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。     

Effects of W Concentration on Creep Microstructure and Property of Novel Co-Based SuperalloysEI北大核心CSCD

以γ’相强化的Co-Al-W高温合金(Co-9Al-xW,x=8、9、10,原子分数,%)为研究对象,耦合CALPHAD和晶体塑性本构关系,建立了高温加载时微观组织演化的三元弹塑性相场模型,考察了W含量对蠕变过程中γ’相演化行为和蠕变性能的影响。结果表明,随W含量增加,γ’相体积分数增加,γ基体塑性变形降低,筏化形成并提前,导致蠕变性能提高。不变矩分析表明,9W和10W合金中筏组织形成是出现稳态蠕变阶段的主要原因。应力/应变分析表明,高W合金γ基体中较大的错配应力减小了塑性变形。     

镍基单晶高温合金的研发进展EI北大核心CSCD

单晶高温合金是先进航空发动机、燃气轮机的核心热端材料,单晶叶片要求高、制造工艺复杂、容错空间小,在高温、复杂应力、氧化和热腐蚀等苛刻环境下工作。本文概述了近几年镍基单晶高温合金在合金研制、组织性能演化和表征、近服役环境下力学行为评价以及叶片制造工艺等方面的研发进展,并简单介绍了难熔高熵合金等“下一代”新型高温结构材料的研发情况。     

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。     

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。     

Effect of Heat Treatment on Microstructure and Microhardness of Directionally Solidified NiAl-Cr（Mo）-Hf Intermetallic Alloy

The effect of heat treatment on microstructure and microhardness of directionally solidified (DS)NiAl-Cr(Mo)-Hf alloy is investigated. The results indicate that there is little change in the morphology of NiA1 and Cr(Mo) phases after heat treatment. However, the semicontinuous Heusler phase which is originally located at grain boundaries is partially reduced and fine Heusler particles re-precipitate within NiA1 matrix. Furthermore, NiA1 precipitates coarsen in Cr(Mo) phase and dislocations appear in Cr(Mo) phase. Microhardness of as-grown (DS)NiAl-28Cr-5Mo-1Hf alloy is much higher than that of (DS)NiAl-28Cr-6Mo and basically has no change after heat treatment.     

Oxidation and Microstructure Evolution of CoAl Coating on Directionally Solidified Ni-Based Superalloys DZ466EI北大核心CSCD

Alumimde coatings are widely employed to protect internal cooling channels of high grades blades and buckets in gas turbines have always been in severe conditions including high temperature oxidation and hot corrosion. There is a major concern for the application of aluminide coatings that refer to the inter-diffusion between alumimde coating and superalloy substrate at high temperatures. Diffusion of Al from the coating to the underlying substrate usually leads to depletion of Al in the coating, resulting in inferior oxidation resistance of the coating. Accordingly, Ni declines to diffuse counter currently from the substrate into the coating, as well as other refractory elements, such as Cr, Mo and W etc.. The inter-diffusion between alumimde coating and superalloy substrate results in degradation or various evolution behaviors of alumimde coatings, in other words, substrate composition significantly affects the properties of aluminide coatings. CoAl coating was prepared on directionally solidified superalloy DZ466 by low pressure chemical vapour deposition (LP-CVD). Oxidation behavior and microstructure evolution of CoAl coating was investigated during long term (about 5000 h) exposure at 900 degrees C. Results suggested that, high concentration of aluminum did help to form Al2O3 on the surface of coating, improving oxidation resistance of DZ466 at 900 degrees C. Evolution of matrix phase and precipitates in the CoAl coating during exposure was displayed, beta-NiAl/CoAl phase in the coating transformed gradually to gamma'-Ni3Al phase, higher transformation rate for the gamma phase closed to the substrate due to the diffusion between the coating and the sub strate superalloy. During exposure, alpha-Cr phase precipitated in the middle layer, which inclined to form close to carbides and grow by consuming them. Needle like TCP phase (mu phase) grew in the inner layer that arranged in order, which was due to the cubic microstructure of gamma/gamma'. Heredity-effect was in company with the precipitates evolution.     

一种定向凝固高温合金的薄壁效应研究

采用定向凝固无余量精铸工艺，制造了接近于空心涡轮叶片的２和壁厚的管状薄壁试样，研究了一种定向凝固高温合金的薄壁性能，结果表明，薄壁试样的持久性能与标准试样相比没有明显的下降，说明在本试验条件下，该合金的薄壁效应不明显，从工艺因素和显微组织方面讨论了试验结果。     

定向凝固高温合金DZ125的修复热处理

对实际生产中叶片在真空热处理工序可能出现的问题，如设备故障造成的欠温、保温不足、冷却速度偏低等进行了模拟，研究了修复热处理对DZ125合金组织和性能的影响。结果表明，出现上述情况时，可以对DZ125合金叶片进行修复热处理。     

定向凝固铸造高温合金DZ125热处理工艺的研究

研究了一步和三步两种热处理工艺对DZ125合金组织及性能的影响。结果表明：采用三步热处理工艺可明显改善显著组织。1180℃预处理消除了合金中的低熔点相，有效地抑制了合金的初熔，提高了合金的固溶温度，随着固溶温度的提高，元素枝晶偏析减轻，共晶中的γ′相和初生的粗大γ′相固溶量增加，在随后的冷却和时效过程中析出较多细小γ′相，1100℃高温时效调整了细小γ′相的尺寸和形状，使合金中温，高温持久寿命比一步热处理有不同程度提高。     

高温合金定向凝固技术研究进展

首先回顾了定向凝固的发展历史,重点分析了液态金属冷却定向凝固的技术特点。总结了高温度梯度下制备的定向凝固法单晶高温合金在组织和性能方面的研究现状,结合作者在本领域的研究,着重分析了定向凝固温度梯度、凝固速率、晶体取向、熔体超温处理、熔体对流控制对组织和性能的作用规律和机制,认为高温度梯度定向凝固是细化组织、减少缺陷、提高合金性能的重要途径。最后展望了高温合金定向凝固的发展趋势。     

下引连铸Cu-Ag合金拉拔过程中组织与性能演变EI北大核心CSCD

采用下引连铸工艺制备了d 11 mm Cu-4%Ag合金棒坯,棒坯具有高轴向取向柱状晶组织,表面质量良好,断后伸长率达到35.0%,可直接进行拉拔加工成d 0.04 mm微细丝,无中间退火的累积冷变形量达99.998%。拉拔过程中,在一般变形阶段,棒材变形机制为位错滑移,形成了较多的位错胞,晶内发生微区晶体转动,随应变量增大,剪切变形程度加剧,晶内形成了较多的切变带;在大变形阶段,通过各种动态回复机制如纳米孪晶、位错墙的亚晶化,层状界面,实现大塑性变形。随着变形量的增加,棒材的抗拉强度和硬度分别由铸态的245 MPa和63.4 HV增加至变形量99.7%时的655 MPa和187 HV,而导电率由90.3%IACS则降低至82.2%IACS。上述下引连铸-连续拉拔加工方式制备Cu-Ag合金微细丝材具有短流程、高效率的优势,提供了新的思路。     

Effect of liquid-liquid structure transition on solidification of Sn-Bi alloys

The effect of the liquid-liquid structure transition（L-LST） on the solidification behaviors and morphologies of Sn-Bi alloys was studied further. The results show that the undercooling of the primary and eutectic phase increases and the microstructure becomes finer after solidifying from the melt experiencing the L-LST. In the meantime, in hypoeutectic alloy, when solidifying from the melt experiencing the L-LST, the morphology of primary phase changes from the fir-tree crystal into the equiaxed crystal, and less primary phase and more eutectic structure are observed. Moreover, in eutectic alloy, the spacing of eutectic phase decreases markedly. These investigations would be beneficial to further exploration of the correlation between the melt structure and the micro mechanism of solidification.     

Microstructure,Microsegregation, and Mechanical Properties of Directional Solidified Mg–3.0Nd–1.5Gd Alloy

The microstructure, microsegregation, and mechanical properties of directional solidified Mg–3.0Nd–1.5Gd ternary alloys were experimentally studied. Experimental results showed that the solidification microstructure was composed of dendrite primary a(Mg) phase and interdendritic a(Mg) ? Mg12(Nd, Gd) eutectic and Mg5 Gd phase. The primary dendrite arm spacing k1 and secondary dendrite arm spacing k2 were found to be depended on the cooling rate R in the form k1= 8.0415 9 10-6R-0.279 and k2= 6.8883 9 10-6R-0.205, respectively, under the constant temperature gradient of40 K/mm and in the region of cooling rates from 0.4 to 4 K/s. The concentration profiles of Nd and Gd elements calculated by Scheil model were found to be deviated from the ones measured by EPMA to varying degrees, due to ignorance of the back diffusion of the solutes Nd and Gd within a(Mg) matrix. And microsegregation of Gd depended more on the growth rate, compared with Nd microsegregation. The directionally solidified experimental alloy exhibited higher strength than the non-directionally solidified alloy, and the tensile strength of the directionally solidified experimental alloy was improved,while the corresponding elongation decreased with the increase of growth rate.     

Effect of Surface Roughness on the Oxidation Behavior of a Directionally Solidified Ni-Based Superalloy at 1,100℃

The effect of surface roughness on the oxidation behavior of a directionally solidified Ni-based superalloy was investigated by surface mapping microscope,scanning electron microscope and X-ray diffraction.It was found that specimens with surface roughness of 0.05 urn exhibit the best oxidation resistance,while specimens with surface roughness of 0.14 μm behave worse than specimens with surface roughness of 0.83 μm.The specimens with surface roughness of 0.05 μm have the best oxidation resistance,which is mainly due to the smallest surface area exposed in air and thinnest work-hardening layer.The Al_2O_3 layer alleviates the oxidation process of the specimens with surface roughness of 0.83 μm,and this is the possible reason for the better oxidation resistance of samples with surface roughness of 0.83 μm than samples with surface roughness of 0.14 μm.     

Incipient Melting Behavior and Its Influences on the Mechanical Properties of a Directionally Solidified Ni-Based Superalloy with High Boron ContentEI北大核心CSCD

A new directionally solidified Ni-based superalloy is developed for industrial gas turbine applications, which has high strength and excellent hot corrosion resistance at high temperatures. The high strength of the alloy is primarily derived from precipitation hardening by ordered L12 gamma' phase. To achieve a uniform distribution of precipitated gamma' particles for optimized mechanical properties, the suitable heat treatments are used. However, the heat treatment temperature in Ni-based superalloys is limited by the problem of incipient melting. Incipient melting microstructrue evolution during heat treatment has been hardly reported. Therefore, the behaviors of incipient melting and its effect on mechanical properties in the new directionally solidified superalloy DZ444 with high boron have been investigated in this work. The results show that some interdendritic regions of the as-cast DZ444 sample exhibit many of gamma'/gamma eutectic, MC carbides and multi-phase eutectic-like constituent which are composed of boride, Ni5Hf and eta phases. During solution treatments, incipient melting does not occur in boride or Ni5Hf phase with low melting point firstly, but appears in gamma matrix around multi-phase eutectic-like constituent which is affected significantly by borides. Compared to DZ444 alloy with the normal boron content, incipient melting occurs at the lower temperature in the range between 1160 degrees C and 1170 degrees C. Incipient melting can occur significantly with the increase of the solid solution temperature or time. Incipient melting consists of typical gamma dentrites and a lot of tiny precipitation particles after the water quenching (WQ) method following solution treatment. However, incipient melting forms multi-phase eutectic-like constituent, gamma matrix and gamma'/gamma eutectic successively during air cooling (AC) following solution treatment, and the morphology of multi-phase eutectic-like constituent is similar to that of as-cast alloy. Firstly, a so-called incipiently melted circle (IMC) forms around multi-phase eutectic-like constituent; with the increase of the solid solution temperature or time, IMC extends inwards which makes gamma matrix and multi-phase eutectic-like constituent in this circle melt successively. Finally, a incipiently melted pool forms gradually. Incipient melting is limited to the IMC below 1200. and the area of incipient melting changes with temperature or time correspondingly. However, incipiently melted region (IMR) expands outwards continuously which makes gamma matrix outside the incipiently melted circle melt when the temperature is higher than 1210 degrees C. Especially, IMR swallows up plenty of gamma matrix, and many matrix islands, regions unmelted, exist in IMR above 1250 degrees C which destroys the continuity of the matrix significantly. The incipient melting has a minor effect on the tensile properties, nevertheless, decreases the creep-rupture properties remarkably. The degradation of mechanical properties mainly results from the increasing of the incipient melting area fraction and size.     

Effect of Nb on oxidation behavior of NiTiNb alloys

The oxidation behavior of NiTi and NiTiNb alloys containing different amounts of Nb （7%, 9%, mole fraction） were studied at 800℃ in air. It is found that the oxidation resistance of NiTi alloy can be effectively increased by the Nb addition. Under the same oxidation condition, the mass gain of NiTi is about 7 mg/cm^2, while the inass gains are only 3 mg/cm^2 for Ni47Ti44Nb9 alloy and 2.4 mg/cm^2 for Ni52Ti41NbT. Moreover the oxidation resistance of single phase NiTiNb alloy is better than that of the dual-phase alloy with large amount of Nb precipitates. On the basis of thermodynamics and kinetics of oxidation, the effect of Nb alloying element on the oxidation behavior of NiTi-based alloys was discussed.     

CMSX-2合金位错组态与凝固组织的关系

对CMSX 2合金粗枝晶、细枝晶以及超细枝晶单晶持久变形后的位错组态进行了研究。结果表明 :超细枝晶单晶持久变形过程中在γ/γ′相界上形成的位错网络比粗枝晶单晶的均匀 ;γ相内部形成的位错密度减小 ,分布也更加均匀 ,多位错滑移程度降低 ;切过γ′的位错明显减少 ,反映出高温下合金抵抗蠕变变形的能力提高。     

Effect of Brazing Coating Induction Parameters on Temperature FieldEI北大核心CSCD

重熔处理技术作为提高基体与涂层界面结合强度的方法之一,具有加热速度快、工作环境清洁等优点,但仍存在热影响偏大时造成基体损伤的风险。以感应重熔涂层为研究对象,建立二维有限元传热模型,研究涂层感应重熔过程中的温度场变化规律。以 TC11 钛合金基体表面感应重熔钛基涂层 Ti49Zr49Be 为典型材料,研究发现：集肤深度分别为 4.0 mm、1.5 mm 和 0.6 mm 时,三种集肤深度下涂层熔化界面推移方式均不同,且基体热敏感温度区深度和持续时间随着集肤深度的减小呈现递减趋势;感应重熔功率分别为 35 kW、45 kW 和 55 kW 时,发现界面推移方式相同,均为涂层表面、涂层 / 基体界面处向涂层内部双向推移,且基体热敏感温度区深度和持续时间随加热功率的增加呈现递减的趋势。涂层感应重熔过程传热模型和温度场变化规律研究表明,增大功率、减小集肤深度有助于工程应用中抑制基体热影响。