硼对第二代镍基单晶高温合金显微组织的影响

硼(B)是强化镍基单晶合金小角度晶界的重要微量元素,但目前关于B对镍基单晶合金显微组织影响的系统报道非常有限。通过对3种不同B含量(质量分数分别为0、0.01%、0.02%)的第二代镍基单晶合金DD11铸态及热处理态组织定量表征,研究了B对相转变温度、(γ+γ′)共晶组织、硼化物的影响。结果表明:B显著降低合金的固液相线,提高铸态共晶组织体积分数;0.01%B的加入,合金中未出现M3B2型硼化物相;而0.02%B的加入,显著促进了骨架状硼化物的形成,降低合金初熔点,引起残余共晶含量的大幅度提高;骨架状硼化物吸收较多的Cr、Mo和W等元素,降低合金的固溶强化效果,可导致单晶合金基体的蠕变性能大幅度降低。研究结果对认识单晶合金中微量元素B的作用机理及优化B成分范围具有理论指导意义。     

Effect of temperature and stress on high temperature creep behavior of Ni3Al-based single crystal superalloy

The creep behavior and dislocations mechanism of the Ni₃Al-based single crystal alloy IC6SX with [001] orientation were investigated under the testing conditions of 1100 ?°C/137 ?MPa, 1100 ?°C/120 ?MPa and 1070 ?°C/137 ?MPa. It was observed that the temperature and stress had a significant effect on the high temperature creep life of the single crystal alloy. As the temperature was reduced from 1100 ?°C to 1070 ?°C, the creep life increased from 65.07 ?h to 313.8 ?h. As the stress was reduced to 120 ?MPa, the creep life increased to 243.3 ?h. Under the high temperature and low stress condition the dislocations entered the γ′ phase by climbing caused by the atomic diffusion, instead of slipping.     

Effects of Mo addition on microstructure of a 4th generation Ni-based single crystal superalloy

The effects of Mo addition on the microstructure of a 4th generation Ni-based single crystal superalloy were investigated. Mo addition significantly promoted elements Mo, W and Re partition into γ phase and enhanced absolute lattice misfit at 1100 ?°C. The increase of Mo concentration from 2 ?wt% to 4 ?wt% also decreased the content of eutectic islands and the segregation ratios of other alloying elements in the as-cast state, especially Re and W. After heat treatments, the size of γ′ phase and width of γ channels decreased slightly with higher Mo content. More Mo additions slightly enhanced the segregation behavior of Re while reducing the segregation behavior of Mo. However, it revealed that the primary and secondary dendrite arm spacings were barely affected by Mo addition.     

High temperature creep mechanisms of a single crystal superalloy: A phasefield simulation and microstructure characterization

The high temperature creep behavior of a single crystal Ni-based superalloy was studied by combined experimental and numerical methods. The creep test results showed that the creep curves exhibited a three-stage feature. The qualitative explanations for each stage of the creep curves were carried out based on the microstructure characterizations of γ/γ′ phases and dislocations. An elastoplasticity incorporated phase-field model was developed to provide quantitative understanding on directional coarsening(rafting) of γ′ phase. The simulation results showed that the directionality of γ′ coarsening was induced by both dislocation activity in γchannels and elastic inhomogeneity between γ and γ' phases, therein the dislocation activity played a major role.This findings provide new insights into the design of novel single crystal superalloys with improved creep properties.     

热等静压技术在镍基单晶高温合金中的应用研究进展

通过总结目前热等静压对镍基单晶高温的应用研究进展,阐释了热等静压对镍基单晶高温合金中的疏松等孔洞类缺陷的消除作用,分析了孔洞的愈合机理,并分别展示了应用热等静压后合金的拉伸、持久、疲劳等力学性能变化情况,同时介绍了热等静压对已服役制件组织力学性能恢复处理的研究进展、计算模拟在指导热等静压工艺研究中的作用,展望了中国热等静压技术的未来发展。     

固溶热处理对一种第三代镍基单晶高温合金组织及高温持久性能的影响

研究了固溶热处理对一种Re含量为6.5%(质量分数)的第三代单晶高温合金组织及持久性能的影响,实验结果表明:合金铸态下存在明显的凝固偏析,枝晶间区域存在大量的(γ+γ′)共晶组织。固溶过程中,共晶组织在1 335℃以上开始快速溶解,但难熔元素,尤其是Re元素的偏析需要在1 360℃以上才能有明显改善;经过1 365℃固溶后疏松含量增加至0.21%(体积分数),接近铸态下疏松含量的5.2倍。铸态及经1 360℃和1 365℃固溶热处理后合金的持久性能测试结果显示:固溶热处理显著改善了合金的持久性能,且固溶温度越高,持久性能越高。在高温持久加载过程中,铸态合金的裂纹主要沿枝晶间分布,在(γ+γ′)共晶组织处萌生;当固溶温度较低时,且枝晶干处析出了较多的TCP(topologically close-packed)相,未能充分降低Re元素的偏析可能是导致枝晶干处TCP相大量析出的主要原因;当固溶温度较高时,TCP相析出量较少。     

Effect of long term aging on microstructure and stress rupture properties of a nickel based single crystal superalloy

A nickel based single crystal superalloy with [001] orientation was prepared by the screw selecting method in the directionally solidified furnace. The long term aging (LTA) of the alloy after full heat treatment was performed at 1040 1C for 200–800 h. The microstructure and stress rupture properties at 980 1C/250 MPa and 1070 1C/140 MPa of the alloy long term aged (LTAed) for different time were investigated. The coarsening g0 phase and the broadening g matrix channel are observed in the samples LTAed at 1040 1C, but the g0 morphology is still in cubic shape after LTA for 800 h. No TCP phase precipitates after LTA for 400 h, while needle shaped and granular TCP phase forms in dendritic core of the alloy after LTA for 600 h. With increasing aging time, the volume of the TCP phase increases and it grows from the dendritic core to the interdendritic region along a fixed direction. The composition of the TCP phase is mainly composed of Re and W. With increasing aging time the rupture life of the alloy at 980 1C/250 MPa and 1070 1C/140 MPa all turns shorter. Finally, the relationship between the microstructure and the stress rupture properties is discussed.     

Effects of temperature and stress on the creep behavior of a Ni3Al base single crystal alloy

The creep behavior and microstructure of a Ni3Al base single crystal alloy IC6SX with [001] orientation under the testing conditions of 760 ℃/593 MPa, 980 ℃/205 MPa, and 1100 ℃/75 MPa were investigated. The experimental results showed that Alloy IC6SX had good creep resistance and its creep resistance at elevated temperatures was similar to the second generation nickel-base single crystal alloy containing Re. TEM analysis indicated that the dislocation configuration and movement pattern were different under different temperature and stress conditions. It has been found that under the test condition of 1070 ℃/137 MPa the dislocations moved within the γ channel during the primary creep stage, and the motion of dislocations were prevented by the matrix of γ′ phase, which reduced the creep rate of the alloy. In the secondary creep stage, dislocations cut into the γ′ phase from the γ/γ′ interface. However in the third creep stage, the dislocation pileups were observed in both γ and γ′ phase, and dislocation multiplication occurred when the dislocations with different Burgers vector met and reacted each other.     

一种[011]取向镍基单晶合金的蠕变断裂

研究了[011]取向的镍基单晶高温合金在750~980℃温度范围和200~680 MPa应力下的蠕变断裂特征。在扫描电镜上对各种实验状态下的蠕变断口和纵向剖面进行了详细观察。研究发现：在低温750℃和中温870℃不同初始蠕变应力条件下,枝晶间区亚晶界处不规则γ＇/γ界面是裂纹主要萌生场所,这些已萌生的裂纹在与外加应力轴垂直的(011)面上沿＜110＞和＜100＞两个方向扩展;980℃不同初始应力条件下,裂纹主要在合金中显微疏松孔洞处萌生,沿与外应力轴垂直的方向扩展。观察750℃和870℃不同应力状态蠕变试样的纵向剖面,对亚晶界区不规则γ＇相面积分数的测量和计算表明,用面积分数表征该合金[011]取向在中低温状态下的蠕变损伤程度是可行的。     

一种新型镍基单晶高温合金拉伸性能研究

研究了一种新型镍基单晶高温合金拉伸性能及断裂模式。采用[001]取向制备拉伸性能试样,以平行于单晶[001]取向作为应力轴方向,采用扫描电镜观察断口形貌。实验结果表明,随着拉伸温度升高,合金的抗拉和屈服强度逐渐升高,并分别在760℃和850℃达到峰值,之后随着温度升高迅速下降。塑性随温度的变化则相反,在760℃塑性指标达到最低值,随后断面收缩率急剧增加,而延伸率在850℃达到峰值后迅速下降。合金的拉伸断裂模式在980℃从准解理断裂转向塑韧性断裂。     

Effect of trace boron on microstructural evolution and high temperature creep performance in Re-contianing single crystal superalloys

The effect of trace B on the microstructure and creep properties under 1100 °C/130 MPa in three single crystal superalloys with various levels of B(0, 0.01 and 0.02 wt %) additions was investigated. Compared with the boron-free alloy, the creep rupture life decreased slightly for the alloy with 0.01 wt % B, but dropped obviously for the 0.02 wt% B contained alloy. The low B addition had a slight effect on the main element compositions ofγ/γ′ by the high precision atom probe tomography(APT) analysis and no significant change of γ/γ′ misfit was observed. However, the contents of Re, Mo, Cr in γ phase were decreased with the high B addition, resulting in the decrease of γ/γ′ misfit and increase of the spacing of γ/γ′ interfacial dislocation networks. Meanwhile, the residual(γ+γ′) eutectics and borides with a large volume fraction obviously decreased the creep rupture properties in the high B addition alloy. This study is helpful for understanding the boron's role of strengthening mechanism in high temperature creep of Ni-base single crystal superalloys.     

一种镍基单晶高温合金的反相热机械疲劳行为

采用扫描电镜、透射电镜等方法研究了一种镍基单晶高温合金在600～900℃的反相位热机械疲劳行为,旨在探讨单晶合金的热疲劳变形及断裂机制,丰富单晶合金热机械疲劳理论。试验结果表明:当应变幅由0.6%增大至0.9%时,热机械疲劳寿命下降,塑性应变量和应力范围增大。合金的循环应力响应曲线在低温半周表现为循环硬化行为,而在高温半周则表现为循环软化行为。合金主要的变形特征是局部区域滑移带的运动。热机械疲劳裂纹起始于试样表面应力集中处,并沿着滑移带在{111}面向合金内部扩展,拉应力对合金的断裂起到了主导作用。     

Deformation and damage features of a Re/Ru-containing single crystal nickel base superalloy during creep at elevated temperature

The deformation and damage features of a 4.5%Re/3.0%Ru-containing single crystal nickel-based superalloy during the creep in the temperature range of 1040–1070 °C and stress range of 137–180 MPa was investigated by means of creep properties measurement and contrast analysis of dislocation configuration. The results showed that the alloy exhibited a better creep resistance in the range of the testing temperatures and stresses, the deformation mechanism of the alloy during steady state creep was dislocations climbing over the rafted γ′ phase.In the latter period of creep, the deformation mechanism of the alloy was dislocations shearing into the rafted γ′phase. It is believed that the dislocations shearing into γ′ phase may cross-slip from {111} to {100} planes for forming the K-W locks to restrain the slipping and cross-slipping of dislocations on {111} plane. As the creep goes on, the alternate slipping of dislocations results in the twisted of the rafted γ′ phase to promote the initiation and propagation of cracks along the γ/γ′ interfaces up to creep fracture, which is considered to be the damage and fracture feature of alloy during creep at high temperature.     

镍基单晶高温合金蠕变机制研究进展

叶片在服役过程中主要承受〈001〉轴向的离心载荷,由离心应力导致的蠕变损伤是叶片的主要失效机制之一。基于单晶叶片的典型服役条件,总结了国内外关于高温低应力和中温高应力蠕变变形损伤机制的研究现状,指出深入开展含典型缺陷单晶高温合金蠕变行为、氧化和热腐蚀对单晶合金蠕变-疲劳变形损伤机制影响研究十分必要。     

Effect of Mo content on microstructure and stress-rupture properties of a Ni-base single crystal superalloy

The additional 1.5 wt% Mo was added in a Ni-base single crystal(SC) alloy with the composition of Ni–6.5Al–8.0Mo–2.4Cr–6.2Ta–4.9Co–1.5Re–(0.01–0.05)Y(wt%) to study the effect of Mo content on the microstructure and stress-rupture properties. The creep and stress-rupture tests under the conditions of 850 °C/500 MPa and 1100 °C/130 MPa were conducted, and the microstructure of as-cast, heat treated and stress ruptured specimens were analyzed. It was found that the 1.5 wt% Mo addition enhanced the stress-rupture lives at both intermediate(850 °C) and high(1100 °C)temperatures. The microstructure analysis showed that adding 1.5 wt% Mo in the basic alloy affected the microstructure dramatically, i.e., the Mo-rich phases formed in the specimens of as-cast and stress-ruptured specimens. It is considered that the improvement of the stress-rupture lives is duo to the strengthening effect of Mo to both γ and γ'phases and the decrease of stacking fault energy, diffusion constant and dislocation spacing. The Mo-rich phases precipitated under condition of 1100 °C/130 MPa did not affect the creep and stress-rupture properties obviously in the present study.     

Influence of withdrawal rate on the porosity in a third-generation Ni-based single crystal superalloy

The influence of withdrawal rate on the porosity in a third-generation Ni-based single crystal superalloy was investigated by a quantitative evaluation method. The results showed that the withdrawal rate obviously effected on the average area fraction, number and diameter of porosities except their radius ratios. In consideration of the microstructure observation for dendrite arms, an optimized withdrawal rate was obtained with a minimum porosity level as about 125 μm s~(-1). Simultaneously, a threshold value for the acceptance level of porosities might be set as about 0.1% in order to fulfill the requirements for Ni-based single crystal casting in laboratory scale. Finally, the formation reason of porosity was discussed and it was concluded that the feeding for the volume shrinkage of the last solidified eutectic liquids from the residual liquids and the isolating effect of the morphologies of dendrite arms might be two key factors in controlling the porosities level in Ni-based single crystal superalloy.     

Alloying effects of refractory elements in the dislocation of Ni-based single crystal superalloys

The alloying effects of W, Cr and Re in the [100] (010) edge dislocation cores (EDC) of Ni-based single crystal superalloys are investigated using first-principles based on the density functional theory (DFT). The binding energy, Mulliken orbital population, density of states, charge density and radial distribution functions are discussed, respectively. It is clearly demonstrated that the addition of refractory elements improves the stability of the EDC systems. In addition, they can form tougher bonds with their nearest neighbour (NN) Ni atoms, which enhance the mechanical properties of the Ni-based single crystal superalloys. Through comparative analysis, Cr-doped system has lower binding energy, and Cr atom has evident effect to improve the systemic stability. However, Re atom has the stronger alloying effect in Ni-based single crystal superalloys, much more effectively hindering dislocation motion than W and Cr atoms.     

热处理对第二代镍基单晶合金DD11显微组织及持久性能的影响

采用光学显微镜和扫描电镜对铸态、固溶态和时效态的第二代镍基单晶合金DD11的显微组织进行定量表征,并测试了不同一级时效处理后的合金1 100℃/140MPa和980℃/250MPa条件下的持久性能。结果表明:合金经过1 320℃/6h固溶处理后,(γ+γ′)共晶相全部溶解,凝固偏析显著降低,合金组织均匀。一级时效温度低于1 160℃时,γ′相为方形,一级时效温度高于1 180℃时,γ′相为球形,γ通道显著变宽,并在通道内析出细小的γ′相。随一级时效温度提高,合金的持久寿命先增大后降低。持久性能与γ′相尺寸、体积分数及形态密切相关。     

应力时效对镍基单晶合金DD11组织及持久性能的影响

研究了1070℃/100MPa应力时效对镍基单晶合金DD11组织和持久性能的影响,通过扫描电镜和透射电镜对应力时效前后的组织进行了观察和分析,并对合金应力时效前后的持久性能进行了测试与分析。实验结果表明:应力时效前50h是形筏阶段,竖直方向的γ相通道逐步变窄或消失,水平方向的γ相通道宽度逐步增加,立方状γ′相在应力的作用下向水平方向扩展并相互连接在一起逐渐形成筏排形貌。应力时效50h后是筏排粗化阶段,随应力时效时间的延长,组织缓慢粗化,但是筏排形貌基本保持稳定。对应力时效后的组织进行1 070℃/140 MPa持久性能测试,结果表明,在形筏阶段持久寿命基本保持稳定,在筏排粗化阶段,持久寿命逐步下降。应力时效500h后,持久寿命由热处理后初始状态的360h左右降低至140h左右。研究结果表明,γ相通道尺寸的粗化和组织的筏排化是持久性能衰退的主要因素。