单晶高温合金(如无特殊说明，文中单晶高温合金均指镍基单晶高温合金)主要用于制造航空发动机、燃气轮机热端涡轮叶片，其承温能力是提升发动机性能、效率、可靠性的关键技术指标。自20世纪七八十年代以来，国内外针对单晶高温合金的成分设计、组织性能表征、缺陷形成机理和控制、单晶叶片制造工艺优化等开展了大量研究。单晶高温合金已经发展到了工作温度超过1100℃的第四代合金，单晶叶片的结构也越来越复杂。尽管如此，结合高温合金在工程应用中的新需求、新现象、新问题，上述几方面的工作仍然是近几年研发人员关注的重点——在过去3年召开的高温合金领域2个最重要的国际会议(美国高温合金会议(Superalloys 2021)和欧洲高温合金会议(Eurosuperalloys 2022))中，单晶高温合金的相关报道接近一半，工作也始终集中在上述几个方面。
本文在2019年单晶高温合金研发进展概述[1]的基础上，重点总结了单晶高温合金近几年的发展，介绍了单晶合金成分设计，组织、性能及相关机理方面新的研发工作，单晶叶片制造工艺的新进展，以及“下一代”高温结构材料的研发情况。
1 单晶高温合金研制
图1
近几年，除了不断深入理解关键合金元素的作用机理[11]，在先进单晶高温合金的研发中，人们一方面仍在寻求新的合金强化方法，希望开发高温强度更高的单晶合金；另一方面，也在降低成本、开发定制化单晶合金方面开展了大量工作。此外，在痕量和微量元素的影响方面也开展了较深入的研究工作，主要目的是针对不同应用场景，优化和细化母合金有害杂质元素标准以及单晶铸件的验收标准，兼顾成本和服役安全性、可靠性。
1.1 提高高温强度
单晶高温合金中最重要的强化相(γ'相)通常在1150℃以上会迅速固溶，因此限制了单晶合金的高温强度。由于Pt元素倾向于在γ′相中偏聚，可以提高γ'相的高温稳定性[12]，Rame等[13]设计了含2%Pt (质量分数，下同)的新型单晶合金TROPEA，Re含量控制在1% (第二代单晶高温合金的Re含量约为3%)，合金950~1150℃的蠕变性能与第二代单晶合金相当，1200℃以上的蠕变性能与第三代单晶合金相当。但是，含Pt合金的成本与第三代单晶合金相比显著提高。除了添加新合金元素，近几年研究人员也通过不断优化合金成分，例如适当增加Al、Ta含量，提高γ'相体积分数和溶解温度[14,15]，通过调整Co、Mo等合金元素含量平衡第四代单晶高温合金的组织稳定性和持久性能等[16,17]，来进一步提升合金的高温性能。
1.2 定制合金
由于先进单晶高温合金的成分设计空间越来越窄，基于合金的服役环境和具体用途，人们提出了“定制化”合金设计理念。先进涡轮工作叶片必须使用热障涂层(TBC)，而涂层基体间出现的二次反应区(SRZ)会显著影响薄壁叶片的性能，为应对这一问题，2020年Rame等[18]报道了第三代单晶高温合金AGAT。AGAT合金设计时综合考虑了基体-粘结层-TBC结构，提出的关键性能指标包括：密度< 8900 kg/m3、耐高温蠕变性能、抗氧化性能、组织稳定性、涂层兼容性和铸造性能等。与第三代单晶合金CMSX-10K比较，AGAT合金中Re含量降低(降低成本、密度，防止有害相和SRZ出现)，提高了Cr含量来改善抗氧化性能，Co含量升高到8.5%，进一步提高组织稳定性。此外，合金还添加了微量的Si和Hf，严格控制S含量(0.4 × 10-6)，保证TBC的稳定性。
1.3 微量和痕量元素控制
单晶高温合金中的杂质元素需要严格限制，但是不同合金、不同应用场景对杂质元素的控制标准并不一致，在深入研究相关机理和大量数据积累的基础上，早期沿用的部分母合金相关标准可能仍有较大的优化空间。例如Horie等[21]发现(0.04~31) × 10-6的Pb对TMS-238合金800~1150℃蠕变性能无明显的影响；氧化和热腐蚀初期，S会迅速向合金表面扩散，破坏合金表面保护性Al2O3/Cr2O3的连续性，2 × 10-6和5 × 10-6的S对单晶合金的高温氧化和热腐蚀性能影响相近，而10 × 10-6的S会显著恶化合金的热腐蚀性能[22,23]。当S含量在(2~3) × 10-6时，使用CaO坩埚熔炼单晶合金并没有降低S含量，但似乎延缓了高温氧化时S向合金表面的富集，并因此改善了合金的高温氧化性能[24]。由于杂质元素含量很低，深入理解其作用机理仍需开展大量工作，而且对检测和表征手段提出了很高的要求。
1.4 合金设计方法
在成分、组织和基础性能等数据库的支撑下，运用人工智能以及机器学习等新方法对单晶高温合金进行成分设计、组织结构分析、服役评估和预测，大大缩短了实验周期，降低了研发成本，是解决材料研发中复杂任务的有效科学工具。在合金设计方面，Liu等[25]通过机器学习同时优化多组分钴基高温合金的多个目标性能，如显微组织稳定性、γ′溶解温度、γ'体积分数、密度、抗氧化性能等，从约2.1 × 104个候选材料中筛选并成功实验合成了一系列新型钴基高温合金金。在组织结构分析方面，研究人员利用机器学习的方法研究了单晶高温合金的γ/γ′微观结构[26]、拓扑密堆(TCP)相[27]等。Thome等[28]基于上千个高清截面上枝晶结构的大数据累积，开发了一个自动化的程序，分析单晶高温合金的三维枝晶生长。在服役评估和预测方面，Liu等[29]结合CALPHAD (计算相图)方法和材料的基础组织-性能关系，开发了一种自适应学习方法(divide-and-conquer self-adaptive，DCSA)，这种DCSA模型可以实现对单晶高温合金蠕变寿命的准确、高效预测(误差在2.8%~5.8%范围内)。
2 组织和性能
2.1 蠕变
单晶高温合金在不同温度范围内的蠕变机制，国内外已经开展了大量研究，图2简单总结了目前观察到的几种单晶高温合金的主要蠕变变形机制。Xia等[30]最近综述了单晶高温合金蠕变中的组织演化和相关变形机制。近几年，与单晶高温合金工程应用密切相关的因素，如单晶晶体取向[31,32]、热腐蚀[33]、气膜孔[34]、再铸层[35]等对单晶高温合金蠕变性能的影响越来越受到关注。例如，Heep等[32]解释了不同取向单晶合金中温高应力蠕变的各向异性行为(当晶体取向位于[001]-[111]边时，蠕变寿命可能超过严格的[001]取向单晶，而当取向位于[001]-[011]边时，蠕变性能极低)，提出单晶叶片仍有必要细化取向偏离角相关的技术要求；对含孔样品的蠕变性能分析表明，980℃、300 MPa蠕变条件下，[001]、[011]和[111] 3个取向含气膜孔的薄壁样品均表现出更好的持久性能，其中[011]取向样品的强化效应最明显[34]，这为单晶叶片的设计和强度分析积累了数据。
图2
图2
单晶高温合金蠕变变形机制示意图
Fig.2
Schematics of temperature dependence of creep deformation mechanisms for single crystal superalloys (a-c) at medium temperature, the main deformation mechanism is matrix dislocation reaction (a), and at higher stress, dislocation dissociation is also activated (b), both mechanisms in Figs.2a and b lead to the formation of stacking faults (SFs) (c) (σ—applied stress) (d-f) at high temperature, the applied stress and misfit stress impel matrix dislocation reaction at γ/γ' interface during primary creep, and then dislocation networks generate (d), as creep in progress, high local stress and interaction of interfacial dislocations result in the formation of superdislocations, including a<110> type and a<100> type (e), the former is antiphase boundary (APB) coupled with dislocation pair, and the latter is dislocation pair with non-compact core originating from interfacial dislocations. In latter stage, γ'-raft cutting by superdislocations occurs (f)
2.2 疲劳
叶片服役过程中，疲劳是主要的失效方式。近几年，超高周疲劳、热机械疲劳等研究逐渐增加。而且，为了保障叶片的安全服役，热腐蚀环境下的疲劳和微动疲劳等特种疲劳也越来越受到关注。
除了传统的疲劳行为和机理研究[41]，不少工作报道了制造和服役环境相关的疲劳行为研究。例如，温激光冲击喷丸(warm laser shock peening，WLSP)表面处理可以显著提升单晶合金的低周疲劳寿命[42]；1150~1300℃高温时效后，γ/γ'界面弱化，位错更易切过γ'相，相比于时效时间，时效温度对低周疲劳性能的不利影响更明显[43]；含孔的单晶合金短时高温蠕变后，孔周γ'相发生不规则筏化，显著降低了低周疲劳性能[44]。此外，单晶高温合金的低周疲劳寿命的评估进一步发展为多因素考量的综合评估，例如，考虑各向异性、驻留时间和不均匀筏化微观组织的低周疲劳寿命预测[45,46]。同样，在高周疲劳方面，研究工作报道了晶体取向[47]、宏观腐蚀[48]、打孔方式[49]等因素的影响。例如，宏观腐蚀4次后，样品表面粗糙度Ra值达到0.2，对760℃高周疲劳性能的影响大于980℃，且在高应变幅时的影响更大，高应变幅和多次腐蚀后，裂纹源于腐蚀坑[48]。
近几年针对超高周疲劳中裂纹的萌生机制[50]、萌生位置[51,52]、萌生各向异性[53]等开展了较系统的研究。研究[52]发现，当疲劳寿命超过107 cyc后，在传统定向凝固工艺和高梯度液态金属冷却(liquid metal cooling，LMC)定向凝固工艺制备的单晶样品中，疲劳裂纹仍然萌生于微孔；热等静压消除了孔洞，疲劳裂纹主要萌生于样品表面或内部残余共晶；随疲劳寿命提高，LMC样品的疲劳寿命逐渐接近热等静压样品，当循环周次超过1010 cyc时，LMC样品的疲劳寿命与热等静压样品相当，疲劳裂纹萌生位置也从微孔转变为残余共晶。热等静压可以使传统定向凝固工艺制备的单晶合金的超高周疲劳寿命提高2~4个数量级，热等静压过程产生的亚晶和初熔对超高周疲劳寿命影响不大[54]。
2.3 单晶高温合金组织和性能的先进表征技术
在单晶高温合金的组织和力学性能研究中，近几年先进表征手段也在不断发展。例如，Reinhart等[64]在欧洲同步辐射中心(ESRF，Grenoble，France)结合高温定向凝固炉、强单色光束和高速X射线敏感相机实现了CMSX-4单晶高温合金糊状区枝晶生长过程中溶质对流行为的直接观察。Perry等[65]使用原位电热机械测试技术(ETMT)测量电阻随时间的演化，以跟踪回复和再结晶的过程。研究人员还利用三维X射线断层扫描(CT)技术及三维X射线纳米CT原位或准原位观测了单晶高温合金的应力腐蚀行为[66]，以及拉伸[67]、疲劳[68,69]、蠕变[70]中微孔、裂纹及TCP相[71]等的演化。此外，数字图像相关技术(digital image correlation，DIC)也被广泛用于单晶高温合金的力学行为研究。如Ren等[72]采用原位扫描电镜-DIC研究不同取向第二代单晶高温合金室温疲劳行为与组织演变的关系，可以准确识别应变集中，预测裂纹萌生区域。Duan等[73]提出了一种新的基于双棱镜的单镜头三维数字图像相关畸变标定技术(BSL-3D-DIC)，用于高精度实时表征单晶高温合金980℃疲劳裂纹扩展过程中裂纹尖端附近的三维变形行为。Shang等[74]利用紫外-DIC技术原位表征了单晶高温合金980℃蠕变行为。同时，工作温度在1000℃以上并同时提供清晰图像的原位测试(拉伸[75]、疲劳[76]、蠕变[77]等)实验装置的发展，匹配扫描电镜-DIC技术准确记录测试过程中样品的全场动态位移和应变，为深入理解单晶合金的高温变形和损伤行为提供了全新的平台。
3 单晶高温合金的近服役环境行为
近年来，研究者们除了不断深入研究单晶高温合金蠕变、疲劳损伤机制外，还重点围绕单晶高温合金近服役损伤行为开展了大量研究工作。这些研究主要包括以下2个方面。
3.1 蠕变-疲劳-环境耦合效应
针对单晶叶片服役工况，考虑长时热-力-化耦合因素，开展了单晶高温合金长时蠕变、氧化/热腐蚀-蠕变/疲劳、蠕变-疲劳等变形行为与损伤机制研究，重点分析了动态加载过程中蠕变-疲劳-环境等交互作用对单晶高温合金裂纹扩展、组织损伤和性能恶化的影响机理。
此外，为了保证燃气轮机及近海发动机用单晶叶片安全服役，热腐蚀对单晶合金蠕变/疲劳行为的影响越来越受到重视。相比高温氧化，热腐蚀对合金蠕变/疲劳性能的影响更明显[33,82]。研究[83]发现，S、Cl等会加速合金非保护性氧化膜的形成，在应力的协同作用下，S、Cl快速扩散到合金基体，导致合金局部脆化进而诱发裂纹。为了进一步理解应力-热腐蚀对裂纹扩展的影响，研究者[84]还发展了一种可在高温热腐蚀环境下测量裂纹扩展的新技术，建立了不同电位差信号与裂纹开裂面积的对应关系，发现热腐蚀会促进裂纹萌生、加快早期裂纹扩展速率。研究还发现，热腐蚀还会导致合金表面产生再结晶，加速组织退化。近期，国内外研究者都开始着手搭建带热腐蚀气氛的蠕变/疲劳测试平台，进而实时反映热腐蚀与机械载荷的交互作用，开展合金热腐蚀-蠕变/疲劳耦合作用下的失效机理研究。
单晶叶片在高温环境下承受复杂应力，损伤模式往往是蠕变疲劳的叠加。目前国外主要通过保载疲劳实验，研究蠕变-疲劳交互作用对单晶高温合金裂纹扩展和变形损伤机制的影响[85]。结果表明，蠕变导致的变形累积损伤控制着裂纹扩展，进而加重疲劳损伤。与压缩保载疲劳相比，拉伸保载更易恶化合金疲劳性能。通常，随着保载载荷和时间的增加，裂纹尖端蠕变孔洞、γ′相贫化区和氧化程度不断增加，裂纹扩展速率不断增加。但是，也有研究工作[86]发现较长的保载时间会降低裂纹扩展速率，这可能与蠕变变形导致的应力释放和裂纹尖端钝化以及氧化诱发的裂纹闭合有关。拉伸保载虽然降低了裂纹扩展速率，但在恢复疲劳加载的下一循环周次，裂纹会继续萌生。Wang等[87]和Okamoto等[88]通过原位观察、数字图像技术和裂纹尖端应变场有限元计算，发现随着拉伸保载时间的增加，合金疲劳寿命先降低后缓慢增加，这主要与循环变形过程中非弹性应变的不断累积、材料退化以及位错组态有关。另外，随着保载时间的增加，合金位错网结构特征越来越明显，断口由单裂纹源萌生转为多裂纹源，且非晶体面断裂部分不断增加(蠕变损伤的特征逐渐增加)。Cervellon等[89]认为，当疲劳测试应力比R为0.3时，其损伤模式为典型的蠕变-疲劳-氧化交互作用。
3.2 叶片结构和涂层的影响
近年针对单晶叶片空心薄壁、涂覆先进涂层和气膜冷却等结构特征，国内外持续开展相关研究，包括单晶高温合金薄壁效应研究，涂层对单晶高温合金蠕变和疲劳性能的影响，气膜孔周围应力-应变分布及裂纹萌生和扩展行为等。
4 单晶叶片制造工艺
4.1 定向凝固工艺及缺陷控制
高速凝固定向凝固(high rate solidification，HRS)技术是国内外广泛采用的定向凝固工艺，但在结构复杂、合金化程度高的高代单晶叶片和大尺寸燃机叶片制造中，往往容易出现晶粒缺陷[100]、偏析严重、热处理困难[101]等问题。LMC技术是俄罗斯、乌克兰广泛应用的定向凝固工艺，国内也实现了初步的工程化应用。近年来，围绕定向凝固技术开展的工作主要集中于优化和改进工艺参数。例如，针对复杂结构单晶叶片形状，通过多段调整抽拉速率，控制凝固过程中固/液界面位置和形状，以获得完整的单晶结构[102]。另外，单晶叶片的主取向通常是[001]方向，目前广泛应用的选晶法无法控制铸件的第二取向(与主取向垂直的晶体取向)。由于单晶铸件第二取向对性能有影响，因此能准确控制第二取向的籽晶法近年越来越受到关注，例如籽晶材质与铸件材料的匹配关系[103]，籽晶氧化对单晶生长的影响[104]，工艺参数对籽晶法的影响[105]，选晶和籽晶工艺的对比[106]等。
Git等[107]报道了流态床冷却(fluidized bed cooling，FBC)定向凝固工艺的新进展，对比每组5个35 mm × 12 mm × 170 mm的CMSX-4单晶铸件，与传统HRS工艺比较，FBC工艺获得的单晶样品微孔尺寸、体积分数明显降低，一次枝晶间距细化。表1对比了本课题组利用LMC工艺制备的尺寸相近的第三代单晶合金DD33铸件的组织参数。可见FBC工艺获得的单晶样品已经可以很好地控制显微疏松，平均微孔尺寸已经接近LMC工艺的水平。除了不断优化工艺提高单晶叶片的合格率之外，如何提升单晶叶片的制造效率也是定向凝固工艺优化的重要课题[108]。本课题组[109]根据LMC工艺中低熔点冷却介质可流动、散热效率高且恒定，模组冷却无遮挡效应的特点，探索了密排多层单晶模组的高效制备技术，该方法的推广应用将显著提高单晶叶片的制造效率。
表1 不同定向凝固工艺制备的单晶铸件组织对比
Table 1
| DS process | Alloy | PDAS / μm | Volume fraction of porosity / % | Average porosity size / μm2 |
|---|---|---|---|---|
| HRS | CMSX-4 | 333-384 | 0.072-0.102 | 18.9 |
| FBC | CMSX-4 | 248-275 | 0.022-0.045 | 8.8 |
| LMC | DD33 | 180 | 0.02 | 6.8 |
单晶叶片中常见缺陷的形成机理和控制技术始终是单晶叶片制造中的关键问题。
条纹晶是定向凝固过程中出现在单晶叶片表面的一种缺陷。目前，多数研究者[110~112]认为条纹晶的出现是糊状区的枝晶变形所致，其诱因主要与凝固收缩应力有关。最近，有文献报道铸件中的夹杂会诱发条纹晶[113,114]，型壳面层形貌对枝晶断裂和条纹晶产生也影响较大[115]。无论哪种机制，本质都与枝晶变形有关。因此，控制条纹晶的关键在于：增强糊状区枝晶强度，如提高温度梯度，以及通过控制工艺降低热应力，避免夹杂。雀斑出现在单晶叶片表面，由取向随机的细碎等轴晶粒组成。雀斑产生与糊状区的熔体对流有关，且受到合金成分[116]、凝固工艺[117]、铸件形状[118,119]等的影响。结合实验和模拟仿真可以较好地预测雀斑的形成[119~121]。提高温度梯度以细化枝晶，以及控制局部冷却来降低固/液界面曲率，可以缓解或消除雀斑[122]。由于单晶叶片结构复杂以及定向凝固过程中溶质场、温度场的不稳定，不同区域枝晶在分支生长过程中，受到热应力作用发生变形或绕生长方向[001]旋转，都会造成晶体取向的微小偏离，这种取向偏离逐渐累积，当枝晶再次汇聚时可能会形成小角度晶界[123,124]。由于缺乏枝晶变形和取向偏离过程的直观观测，目前相关机理仍然不完全清楚。
再结晶是单晶叶片固溶热处理后的常见缺陷，近几年本课题组[128]在过去相关工作的基础上，结合计算模拟和实验，发现定向凝固中的高温塑性变形是单晶铸件再结晶的关键诱因之一。对比观察诱发再结晶的铸态变形组织和铸态单晶样品不同温度的变形组织(位错组态、γ′相形态等)，以及它们热处理后的再结晶情况，可以判定诱发DD413单晶铸件再结晶的塑性变形发生在1150~1200℃之间，再结晶发生的临界塑性变形量在3.5%左右[128,129]。在大量材料性能数据(室温到高温的弹性模量、Poisson比、各向异性屈服强度等)的支撑下，模拟仿真技术可以比较准确地预测定向凝固中单晶铸件的塑性变形。例如，Long等[130]在修正的合金热膨胀系数基础上，得出了单晶合金AM1在不同温度下产生再结晶的临界塑性应变；本课题组[131]结合ProCast和Abaqus软件，准确预测了单晶铸件中再结晶的易发位置。
4.2 热等静压
热等静压可以闭合单晶铸件中由于凝固和固溶处理形成的微孔，显著提升疲劳性能。国外单晶铸件生产中，热等静压技术的应用已经比较成熟[132]，近年来还开展了热等静压技术在单晶铸件修复方面的探索[69,133]。最近，Lopez-Galilea等[134]报道了热等静压高温固溶处理技术——在超过合金固相线的温度进行热等静压处理。更高的固溶处理温度显著提高了偏析元素的扩散速率，第三代单晶合金CMSX-10K的固溶处理时间从传统工艺的45 h缩短到10 h，同时压力的作用阻止了凝固或固溶孔的形成。Ruttert等[135]还报道了利用热等静压替代固溶和时效热处理的工艺，在100 MPa压力下进行高温固溶和两级时效处理，显著提升了合金的蠕变和低周疲劳性能。
4.3 单晶铸件的强化和修复
单晶叶片在长期服役过程中组织逐渐退化，影响叶片甚至发动机的使用寿命[143]。近年的研究结果[143~147]表明，通过适当的恢复处理工艺，可以恢复单晶合金的显微组织，基本消除位错网，使持久与疲劳性能得到恢复。具有气淬功能的热等静压技术应用于恢复处理，可以在恢复显微组织的同时，消除内部孔洞[145,148]。但是，恢复处理中的再结晶控制技术，以及可恢复处理的变形量或蠕变速率阈值还需要进行深入研究。单晶叶片在服役过程中还会出现裂纹或蚀坑等损伤，为修复受损叶片，研究人员探索了等离子喷涂[149]、钎焊[150]、熔焊(电子束或激光)[150~153]、瞬时液相扩散焊[153]等修复工艺。这些单晶叶片修复技术具有各自的优缺点，例如电子束或激光熔覆可以获得单晶结构，但是杂晶与裂纹很难避免；钎焊可以焊接长裂纹，但是焊料中的B元素会扩散到基体中降低材料性能；瞬时液相扩散焊可实现修复区成分均匀化和结构单晶，但是工艺性稍差。
4.4 数值模拟
在宏观物理场方面，复杂形状单晶叶片定向凝固过程的仿真模拟已经开展了大量工作，可以辅助进行杂晶、雀斑、小角度晶界等缺陷的控制、籽晶优化设计、缩孔预测等方面的工作[158~160]。例如，结合数值模拟提出单晶叶片抽拉速率的调控准则[161]；利用模拟仿真获得的单晶叶片温度场，进行定向凝固过程中的应力、应变模拟，预测再结晶行为[128~130]；分析铸造过程中陶瓷型芯的位移以及壁厚的演化[162]等。近期，本课题组[163]针对LMC定向凝固中，商用软件无法同时模拟2种流体(高温合金熔体和低熔点冷却介质熔体)的问题，利用ANSYS进行冷却介质流动模拟，利用ProCAST进行单晶铸件凝固模拟，通过2者耦合计算，初步实现了对LMC工艺定向凝固过程中铸件/模壳与金属冷却介质相对运动时复杂传热传质过程的精确模拟。如图3所示，模型在计算铸件温度场时，不再采用预先设定低熔点冷却介质Sn温度的办法，而是充分考虑了低熔点冷却介质在定向凝固模壳向下抽拉过程中的内部对流，进一步提高了LMC工艺模拟的准确性。
图3
图3
液态金属冷却(LMC)工艺过程的数值模拟
Fig.3
Simulation of the temperature field during LMC process
元胞自动机方法(CA)和相场法是介观组织模拟的常用手段。与相场法比较，CA在计算量和计算尺度上具有明显的优势，目前在商用软件如ProCAST中的应用已经很成熟，可以计算单晶铸件定向凝固的晶粒组织。相场法主要用来计算单晶铸件的树枝晶结构，但目前受计算量的制约，大多将多元高温合金简化为二元合金，并且仅计算单晶铸件某一截面或者局部树枝晶的演化[157]。
目前，单晶铸件的数值模拟主要依靠单一商用软件或几种商用软件的联合应用。开发不同软件间的数据传输方法、测定宽温域内不同晶体取向的材料物理性能(弹性模量、Poisson比、热传导系数等)以及力学性能(拉、压屈服强度)等边界条件，是获得准确模拟仿真结果、指导单晶铸件工艺优化的关键。
4.5 单晶高温合金增材制造
近年科研人员利用选区电子束熔化(SEBM)、选区激光熔化(SLM)以及激光直接能量沉积(LDED)等技术探索了单晶高温合金的增材制造[166]。受限于工艺水平，已报道的增材制造单晶合金大多形状简单(如棒状或者立方状)，尺寸较大的样品单晶性较差且易开裂[167]。增材制造单晶合金由于枝晶组织更细、偏析更少，往往表现出与常规铸造合金相当甚至更优异的拉伸、蠕变持久及疲劳性能[168~170]。另外，为优化工艺，一些新方法和计算模拟手段近来也应用在增材制造单晶合金研究上，例如Bäreis等[171]利用光电原位成像技术实现了SEBM成形过程中点坑和裂纹等表面缺陷的实时监测；Tinat等[172]利用宏观计算流体力学模拟(CFD)分析了单晶合金SEBM成形过程的熔池动力学行为。
通过散焦电子束预热粉末床等办法，Ramsperger和Eichler[173]利用SEBM工艺成功制备了合金化程度很高的247合金叶片，叶片壁厚1.5 mm，冷却孔直径0.6 mm，叶片组织可以从等轴晶到细柱晶调控，未来还可能实现单晶叶片的增材制造。
5 展望：材料与工艺
先进单晶高温合金的工作温度已经接近其初熔温度的90%，科研人员也因此不断探索承温能力更高的“下一代”高温结构材料，例如新型γ'相强化钴基高温合金、高温/难熔高熵合金、颗粒/纤维增强高温合金、Nb-Si/Mo-Si合金、陶瓷基复合材料等。但到目前为止，上述材料在某些方面仍然存在短板，如塑性低、抗氧化腐蚀性能差、高温组织不稳定等等，未来为替代高温合金实现工程应用，仍需开展大量的研发工作。
以高熵合金为例，近年来国内外针对单相和双相、多相高温和难熔高熵合金的成分设计、组织和性能都开展了大量研究工作[174]。目前，公开报道的资料普遍通过高温压缩实验来评价高熵合金的屈服强度和塑性[174,175]，高熵合金压缩屈服强度一般高于镍基高温合金(图4a[176~187])，部分高熵合金在1200℃以上的压缩强度较高温合金优势明显。受样品尺寸和脆性的限制，高熵合金的高温拉伸性能数据报道较少，图4b[188~193]对比了6种高熵合金与典型单晶高温合金的高温拉伸屈服强度，其中仿高温合金成分的析出强化型(fcc + L12)高熵合金与单晶合金性能较接近，而fcc单相结构、bcc + fcc结构和B2 + A2结构的高熵合金性能较差。高温蠕变性能目前仅有Gadelmeier等[194]对比了难熔高熵合金TiZrHfNbTa (单相bcc)与第二代单晶合金CMSX-4的拉伸蠕变性能，单晶合金980和1100℃蠕变性能较高熵合金分别高出25倍和70倍，单晶合金基体成分的蠕变性能也明显高于高熵合金。
图4
总体而言，高熵合金具有广阔的成分设计空间，仍有很大一部分领域未进行探索。随着高通量计算和实验、大数据驱动的合金研发模式的应用，未来：(1) 使用温度更高(> 1200℃)的析出强化型高熵合金可能具有较大的开发潜力；(2) 由于高温合金的拉压不对称，目前尚无法通过小样品压缩实验准确评估材料的拉伸性能[195]，评价“下一代”替代材料性能的关键仍是高温拉伸蠕变测试。
制造工艺方面，随着单晶叶片冷却结构、服役工况越来越复杂，一方面仍需紧扣需求，(1) 不断凝练、细化和深入理解从母合金(新料和返回料)冶炼、定向凝固，到热处理、涂层、钎焊等部件生产过程，以及恢复热处理、修复等工艺过程中的关键科学和技术问题，特别是涉及高温合金-涂层、高温合金-陶瓷材料(坩埚、型芯、型壳)交互作用等的应用基础研究应进一步重视；(2) 随工艺装备和制造水平的提升，新技术(如多模组密排叶片定向凝固、热等静压等)的评估和应用，以及(3)模拟仿真、数字孪生等技术在优化工艺和过程控制等方面的推广应用，都将显著推动单晶叶片制造水平的提升，提高合格率和生产效率，降低单晶铸件的成本。另一方面，单晶部件的服役损伤、寿命评估和预测等方面的工作也迫切需要设计、考核、制造和材料相关人员密切合作，开展系统深入的研发工作。
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Predicting creep rupture life of Ni-based single crystal superalloys using divide-and-conquer approach based machine learning
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Microstructural evolution and creep mechanisms in Ni-based single crystal superalloys: A review
[J].
Creep anisotropy of a 3rd generation nickel-base single crystal superalloy at 850oC
[J].
The effect of deviations from precise [001] tensile direction on creep of Ni-base single crystal superalloys
[J].
Experimental and chemo-mechanical analysis of hot corrosion influence on creep properties of DD6 single crystal superalloy in molten NaCl salt
[J].
Anisotropic creep fracture mechanism and microstructural evolution in nickel-based single crystal specimen with a center film hole
[J].
Oxidation behavior of recast layer of air-film hole machined by EDM technology of Ni-based single crystal blade and its effect on creep strength
[J].
Creep of single-crystals of nickel-base γ-alloy at temperatures between 1150oC and 1288oC
[J].
Facilitating effect of interfacial grooves on the rafting of nickel-based single crystal superalloy at high temperature
[J].
Role of interfacial dislocation networks during secondary creep at elevated temperatures in a single crystal Ni-based superalloy
[J].
On dislocation networks and superdislocations in Re-containing nickel-based SX superalloy under different creep conditions
[J].
On the role of topological inversion and dislocation structures during tertiary creep at elevated temperatures for a Ni-based single crystal superalloy
[J].
Tensile, low cycle fatigue, and very high cycle fatigue characterizations of advanced single crystal nickel-based superalloys
[A].
Effect of warm laser shock peening on the low-cycle fatigue behavior of DD6 nickel-based single-crystal superalloy
[J].
Effect of overheating events on microstructure and low-cycle fatigue properties of a nickel-based single-crystal superalloy
[J].
Low cycle fatigue behaviour of a single crystal Ni-based superalloy with a central hole: Effect of inhomogeneous rafting microstructure
[J].
Damage-based low-cycle fatigue lifetime prediction of nickel-based single-crystal superalloy considering anisotropy and dwell types
[J].
Fatigue life evaluation for notched single-crystal Ni-based superalloys considering inhomogeneous rafting microstructure
[J].
Orientation dependence of high cycle fatigue behavior of a<111> oriented single-crystal nickel-based superalloy
[J].High cycle fatigue failure has been recognized as one of the major forms of failure of aero-engine blades. This paper presents the high cycle fatigue testing of a Ni-based superalloy near <111> orientation at 800 °C. The fracture morphology and dislocation configuration were analyzed in detail by scanning electron microscopy (SEM) and transmission electron microscopy (TEM) to indicate the influence of orientation deviation degree on the high cycle fatigue properties. The results show that the orientation deviation significantly affects the initiation of the slip systems, which is closely related to fatigue performance. The best fatigue life appears on the precise <111> orientation, and the deformation behavior is controlled by multiple sets of equivalent <110> {111} slip systems. With the increase in orientation deviation, the fatigue properties of the alloy degenerate significantly. On the boundary of <111>-<001>, two groups of <110> {111} slip systems with the maximum Schmid shear stress dominate the deformation behavior. On the other hand, on the <111>-<011> boundary, the formation of stacking faults and rapid cutting of γ’ precipitates results in a negative effect on the fatigue life.
Effect of etching on fatigue properties of DD6 single-crystal superalloy
[J].
Effect of different drilling techniques on high-cycle fatigue behavior of nickel-based single-crystal superalloy with film cooling hole
[J].The flat plate specimens of nickel-based single-crystal superalloy with 14 film cooling holes, which made by different drilling techniques, were used to study the high-cycle fatigue (HCF) properties at 980°C in an ambient atmosphere. At the same time, the electrical discharge machining (EDM) specimens with a single hole were also used to study the HCF properties under different temperatures. The hole and fracture micrographs were analyzed by scanning electron microscope. The results indicated that different drilling techniques have a great influence on HCF life. The fatigue limit of the millisecond laser drilling is 353 MPa, while the EDM is 359 MPa and the electro-stream machining (ESM) is 378 MPa. The fatigue life decreases gradually with the temperature increasing. The fatigue limit of EDM specimens with a single hole at 900°C, 980°C, and 1,050°C are 472, 430, and 293 MPa, respectively. The destruction of the specimens is a typical multisource rupture, and the fracture morphology includes three parts: the cracks sources around the film cooling hole, the propagation zone along the {001} planes, and instant rupture zone along the {111} planes.
Crack initiation mechanisms during very high cycle fatigue of Ni-based single crystal superalloys at high temperature
[J].
Transition from internal to surface crack initiation of a single-crystal superalloy in the very-high-cycle fatigue regime at 1100oC
[J].
High temperature VHCF of a 3rd generation Ni-based single crystal superalloy with different casting pore sizes
[J].
Crack initiation anisotropy of Ni-based SX superalloys in the very high cycle fatigue regime
[J].
On the impact of an integrated HIP treatment on the very high cycle fatigue life of Ni-based SX superalloys
[J].
Anisotropic thermomechanical fatigue of a nickel-base single-crystal superalloy Part I: Effects of crystal orientations and damage mechanisms
[J].
Influence of Ru on the thermomechanical fatigue deformation behavior of a single crystal superalloy
[J].
The role of Ru on the deformation mechanism of a single crystal superalloy during thermomechanical fatigue
[R].
Assessment of thermo-mechanical fatigue in a nickel-based single-crystal superalloy CMSX-4 accounting for temperature gradient effects
[J].
Thermomechanical fatigue damage mechanism and life assessment of a single crystal Ni-based superalloy
[J].
Lifing the effects of crystallographic orientation on the thermo-mechanical fatigue behaviour of a single-crystal superalloy
[J].Thermo-mechanical fatigue (TMF) is a complex damage mechanism that is considered to be one of the most dominant life limiting factors in hot-section components. Turbine blades and nozzle guide vanes are particularly susceptible to this form of material degradation, which result from the simultaneous cycling of mechanical and thermal loads. The realisation of TMF conditions in a laboratory environment is a significant challenge for design engineers and materials scientists. Effort has been made to replicate the in-service environments of single crystal (SX) materials where a lifing methodology that encompasses all of the arduous conditions and interactions present through a typical TMF cycle has been proposed. Traditional procedures for the estimation of TMF life typically adopt empirical correlative approaches with isothermal low cycle fatigue data. However, these methods are largely restricted to polycrystalline alloys, and a more innovative approach is now required for single-crystal superalloys, to accommodate the alternative crystallographic orientations in which these alloys can be solidified.
The mechanism of thermal corrosion fatigue (TCF) on nickel-based single crystal superalloy and the corresponding structure shape effect
[J].
Improving high temperature fretting fatigue performance of nickel-based single crystal superalloy by shot peening
[J].
Fretting fatigue life extension for single crystal Ni-based superalloy by applying optimized surface texturing
[A].
Impact of solute flow during directional solidification of a Ni-based alloy: In-situ and real-time X-radiography
[J].
An in situ resistance-based method for tracking the temporal evolution of recovery and recrystallization in Ni-base single-crystal superalloy at super-solvus temperatures
[J].
An in situ X-ray tomography study on the stress corrosion behavior of a Ni-based single-crystal superalloy
[J].
In-situ X-ray tomography investigation of pore damage effects during a tensile test of a Ni-based single crystal superalloy
[J].
Initiation of fatigue cracks in a single-crystal nickel-based superalloy at intermediate temperature
[A].
Combining 2D and 3D characterization techniques for determining effects of HIP rejuvenation after fatigue testing of SX microstructures
[J].
Evolution of micro-pores in a single crystal nickel-based superalloy during 980oC creep
[J].
Deformation behaviour of TCP-phases in an additively manufactured Ni-base superalloy studied by 3D X-ray nanotomography and micro-compression tests
[R].
In-situ fatigue behavior study of a nickel-based single-crystal superalloy with different orientations
[J].
Study on fracture behavior of nickel-based single crystal superalloy subjected to high temperature fatigue using digital image correlation
[J].
In situ creep behavior characterization of single crystal superalloy by UV-DIC at 980oC
[J].High temperature creep resistance is a critical characteristic of Ni-based single crystal (SX) superalloys. In this work, the creep behavior of a Ni-based SX superalloy was in situ characterized at 980 °C by ultraviolet (UV) imaging combined two-dimensional digital image correlation (DIC) in vacuum environment. The surface pattern was fabricated to maintain stable over 65 h at 980 °C. The pattern images captured by UV imaging were analyzed using mean gray value and the full-field strain map of creep deformation was obtained. A laser displacement senor (LDS) was employed for measuring the creep strain on the specimen for comparison. The creep deformation result shows a good agreement between DIC and LDS, the microstructure of the different creep areas on the specimens also demonstrate that the results of DIC are reliable. The in situ creep characterization by UV-DIC shows a great potential for investigating creep behaviors at high temperatures.
A novel instrument for investigating the dynamic microstructure evolution of high temperature service materials up to 1150oC in scanning electron microscope
[J].
In-situ SEM study on fatigue crack behavior of a nickel-based single crystal alloy at 950oC and 1050oC
[J].
In situ scanning-digital image correlation for high-temperature deformation measurement of nickel-based single crystal superalloy
[J].
On the mechanism of oxidation-fatigue damage at intermediate temperatures in a single crystal Ni-based superalloy
[J].
Stress effect on rupture mechanisms of a third generation single crystal superalloy crept at ultra-high temperature
[J].
应力对第3代单晶高温合金超高温蠕变断裂机制的影响
[J].
Oxidation-induced recrystallization and damage mechanism of a Ni-based single-crystal superalloy during creep
[J].
Phenomenological modeling of the effect of oxidation on the creep response of Ni-based single-crystal superalloys
[A].
Analysis of combined static load and low temperature hot corrosion induced cracking in CMSX-4 at 550oC
[J].
Investigation into the effects of salt chemistry and SO2 on the crack initiation of CMSX-4 in static loading conditions
[A].
Measurement and evaluation of co-existing crack propagation in single-crystal superalloys in hot corrosion fatigue environments
[A].
Creep-fatigue interaction behavior of high temperature alloys: A review
[J].
Fatigue crack retardation associated with creep deformation induced by a tension hold in a single crystal Ni-base superalloy
[J].
Creep-fatigue interaction behavior of nickel-based single crystal superalloy at high temperature by in-situ SEM observation
[J].
Evolution of short-term creep strain field near fatigue crack in single crystal Ni-based superalloy measured by digital image correlation
[J].
Creep, fatigue, and oxidation interactions during high and very high cycle fatigue at elevated temperature of nickel-based single crystal superalloys
[A].
Thickness debit effect in Ni-based single crystal superalloys at different stress levels
[J].
Low cycle fatigue lifetime and deformation behaviour prediction of nickel-based single crystal superalloy considering thickness debit effect
[J].
Stress state mechanism of thickness debit effect in creep performances of a Ni-based single crystal superalloy
[J].
Effect of Pt-Al bond-coat on the tensile deformation and fracture behaviors of a second-generation SX Ni-based superalloy at elevated temperatures
[J].
Coating-assisted deterioration mechanism of creep resistance at a nickel-based single-crystal superalloy
[J].
Damage mechanisms during very high cycle fatigue of a coated and grit-blasted Ni-based single-crystal superalloy
[J].
Effect of thermal barrier coatings on the fatigue behavior of a single crystal nickel-based superalloy: Mechanism and lifetime modeling
[J].
High temperature tensile behavior of a thin-walled Ni based single-crystal superalloy with cooling hole: In-situ experiment and finite element calculation
[J].
Experimental and analytical investigation on service life of film cooling structure for single crystal turbine blade
[J].
Creep rupture mechanism and microstructure evolution around film-cooling holes in nickel-based single crystal superalloy specimen
[J].
Common solidification defects and inhibition methods in single crystal superalloy turbine blades
[J].
单晶高温合金涡轮叶片的常见凝固缺陷及控制方法
[J].近年来，随着单晶涡轮叶片气冷结构的复杂化和尺寸的大型化，单晶叶片制备过程中容易出现选晶失败、取向偏离、条纹晶、大角晶界、缘板杂晶、再结晶、雀斑等多种凝固缺陷，导致单晶高温合金性能降低，造成叶片报废。为了减少这些凝固缺陷，提高单晶品质，获得结构完整的单晶组织，熟悉单晶叶片生长过程中的各种常见缺陷，掌握凝固缺陷的形成规律及其控制技术就显得尤为重要。针对这一现状，系统地介绍了单晶叶片生长过程中常见的凝固缺陷，以及凝固缺陷形成规律和控制方法，为减少单晶叶片缺陷，提高叶片品质和合格率奠定了坚实的理论基础，提供了有力的技术保障。
Directional solidification under high thermal gradient and its application in superalloys processing
[J].<p>Industrial gas turbines (IGTs) are the key equipment to achieving energy strategy, such as energy conservation and clean power generation. When the large and complex IGT blades are fabricated by the conventional Bridgman directional solidification process, the thermal gradients at the solidification front are low and unstable, resulting in some disadvantages: the coarse dendrite structure with severe dendritic segregation, the increased occurrence of casting defects and the poor performance of mechanical properties. These disadvantages provide a good opportunity for rapid development of the directional solidification with high thermal gradient (HG), such as the liquid metal cooling (LMC). In the present work, the physical basis of HG process, the microstructure, mechanical properties, solution heat treatment, and casting defects of the superalloys processed by HG process, have been reviewed. The HG process increases the thermal gradient and the cooling rate, thus permitting microstructural improvements including a more homogeneous fine-dendrite structure with lower elemental segregation and shrinkage porosity, and refinement of carbide, <i>γ′</i> phase and eutectic, reducing the volume fraction of eutectic and shrinkage porosity. During the solution heat treatment, the HG process increases the incipient melting temperature and reduces the residual segregation as well as the content of solution pore. The HG process could effectively inhibit the formation of freckle chains, increase the critical withdrawal rate of the stray grain formation, and decrease the degree of the misorientation of the <001> grain orientation from the casting axis. Moreover, the HG process could improve the mechanical properties including the stress rupture life, low-cycle fatigue (LCF), high-cycle fatigue properties and short-term strength, but the improvement might be reduced at higher temperature or under the oxidation condition.</p>
高梯度定向凝固技术及其在高温合金制备中的应用
[J].
Influence of seeding materials on epitaxial growth of single crystal superalloys
[J].
高温合金籽晶材料对单晶外延生长的影响
[J].
Effect of seed oxidation on solidification process of Ni-based single crystal superalloy
[J].In this study, an industrial directional solidification furnace was used to study the structure of the oxidation of the seed and its effect on the formation of solidification defects in the Ni-based single crystal superalloys. The results show that the oxide layer on the top of the seed was mainly composed of Al<sub>2</sub> O<sub>3</sub> and NiO. The oxide layer isolated the molten seed from the cast alloy, resulting in the formation of the solidification defects, such as low-angle grain boundary, stray grain and so on, during the directional solidification process.
镍基单晶高温合金籽晶氧化对凝固过程的影响
[J].
Effect of temperature gradient on solidification microstructure of seeding preparation process for Ni-based single crystal superalloy DD6
[J].
温度梯度对籽晶法制备镍基单晶高温合金DD6凝固组织的影响
[J].采用液态金属定向凝固试验研究了温度梯度对籽晶法制备镍基单晶高温合金DD6凝固组织的影响规律。结果表明:温度梯度约为50℃/cm的情况下,定向凝固过程中籽晶段可分为完全熔化区、糊状区、热影响区和原始组织区。将温度梯度提高到200℃/cm,可以缩小糊状区和热影响区范围,提高热影响区元素的均匀化程度。高温度梯度可以消除籽晶原始组织对一次枝晶间距的影响,抑制籽晶回熔区杂晶的形成。
Effects of single crystal growth techniques on dendritic microstructures and small angle orientation defects in Ni-based superalloys
[R].
Fluidized carbon bed cooling of Ni-based superalloy CMSX-4
[R].
On the origin of sliver defects in single crystal investment castings
[J].
Formation of lateral sliver defects in the platform region of single-crystal superalloy turbine blades
[J].
Formation of sliver defect in Ni-based single crystal superalloy
[J].
Formation of sliver defects in single crystal castings of superalloys
[J].
高温合金单晶铸件中条纹晶的形成机制
[J].对高温合金单晶叶片铸件中条纹晶起源处的微观组织进行了观察分析，提出了这种晶粒缺陷的产生机理。通过微观检测确认了条纹晶的出现是由于铸件表面单个枝晶主干在糊状区内被撕断，但又被残余液体焊合，呈现出明显的起点。导致这种撕裂的主要原因是型壳粘连引起的枝晶收缩严重受阻或是由夹杂切割引起的枝晶强度严重受损，从而形成了条纹晶缺陷产生的2种主要机制。撕裂后的枝晶会发生一定程度的整体偏转，在基体组织上形成一个由小角度晶界封闭而成的狭长晶粒。比较了条纹晶与其它晶粒缺陷的相似和不同之处，并讨论了减少条纹晶缺陷应该采取的工艺措施。
Sliver defect formation in single crystal Ni-based superalloy castings
[J].
Generation mechanism and motion behavior of sliver defect in single crystal Ni-based superalloy
[J].Sliver is a common but easily neglected defect in single crystal Ni-based superalloy castings. To date, there is still no unified viewpoint on its formation mechanism and generation causes. In this work, the orientation discontinuity and motion behavior of sliver defects were studied through experiments and numerical simulations. The ultrathin wedge-shaped specimen containing the grain boundary of the sliver and the matrix was prepared at the initial position of the sliver defect for the observation of equal thickness fringes. The discontinuity of equal thickness fringes on both sides of the grain boundary was observed through a transmission electron microscope, which directly confirms the abrupt change in the orientation between the sliver and matrix from the nanoscale. The crystal lattices at the smooth area and the bulging area of the grain boundary were found to have unusually different arrangements. The irregular lattice arrangement at the bulging area shows that the grain boundary has experienced high-stress deformation. Statistical results of sliver orientation deviation with a further composition analysis show the micro protuberance of the mold shell has a noticeable inductive effect on the sliver generation. Furthermore, a self-developed three-dimensional phase-field simulation model coupled with the spatial topology algorithm is established to simulate the orientation deflection behavior and orientation deviation threshold of fractured dendrites. The simulation results indicated that there is an upper limit of the cross-section solid fraction at the fracture position for the motion of the fractured dendrites. When the cross-section solid fraction at the fracture position is higher than this upper limit, it will be difficult to produce large deviation slivers due to the structural limitation of surrounding dendrites. This upper limit does not change with the solidification temperature gradient.
A review on the influence of carbon addition on the solidification defects in nickel-based single crystal superalloys
[J].
碳对镍基单晶高温合金凝固缺陷影响的研究进展
[J].
Research progress in freckles of single crystal superalloys
[J].
单晶高温合金雀斑研究进展
[J].
Effect of casting geometry on the freckle formation during single crystal solidification of superalloys
[J].
高温合金单晶铸件中形状因素对雀斑缺陷的影响
[J].
Investigation on freckle formation of nickel-based single crystal superalloy specimens with suddenly reduced cross section
[J].
Insight into the sensitivities of freckles in the directional solidification of single-crystal turbine blades
[J].
Digital twin for directional solidification of a single-crystal turbine blade
[J].
Reduction of freckle defect in single-crystal blade root by controlling local cooling conditions
[J].
Formation of low angle boundaries in Ni-based superalloys
[J].
Variation of crystal orientation and dendrite array generated in the root of SX turbine blades
[J].The variation of the crystal orientation and the dendrite array generated in the root of the single-crystalline (SX) turbine blades made of CMSX-4 superalloy were studied. The blades with an axial orientation of the [001] type were solidified by the industrial Bridgman technique using a spiral selector at a withdrawal rate of 3 mm/min. The analysis of the crystal orientation and dendrite arrangement was carried out using scanning electron microscopy, X-ray diffraction topography, and Laue diffraction. It was found that the lateral growth of such secondary dendrite arms, which are defined as “leading” and grow in the root at first, is related to the rotation of their crystal lattice, which is the reason for creation of the low-angle boundary (LAB) type defects. The primary crystal orientation of the selector extension (SE) area determines the areas and directions of the lateral growth of the leading arms. Additionally, it was found that in the SE areas of the root, near the connection with the selector, the spatial distribution of the [001]γ′ crystallographic direction has a complex wave-like character and may be related to the shape of the crystallization front.
Sand-burning reaction of ceramic shell for directional solidification of nickel-based superalloy
[J].
镍基高温合金定向凝固用陶瓷型壳粘砂反应
[J].
Interfacial reaction mechanism between ceramic mould and single crystal superalloy for manufacturing turbine blade
[J].Single crystal superalloys are the preferred materials for manufacturing turbine blades of advanced aero-engines, due to their excellent high temperature comprehensive performance. The interfacial reaction between alloys and ceramic mould are an important factor to influence the surface quality and service performance of the turbine blade. It is very important to reveal the interfacial reaction mechanism to improve turbine blade quality and yield rate. In this paper, the interfacial reactions between DD6 single crystal superalloy and ceramic mould were investigated by scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS) and X-ray diffraction analysis (XRD). The results show that the main reaction products were HfO2, Al2O3 and Y3Al5O12 when the yttrium oxide powders were the prime coat materials, while alloy surface suffered undesirable sand fusion; the thicknesses of the reaction layers were over 20 μm. The reaction layer can be divided into two layers, the layer close to the alloy was mainly composed of Al2O3 and Y3Al5O12, and the layer close to the mould was composed of SiO2, Al2O3 and Y3Al5O12. Avoiding the formation of Y2O3-Al2O3-SiO2 ternary low-melts can solve the interfacial reaction between DD6 alloy and yttrium oxide mould.
Pore formation in single-crystal turbine rotor blades during directional solidification
[J].
On the inducement of recrystallization in single-crystal superalloy
[J].
The effect of deformation temperature on recrystallization in a Ni-based single crystal superalloys
[J].
A new experimental and simulation methodology for prediction of recrystallization in Ni-based single crystal superalloys during investment casting
[J].
Recrystallization in Ni-based single crystal superalloys
[R].
Research and applications of hot isostatic pressing technology in nickel-based single crystal superalloy
[J].
热等静压技术在镍基单晶高温合金中的应用研究进展
[J].
On the rejuvenation of crept Ni-base single crystal superalloys (SX) by hot isostatic pressing (HIP)
[J].
Super-solidus hot isostatic pressing heat treatments for advanced single crystal Ni-base superalloys
[J].
An integrated HIP heat-treatment of a single crystal Ni-base superalloy
[A].
Enhanced high temperature elongation of nickel based single crystal superalloys by hot isostatic pressing
[J].
Mechanism of improved intermediate temperature plasticity of nickel-base single crystal superalloy with hot isostatic pressing
[J].
Effects of hot isostatic pressure on microdefects and stress rupture life of second-generation nickel-based single crystal superalloy in as-cast and as-solid-solution states
[J].Due to the excellent high temperature comprehensive performance and cost effective, the second-generation nickel-based single crystal superalloy has been widely used in the high-pressure turbine blades of advanced aero-engines. Microdefects such as micropores and interdendritic eutectic are seriously harmful to the high temperature mechanical properties of nickel-based single crystal superalloys. Hot isostatic pressure (HIP) technology, which has been widely used in powder and casting superalloys, can effectively reduce the micropores, interdendritic eutectic and other structural defects formed in the turbine blades during manufacturing, and improve the service reliability of turbine blades. However, the effect of HIP process on the high temperature stress rupture life of nickel-based single crystal superalloys is still controversial, especially with regard to the initial microstructure state of the nickel-based single crystal superalloys, i.e. the as-cast microstructure state or the as-solid-solution state. In this work, a kind of second-generation nickel-based single crystal superalloy with as-cast state or as-solid-solution state was selected as the research object. Through two-stage heat/booster type heat treatment process, in combination with microdefects quantitative analysis, quantitative characterization of alloying element segregation and high temperature stress rupture tests at 980 ℃ and 250 MPa, the effects of HIP process on the microdefects and high temperature stress rupture life of the used superalloy with different initial microstructures were studied. The results indicated that the solid-solution treatment can significantly promote the diffusion of alloying elements, such as Re, W, Al, and Ta, reduce the area fraction of interdendritic eutectic, but significantly increase the average area fraction and size of micropores in the used alloy with as-cast state. While, HIP process can effectively reduce the average area fraction and size of microspores in the used alloy with as-cast state or as-solid-solution state, but cannot eliminate the interdendritic eutectic as remarkable as the solid-solution treatment. By HIP process of the used alloy with as-solid-solution state, the area fraction of micropores is reduced to 0.005%, the eutectic structure is basically eliminated, and the dendrite segregation of Re, W, Al, Ta and other elements is significantly alleviated, resulting in the higher stress ruputure life of the used alloy, about 40% over that of the used alloy with the standard heat treatment state. Performing HIP process on nickel-based single crystal superalloy alloy with as-solid-solution state is of benefit to the high temperature stress rupture life due to the reduction of microdefects and the homogenization of alloying elements, in comparison with performing HIP process directly on the alloy with as-cast sate.
热等静压对铸态及固溶态第二代镍基单晶高温合金显微缺陷及持久性能的影响
[J].以初始组织分别为铸态组织和固溶态组织的第二代镍基单晶高温合金为研究对象，通过进行1300 ℃、30 MPa、2 h+1300 ℃、100 MPa、3 h两阶段的热等静压处理，对比热等静压前后显微缺陷及微观组织的变化，并在980 ℃、250 MPa条件下进行高温持久性能实验，明确了热等静压处理对不同初始组织状态下镍基单晶合金组织状态及持久性能改善的影响机制。结果表明：固溶处理显著促进Re、W、Al、Ta等合金元素的扩散，降低铸态组织共晶面积分数但显著提高显微孔洞平均面积分数及平均尺寸。热等静压处理可以显著降低显微孔洞平均面积分数及平均尺寸且对固溶态组织的作用更为显著，但热等静压对共晶组织的消除作用不如固溶处理明显。固溶态组织经热等静压处理后，显微孔洞面积分数降低至0.005%；共晶组织基本消除；Re、W、Al、Ta等元素枝晶偏析程度显著缓解；其980 ℃、250 MPa高温持久寿命相比未经热等静压处理的标准热处理态合金提高了40%左右。对固溶态组织进行热等静压处理的工序安排有利于提高显微孔洞闭合作用，促进成分均匀化并显著提高合金高温持久寿命。
Effect of hot isostatic pressing on microstructure of a third-generation single crystal superalloy DD33
[J].The third generation DD33 single crystal superalloy was subjected to standard heat treatment and hot isostatic pressing respectively, and then to different post-solution and -aging treatments. Hereafter, the effect of hot isostatic pressing and heat treatment on the microstructure and durability of the alloy were investigated by means of high-temperature endurance tests at 850℃/650 MPa and 1100℃/170 MPa, as well as metallographic microscope (OM), scanning electron microscope (SEM) and X-ray three-dimensional imaging (XCT). The results show that after proper hot isostatic pressing and subsequent heat treatment, the as-cast DD33 single crystal superalloys present more or less the same microstructure of (γ' phase size, volume fraction and cubic degree) as those subjected to standard heat treatment. Compared with the standard heat treated alloy, the volume fraction and size of the micropores of the alloy decreased significantly after hot isostatic pressing, from 0.0190% to 0.0005%, and the maximum equivalent diameter of the micropores decreased from 36.9 μm to 14.2 μm. The durable life of the alloy subjected to hot isostatic pressing was significantly prolonged when testing by 850℃/650 MPa and 1100℃/170 MPa. These results show that proper hot isostatic pressing and heat treatment can eliminate the micro voids, therewith, improve the durability of the alloy.
热等静压对第三代单晶高温合金DD33显微组织和持久性能的影响
[J].对第三代DD33单晶高温合金进行标准热处理、热等静压以及不同制度的后续固溶和时效处理,并在850℃/650 MPa和1100℃/170 MPa条件下进行高温持久性能实验,使用金相显微镜(OM)、扫描电子显微镜(SEM)和X射线三维成像技术(XCT)等手段观察和表征不同状态的样品,研究了热等静压和热处理对这种合金显微组织和持久性能的影响。结果表明：铸态DD33单晶高温合金经过适当的热等静压和后续热处理工艺后,样品的组织形貌(γ′相尺寸、体积分数与立方化程度)与标准热处理态基本相同。与标准热处理态合金相比,热等静压处理后合金显微孔洞的体积分数和尺寸均显著降低,其体积分数从0.0190%降低到0.0005%,最大孔等效直径从36.9 μm减小到14.2 μm。在850℃/650 MPa和1100℃/170 MPa条件下热等静压后的样品持久寿命均显著延长。这表明,适当的热等静压和热处理能消除合金内部的显微孔洞缺陷,使其持久性能显著提高。
Research progress and prospect of laser shock peening technology in aero-engine components
[J].Because of hostile service environment and complicated working loads, aero-engine components are apt to high-cycle fatigue fracture, which seriously affects security and reliability of aero-engine. Laser shock peening, LSP, is a novel surface plastic-strengthening technology. High-cycle fatigue performance can be improved effectively under the action of compressive residual stress and microstructural modification. LSP has been widely applied in the batch production and maintenance of aero-engine components. Research status, application situation and problems to be resolved of laser shock peening on fan/compressor blade, turbine blade, turbine disk, cartridge receiver, hydraulic actuator, pipe, gear and so on were discussed deeply. The development tendency of laser shock peening research on aero-engine components in recent years was analyzed and concluded. The future work on equipment, technique, mechanism and application of laser shock peening on aero-engine components was taken an outlook. In future, hope that the large-scale industrial application of laser shock peening on aero-engine components could be realized under the synergistic action of the whole industry and the whole technology chain.
航空发动机部件激光冲击强化研究进展与展望
[J].航空发动机部件服役环境恶劣、工作载荷复杂，容易发生高周疲劳断裂，严重影响发动机安全可靠性。激光冲击强化是一种新兴的表面塑性强化技术，可通过残余压应力预制和微观组织改善显著提升金属材料高周疲劳性能，已在航空发动机部件生产和修理中实现了批量化应用。将深入讨论风扇/压气机叶片、涡轮叶片、涡轮盘、机匣、作动筒、导管、齿轮等部件激光冲击强化研究进展和应用情况及有待解决的问题，分析总结近年来航空发动机部件激光冲击强化研究历程及特点，并就未来设备、机理、工艺和应用等方面研究进行展望，希望通过全行业、全技术链的力量创新协同，推动激光冲击强化技术在我国航空发动机部件上的规模化工业应用。
Nonuniformity of morphology and mechanical properties on the surface of single crystal superalloy subjected to laser shock peening
[J].
Evolutions of microstructure, phase, microhardness, and residual stress of multiple laser shock peened Ni-based single crystal superalloy after short-term thermal exposure
[J].
Microstructural rejuvenation in a Ni-based single crystal superalloy
[J].
Rejuvenation of directionally solidified and single-crystal nickel-base superalloys
[J].
VHCF life of AM1 Ni-based single crystal superalloy after pre-deformation
[J].
High-temperature pre-deformation and rejuvenation treatment on the microstructure and creep properties of Ni-based single-crystal superalloys
[A].
Effect of solution temperature in recovery heat treatment on microstructure and stress rupture properties of single crystal alloy DD11
[J].
恢复热处理固溶温度对DD11单晶合金组织及持久性能的影响
[J].为修复单晶合金涡轮叶片退化组织及提高其持久性能,对一种第二代镍基单晶合金DD11进行1 070℃/3 000 h长期时效处理后再进行恢复热处理,研究了恢复热处理工艺中固溶温度对DD11单晶合金组织和持久寿命的影响。结果表明:恢复热处理可以有效消除长期时效处理试样中粗大γ'相组织并恢复其体积含量,使合金的组织和持久寿命恢复到完全热处理状态。
Rejuvenation of single-crystal Ni-base superalloy turbine blades: Unlimited service life
[J].
Repair of Ni-based single-crystal superalloys using vacuum plasma spray
[J].
Diffusion braze homogenisation and contraction during re-repair heat treatments of a single crystal nickel-based superalloy
[J].
Epitaxial laser deposition of single crystal Ni-based superalloys: Repair of complex geometry
[J].
The inhibition and repairation of the solidification cracks in laser cladded nickel-based single crystal alloy
[J].
激光熔覆镍基单晶合金凝固裂纹抑制与修复技术研究
[J].
Research progress of repair technology for surface defects of single crystal superalloy
[J].
单晶高温合金表面缺陷焊接修复的研究进展
[J].
Research progress on numerical simulation of directional solidification of nickel-based superalloy turbine blade
[J].
镍基高温合金叶片定向凝固过程宏微观数值模拟研究进展
[J].
Multi-scale modeling of liquid-metal cooling directional solidification and solidification behavior of nickel-based superalloy casting
[J].Liquid-metal cooling (LMC) process can offer refinement of microstructure and reduce defects due to the increased cooling rate from enhanced heat extraction, and thus an understanding of solidification behavior in nickel-based superalloy casting during LMC process is essential for improving mechanical performance of single crystal (SC) castings. In this effort, an integrated heat transfer model coupling meso grain structure and micro dendrite is developed to predict the temperature distribution and microstructure evolution in LMC process. An interpolation algorithm is used to deal with the macro-micro grids coupling issues. The algorithm of cells capture is also modified, and a deterministic cellular automaton (DCA) model is proposed to describe neighborhood cell tracking. In addition, solute distribution is also considered to describe the dendrite growth. Temperature measuring, EBSD, OM and SEM experiments are implemented to verify the proposed model, and the experiment results agree well with the simulation results. Several simulations are performed with a range of withdrawal rates, and the results indicate that 12 mm·min -1 is suitable for LMC process in this work, which can result in a fairly narrow and flat mushy zone and correspondingly exhibited fairly straight grains. The mushy zone length is about 4.8 mm in the steady state and the average deviation angle of grains is about 13.9° at the height 90 mm from the casting base under 12 mm·min -1 withdrawal process. The competitive phenomenon of dendrites at different withdrawal rates is also observed, which has a great relevant to the temperature fluctuation.
Multi-scale simulation of single crystal hollow turbine blade manufactured by liquid metal cooling process
[J].
Dendrite growth and defects formation with increasing withdrawal rates in the rejoined platforms of Ni-based single crystal superalloys
[J].
Investigation on freckle formation and evolution of single-crystal nickel-based superalloy specimens with different thicknesses and abrupt cross-section changes
[J].
Identifying critical defect sizes from pore clusters in nickel-based superalloys using automated analysis and casting simulation
[J].
Research on displacement and wall thickness evolution of gas turbine blade during casting process based on cantilever structure core
[J].
基于悬臂结构型芯的燃机叶片铸造过程位移、壁厚演化研究
[J].
A coupled numerical scheme for simulating the process of liquid metal cooling process
[R].
Phase-field modeling of γ' and γ″ precipitate size evolution during heat treatment of Ni-based superalloys
[A].
Review on additive manufacturing of single-crystal nickel-based superalloys
[J].
Microstructure and grain growth direction of SRR99 single-crystal superalloy by selective laser melting
[J].
Microstructure and stress-rupture property of DD32 nickel-based single crystal superalloy fabricated by additive manufacturing
[J].
Creep properties of single crystal Ni-base superalloys (SX): A comparison between conventionally cast and additive manufactured CMSX-4 materials
[J].
Very high cycle fatigue durability of an additively manufactured single-crystal Ni-based superalloy
[J].
Electron-optical in-situ crack monitoring during electron beam powder bed fusion of the Ni-Base superalloy CMSX-4
[J].
Numerical simulations to predict the melt pool dynamics and heat transfer in additive manufacturing process of Ni-based superalloy (CMSX-4)
[R].
Electron beam based additive manufacturing of Alloy 247 for turbine engine application: From research towards industrialization
[J].
Refractory high-entropy alloys: A focused review of preparation methods and properties
[J].In recent years, high-entropy alloys (HEAs) have become prominent metallic materials due to their unique design strategies and excellent mechanical properties. The HEAs-inherent high-entropy, lattice-distortion, sluggish-diffusion, and cocktail effects make HEAs maintain high strength, oxidation resistance, corrosion resistance, wear resistance, and other excellent comprehensive properties, showing stronger competitiveness relative to traditional alloys. Refractory high-entropy alloys (RHEAs) are considered as a new kind of high-temperature materials with great application prospects due to their excellent mechanical properties and have the potential to replace nickel-based superalloy as the next generation of high-temperature materials. We reviewed the research status and preparation methods of RHEAs in recent years, including the metallurgical smelting, powder metallurgy, magnetron sputtering, and additive manufacturing technologies. The microstructure and phase-transformation process of RHEAs were analyzed. The mechanical properties and main strengthening and toughening mechanisms of RHEAs, such as solid-solution strengthening, precipitation strengthening, and the transformation-induced plasticity (TRIP), were discussed, and the deformation mechanism of RHEAs was revealed. The properties of RHEAs, including high strength, oxidation resistance, corrosion and wear resistance were reviewed. RHEAs will meet the huge market demand in the engineering materials field, but there are still many challenges, such as the trade-off between high strength and high ductility, structural design, and performance optimization of RHEAs with brittle BCC structures. We believe that this combination of knowledge may shape the future of RHEAs and break through the mutually exclusive conundrum of high strength and high toughness for RHEAs.
Development and property tuning of refractory high-entropy alloys: A review
[J].
Design of novel low density refractory high entropy alloys for high-temperature applications
[J].
Effect of Al addition on structural evolution and mechanical properties of the Al x HfNbTiZr high-entropy alloys
[J].
High-strength NbMoTaX refractory high-entropy alloy with low stacking fault energy eutectic phase via laser additive manufacturing
[J].
Structure and mechanical properties of an in situ refractory Al20Cr10Nb15Ti20V25Zr10 high entropy alloy composite
[J].
Superior mechanical properties and strengthening mechanisms of lightweight Al x CrNbVMo refractory high-entropy alloys (x = 0, 0.5, 1.0) fabricated by the powder metallurgy process
[J].
Compositional variation effects on the microstructure and properties of a refractory high-entropy superalloy AlMo0.5NbTa0.5TiZr
[J].
Mechanical performance of the Al x CoCrCuFeNi high-entropy alloy system with multiprincipal elements
[J].
Microstructure and mechanical properties of refractory HfMo0.5NbTiV0.5Si x high-entropy composites
[J].
Contribution of lattice distortion to solid solution strengthening in a series of refractory high entropy alloys
[J].
Effect of Ti additions on mechanical properties of NbMoTaW and VNbMoTaW refractory high entropy alloys
[J].
The effect of Ti on the sintering and mechanical properties of refractory high-entropy alloy Ti x WTaVCr fabricated via spark plasma sintering for fusion plasma-facing materials
[J].
A critical review of high entropy alloys and related concepts
[J].
High temperature tensile properties of as-cast and forged CrMnFeCoNi high entropy alloy
[J].
Tensile properties of an AlCrCuNiFeCo high-entropy alloy in as-cast and wrought conditions
[J].
Compressive vs. tensile yield and fracture toughness behavior of a body-centered cubic refractory high-entropy superalloy Al0.5Nb1.25Ta1.25TiZr at temperatures from ambient to 1200oC
[J].
A novel single-crystal L12-strengthened Co-rich high-entropy alloy with excellent high-temperature strength and antioxidant property
[J].
The high temperature tensile and creep behaviors of high entropy superalloy
[J].This article presents the high temperature tensile and creep behaviors of a novel high entropy alloy (HEA). The microstructure of this HEA resembles that of advanced superalloys with a high entropy FCC matrix and L12 ordered precipitates, so it is also named as “high entropy superalloy (HESA)”. The tensile yield strengths of HESA surpass those of the reported HEAs from room temperature to elevated temperatures; furthermore, its creep resistance at 982 °C can be compared to those of some Ni-based superalloys. Analysis on experimental results indicate that HESA could be strengthened by the low stacking-fault energy of the matrix, high anti-phase boundary energy of the strengthening precipitate, and thermally stable microstructure. Positive misfit between FCC matrix and precipitate has yielded parallel raft microstructure during creep at 982 °C, and the creep curves of HESA were dominated by tertiary creep behavior. To the best of authors’ knowledge, this article is the first to present the elevated temperature tensile creep study on full scale specimens of a high entropy alloy, and the potential of HESA for high temperature structural application is discussed.
High-temperature tensile strength of Al10Co25Cr8Fe15Ni36Ti6 compositionally complex alloy (high-entropy alloy)
[J].
Creep strength of refractory high-entropy alloy TiZrHfNbTa and comparison with Ni-base superalloy CMSX-4
[J].
