相当,随着固溶温度的提高,持久寿命有所提高但不明显。而760℃/840MPa条件下,随着固溶温度的提高,持久寿命有明显提高。
实验合金经过室温拉伸变形后,发生脆性断裂。铸态、时效处理的样品中,共晶、碳化物为主要裂纹源。固溶温度为1260?C的样品,碳化物为主要裂纹源。
合金热裂敏感性研究表明,HRS和LMC工艺下,距定向凝固冷端距离分别为40mm、80mm、120mm三个位置的晶粒个数相当。HRS条件下的三个不同位置的一次枝晶间距分别为266±26μm, 352±28μm, 290±27μm,共晶含量分别为1.21±0.14%, 1.52±0.13%, 1.44±0.15%,LMC工艺下的三个不同位置的一次枝晶间距分别为103±6μm, 109±5μm, 110±5μm,共晶含量分别为0.41±0.17%,0.45±0.13%,0.45±0.1%。
LMC条件下的热裂敏感因子较HRS要小很多。LMC工艺降低了合金热裂敏感性。
"
Other Abstract"In this dissertation, the as-cast and heat-treated microstructure of a directionally solidified (DS) superalloy produced by both high rate solidification (HRS) process and the advanced liquid metal cooling (LMC) process has been studied. The tension property and rupture properties, as well as the hot cracking tendency of the alloy processed by different DS techniques have been systematically compared. Metallographic microscope (OM), scanning electron microscope (SEM), energy dispersive spectroscope (EDS), and electron probe microanalysis (EPMA) were employed in the present study. The main results are as follows:
The primary dendritic arm spacing (PDAS) of the as-cast microstructures by the HRS and LMC processes are approximately 327±5μm and 135±3μm and the volume fractions of eutectic are 4.2±0.04% and 3.2±0.08% respectively. Different solution temperatures (1220℃, 1240℃, 1250℃ and 1260℃) were used in the experiments. It was found that the volume fraction of the residual eutectic in HRS process is higher than in the LMC sample after the same heat treatment. In addition, the incipient melting was observed in the HRS sample when the solution temperature is 1260℃. As a comparison, the as cast eutectics in the LMC sample disappeared and full solutionized alloy was obtained at the solution temperature of 1260℃Different fracture surface characterization was observed after stress rupture test at 760℃/850MPa and 980℃/235MPa. The initiation of cracks of different samples (as-cast, aging, solution and aging) was different. When the residual eutectic existed in the sample, the cracks initiated at the eutectic/matrix interface, while the cracks initiated at the carbide/matrix interface when there is no residual eutectics.. By the same heat treatment procedure, the stress rupture life at 980℃/235MPa was comparable for both HRS and LMC samples. The stress rupture life at 760?C/850MPa significantly increased as increasing the solution temperature was increased.
The fracture surface was brittle rupture after the room temperature tension test. The majority cracks in the as-cast and aging samples initiated at the carbide/matrix and/or eutectic/matrix interface. The cracks mainly initiated at the carbide/matrix interface when the solution temperature was 1260℃.
The results of the hot-tearing castability tests showed that: there was no essential difference in the number of grains at the bottom, middle and top (40mm, 80mm and 120mm from the cooling chill, respectively) sections of the castings obtained by HRS and LMC processes. The PDAS was greatly refined in LMC castings, i.e. The PDAS in HRS sample at the three different places was measured as: 266±26μm, 352±28μm and 290±27μm, respectively, while the PDAS of the LMC sample at the three different places was 103±6μm, 109±5μm, and 110±5μm. The volume fractions and size of the eutectic were also reduced in the LMC samples.
The crack ratio of the LMC castings was smaller than that of the HRS castings, and the hot tearing sensitivity was reduced by the LMC process.
"
Document Type学位论文
Identifierhttp://ir.imr.ac.cn/handle/321006/64549
Collection中国科学院金属研究所
Recommended Citation
GB/T 7714
李凯文. 液态金属冷却定向凝固工艺对定向合金组织与性能的研究[D]. 北京. 中国科学院金属研究所,2012.
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