Langhong Lou

Anisotropic Creep Rupture Properties of a Nickel-base Single Crystal Superalloy at High Temperature

The creep rupture properties of a single crystal superalloy were tested at 975°C/255 MPa as a function of the deviation degrees from [001]. The misorientation of the specimens away from [001] distributed approximately along a line between [001]–[011] and [001]–[111] boundaries in the triangle of the stereographic projection. Creep rupture lifetimes of the specimens were not sensitive to the misorientation until the deviation degree exceeded ∼30 deg. Two steps of lattice rotation were found in all specimens during creep, first towards the [001]–[111] boundary, and then to [001] or [111] along the boundary. Single slip and strong asymmetric deformation were observed during the first stage of lattice rotation in specimens with large misorientation. The rotation mechanism was associated with the activated slip systems according to the calculated Schmid factors. The impact of lattice rotation on the rupture properties was also discussed.

Improved Castability of Directionally Solidified, Ni-Based Superalloy by the Liquid Metal Cooling Process

The hot-cracking susceptibility of directionally solidified, Ni-based superalloys obtained by different casting technologies, such as high-rate solidification (HRS) and liquid metal cooling (LMC), are compared in this article. Refined microstructures and the improved castability of LMC castings has been found. The improved hot-cracking resistance in castings using the LMC process can be attributed to the decreased length of the uncompensated zone and to the increased grain boundary cohesion in this zone.

Effect of Casting Modulus on Microstructure and Segregation in K441 Superalloy Casting

Four castings with different modulus have been designed to simulate different thickness sections of a turbine blade during casting process. The microstructure has been observed by optical microscopy (OM) and scanning electron microscopy (SEM). The micro-segregation was tested by energy dispersive spectroscopy (EDS) and the macro-segregation was analyzed by using Metalscan 2500 spectrometers. The experimental results show that the microstructure of casting is affected by modulus apparently. The smaller the modulus, the finer the microstructure. The average grain size of castings with modulus of 0.29, 1.57, 3.16 and 5.0 mm is 0.3, 1.5, 2.7 and 4.3 mm, and the volume fraction of eutectic is 0, 0.1%, 0.2% and 1.0%, respectively. The micro/macro segregation is affected by the modulus apparently. The smaller the modulus, the lower the segregation level. When the modulus of casting increases, the content of Al increases, while the content of W, Cr and Mo decreases both in inner grain and near grain boundary. The content of Al and Mo in inner grain is higher than that near grain boundary, while the content of Cr and W in inner grain is lower than that near grain boundary.

The Effect of Long-Term Thermal Exposure on the Microstructure and Stress Rupture Property of a Directionally Solidified Ni-Based Superalloy

Microstructural degradation and microstructure-property relationship during long-term thermal exposure in a directionally solidified Ni-based superalloy are systematically studied. The coarsening kinetics of γ′ precipitation conforms well to the LSW model during the long-term thermal exposure. The detailed time dependence of MC decomposition during the long-term thermal exposure is revealed. Grain boundary coarsening was mainly facilitated by γ′ and M23C6 precipitates coarsening in GBs region, and the GB coarsening kinetics conforms well to the JMAK theory. During different stages of the thermal exposure, dominant factors for the decrease of stress-rupture lifetime vary due to the evolution of multiple microstructures (γ′ coarsening, MC decomposition, and grain boundary coarsening).

Effect of Secondary Orientation on Thermal Fatigue Behavior of a Nickel-base Single Crystal Superalloy DD33

The effect of secondary orientation on thermal fatigue behavior of a third generation nickelbased single crystal superalloy DD33 was investigated. Samples with different secondary orientations were machined along(100) and(110) plane, respectively, and thermal fatigue test was performed cyclicly between room temperature and 1100 oC. It was found that different initiation sites and propagation orientations of the thermal fatigue cracks were observed in samples with different secondary orientations. In samples with secondary orientation of [100], thermal cracks initiated at the edge of the holes along a direction with 45 oincline to the directional solidification(DS) direction and propagated along also the derection with 45 oincline to the DS direction. While in samples with secondary orientation of [110], thermal cracks initiated at the edge of the holes vertical to the DS direction and propagated along the DS direction. In general, samples with secondary orientation of [100] exhibited better thermal fatigue properties than that of [110] samples.

CORROSION BEHAVIOR OF GH3535 SUPERALLOY IN FLiNaK MOLTEN SALT

As one of the most promising next generation reactors, the molten salt breeder reactor (MSBR) with excellent inherence security has attracted more and more attentions in recent years due to energy shortage and the security problem of traditional nuclear reactor. The most significant service characteristic of the structural material used in MSBR is the existence of FLiNaK molten salt compared with other nuclear reactors. FLiNaK molten salt is very corrosive to the structural material in the reactor, and affects the safety operation of nuclear power plants. A polycrystalline Ni-Mo-Cr-Fe superalloy was developed and used as an important structural material in MSBR at Oak Ridge National Laboratory (ORNL), but the corrosion mechanism of the alloy in FLiNaK molten salt has not been determined since the study terminated in 1970s as some politic reasons. Alloy served in harsh environments, often using protective coating to improve the corrosion properties. While few works about the coating. corrosion resistance in FLiNaK molten salt were reported at present. Al2O3 and Cr2O3 coatings usually have excellent corrosion resistance in molten salt, such as sulphate, nitrate and halide molten salt. But, whether the oxide film has corrosion resistance in FLiNaK molten salt has not been determined. In this work, the corrosion mechanism of alloy in FLiNaK molten salt was studied by using immersion corrosion experiment through the method of SEM, EDS and XRD. The influence of Al2O3 coating on corrosion resistance in FLiNaK molten salt was also investigated. The results show that the Al2O3 coating does not affect the exsolution corrosion characteristics of Cr and Mo elements in FLiNaK molten salt at 700 degrees C for 400 h. The different is that naked alloy exhibits intergranular corrosion characteristic, and the alloy with Al2O3 coating exhibits spot corrosion characteristic. The Al2O3 coating cannot improve the corrosion resistance of the alloy in FLiNaK molten salt. The Al2O3 film dissolved in molten salt and resulted in the exposure of the alloy surface. The corrosion rate was increased since the formation of corrosion cell between oxide film and the exposed alloy surface.

Microstructure and Stress-Rupture Property of Large-Scale Complex Nickel-Based Single Crystal Casting

The microstructure and stress-rupture property of the large-scale complex single crystal (SX) casting DD10 were investigated in high-rate solidification process. It is found that the primary dendrite arm spacing (PDAS) does not increase monotonically with the height increase. When across the platform, the temperature gradient increases due to the effect of platform, and the corresponding PDAS decreases. The distribution of eutectic volume fraction in large-scale complex SX casting is affected by PDAS, solid back diffusion, and the development of high order dendrites. The eutectic volume fraction contained in the sample taken below the platform decreases with the height increase. While the eutectic volume fraction contained in the sample taken upper the platform increases gradually with the height increase. After heat treatment, most of the γ/γ′ eutectics are eliminated and the components are distributed uniformly. The similar stress rupture properties of the samples at different heights in the same direction are obtained.

Effect of silica fiber on the mechanical and chemical behavior of alumina-based ceramic core material

Abstract In order to improve the chemical leachability, the alumina-based ceramic core material with the silica fiber was injected and sintered at 1100 °C/4 h, 1200 °C/4 h, 1300 °C/4 h and 1400 °C/4 h, respectively. The micrographs of ceramic core materials at sintered and leached state were characterized by scanning electron microscopy (SEM). The phase composition of ceramic core material after sintering and the leaching product after leaching were detected by X-ray diffraction (XRD). The porosity, room temperature bend strength, creep property at elevated temperature and the leaching rate in aqueous caustic solution were studied. The experimental results show that the ceramic core material with silica fiber obtain a fair balance between mechanical and chemical properties at sintering of 1300 °C/4 h. Specifically, the leach rate of ceramic core material with silica fiber is increased apparently. High leaching surface and weak adhesive strength between agglomerated alumina particles are the reasons that responsible for the ceramic core material with silica fiber be leaching fast than that of the ceramic core material without fiber.

EFFECT OF Re AND W ON RECRYSTALLIZATION OF AS-CAST Ni-BASED SINGLE CRYSTAL SUPERALLOYS

Ni-based single crystal (SX) superalloys have been used as blades in aero-space industry and landbased applications due to their excellent high-temperature properties. However, residual strain is introduced into ascast SX superalloy blades during the manufacturing process, such as casting, grinding or shot peening, and so on. Recrystallization (RX) occurs easily during subsequent high temperature heat treatment. In previous work, it is believed that RX has detrimental effect on the mechanical properties of SX superalloy. Furthermore, in order to improve the mechanical properties, more and more refractory elements, such as W, Re, Mo, Ta, are added into SX *国家自然科学基金项目50901079,国家重点基础研究发展计划项目2010CB631201及国家高技术研究发展计划项目2012AA03A513资助 收到初稿日期: 2015-08-26,收到修改稿日期: 2015-10-30 作者简介:濮 晟,男, 1981年生,工程师 DOI: 10.11900/0412.1961.2015.00456

Orientation Dependence of Stress Rupture Properties of a Ni-based Single Crystal Superalloy at 760 °C

The orientation dependence of creep rupture lives of a single crystal superalloy at 760 degrees C/760 MPa was investigated. The orientations of the specimens tested were about 30 away from [001]. The results showed that specimens with orientations on the [001]-[011] boundary had the longest rupture life. The deformation of these specimen were controlled by a/2 slip and a few stacking faults with two orientations were observed. On the other hand, specimens with orientations near the [001]-[011] boundary or on the [001]-[111] boundary showed short rupture lives, and stacking faults with single orientation were observed in these specimens. The rupture properties and the deformation mechanisms were discussed based on the dislocation pattern and the calculated Schmid factors for different specimens.