Threshold Stress Creep Behavior of Alloy 617 at Intermediate Temperatures

Creep of Alloy 617, a solid solution Ni-Cr-Mo alloy, was studied in the temperature range of 1023 K to 1273 K (750 °C to 1000 °C). Typical power-law creep behavior with a stress exponent of approximately 5 is observed at temperatures from 1073 K to 1273 K (800 °C to 1000 °C). Creep at 1023 K (750 °C), however, exhibits threshold stress behavior coinciding with the temperature at which a low volume fraction of ordered coherent γ′ precipitates forms. The threshold stress is determined experimentally to be around 70 MPa at 1023 K (750 °C) and is verified to be near zero at 1173 K (900 °C)—temperatures directly correlating to the formation and dissolution of γ′ precipitates, respectively. The γ′ precipitates provide an obstacle to continued dislocation motion and result in the presence of a threshold stress. TEM analysis of specimens crept at 1023 K (750 °C) to various strains, and modeling of stresses necessary for γ′ precipitate dislocation bypass, suggests that the climb of dislocations around the γ′ precipitates is the controlling factor for continued deformation at the end of primary creep and into the tertiary creep regime. As creep deformation proceeds at an applied stress of 121 MPa and the precipitates coarsen, the stress required for Orowan bowing is reached and this mechanism becomes active. At the minimum creep rate at an applied stress of 145 MPa, the finer precipitate size results in higher Orowan bowing stresses and the creep deformation is dominated by the climb of dislocations around the γ′ precipitates.     

Influence of Grain Boundary Character on Creep Void Formation in Alloy 617

Alloy 617, a high-temperature creep-resistant, nickel-based alloy, is being considered for the primary heat exchanger for the Next Generation Nuclear Plant (NGNP), which will operate at temperatures exceeding 760 °C and a helium pressure of approximately 7 MPa. Observations of the crept microstructure using optical microscopy indicate creep stress does not significantly influence the creep void fraction at a given creep strain over the relatively narrow set of creep conditions studied. Void formation was found to occur only after significant creep in the tertiary regime (>5 pct total creep strain) had occurred. Also, orientation imaging microscopy (OIM) was used to characterize the grain boundaries in the vicinity of creep voids that develop during high-temperature creep tests (900 °C to 1000 °C at creep stresses ranging from 20 to 40 MPa) terminated at creep strains ranging from 5 to 40 pct. Preliminary analysis of the OIM data indicates voids tend to form on grain boundaries parallel, perpendicular, or 45 deg to the tensile axis, while few voids are found at intermediate inclinations to the tensile axis. Random grain boundaries intersect most voids, while coincident site lattice (CSL)–related grain boundaries did not appear to be consistently associated with void development. Similar results were found in oxygen-free, high-conductivity (OFHC) copper, severely deformed using equal channel angular extrusion, and creep tested at 450 °C and 14 MPa.     

Evolution of Precipitate Microstructure During Creep of an AA7449 T7651 Aluminum Alloy

Creep forming is a process where plastic deformation is applied at the material’s aging temperature. It enables to obtain parts of complex shape with reduced internal stresses and finds applications, for instance, in the aerospace industry. In this article, we report in-situ small-angle X-ray scattering measurements during creep experiments carried out on an AA7449 Al-Zn-Mg-Cu alloy in the T7651 temper. In the range of temperatures of 413 K to 453 K (140 °C to 180 °C), we show that the initial microstructure is not stable with respect to the applied stress/strain. Accelerated precipitation coarsening is shown to occur, clearly related to the plastic deformation. This strain-induced microstructure evolution is shown to happen even at temperatures well below the aging temperature that has led to the initial temper.     

Ti-Al-Mo合金的时效析出行为

研究了一种新型Ti-Al-Mo近β型钛合金在900℃×30 min/WQ固溶处理后,在不同条件时效处理后的析出相、显微硬度及力学性能。研究结果表明,在500℃时效4 h后的合金显微维氏硬度最高,为4 273 MPa;时效温度在400~700℃范围内时,随着时效温度的升高,析出的片层状α相尺寸逐渐增大,体积分数先增加后降低。由于加入了β稳定元素Mo,能提高强度但也会降低塑性,为了获得较好的强塑性匹配,在时效时间一定的前提下,时效温度应选取500℃左右;而为了得到较高的塑性和断裂韧性,在600~700℃之间时效较为适宜。     

Precipitate Rafting in a Polycrystalline Superalloy During Compression Creep

Rafting is an industrially and scientifically important phenomenon for precipitate-strengthened alloys utilized at high temperatures. Although this phenomenon is observed in polycrystalline alloys as well, the literature lacks scientific work on rafting in polycrystals. Scientific work is usually conducted on single-crystal superalloys. Being one of the many polycrystalline nickel-base superalloys, IN738LC has a good high-temperature strength and hot corrosion resistance. Coherency strains between the FCC gamma matrix (γ)- and L1₂ gamma prime (γ′)-precipitate phase particles mainly provide the high-temperature strength in IN738LC. Conical IN738LC specimens have been aged under compression for various times [24, 192, 480, and 960 hours at 1223 K (950 °C) and 12, 24, 192, and 480 hours at 1323 K (1050 °C)] in order to observe the morphological evolution of the γ′ precipitate microstructure. Dislocations play a determining role in morphological changes. Fingerprints of matrix dislocations in the form of indentations on γ′ precipitates have been identified by scanning electron microscope. Precipitate morphology has become more complex through dissolution/merging as temperature, aging time, and stress have increased. The precipitate morphology has evolved toward rafting at appropriate strain, temperature, and time. Localized slip bands have marked the beginning of rafting. The rafts have been observed at around a 45 deg angle away from the load direction. For higher stress positions, there is a trend toward N-type rafting which is expected of a positive misfit alloy under compression. Rafts eventually have collapsed due to severe creep deformation.     

合金凝固过程的微分型本构方程及其蠕变与松弛特性

指出了加强合金凝固温区力学特性研究的工程意义基于作者近来的实验研究结果及所提出的流变模型,进一步揭示了合金凝固过程相应的微分型本构方程及其蠕变与松驰特性。     

Precipitate Size in the Superalloy IN738LC During Compression Creep

Superalloys are high-temperature materials that are used widely in many industries especially in several engine components in gas turbines. One of the many nickel-base superalloys is IN738LC, which possesses a good high-temperature strength and hot corrosion resistance. It is employed mainly in nozzle guide vanes and blades of land-based turbines. The strengthening is provided mainly by coherent γ′ precipitates. The current research is undertaken to determine precipitate size in IN738LC specimens aged under compressive stress. To observe the influence of magnitude of stress, a constant compressive load is applied to conical specimens that are aged in an inert atmosphere for various times [24, 192, 480, and 960 hours at 1223 K (950 °C) and 12, 24, 192, and 480 hours at 1323 K (1050 °C)]. Thus, the study of microstructural changes due to various stress levels becomes possible with a single specimen in the same experimental condition. A single-size distribution of precipitates has been observed except for the 192-hour and 480-hour experiments at 1323 K (1050 °C). The results indicate that the average precipitate size is directly proportional to the temperature but inversely proportional to the applied stress, although in some specimens, maximum size is observed at medium-stress levels. Strain becomes effective at most severely crept specimen by causing a merging of precipitates. Raft degeneration is also observed in this specimen. The growth exponent is found to vary with stress and temperature between 2.97 and 3.78.     

Characterization of Creep-Damaged Grain Boundaries of Alloy 617

Intergranular cracking and void nucleation occur over extended periods of time in alloy 617 when subjected to stress at high temperatures. Damage occurs inhomogeneously with some boundaries suffering failure, while others are seemingly immune to creep. Crack propagation associated with grain size, and grain boundary character was investigated to determine which types of grain boundaries are susceptible to damage and which are more resistant. Electron backscatter diffraction and a stereological approach to obtain the five-parameter grain boundary distribution were used to measure the proportions of each type of boundary in the initial and damaged structures. The samples were crept at 1273.15 K (1000 °C) at 25 MPa until fracture. It was found that in addition to low-angle and coherent twin boundaries, other low index boundary plane grain boundaries with twist character are relatively resistant to creep.     

617合金的组织稳定性

 对先进超超临界燃煤发电机组锅炉高温部件候选材料617合金在750℃下时效1000,2000,3000和5000h后试样的组织稳定性进行了研究。通过OM,SEM,TEM和X射线衍射等方法对时效后晶界及晶粒内的析出物进行了分析。研究发现,时效后试验材料的晶界析出M23C6碳化物;晶内析出γ′相,且γ′相呈颗粒状弥散分布,γ′相尺寸随时效时间的延长而增长。经1000h时效后,试验材料的强度开始增加,并且随着时效时间的延长,强度开始呈现平稳趋势。随着时效时间的延长,试验材料的韧性呈下降趋势。     

M963合金高温蠕变过程中显微组织的演化及断裂机理

研究了M963合金在975℃×225MPa条件下蠕变过程中的组织演化及断裂机理.结果表明M963合金的蠕变曲线呈现出明显的3个阶段且稳态蠕变速率较低;蠕变过程中,γ'相粒子逐渐筏形化,由初始阶段分布在γ基体中的立方状孤立相转变为蠕变后期包围γ相的连续相;在枝晶干上有颗粒状M6C碳化物析出;蠕变变形机制从初始阶段的Orowan绕过γ'相粒子变为蠕变后期的位错切过γ'相粒子.     

Ti-Cr合金中片状相表面浮突的研究

利用干涉显微镜和原子力显微镜观察研究了Ti 8%Cr(质量分数 )合金中析出的片状相的表面浮突。发现析出的片状相的浮突类型为“帐篷型” ,中间存在一条“中脊” ,析出相在其两边生长 ,观察到的最大浮突高度为140nm ,最大的浮突角为 9 6°。实验值和理论计算值能够比较好地吻合。     

M_2C Precipitate in Isothermal Tempering of High Co-Ni Alloy Steel

Secondary hardening alloy has been studied fordecades and emphases are focused on the mechanismof secondary hardening and carbides precipitationduring second hardening reaction.It has got to awide common view that alloy carbides replace ce-mentite during the aging time reaction in secondaryhardening alloy steel[1] .Many studies were conduct-ed on the alloy carbide precipitation mechanism[2 ,3]and it is well established that the secondary harden-ing is accomplished by the precipitation of fine …     

油膜轴承巴氏合金蠕变ANSYS分析

基于ANSYS有限元分析软件,结合巴氏合金承载区运行温度测试、油膜压力的计算程序以及巴氏合金拉伸试验,分析了轧机油膜轴承在典型工况下承载区蠕变的整个过程,得到了不同温度、不同压力下巴氏合金蠕变曲线以及最大蠕变值点的应力松弛曲线,得出温度与应力对巴氏合金的影响关系以及巴氏合金蠕变未达到蠕变破坏阶段的结论。     

Ti-600合金蠕变性能及硅对其蠕变性能的影响

研究了Ti-600合金在3种温度(550、600、650℃)、5种应力(150、200、250、300、350 MPa)下的蠕变性能,并分析了硅化物对合金蠕变性能的影响。研究结果表明,Ti-600合金具有较小的稳态蠕变速率及较大的蠕变激活能,反映出该合金具有较好的蠕变抗力。当温度升高、应力增大时,Ti-600合金的稳态蠕变速率增大。600℃下,当蠕变应力高达350 MPa时,Ti-600合金的稳态蠕变速率低至3.72×10-7s-1。Ti-600合金的蠕变激活能最高可达574.6kJ·mol-1,最低为332.7 kJ·mol-1。在蠕变过程中,Ti-600合金内析出了S2型(TiZr)6Si3硅化物,能够钉扎位错、阻碍位错滑移,提高合金的蠕变抗力。     

热时效和辐照环境下Fe-Cr-Mn奥氏体钢晶界溶质元素偏聚和相析出

研究了含2%W和V的Fe-Cr-Mn合金经热时效处理和电子辐照条件下,溶质元素晶界偏聚和析出相的行为。实验结果表明:在含W、V合金中热时效引起Cr、Mn晶界贫化,它决定于富Cr碳化物M₂₃C₆中组元组成。辐照条件下,两种合金中Cr、Mn都发生晶界贫化,它决定于辐照产生空位浓度和空洞体胀量,辐照诱起晶界元素偏聚影响析出相成分。