Al、Ti、Ta对镍基定向高温合金组织与性能的影响

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	Al、Ti、Ta对镍基定向高温合金组织与性能的影响
	韩汾汾
学位类型	博士
导师	孙晓峰 ; 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	定向柱晶高温合金持久拉伸性能热裂氧化热腐蚀 Directionally Solidified Ni-base Superalloy Creep And Tensile Property Hot Tearing Oxidation Hot Corrosion
摘要	"本论文采用扫描电子显微镜（SEM）、电子探针（EPMA）、透射电子显微镜（TEM）、能谱分析（EDS）等手段对不同Ti/Al比和Ta含量的某种定向柱晶高温合金铸态组织、热处理态组织、典型条件下的持久和拉伸性能、铸造性能、抗氧化及抗热腐蚀性能进行了系统的研究，对比分析了Ti/Al比和Ta对定向柱晶高温合金组织与性能的影响及其机理。随着Ti/Al比和Ta含量增加，合金铸态组织中共晶含量显著增加。Ti/Al比增加时，合金初熔温度降低，铸态组织中共晶周围有η相生成；相同热处理条件下γ′相尺寸增加，立方度降低，γ′相形貌逐渐不规则，合金持久性能变化不大，但拉伸性能有所降低。而Ta含量增加时，合金铸态组织中共晶周围有M23C6形成；相同热处理条件下γ′相体积分数增加且立方度增加，合金的持久性能显著提高，而拉伸性能变化不大。随着Ti/Al比和Ta含量增加，合金的热裂倾向性增加。高Ti/Al比和高Ta含量合金，凝固末期枝晶间的残余液体含量较多，导致凝固收缩应变增大，且凝固后期的共晶熔体含量增加，二次枝晶臂搭桥面积降低，从而降低了晶界结合力，造成合金在铸造过程中产生热裂。在1000℃氧化过程中，由于高Ti/Al比合金表面不能生成较为连续致密的Al2O3氧化膜，导致合金的抗氧化性能显著降低。而Ta含量增加，合金在1000℃的抗氧化性能得到一定改善，这主要归因于富Ta氧化物层的厚度增加且更加连续，有效降低了离子迁移速率。但Ta对抗氧化性能的贡献有限，Ta含量过高，降低Al2O3的稳定性。 750℃低温热腐蚀条件下，虽然高Ti/Al比促使合金表面有更多的富Ti氧化物生成，但Ti/Al比对合金的抗热腐蚀性能影响不大。900℃热腐蚀条件下，Ti/Al比增加，合金的抗热腐蚀性能有所降低；不同Ti/Al比合金的热腐蚀增重随腐蚀时间延长都显著增加，但高Ti/Al合金腐蚀增重始终大于低Ti/Al合金，且合金表面有更多富Ni、Ti的腐蚀产物生成，而低Ti/Al比合金表面富Cr氧化物更多。950℃热腐蚀条件下，Ti/Al比增加，合金的抗热腐蚀性能显著降低；不同Ti/Al比合金热腐蚀100h后都出现失重现象，但高Ti/Al比合金表面的NiO不断生成和剥落，导致合金急剧失重，而低Ti/Al比合金表面有大量Cr2O3生成，且具有连续致密的Al2O3层。分析认为，在Al+Ti质量和一定的条件下，Ti/Al比增加导致合金表层的富Ni、Ti氧化物含量增多，而富Cr氧化物含量降低，氧化物之间的协同溶解效应是两种合金热腐蚀行为有所差别的主要原因。 Ta含量增加，合金的抗热腐蚀性能显著改善。低Ta合金的热腐蚀产物主要为(Ni, Co)O、Ni3TiO5、NiTiO3、CoCr2O4和NaTaO3，而高Ta合金表面腐蚀产物组成主要为Cr2O3、(Cr, Ti)TaO4、TiO2和(Ni, Co)Cr2O4。高Ta合金表面除了有连续的Cr2O3层形成外，在Cr2O3和Al2O3两层之间还有连续致密的(Cr, Ti)TaO4层形成，两者的共同作用保证了合金具有良好的抗热腐蚀性能。"
其他摘要	"In this dissertation, the as-cast and heat-treated microstructure, creep and tensile properties, castability, as well as the oxidation and hot corrosion resistance of a directionally solidified Ni-base superalloy with different Ti/Al ratio and Ta content have been systematically investigated using scanning electron microscopy (SEM), electron probe microanalysis (EPMA), transmission electron microscopy (TEM), and energy dispersive spectroscopy (EDS). The influence of Ti/Al ratio and Ta content on the microstructure and properties is analyzed and discussed. The volume fraction of g/g¢ eutectics increase with increasing of Ti/Al ratio or Ta content. A high Ti/Al ratio in the alloy prompts the formation of η phase around the as-cast eutectics, decreases the incipient melting temperature of the alloy, increases the size of γ′ phase and promotes the precipitation of irregular γ′ phase in the heat treated samples, but no obvious effect of Ti/Al ratio is found on the creep and tensile properties. A higher Ta content in the alloy prompts the formation of M23C6 phase around the as-cast eutectics, increases the volume fraction of γ′ phase in the heat treated samples, and at the meantime, optimises the γ′ morphology. Ta has beneficial effect on creep property, but shows little effect on tensile property. Alloys with high Ti/Al ratio and high Ta content show increased hot cracking propensity. The volume fraction of remaining liquid is increased in interdendritic regions at the final stage of solidification. A dramatic change in the fraction of liquid results in high strain and strain rates during this stage of solidification, which leads to crack formation and bad castability. In addition, the volume fraction of eutectics melt at the grain boundary is increased with increasing of Ti/Al ratio or Ta content in the alloy, that may prevent the bridging of sencondary dendrite arms, and therefore results in the lower grain boundary cohesion strength. At 1000oC, the oxidation resistance of the alloy reduces with increasing of Ti/Al ratio. Discontinuous and incompact Al2O3 layer formed on the surface of alloy with high Ti/Al ratio results in the gerneration of mass oxidation products like (Ni, Co)O and spinels. The formation of such non-protective oxidation scales is the main reason for the poor oxidation resistance. While higher Ta content in the alloy improves the oxidation resistance of the alloy at 1000oC. This can be attributed to the formation of Ta-rich oxides layer which could effectively inhibit the diffusion rate of ions. However, the beneficial effect of Ta on oxidation in the alloy is limited, probably due to the decreasing stability of Al2O3 layer. At 750oC, although higher Ti/Al ratio prompts the formation of more Ti-rich oxides on the surface of the alloy, there shows almost no effect of high Ti/Al ratio on the hot corrosion resistance of the alloy. However, at 900oC and 950oC, the hot corrosion resistance of the alloy reduces with increasing of Ti/Al ratio, and the weight gain of the alloy with high Ti/Al ratio increases significantly with the temperature. The formation and spallation of NiO lead to rapid weight loss of the alloy with high Ti/Al ratio after corroded at 950oC for 100h. With the increasing of Ti/Al ratio (at an approximately constant Ti plus Al level of 7.7), the amounts of Ni-rich and Ti-rich oxides are increased on the surface of the alloy, while those of Cr-rich oxides decrease. Probably increasing of synergistic dissolution of oxides in molten salt is the main reason for the worse hot corrosion resistance of the alloy with high Ti/Al ratio. At 900oC, the hot corrosion resistance of the alloy is improved with increase of Ta content. Besides the Cr2O3 layer, a relatively continuous and compact (Cr, Ti)TaO4 layer is produced on top of Al2O3 with increasing of Ta content. Both of the Cr2O3 and (Cr, Ti)TaO4 layers acted as a protective scale improve the hot corrosion resistance of the alloy."
文献类型	学位论文
条目标识符	http://ir.imr.ac.cn/handle/321006/64447
专题	中国科学院金属研究所
推荐引用方式 GB/T 7714	韩汾汾. Al、Ti、Ta对镍基定向高温合金组织与性能的影响[D]. 北京. 中国科学院金属研究所,2012.