【作者】 徐仰涛；

【导师】 夏天东；

【作者基本信息】 兰州理工大学 ， 材料加工工程， 2010， 博士

【摘要】 传统沉淀强化机制认为,钴基高温合金中γ′强化相易呈脆性形态沉淀析出,γ′相与合金基体错配度较大,造成钴基合金不良的高温性能。Co-Al-W合金是由γ′-Co₃（Al,W）相沉淀强化的新型钴基高温合金,由于合金具有在某一高温范围内强度随温度升高而增加的“反常”变化现象。对Co-Al-W合金进行成分设计和制备,研究合金耐NaCl溶液电化学腐蚀、耐75%Na2SO₄+25%NaCl熔盐热腐蚀和空气中抗800、900℃高温氧化行为,合金在不锈钢基体上堆焊性能成为学者关注的问题。本论文在对燃烧合成和真空感应熔炼制备钴基Stellite 6合金微观组织、碳化物类型及分布等预实验及对Co-Al-W合金固溶温度、强化相类型、微观组织结构、硬度等国内外已有研究工作的基础上,通过合金成分设计,对燃烧合成和真空感应熔炼制备Co-Al-W合金固溶温度、强化相类型、微观组织结构、硬度等进行研究,进而对合金电化学腐蚀、热腐蚀和高温氧化性能及Co-Al-W合金在不锈钢基体上堆焊性能研究,为合金设计和应用提供理论依据和科学指导。研究发现:（1）燃烧合成和真空感应熔炼制备Co-Al-W合金微观组织由γ-Co基体及其上γ′-Co₃（Al,W）强化相和少量碳化物组成。钨含量增加,合金固溶温度升高,在γ-Co基体上γ′相数量和体积分数增加。合金元素Ta、Nb、Mo、Ti与γ-Co基体形成A3B型相Co₃（Ta,Nb,Mo,Ti）可提高合金中γ′-Co₃（Al,W）相和γ-Co基体的结晶度,对Co-Al-W合金中γ′相起到不同程度的稳定作用,合金的固溶温度和硬度适度提高。（2） Co-Al-W合金在NaCl溶液中电化学腐蚀时,由于Cl-穿透钝化膜导致“闭塞腐蚀电池”效应,在晶界处发生点蚀。加入合金元素Mo、Nb、Ti和Ta可以提高Co-Al-W合金耐NaCl溶液的电化学腐蚀能力。（3） Co-Al-W合金在800℃75%Na2SO4+25%NaCl熔盐中腐蚀后腐蚀膜分三层,即呈蓬松状由钴氧化物Co₃O₄组成的最外层,由Co、Al、W和合金元素复杂氧化物组成的中间过渡层和由Al、Co氧化物组成较致密的最内层。合金元素在氧化性气氛中形成的腐蚀氧化膜对基体起良好保护作用。（4） Co-Al-W合金在800℃和900℃空气中静态氧化后氧化膜大致分为三层,即以钴氧化物Co3O4形式存在且厚度基本均匀的氧化膜最外层;基体和表面层间不连续的中间过渡层主要由W、Al和合金元素氧化物组成;氧化膜内层主要是Co、Al氧化物。Co-Al-W合金中Ta、Nb、Mo、Ti元素的加入可提高合金在高温空气中静态氧化的氧化激活能,减少合金氧化增重,提高合金抗高温氧化能力。（5）在304不锈钢基体表面用TIG电弧对Co-8.8Al-9.8W（at.%）合金混合粉末进行堆焊,能获得表面成形及与母材结合良好的堆焊层。堆焊电流和堆焊速度都会对堆焊层熔宽、熔深和稀释率产生影响。堆焊层合金微观组织由γ-Co基体及其上碳化物和复杂Co、Al元素金属间化合物组成。堆焊层硬度较高,平均硬度约为HRC53.1,显微硬度最高Hv50可达1050。（6）燃烧合成和真空感应熔炼方法制备Stellite 6合金微观组织均由γ-Co基体及其上对基体起强化作用的碳化物组成。由于燃烧合成制备合金时热量聚集和散失不均匀,合金并不是过饱和固溶体,碳化物处于亚稳态且不能均匀析出。燃烧合成制备Stellite 6合金耐中性NaCl溶液腐蚀能力较强。综合以上研究结果,通过合金成分设计、合金化可显著提高Co-Al-W合金固溶温度和硬度,改善合金耐NaCl溶液腐蚀能力,提高合金耐75%Na2SO4+25%NaCl熔盐热腐蚀和合金在800、900℃空气中的抗氧化能力,进而根据需要对合金进行有目的的设计,为合金广泛应用提供科学和理论基础。更多还原

【Abstract】 Co-Al-W superalloy is strengthened by a ternary compoundγ′-Co₃（Al,W） phase with the precipitation strengthening onγ-Co matrix novel Co-base superalloys. Conventional Co-base superalloys lack effective precipitation strengthening by intermetallic compounds, and depend on alloying elements and precipitation of low volume fraction of carbides for their strength. The microstructures,carbides typology and distribution have effect on characteristics and properties of Stellite 6 alloy by combustion synthesis （CS） and vacuum induction melting（VIM）, and on top of this we study on microstructures and properties of tungsten content and alloying elements have effect onγ′strengthening phase of novel Co-Al-W superalloy by combustion synthesis （CS） and vacuum induction melting（VIM）.In addition, tungsten content and alloying elements, namely Tantalum、Niobium、Titanium、Molybdenum, have effect on electrochemistry characteristic, hot corrosion and high temperature oxidation behavior of Co-Al-W superalloy by vacuum induction melting in this paper. Finally, Tungsten Inert Gas （TIG） welding was used to deposit Co-8.8Al-9.8W （at.%） superalloy on 304 austenite stainless steel plate and cladding layer shape, dilution, Vickers hardness, microstructure and distribution of alloying elements were investigated. At the same time, the microstructure and corrosion behavior of cobalt-base Stellite 6 alloy by combustion synthesis and vacuum induction melting.The following results can be obtained:（1） The effect of alloying elements,such as Tantalum, Niobium, Titanium, Molybdenum onγ′precipitation strengthen phase of Co-Al-W superalloys by combustion synthesis and vacuum induction melting. The microstructures of Co-Al-W superalloy are composed of richγ-Co matrix,γ′-Co₃（Al,W） phase and few carbides. Tungsten stabilizeγ′phase with precipitation strengthening, the melting temperatures of novel Co-Al-W superalloy gradually increasd with tungsten content increasing. Positive effects of alloying elements Tantalum, Niobium, Titanium and Molybdenum are made of A3B-Co₃（Ta,Nb,Mo,Ti） strengthen phase withγ-Co matrix stabilizeγ′-Co₃（Al,W） phase, promoted the crystallization ofγ′-Co₃（Al,W） phase andγ-Co matrix, increased solvus temperature of Co-Al-W superalloy.（2） Corrosion behaviors of Co-Al-W superalloy additional alloying elements are investigated in different pH value NaCl solutions at room temperature by electrochemical techniques. Different Tungsten contents and alloying Co-Al-W superalloys completely suffered from serious pitting corrosion and pits located at grain boundaries. The pitting corrosion resulted from the electro-migration of Cl- into the pits from the buck solution by Occluded Corrosion Cell（OCC）. The pitting corrosion resistance of Co-Al-W superalloy gradually increased with addition to alloying elements, such as Tantalum, Niobium, Titanium, Molybdenum.（3） The kinetic of hot corrosion at 800℃in 75%Na₂SO₄+25%NaCl molten salt of Co-Al-W superalloy with tungsten content and alloying elements, namely, Molybdenum, Niobium,Tantalum and Titanium. The results show the hot corrosion oxide scale of Co-Al-W superalloy are made up of three layers, that is the external layer consists of Co oxide Co₃O₄, the intermediate mixed oxides layer is composed of complex oxide and nonuniform-barren oxide layer of Co, Al, W and alloying elements.The internal attacked layer with different compounds of Co, Al and O. When added to alloying elements, Molybdenum, Niobium, Tantalum and Titanium the hot corrosion resistance of Co-Al-W superalloys provides added protection against corrosion and the film oxidation compactness is gradually increased in NaCl solutions.（4） The kinetic data of weight gain and the cyclic oxidation behavior of Co-Al-W superalloy were measured and investigated at 800 and 900℃in air.The rate constants at different temperatures were determined by the relevant linear treatment. According to Arrhenius relation of rate constants of oxidation, the activation energy of oxidation was derived for Co-Al-W superalloy and the specified expression for the rate constant was further obtained in the case of oxidation at different temperatures. The results show that the oxide scale at different temperature exhibited a multi-layered structure including an outer layer of Co₃O₄ oxide, a layer is composed of complex oxide and non-uniform Co-barren oxide layer, an intermediate mixed oxides layer and an internal attacked layer with complicated oxides of Co and Al. The oxidation film of Co-Al-W superalloy surface appears agglomeration, oxide film crack propagation and oxide layer deterioration phenomena at different temperatures. Adding to alloying elements Molybdenum, Niobium, Tantalum and Titanium of Co-Al-W superalloy can reduce weight gain and increase activation energy of oxidation. Positive effects of alloying Co-Al-W superalloy provide added protection against hot corrosion at different temperatures.（5） Tungsten Inert Gas （TIG） welding was used to for deposit Co-8.8Al-9.8W（at.%） superalloy on 304 austenite stainless steel plate and cladding layer shape, dilution, Vickers hardness, microstructure and distribution of alloying elements were investigated. It was found that TIG cladding layer has the characteristics of large dilution rate, fine microstructure, narrow heat-affected zone （HAZ）, narrow alloying elements segregation, high Vickers hardness, high contents and low contents of Fe in the cladding layers.TIG cladding layer dilution of Co-8.8Al-9.8W（at.%） superalloy on 304 stainless steel is about 12% when current and Voltage are 100A and 12V,respectively. High Vickers microhardness can be available in cladding layer, which arrives at 1050 （Hv50）. The average Rockwell hardness value of cladding layer arrives at HRC53.1.（6） This investigation is undertaken to microstructures and corrosion behavior of Stellite 6 alloy by combustion synthesis and vacuum induction melting. The microstructures of Stellite 6 alloy by combustion synthesis （CSed Stellite 6） and vacuum induction melting （VIMed Stellite 6） are composed ofγ-Co matrix and primarily carbides at grain boundaries. Carbides existγ-Co matrix in the form of carbide mixture for the primary carbide and secondary eutectoid carbide of CSed Stellite 6, yet the carbides of VIMed Stellite 6 are single form at grain boundary for secondary eutectoid carbide. Comparing corrosion resistance of CSed Stellite 6 with VIMed Stellite 6 in NaCl neutral solutions, the former corrosion resistance is superior to the later.更多还原