Subjects: Materials Science >> Materials Science (General) submitted time 2023-03-19 Cooperative journals: 《金属学报》
Abstract： Microelement Hf added in Ni-based powder metallurgy (PM) superalloy can modify microstructure and improve mechanical properties, such as stress-rupture life, creep resistance and crack growth resistance, and also benefit to eliminate notch sensitivity. So systematically studying the effect of microelement Hf on PM superalloy microctructure will help to comprehend its corresponding mechanism. The effects of microelement Hf on the morphologies, chemical compositions and content of g' phase and MC carbide in FGH97 PM Superalloy were investigated by means of SEM and physiochemical phase analysis. The results showed that Hf facilitated the precipitations of g' phase and MC carbide, and changed chemical compositions of g' phase and MC carbide, the effect of Hf on the size and morphology of MC carbide was not obvious, while Hf greatly affected the size and morphology of g' phase and accelerated the splitting of g' phase from one instable cubic g' particle to stable octet of cubes. As Hf affected the lattice misfit of g'/g phase (d), modifying Hf content changed the critical splitting size of g' phase (Dc). The relationship between Dc and Hf content (w(Hf)) was found to be Dc=315.4+640.2w(Hf)-358.2[w(Hf)]2. With Hf content increased, the absolute value of d decreased and Dc increased. Cubic g' particle split into an octet of cubes when g' phase grew up to the critical splitting size.
Subjects: Materials Science >> Materials Science (General) submitted time 2023-03-19 Cooperative journals: 《金属学报》
Abstract： GH4169 alloy is widely used to make aero engine, gas turbine as it is one of the most important superalloy. g'' phase is the main strengthen phase, however, the metastable g'' phase will transform to stable d phase during aging or servicing for certain time. d phase is significance in the alloy, its precipitating and distributing behavior have an effect on the properties of the alloy. In recent year, researchers pay more attention on electric field treatment (EFT), this is because of high energy density, accurate controlling, clean and safety. EFT is one of the most important energy field except temperature field and stress field. In this work, EFT was performed on GH4169 superalloy to investigate the influence of EFT on precipitation behavior of d phase in the alloy, and the mechanism of the effect of EFT on the phase transformation was also discussed. The results show that d phases precipitate on the grain boundaries after EFT with 8 kV/cm at 850 ℃ for 15 min, and large amounts of g'' phases precipitate inside the grains. With the increasing of EFT time, both the volume fraction and the size of d phase increase, at the same time the size of g'' phase increases. The volume fraction of d phase is less and the size of d phase is smaller, and the volume fraction of g'' phase is higher by EFT, compared with that by aging treatment (AT) for the same time. In addition, the Nb content on the grain boundary decreases and both Fe and Cr content increase, meanwhile the lattice parameters of c decreases and a, b increase. The vacancy concentrations can be accelerated by EFT, so that the diffusion of Fe and Cr atoms can be promoted. Meanwhile, the Nb atoms in d phases on the grain boundaries can be displaced by Fe atoms and Cr atoms, therefore the Nb atoms are dissolved into the grain. The nucleation rate of g'' phases increases with the increasing of vacancy concentrations. The vacancies relax coherent distortion between g phases and g'' phases, and suppress g'' phases to transform to d phases. Thus the stabilization of g'' phases is enhanced.
Subjects: Materials Science >> Materials Science (General) submitted time 2023-03-19 Cooperative journals: 《金属学报》
Abstract： Hafnium (Hf) is one of the most important microelements in powder metallurgy (P/M) superalloy. Hf modifies the microstructure and drastically improves mechanical properties in P/M superalloy. The effect of Hf in a nickel-based P/M superalloy was systematically studied by means of FEG-SEM, TEM, AES, EDS and physical and chemical phase analysis. Hf mainly distributes at interdendritic region of the solidification powder in form of solid solution, which is helpful to reduce prior particle boundary (PPB). Hf facilitates morphology of γ′ phase to be unstable and enhances the large cubic γ′ phase to split into smaller ones, so the γ′ phase turns into a stable state with a lower energy faster. Hf is mainly distributed in γ′ phase and MC carbides, which changes the distribution of element between the γ′ phase, MC and g solid solution, which is beneficial to eliminate notch sensitivity and improves overall mechanical properties of the alloy.
Subjects: Materials Science >> Materials Science (General) submitted time 2023-03-19 Cooperative journals: 《金属学报》
Abstract： Much attention has been paid to the development of more advanced materials for high- pressure compressor and turbine discs of gas turbine engines. A high performance wrought superalloy GH4065 for disc applications has been recently developed based on the comprehensive evaluation of a series of model alloys with characteristic chemical composition, lattice parameter, particularly g′ volume fraction. The concentration of major alloying elements of GH4065 is closely similar with René 88 DT and specifically optimized considering the demands of ingot metallurgy technologies. Therefore, GH4065 can be considered as an ingot metallurgy version of powder metallurgy René 88 DT. Large scale vacuum arc remelting (VAR) ingots of GH4065 alloy with diameter up to 508 mm have been produced via standard triple melting techniques. Micro-scale segregation of alloying elements on large VAR ingot has been effectively suppressed due both to optimized alloying elements concentration and to improved melting techniques. Ultra-low carbon content (less than 0.02% in mass fraction) significantly decreases the dendritic segregation tendency of certain alloying elements and promotes the uniformity of microstructures. VAR ingot of GH4065 exhibits extraordinary hot plasticity, ingot conversion can be accomplished using conventional open die forging procedure. Fine and uniform g+g′ duplex structures can be obtained on billets and disc forgings via a newly developed multi-cycle thermomechanical processing method. The flow stress data show that the formation of g+g′ microduplex results in a significant decrease of flow stress in comparison with g′ dispersion structures under exactly the same deformation conditions. The distribution of strain rate sensitivity m in relationship with temperature and strain rate accurately identifies a specific domain within which g+g′ microduplex exhibits superplasticity. Fullscale turbine discs of GH4065 alloy with diameter of 630 mm achieve an optimal combination of creep resistance, fatigue lifetime and ductility. GH4065 discs exhibit extraordinary microstructural and property stability during prolonged thermal exposure, which means that dendritic segregation has been successfully restricted to an acceptable level. The results reveal that highly alloyed disc alloys produced via ingot metallurgy techniques exhibit lower costs and higher productivity, and can still meet the ever increasing demand of high performance gas turbine engines.
Subjects: Materials Science >> Materials Science (General) submitted time 2023-03-19 Cooperative journals: 《金属学报》
Abstract： GH4169 superalloy is one kind of important metallic materials used for manufacturing turbine discs in aero-engine. In order to meet the demand of higher strength, high ratio alloying elements have to be added, resulting in the complex microstructure evolution during the long- term service at elevated temperature. Furthermore, the turbine disc usually bears overloading which will lead to the low cycle fatigue (LCF) damage in real working and result in fatal security problem. Besides, it is meaningful to decide the relationship between the microstructure evolution and performance degradation. In the present work, microstructure evolution and LCF behavior of GH4169 alloy during long- term aging were investigated. The microstructure evolutions of GH4169 alloy during long-term aging at 750 ℃ for 500, 1000, 1500 and 2000 h and the influences of long-term aging on the LCF behavior were investigated. The results show that the size of g ″ phases increases and the volume fraction decreases with the increase of aging time, compared with the increase of both size and volume fraction of d phases. Both the fatigue strength and fatigue life of the alloy decrease with the increase of aging time. For the specimen aged for the same time, the cyclic stress firstly contributes to cyclic hardening, then cyclic stability, and finally cyclic softening with the increase of cyclic numbers. It is found that the decrease of cyclic stress contribution is slightly effected by the size of g ″ phases increase and volume fraction decrease after long-term aging. Therefore, the LCF life of the alloy decreases since the crack easily propagates along with the long needle-like d phases and the g ″ phases precipitate free zones.
Subjects: Materials Science >> Materials Science (General) submitted time 2023-03-18 Cooperative journals: 《材料研究学报》
Abstract： The precipitation behavior of the strengthening precipitate Cu-rich phase at the early aging stage in 18Cr9Ni3CuNbN austenitic heat-resistant steel, which is commonly used as tubes for superheater and /or reheater at 600℃ of USC power plants, has been investigated by three dimensional atom probe (3DAP), and C curve of Cu-rich phase in 18Cr-9Ni type steel has been given. Experimental results show that the Cu-rich phase precipitates quickly at the early stage of long-term aging treatment at both 650℃ and 700℃. The Cu-rich segregation clusters quickly form and then the Cu atoms diffuse from the gmatrix into the Cu-rich segregation clusters. The atoms of Fe, Cr and Ni from Cu-rich clusters defuse outwards to the g-matrix simultaneously, finally the Cu-rich phase forms.
Subjects: Materials Science >> Materials Science (General) submitted time 2016-11-15 Cooperative journals: 《金属学报》
Abstract：研究了含微量元素Hf(0-0.89%)的FGH97粉末高温合金中拓扑密排μ相的析出动力学、组织形态以及μ相对Hf含量为0.30%的合金盘坯力学性能的影响。结果表明：Hf含量为0.30%和0.89%的合金经750-900℃长期时效后μ相已明显析出，并随着时效温度的升高、时效时间的延长以及Hf添加量的增加，μ相析出量增加和尺寸长大。合金中的μ相主要在晶内以长条片状形态析出。合金盘坯(Hf含量为0.30%)在550-650℃长期时效后没出现μ相，高温拉伸性能和高温持久性能没有降低，组织稳定性良好。750℃长期时效后μ相的析出对合金盘坯的高温拉伸强度无明显影响，但降低拉伸塑性和高温持久寿命，而提高持久塑性约20%。详细讨论了μ相的析出行为、γ固溶体中合金元素的再分配以及合金断裂断口的特征，从理论上解释了μ相对力学性能的影响的脆-韧双重作用的机理，并提出控制和避免μ相大量析出造成性能劣化的措施和方法。
Subjects: Materials Science >> Materials Science (General) submitted time 2016-11-15 Cooperative journals: 《金属学报》
Abstract：采用热模拟实验研究了定向凝固与等轴晶U720Li合金的高温塑性变形行为,利用OM,SEM和EBSD分析了不同变形条件下合金的组织特征及动态再结晶机制.结果表明,定向凝固与等轴晶合金的变形抗力均随变形温度升高而降低.定向凝固U720Li合金沿垂直柱状晶方向变形时变形抗力较低,枝晶间协调变形能力良好,未出现裂纹.与等轴晶合金相比,相同变形条件下定向凝固合金的动态再结晶组织均匀;高温塑性变形过程中,定向凝固合金的动态再结晶主要以晶界弓弯和位错塞积方式形核.定向凝固U720Li合金的变形激活能为766 kJ/mol,比等轴晶合金降低了38.6 %,定向凝固合金呈现出更好的热加工特性.
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