| 摘要 | 本文系统地研究了抗热腐蚀镍基单晶高温合金DD10的铸态组织、凝固顺序、热处理制度,以及合金典型的力学性能、抗氧化和抗热腐蚀性能。
利用液态金属冷却定向凝固工艺，研究了定向凝固速率对DD10合金的显微组织和典型持久性能的影响，发现在较宽的定向凝固抽拉速率范围内均可制备出单晶，但拉速过快时会出现杂晶形核和长大。一次枝晶间距、二次枝晶间距、共晶体积分数和尺寸随定向凝固抽拉速率的增加而减小。
通过差热分析、等温水淬等实验方法确定了DD10合金的凝固析出顺序:基体树枝晶，碳化物，共晶, eta相，硼化物相，gamma prime相和sigma相。分析发现在共晶形成以后,共晶前沿残留液体中Ti、Ta含量较高，从而诱发了eta相生成，但仍然有极少量的残余液体存在。这些液体中富集Cr、Mo和W元素,它们可能与B原子结合生成硼化物；也可形成过饱和固溶体，在随后的冷却过程中析出sigma相。
根据DD10合金中各相的凝固析出规律,制定了分级固溶热处理制度，依次固溶合金中的sigma相、硼化物、eta相和共晶，并采用1100oC/5h, AC + 870oC/24h, AC两级时效制度，实现了合金较为充分的均匀化，得到了尺寸理想的立方状二次gamma prime和球状三次gamma prime。
DD10合金在温度为980~1030oC，应力为235~300MPa的条件下具有良好的持久性能。分析不同持久阶段中断样品的显微组织发现：持久初期，g'相基本保持立方形态，位错通过攀移越过gamma prime；随着持久的进行，gamma prime逐渐形成了垂直应力方向的粗化，少量位错以切入方式进入筏化的gamma prime。
DD10合金在室温、760oC和950oC的拉伸变形机制不同。室温拉伸以滑移为主要变形机制，a/2<110>方向的螺型位错多塞积于与滑移线平行或与略微有小角度的基体通道内。合金760oC拉伸时有多个滑移系先后开动，<110>滑移线与<211>层错存在交互作用。950oC拉伸时位错以绕过机制越过gamma prime粒子，或者以不全位错以切割gamma prime并生成层错的方式通过gamma prime粒子；两个不同<211>晶向的层错相交组成层错锁，提高了合金的变形抗力。
DD10合金具有较好的抗氧化和抗热腐蚀性能。合金在870oC、900oC、980oC和1000oC氧化时，氧化动力学曲线符合抛物线规律,氧化产物分为三层：外层主要由Cr2O3和TiO2组成，中间层是很薄的CrTaO4和Ta2O5，内层是连续的Al2O3。1100oC氧化动力学曲线偏离抛物线规律,生成了NiCr2O4, CoAl2O4, CoNiO2和Co2TiO4等尖晶石相。900℃涂盐(80wt.%Na2SO4+20wt.% K2SO4)热腐蚀实验表明，合金抗热腐蚀性能略低于M38合金，热腐蚀200小时后，DD10合金表层热腐蚀产物主要为Cr2O3和TiO2，内层有硫化物生成。热腐蚀过程中存在着硫化－氧化反应，生成的氧化膜发生了碱性熔融 |
| 其他摘要 | In this dissertation, the microstructure, solidification sequence, heat treatment process, typical mechanical performance, oxidation and hot corrosion behaviors of hot corrosion resistant nickel base single crystal DD10 were investigated.
The effects of directional solidification withdrawal rates on the microstructure and creep rupture strengths of DD10 alloy obtained by liquid metal cooling technique were studied. It was found that single crystal structure could be obtained within a wide range of the withdrawal rates. However, spurious grains would nucleate and grow when the withdrawal rate was too high. With the increasing of withdrawal rates, the primary dendrite arm spacing, secondary dendrite arm spacing, size and volume fraction of the eutectic decreased. Creep rupture life of heat treated DD10 alloy increased with the increasing of withdrawal rates.
Combined the results from differential scanning calorimeter, quenching experiments and observation of the deep-etched samples, the solidification and precipitation sequence of DD10 alloy was determined. The gamma dendrite formed at high temperature, followed by the formation of MC carbides, eutectic, eta and borides. The gamma prime and sigma precipitated from the supersaturated Ni solution afterwards. According to the solidification sequence of DD10, a solution heat treatment procedure was designed to remove the eta, boride, sigma phase and eutectic. After two-step aging heat treatment: 1100oC/5h, AC + 870oC/24h, AC, homogenized microstructure of DD10 alloy was obtained with uniformly distributed secondary gamma prime in cubic morphology and tertiary gamma prime in spherical morphology in the gamma channel was obtained. DD10 had excellent creep rupture properties at 980~1030oC under stress from 235 MPa to 300MPa. Based on the microstructure of samples from different creep stage, it was found that at the initial stage, the gamma prime particles remained cubic shape. The dislocations passed the gamma prime by cross-slip or climbing. During creep, the gamma prime particles rafted, and a small quantity of dislocation cut into the rafted gamma prime.
Different deformation mechanisms operated in tensile tests of DD10 at room temperature, 760oC and 950oC. During tension at room temperature, the a/2<110> screw dislocations piled up in the matrix channels parallel to the sliding lines or with small divergence to them. At 760oC, at least two slip systems operated, and the <110>slip lines interacted with <211> stacking faults during tension. At 950oC, dislocations would bypassed gamma prime or partial dislocation cut into gamma prime with formation of stacking faults. Two intersecting <211> stacking faults formed stacking fault lock and increased the resistance to deformation.
DD10 alloy had excellent resistance to oxidation and hot corrosion. For the oxidation at 870o at the outer layersC, 900oC, 980oC and 1000oC, the mass gain increases and basically obeys a parabolic law due to the formation and growth of three oxide layers: an outer layer of Cr2O3 and TiO2, a thin intermediate layer of CrTaO4 and Ta2O5, and a continuous inner layer of Al2O3. For the oxidation at 1100oC, the kinetics curve deviated from the parabolic law. Some spinels such as NiCr2O4, CoAl2O4, CoNiO2 and Co2TiO4 formed and internal nitridation occurred. The hot corrosion resistance of DD10 was slightly lower than that of M38 at 900oC with a surface coating of 80wt.% Na2SO4+20wt.% K2SO4. After 200 hours, the main hot corrosion products of DD10 were Cr2O3 and TiO2 with some sulfides in the inner layer. Sulfidation-oxidation reaction was observed in hot corrosion process and acid fluxing occurred. |
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