Research Progress on Surface Impact Strengthening Mechanisms and Application of Nickel-Based SuperalloysEI北大核心CSCD

为满足不断攀升的两机涡轮动力系统的快速发展,表面冲击强化技术在涡轮转子用高温合金表面强化的应用及相应机制的研究受到了广泛关注。然而,高温合金表面硬化层在高温服役环境下的回复、再结晶行为难以避免,由此引起的表面强韧化、抗疲劳效果的退化,成为制约表面冲击强化技术在先进高温合金关键部件深入应用的瓶颈。本文总结了近年来镍基高温合金表面冲击强化机制及应用研究进展,分析了表面冲击强化对镍基高温合金表面强韧性及抗疲劳的作用规律,探究了高温合金表面冲击硬化层在高温及长期时效过程中的显微组织、微结构演化及其对高温稳定性的作用机理。以期为发展镍基高温合金表面冲击强化、提高两机涡轮转子疲劳抗力提供基础。     

Effect of Gravity on Directionally Solidified Structure of SuperalloysEI北大核心CSCD

分别利用常规下抽拉法与新型上提拉法进行不同方向的高温合金定向凝固实验,对比研究重力对单晶铸件凝固组织的影响。结果表明,在常规下抽拉法实验的向上凝固过程中,容易出现雀斑、γ/γ’共晶上聚和籽晶回熔紊乱等问题。原因是糊状区内液体由于元素偏析引起密度减小,在重力作用下形成了上重下轻的失稳状态并引起对流。而通过新型上提拉法实现的顺重力凝固过程中,密度减小的液体处于糊状区上端,形成上轻下重的稳定状态,使重力的作用由失稳因素转化为维持稳定的因素,抑制了液体对流的产生与发展。采用新型上提拉法制备的单晶铸件中彻底消除了雀斑缺陷,抑制了γ/γ’共晶组织的向上聚集,也保证了低密度籽晶稳定的回熔和外延生长。顺重力定向凝固技术从根本上消除了重力对高温合金定向凝固的不良影响,有希望发展成为新一代的先进单晶叶片成型技术。     

γ’相强化钴基高温合金成分设计与蠕变机理研究进展EI北大核心CSCD

近年来,随着航空发动机和地面燃机的持续发展,对其关键热端部件的环境抗力和承温能力的要求越来越高,γ′相强化钴基高温合金在抗热腐蚀性能和熔点温度等方面较镍基高温合金具有优势。为了促进此类合金的发展,本文基于国内外在合金开发和蠕变性能等方面的研究成果,结合本课题组的研究工作,总结了该类合金在合金化原理、合金设计方法和蠕变机理等方面的研究现状,凝练出了目前该类合金发展存在的关键基础科学问题,并对未来需要关注的研究方向进行了概述。     

Creep Properties and Solute Atomic Segregation of High-W and High-Ta Type Powder Metallurgy SuperalloyEI北大核心CSCD

研究了最新开发的高W高Ta型粉末高温合金GNPM01优异的蠕变性能和蠕变强化机理。利用球差校正扫描透射电子显微镜(AC-STEM),详细分析了粉末高温合金GNPM01蠕变变形机制和溶质原子在超点阵层错和微孪晶上的偏聚行为,阐明了溶质原子Cr、Co、Mo的偏聚是导致无序微孪晶在晶内扩展的根本原因。GNPM01合金在815℃蠕变过程中,γ’相内孤立的超点阵外禀层错(SESF)处出现了W、Ta、Nb、Co和Ti的Suzuki偏聚,并且偏聚原子具有有序的占位,造成SESF处发生局部微区相变(LPT),形成的[(Ni, Co)₃(Ti, Nb, Ta, W)]有序相η相能有效阻碍微孪晶的形成和扩展,从而降低合金的蠕变速率。     

Recrystallization During Thermo-Mechanical Fatigue of Two High-Generation Ni-Based Single Crystal SuperalloysEI北大核心CSCD

通过SEM和TEM等手段研究了经热机械疲劳变形后的第三代和第四代单晶高温合金的显微组织,了解高温合金在近服役条件下的变形组织,分析单晶高温合金近服役条件下的变形机制。结果表明,第三代和第四代单晶高温合金样品中在不同{111}面上产生了大量的变形孪晶,且在平行的孪晶片层中或者孪晶片层交截周围发现大量再结晶晶粒。再结晶晶粒的界面主要由变形后的孪晶界、小角度晶界以及孪晶相交产生的大角度晶界组成。借助像差校正透射电镜解析了变形后的孪晶界结构以及孪晶诱发动态再结晶的过程,揭示了单晶高温合金热机械疲劳断裂机制。     

微观相场法研究镍基合金相变时的成分演化及界面定向迁移机制

基于耦合外应力场下的微观相场模型,研究了Ni_(75)Al_(7.5)V_(17.5)三元合金相变过程中相分解、生长过程中合金成分演化及应力作用下相间界面的定向迁移规律。研究表明:早期DO_(22)相长大所需的V来自于在L1_2相间有序畴界处,而后期则来自于其内部,且早期DO_(22)相长大的速度高于后期的长大速度;时效过程有5种异相间界面形成,而A类界面的迁移为相迁移的主要类型;相变初期A、B类界面较少,合金形貌变化不大,随着相变的进行,A类界面数量增加,相的生长及分解过程加剧,导致后期A类界面减少、B类界面增多,合金体系达到平衡状态;在压应力作用下,A类界面沿应力方向迁移,促进了DO_(22)相在此方向上的生长,导致合金"筏状"组织的形成。     

Microscopic Creep and Damage of γ-γ＇Two-phase Nickel-base Superalloy under Elevated-temperature

用晶体热粘塑性模型研究了镍基合金中γ与γ′相的非弹性行为,进行了在晶体塑性理论基础上考虑镍基高温合金中的随机分布相结构对其蠕变与损伤过程的数值模拟.模拟假定合金中γ′相的尺寸及分布具有随机性,γ与γ′相在高温下的变形、损伤用温度相关的晶体粘塑性损伤本构关系进行计算,计算结果与周期体胞模型进行了比较.结果表明：γ′相随机分布模型能够反映镍基合金微观变形和应力分布不均匀、局部剪切变形和可能的破坏倾向;该模型的蠕变分析结果较周期体胞模型合理.     

Formation Mechanisms of Hot Cracks in Laser Additive Repairing Single Crystal SuperalloysEI北大核心CSCD

以第二代单晶高温合金DD432为研究对象开展激光增材修复实验,利用实验与理论计算相结合的手段,分析和研究了单晶高温合金在激光增材修复过程中的热裂纹形成机制。结果表明,单晶修复区内热裂纹在大角度晶界处形成,裂纹两侧呈现显著应力集中,裂纹源区域分布大量MC型碳化物。热裂纹的形成取决于液膜的稳定性、应力集中及碳化物析出相的共同作用。液膜稳定性取决于枝晶凝并过冷度,并与相邻晶粒间的晶界角度密切相关。基于Rappaz枝晶凝固过冷理论,计算获得DD432单晶合金形成稳定液膜的最小晶界角为2.9°,即该合金热裂纹形成的临界角;大角度晶界处的枝晶凝并过冷度为395 K,远高于晶粒内部枝晶间液膜的过冷度(29.58 K)以及小角度晶界(3.6°)处的枝晶凝并过冷度(56 K),大角度晶界为开裂提供了稳定液膜;沉积区内部的高水平应力集中驱动了热裂纹的萌生与扩展;MC型碳化物析出相通过“钉扎作用”抑制液相补缩及弱化与基体之间结合强度等作用进一步促进了热裂纹形成。     

Effects of Oxygen Increasing/Decreasing Processes on Surface Characteristics of Superalloy Powders and Properties of Their Bulk Alloy Counterparts: Powders Storage and DegassingEI北大核心CSCD

采用场发射扫描电镜(FESEM)、X射线光电子谱(XPS)、高角环形暗场扫描透射电镜(HAADF-STEM)以及程序升温脱附与质谱联用(TPD-MS)等表征手段研究了不同窄粒度范围镍基高温合金粉末的原始表面状态以及存储和脱气等增/降氧过程对合金组织和性能的影响。结果表明：不同粒度原始态粉末的表面组成均为Ni O/Ni(OH)₂、TiO₂、CoO和Cr₂O₃,0～15μm粒径粉末(细粉)和150～180μm粒径粉末(粗粉)平均氧化膜厚度分别为3.32和10.90 nm。细粉和粗粉在空气环境中存储后氧含量逐渐增大,在3～10 d达到稳定值,分别约为250×10^-6和40×10^-6。存储后0～53μm粉末制备的块体合金氧含量升高,室温、650℃和750℃拉伸强度变化不大,但塑性下降,合金在650℃、890 MPa和750℃、530 MPa的持久性能均下降。0～15μm细粉加热过程中(室温～1000℃)会发生气体脱附,存在明显脱附峰的气体包括CO₂     

Effect of Fe on Microstructure and Coercivity of SmCo-Based MagnetsEI北大核心CSCD

High-temperature permanent magnets have an important application in the aerospace and other high-tech fields, among which 2:17-type SmCo magnets have become the first choice for high-temperature permanent magnets due to the strong magnetic anisotropy and high Curie temperature. Although there are studies on the effect of Fe on the remanence and coercivity, the role that Fe plays on coercivity mechanism of SmCo magnets is still unclear. In this work, Sm(CobalFexCu0.08-0.10Zr0.03-0.033) z (x= 0.10-0.16, z=6.90 and 7.40) magnets are prepared and the magnetic properties under different temperatures are investigated. The magnets with an intrinsic coercivity of 603.99 kA/m and a maximum energy product of 87.30 kJ/m(3) at 500 degrees C. are obtained. It is revealed that at room temperature the coercivity of the magnets increases with increasing Fe content, however, at 500 degrees C. the coercivity shows an opposite dependency on Fe content. Moreover, the effect of Fe on coercivity is more obvious at low z value. The phase structure and composition analyses were characterized by XRD and TEM. The results show that with the increase of Fe content, the size of the 2: 17R cell phase increases, the volume ratio of cell boundary 1: 5H phase decreases, and furthermore, both Fe content in the 2: 17R phase and Cu content in the 1: 5H phase increase. The variations of Fe and Cu contents in both phases lead to the change of the domain wall energy difference. With the increase of Cu content of 1:5H phase, the domain wall energy of 1: 5H phase (gamma(1:6)) drops faster at room temperature, the coercivity is determined by gamma(2:17)-gamma(1:5), so the coercivity increases with increasing Fe content. While at 500 degrees C, due to gamma(1:6) at its Curie temperature, the coercivity is mainly determined by the domain wall energy of 2: 17R phase (gamma(1:17)), which decreases with increasing Fe content. The increase of Fe content at the low z value results in a smaller growth of cell size, which leads to a more significant change in coercivity.     

Effects of Tempering Temperature on Microstructure and Mechanical Properties of Drill Pipe Steel 26CrMoEI北大核心CSCD

The effects of tempering temperature on microstructure and mechanical properties of steel 26CrMo were studied based on mechanical property tests and microstructure observation. The results show that a phase matrix gradually occurs recovery and recrystallization with increasing temperature during 540 similar to 690 degrees C temper process, martensite morphology fades away gradually, flake or rocklike carbides separate out along the martensite boundaries, and then change into granulated dispersed distribution, at 690 degrees C tempering carbides happen aggregation and growth on grain boundaries. With tempering temperature increasing, the strength of 26CrMo steel is gradually reducing, plasticity and toughness are gradually increasing. The tensile property and impact energy can meet all different grade drill pipe requirements in API 5DP standard with different tempering conditions. The total impact energy, crack initiation energy and crack propagation energy of 26CrMo steel are gradually increasing with the tempering temperature rising, the crack propagation energy is three times of crack initiation energy which shows great anti-crack propagation capability, but their ratio has no obvious change. The change of impact property is closely related to the strength and plasticity change, impact toughness stand or fall depends on high or low plasticity.     

Effect of pre-compression on the stress rupture properties of single crystal superalloy：Ⅰmicrostructure and stress rupture properties of pre-compressed samples

研究了单轴预压缩对镍基单晶高温合金DD98的微观组织和持久性能的影响及其原因．在热处理态的基础上，于真空中1050℃下分别实施了180MPa／6h和220MPa／6h两种预压缩、观察到预压缩后的试样中形成了与应力轴平行的P型γ’筏．同时，在竖直通道内的γ／γ界面上形成了位错网．单轴预压缩对持久寿命的影响因测试条件不同而变化，但预压缩试样的延伸率均得到提高．在高温低应力持久时，预压缩形成的P型筏转化为宽度不同的波浪状N型筏，导致位错越过γ’方式有所差异．在低温高应力时，P型筏仅表现出微弱的向N型筏转化的趋势．此时，两种预压态试样中均形成位错缠结．在中等温度及应力时，两种预压态中的P型筏则转化为较混乱的N型筏，其位错特征是不规则位错网．     

钴基高温合金GH5605热压缩变形行为EI北大核心CSCD

利用Gleeble−3500热模拟试验机对真空感应(VIM)+电渣重熔(ESR)所得的GH5605合金铸锭进行热压缩实验,研究其在变形温度为950~1200℃、应变速率为0.001~10 s^(−1)、真应变为0.65时的热变形行为。结果表明:铸态GH5605合金的真应力−应变曲线属于加工硬化+动态回复型,分为3个阶段,即Ⅰ剧烈加工硬化阶段、Ⅱ平缓加工硬化阶段、Ⅲ稳态流变阶段。建立的Arrhenius本构方程相关系数(Rr)和平均相对误差(δ)分别为0.95和11.99%,结合热加工图和变形组织分析得出GH5605合金良好的加工区域为变形温度1055~1200℃、应变速率0.01~0.1 s^(−1)。     

APPLICATION OF d-ELECTRON ALLOY DESIGN THEORY TO DEVELOPMENT OF HOT CORROSION RESISTANT Ni-BASE SINGLE CRYSTAL SUPERALLOYS——Ⅱ. Effects of Refractory Metals Ti, Ta and Nb on Microstructure and Properties

本文系统地研究了Ti,Ta和Nb三个γ′形成元素在抗热腐蚀合金系统Ni-16Cr—9Al-4Co—2W—1Mo—Ti-Ta—Nb(at-％)中的作用该合金系统是根据d-电子合金设计理论和镍基高温合金合金化的特点选出的根据DTA和图象仪分析结果确定了凝固反应温度以及共晶(γ+γ′)体积分数在Ti-Ta—Nb浓度三角形中的等值线分布,与参考合金IN738LC相比较,在所研究的大部分成分范围内,设计合金表现出很低的共晶(γ+γ′)析出(<0.4v-％),较低的凝固范围(<65℃)和较宽的热处理温度范围(>100℃),说明合金具有良好的组织稳定性,单晶铸造性能和热处理工艺性能.经长期时效(900℃/500h)后,无TCP相析出,在很宽的成分范围内,试验合金表现出良好的抗热腐蚀性能(900℃/24h坩埚试验,失重<20mg/cm~2)综合各项结果,给出了可供选择合金成分进行最后单晶生长及力学性能评价的具有较好综合性能的成分范围     

Research Progress on the Crack Formation Mechanism and Cracking-Free Design of gamma' Phase Strengthened Nickel-Based Superalloys Fabricated by Selective Laser MeltingEI北大核心CSCD

传统牌号高强镍基高温合金具有较宽的凝固温度区间、较高比例的低熔点共晶相,在增材制造快速非平衡凝固过程中易产生裂纹等缺陷;同时,热处理过程中残余应力释放和γ’相快速析出导致应变时效裂纹的形成,严重限制了其在激光增材制造领域的应用与推广。基于此,本文综述了近年来国内外研究组及作者团队在选区激光熔化高强镍基高温合金裂纹形成机理与抗裂纹设计(成形工艺参数优化、热处理制度调控以及合金成分设计)领域相关的研究进展,并对激光增材制造γ’相强化镍基高温合金裂纹调控的研究进行了展望。     

新型高温合金热处理及长期时效过程中组织稳定性研究北大核心CSCD

主要研究了三种新型高温合金在铸态热处理后,再经750℃/2 700 h和850℃/2 700 h长时间时效后的组织稳定性。结合相图计算探讨了不同合金设计对长期时效过程中针状组织演变规律的影响。结果表明:Nb+Ti+Ta总含量对铸态偏析组织及其后热处理及时效过程中的析出相演变具有重要影响,高Nb+Ti+Ta含量合金拥有较高的共晶体积分数,并易于在其附近形成不稳定的η相;随时效时间延长和温度升高,共晶体积分数减少,η相的体积分数降低,在组织中逐渐析出更为平衡的TCP相。     

UN核燃料烧结致密化过程的相场模拟EI北大核心CSCD

引入了平移和旋转等作用过程,建立了烧结致密化过程的相场模型,分析了平流通量刚体运动对烧结颈的形成、平衡二面角和烧结致密化过程的影响。模拟结果表明,引入平流通量刚体运动在烧结初期加快了烧结颈的形成,而在烧结后期作用不明显;烧结相场模型中是否引入平流通量不影响晶界平衡二面角的取值;致密化过程的气孔收缩分为3个阶段,分别是烧结初期的表面扩散主导阶段、平流通量取代表面扩散的阶段和致密化完成阶段;增大平移迁移率能加快致密化过程、增加致密化程度,但超过一定阈值后其作用达到饱和;颗粒完成致密化收缩后会形成稳定的三叉晶界(夹角120°),多晶UN烧结模拟形貌演化中三叉晶界的形成和气孔收缩致密化的行为与实验结果一致。     

Microstructure and Property of Cu/Al Joint Brazed with Al-Si-Ge Filler MetalEI北大核心CSCD

Cu/Al brazing has good prospect for applications in the air conditioning and refrigeration industry. A suitable filler metal is the key of Cu/Al brazing. The chemical and physical properties of the filler metal have great influence on the brazing process and parameters. And the strength of the brazing joint is closely related to the properties of the filler metal and the brazing process. While the previous studies have not developed a kind of Cu/Al brazing filler metal which can achieve a tough joint at a low brazing temperature. In this work, the Al-5.6Si-25.2Ge filler metal was first used to braze Cu/Al dissimilar metals, and the melting characteristics of the filler metal, spreading wettability, Cu interfacial structure and strength of brazed joint were investigated systematically. Additionally, the common Zn-22Al filler metal was also used for comparison. The results show that the Al-5.6Si-25.2Ge filler metal possesses low melting temperature (about 541 degrees C) and excellent spreading wettability on Cu and Al base metals. The interfacial structure of Al-5.6Si-25.2Ge/Cu was CuAl2/CuAl/Cu3Al2. The thickness of planar CuAl and Cu3Al2 phases was only 1 similar to 2 mu m, and the thickness of cellular CuAl2 phase was about 3 mu m. The interfacial structure of Zn-22Al/Cu was CuAl2/CuAl/Cu9Al4, but the average thickness of the CuAl2 layer was up to 15 mm. The test results of the shearing strength show that the shearing strength of the Cu/Al joint brazed with Zn-22Al filler metal was only 42.7 MPa, but the shearing strength brazed with Al-5.6Si-25.2Ge filler metal was higher (53.4 MPa).     

High Temperature Tribological Properties of Ti-GLC Films Deposited on Different Bearing Steel SubstratesEI北大核心CSCD

针对航空发动机主轴轴承服役工况恶劣和类石墨碳基薄膜在高温环境下的性能研究不足等问题,采用磁控溅射技术在不同轴承钢基体(M50 钢、M50NiL 钢和 W9Cr4V2Mo 钢)上沉积 Ti-GLC 薄膜,探究在不同温度下的摩擦学性能。采用 SEM、 Raman 分析薄膜的微观结构,采用纳米压痕仪、划痕仪等测试其力学性能,利用 MFT-5000 型多功能摩擦磨损试验机测试所镀薄膜在不同温度下(室温、200 ℃、250 ℃和 300 ℃)的摩擦学性能。结果表明：在三种不同轴承钢基体沉积的 Ti-GLC 薄膜,其硬度和弹性模量变化不大,结合力从大到小依次为 M50＞M50NiL＞W9Cr4V2Mo。随着温度的升高,三种钢基体沉积 Ti-GLC 薄膜的摩擦因数均逐渐增大,而磨损率则先减小后增大,且表现出不同的磨损形式。三种轴承钢基体沉积 Ti-GLC 薄膜的最佳工作温度区间为室温~200 ℃,M50 钢基体所镀薄膜具有更好的力学性能和摩擦学性能,其结合力达到 80 N 以上, 300 ℃时的平均摩擦因数为 0.125,磨损率仅为 3.05×10^?17 m³ /(N·m)。研究成果为类石墨碳基薄膜在高温环境下的实际应用奠定了理论基础。     

Effect of High-Energy and Instantaneous Electropulsing Treatment on Microstructure and Properties of 42CrMo SteelEI北大核心CSCD

42CrMo steel was widely used in many industry fields for its excellent hardenability and high temperature strength. Many transmission mechanisms and fasteners, such as roller and heat-resistant gear, are made of this steel. However, the ductility of 42CrMo steel is relatively low after quenching and tempering. During high tempering Mo riched carbides at grain boundary and undecomposable martensite at low tempering are the main reasons for poor ductility of 42CrMo steel. Grain refinement can enhance both strength and ductility significantly, but traditional refinement technology will cause intergranular oxidation so that strengthening effect was weak. Although thermomechanical treatment can achieve dynamic recrystallization, its refinement effect is unstable. Elecropulsing treatment, which makes significant change in microstructure and properties of metals, has been applied in many fields such as, modification of solidified microstructure of liquid metal, healing of fatigue crack, nanocrystallization of amorphous materials and so on. Moreover, this process can produce superior mechanical properties in metals. In order to improve the mechanical properties of 42CrMo steel better, high-energy and instantaneous electropulsing treatment was applied. In this contribution, 42CrMo steel was subjected to traditional and electropulsing treatment individually. It was found that EPQ treatment (480 ms electropulsing treatment, water cooled) results in finer grain, promoting the formation of retained austenite and twin martensite; EPT treatment (180 ms electropulsing treatment, air cooled) can stabilize retained austenite in EPQ specimen and induce multiphase structure. Mechanical properties results indicate that strength-ductility balance of EPQ and EPQ+EPT specimen are 32% and 13.9% higher than that of TQ (traditional quenched) and EPQ+TT (traditional tempered) specimen respectively.