Study on the preparation and microstructure of a single-crystal hollow turbine blade

This paper studied the structural design of a ceramic core and a blade, ceramic core localization, shell preparation, casting process, core leaching technology, and the heat treatment process of a single-crystal hollow turbine blade. The results show that the single-crystal structure solidification sequence of the blade platform is consistent with the cooling sequence and the pulling-out direction of the blade. The primary dendrites were obviously enlarged with the increase of the blade thickness owing to the change in the local cooling rate. Besides, the γ′ phase had a high uniform size distribution ranging from 0.40 to 0.60?µm after heat treatment, and the cubic degree was more homogeneous in comparison with the as-cast microstructure, which are favorable for the superalloy structure. Moreover, γ′ phase size gradually increased and its quantity gradually reduced owing to the increase of the wall thickness in the growth direction.     

Microstructural changes in a Ni-base superalloy during service

The microstructural changes were examined for a conventionally cast nickel base superalloy, Mar-M247. In particular, the first and seconnd stage turbine blades in an experimental turbogas power plant were used. TEM observations confirmed significant secondary γ′ precipitation in the mid-blade height of the first stage blade. The formation of dislocation net works in the γ phase was evident in the seconnd stage blades. Coarsening of the primary γ′ precipitates also occurred in the first stage blades. Creep rupture, tensile, and high cycle fatigue tests were performed for the first stage blades and showed no deterioration of the mechanical properties.     

单晶高温合金表面缺陷焊接修复的研究进展

单晶高温合金的性能优异,但是制造成本昂贵,因此其表面缺陷的焊接修复一直是单晶高温合金领域的重要研究课题。本文全面阐述了各类焊接修复(包括熔焊、钎焊和瞬时液相扩散焊)方法对修复区的组织和修复后力学性能的影响,分析了各类焊接修复方法的局限性和存在的问题,并指出了单晶高温合金表面缺陷焊接修复的发展方向。     

Stereological characterization of γ′ phase precipitation in CMSX-6 monocrystalline nickel-base superalloy

The purpose of this investigation was to study in detail the means to quantitatively evaluate γ′ phase precipitation. Many of the mechanical properties of superalloys are directly influenced by the presence of the γ′ (gamma prime) precipitate phase dispersed in a γ matrix phase. The γ′ precipitates act as effective barriers to dislocation motion and restrict plastic deformation, particularly at high temperatures. Due to this, it is essential to accurately quantify the γ′ precipitate size, volume fraction and distribution. Investigations based on quantitative metallography and image analysis were performed on a monocrystalline nickel-base superalloy taking into consideration various γ′ precipitate sizes present in that alloy microstructure. The authors of the present paper propose a new method of quantifying the total volume fraction of the γ′ phase applying images of the microstructure with γ′ phase precipitates registered using light microscopy, scanning electron microscopy (at two different magnifications) and scanning transmission electron microscopy.     

In-situ scanning electron microscopy studies of small fatigue crack growth in recrystallized layer of a directionally solidified superalloy

The small fatigue crack behavior in the surface recrystallized layer of a nickel-based directionally solidified superalloy DZ4 was studied by in-situ scanning electron microscopy. Transgranular cracking manner was identified at 350 °C, distinct from the intergranular fracture of recrystallized grains reported formerly. The transition of fracture mode is attributed to the effect of temperature, which should be particularly concerned in the safety of turbine components with recrystallization. The discontinuous crack growth in recrystallized layer is associated with the local microstructure. Carbide can act as crack source, and barrier to the crack growth as well. Effect of twin boundary on impeding the fatigue crack is evident.     

Recrystallization of the hot isostatic pressed nickel-base superalloy FGH4096: I. Microstructure and mechanism

Hot isostatic pressed (HIPed) nickel-based superalloy FGH4096 behaves a unique flow behavior in hot compression process due to the formation of necklace microstructure and leads to its characteristic dynamic recrystallization (DRX). In this process, dislocations activate the occurrence of DRX at prior particle boundaries (PPB) and the PPB is entirely covered with DRX grains, which forms the first layer in necklace structure. In this paper, two nucleation mechanisms, viz., bulge corrugation (BC) nucleation and dislocation induce phase (DIP) nucleation, are proposed. Based on the proposed mechanisms, the recrystallization firstly occurs in PPB via BC mechanism. The DIP nucleation occurs when the hot plastic deformation is carried out at the temperature below the γ′ phase solution temperature. To verify the proposed mechanisms, hot compression experiments were conducted. The models are then verified based on the experiments. Furthermore, the recrystallization activation energy of 922 kJ/mol is determined, which includes the growth energy and the two nucleation energies, viz., BC and DIP nucleation energies.     

Effect of recrystallisation on microstructural evolution and mechanical properties of single crystal nickel base superalloy CMSX-2 Part 1 - Microstructural evolution during recrystallisation of single crystal

Abstract

Nickel base superalloy CMSX - 2 single crystal bars were shot peened to induce surface residual stresses. Partial solutionising at 1300 ° C revealed a relationship between dissolution of the γ ′ phase into the matrix and recrystallisation. Recrystallisation occurred with dissolution of γ ′ phase in the dendritic core at 1300 ° C. Recrystallisation had preferentially begun at the dendritic core exposed at the shot peened surface and then gradually proceeded to the interdendritic regions. Phases more resistant to dissolution such as the coarse γ ′ particles and the γ - γ ′ eutectic phase in the interdendritic region dissolved into the matrix with recrystallisation, even at 1300 ° C, which is lower than the solutionising temperature of the alloy. Residual stresses assisted dissolution of the existing phases. The recrystallised grains grew preferentially in the dendritic core where rapid dissolution of γ ′ phase occurred. The growth of the recrystallised grains was impeded by the coarse γ ′ particles and the γ - γ ′ eutectic phase in the interdendritic region. Full solutionising of the shot peened specimens resulted in well developed recrystallised grains at the surface.     

Residual stress driven cracking in superalloy weldments

Superalloy weldments are normally given post weld heat treatments to homogenize the weld metal microstructure, relieve residual stress, and precipitate strengthening phases. The relationship between microstructure and post weld heat treatment is easily studied; it is less straightforward to study the effects of post weld heat treatment on residual stress relaxation. Using a self-restrained testing procedure, a relatively simple approach was used to investigate the effects of microstructure and post-weld heat treatment on cracking during residual stress relaxation. Candidate superalloys for Advanced Ultra Supercritical steam plants were studied. It was found that cracking due to residual stress relaxation is primarily dependent on grain size, and in cases of intermediate grain size, intragranular precipitation is a controlling factor. These results are in agreement with traditional stress relaxation cracking theories.     

Effects of rejuvenation heat treatment on microstructure and creep property of a Ni-based single crystal superalloy

Both surface and internal microstructures of a second-generation Ni-based single crystal(SX) superalloy were studied after creep and rejuvenation heat treatment(RHT).It is indicated that the microstructures,such as the dislocation network,the γ phase and the γ' phase,can be recovered to those after the standard heat treatment(SHT).It is found that RHT affected zone(RAZ) formed at the surface is composed of theγ'-free layer,the transition layer and the recrystallization(RX),which are less than 20 μm in depth totally.Such depth of the RAZ doesn't affect the properties of the superalloy.The morphology of γ' phase at the RAZ is related to the composition of the elements.The average creep life after RHT is close to the average life after SHT.It is concluded that RHT could effectively repair SX parts and increase the total life of the sample after a damage by creep.     

Einkristalline Gasturbinenschaufeln aus Nickelbasis-Legierungen. Teil I: Herstellung und Mikrogefüge

Single-Crystal Blades for Gas Turbines Part I: Casting Process and Microstructure The article gives an overview of current single-crystal technology used in fabricating highly stressed blades and vanes for gas turbines. After a short outline of the demands placed on gas turbine blades and the resulting necessity for improvement of nickel-base alloys, and following a short definition of the “technical” single-crystal, the principal of directional solidification with suitable methods of grain selection and the realizable solidification front morphologies are described in Part I. The usuable range of casting parameters for industrial production is fixed using the alloy SRR99 as an example. The major microstructural characteristics of single-crystal components made of nickel-base alloys namely crystallographic orientation, microporosity, morphology of the γ′-hardening phase, dendritic segregation and residual eutectic are presented. After casting, the microstructure is not in a condition which yields optimum service properties. Therefore, heat treatment is necessary. Part II describes how, with appropriate heat treatment, residual eutectic can be dissolved, segregations homogenized and the morphology of the γ′-phase optimized. Finally, the mechanical properties are discussed. The dominant influence of the crystallographic anisotropy on elastic and plastic properties are shown as well as the dependence of the creep behaviour on the morphology of the γ′-phase and the correlation between fatigue strength and microporosity.     

Einkristalline Gasturbinenschaufeln aus Nickelbasis-Legierungen. Teil II: Wärmebehandlung und Eigenschaften

Single-Crystal Blades for Gas Turbines Part II: Heat Treatment and Properties The article gives an overview of current single-crystal technology used in fabricating highly stressed blades and vanes for gas turbines. After a short outline of the demands placed on gas turbine blades and the resulting necessity for improvement of nickel-base alloys, and following a short definition of the “technical” single-crystal, the principal of directional solidification with suitable methods of grain selection and the realizable solidification front morphologies were described in Part I. The usuable range of casting parameters for industrial production was fixed using the alloy SRR99 as an example. The major mictrostructural characteristics of single-crystal comoponents made of nickel-base alloys namely crystallographic orientation, microporosity, morphology of the γ′-hardening phase, dendritic segregation and residual eutectic were presented. After casting, the microstructure is not in a condition which yields optimum service properties. Therefore, heat treatment is necessary. Part II describes how, with appropriate heat treatment, residual eutectic can be dissolved, segregations homogenized and the morphologly of the γ′-phase optimized. Finally, the mechanical properties are discussed. The dominant influence of the crystallographic anisotropy on elastic and plastic properties are shown as well as the dependence of the creep behaviour on the morphology of the γ′-phase and the correlation between fatigue strength and microporosity.     

近[001]取向DD6单晶高温合金980℃/250MPa持久性能各向异性研究

通过不同取向DD6单晶高温合金980℃/250MPa持久测试,研究了取向对980℃持久性能的影响。结果表明:[001]取向偏离主应力轴15°以内,DD6单晶高温合金980℃/250MPa持久寿命相当,没有各向异性。这主要是由于近[001]取向DD6单晶高温合金多个〈110〉{111}滑移系共同作用的结果。同时,原子扩散造成的γ′筏排和γ/γ′相界面形成的位错网也降低持久性能各向异性。     

Effect of heat treatment on the micro-indentation behavior of powder metallurgy nickel based superalloy FGH96

The influences of solution treatment temperature on microstructure and micromechanics of P/M nickel-base superalloy FGH96 were investigated by micro-indentation methods. The alloy was heat-treated at the temperatures of 1050 °C, 1150 °C, 1220 °C and 1310 °C, respectively. The micro-indentation tests were conducted in the indenter load range from 500 mN to 4500 mN and the loading rate range from 5.19 mN/s to 103.71 mN/s at room temperature by using a sharp Berkovich indenter. The influence of solution treatment temperature on microstructure was analyzed based on microstructural observations using both optical and scanning electron microscope. The micro-hardness, Young’s modulus and yield stress were obtained by means of Oliver–Pharr method and reverse analysis algorithms, respectively. The results show that both of micro-hardness and Young’s modulus are significantly affected by indentation depth and solution treatment temperature. Based on microstructure analysis, these effects were attributed to the changes of precipitate properties, e.g., size, distribution and morphology, and the relationship between microstructure and micromechanics was established. Then, the deformation mechanism was explained on the basis of dislocation–dislocation and dislocation–precipitate interactions. In this paper, the descending Young’s modulus was related to localized stress concentration and microcrack propagation. The results reveal that the damage variable is high for P/M nickel-base superalloy FGH96 after high temperature solution treatments.     

Influence of α′ martensite on shape memory effect in Fe-14Mn-5Si-9Cr-5Ni alloy

Abstract

The strain induced γ → ε and γ → α′ martensite transformations were examined by X-ray diffraction in an Fe-14Mn-5Si-9Cr-5Ni alloy. From the original microstructures of single phase austenite and two phase austenite and α′ martensite, the alloy was examined after prestraining at room temperature and recovery heating at 673 K, and finally, its shape memory effect was determined. It was shown that the alloy with the original two phase microstructure of autensite and α′ martensite exhibits a higher degree of shape recovery. The presence of α′ martensite in the original microstructure before prestraining can considerably improve the extent of shape recovery.     

Microstructure and tensile properties of superalloy IN100 fabricated by micro-laser aided additive manufacturing

Layer by layer fabrication using micro-laser aided additive manufacturing (micro-LAAM) was successfully implemented on nickel-base superalloy IN100. It is known that IN100, a type of superalloy having high titanium and aluminum contents, has poor weldability due to weld liquation cracking in the heat-affected zone (HAZ) and strain age cracking. In this study, micro-LAAM process was optimized through a set of designed experiments to eliminate crack formation and reduce porosity. It was found that the crack-free deposition can be achieved owing to the fact that micro-LAAM process used in this study had very low heat input. Three distinct sizes of γ′ precipitates were observed on the post heat-treated samples. The volume fractions of γ to γ′ phases were found to be approximately 60–40%. Microstructure and chemical analysis results showed that γ′ phase was embedded within γ-Ni matrix while various carbides (MC, M₂₃C₆ and M₆C) were observed as precipitates at grain boundaries or within grains. Electron backscatter diffraction (EBSD) was used to compare grain morphologies and size distribution of three distinctly different regions on each layer. The achieved ultimate tensile strength and yield strength are much better than the minimum requirements specified in aerospace material specification 5397 for cast IN100.     

Effects of carburization on recrystallization behavior of a single crystal superalloy

The samples of single crystal superalloy DD6 were grit blasted and heat treated in the solution and aging treatment at vacuum atmosphere, then the effects of carburization on recrystallization of single crystal superalloy DD6 were investigated. The results showed that carbon was introduced into the grit blasted samples during carburization, producing a homogenous distribution of MC-type carbides in the form of submicron dispersed precipitates within the grit blasted region. These MC-type carbides hold back the recrystallized grain boundaries. As temperature increases, MC-type carbides dissolve and secondary M₆C-type carbides precipitate. The interaction between these M₆C-type carbides and recrystallized grain boundaries further hinders recrystallization. Thus the average depth of the recrystallization region and number of recrystallized grains observed in carburized grit blasted samples are less than those in the samples which underwent grit blasting process alone. Carburization can significantly reduce recrystallization in single crystal superalloy DD6 during solution heat treatment.     

镍基高温合金690的研究现状

690合金是一种面心立方结构的镍基高温合金,具有优异的高温力学性能和耐腐蚀性能,被广泛应用于核电、石油化工和航空航天等领域.文中对690合金的微观组织演化、热成形特性、高温失塑裂纹和耐应力腐蚀性能等方面的研究现状进行了总结分析.目前,部分合金元素对690合金组织性能的影响较为复杂,成形工艺参数对690合金高温变形行为和...     

Dislocation network formation during creep in Ni-base superalloy GTD-111

The Ni-base superalloy GTD-111 is used as a blading material in the first stage blades of high power gas turbines. The creep-rupture properties of the cast superalloy were studied over a wide range of temperatures and stresses. The observations of dislocation structures during steady-state creep confirmed that the creep mechanism was different in the high and low stress regions. The results showed that in the high stress region, shear mechanisms including stacking fault formation and anti-phase boundary creation were operative and in the low stress region, a by-passing mechanism occurred by either looping or dislocation climb and glide. With increasing exposure time in the high-temperature low-stress region, dislocations formed networks at γ–γ′ interfaces, as well as inside γ′ particles. The transition in the mode of dislocation–γ′ precipitate interaction from shearing to by-passing was found to depend on creep conditions (stress and temperature) and microstructural characteristic of the alloy. The present paper provides microstructural evidence by means of transmission electron microscopy for a high temperature by-passing mechanism operating in the superalloy GTD-111.     

Low-temperature superplasticity of β-stabilized Ti-43Al-9V-Y alloy sheet with bimodal γ-grain-size distribution

The superplasticity of Ti-43Al-9V-0.2Y alloy sheet hot-rolled at 1100 ℃ was systematically investigated in the temperature range of 750-900 ℃ under an initial strain rate of 10-4 s-1.A bimodal γ grain-distribution microstructure of TiA1 alloy sheet,with abundant nano-scale or sub-micron γ laths embed-ded inside β matrix,exhibits an impressive superplastic behaviour.This inhomogeneous microstructure shows low-temperature superplasticity with a strain-rate sensitivity exponent of m =0.27 at 800 ℃,which is the lowest temperature of superplastic deformation for TiAl alloys attained so far.The maximum elongation reaches ～360％ at 900 ℃ with an initial strain rate of 2.0 × 10-4 s-1.To elucidate the softening mechanism of the disordered β phase during superplastic deformation,the changes of phase composi-tion were investigated up to 1000 ℃ using in situ high-temperature X-ray diffraction (XRD) in this study.The results indicate that β phase does not undergo the transformation from an ordered L20 structure to a disordered A2 structure and cannot coordinate superplastic deformation as a lubricant.Based on the microstructural evolution and occurrence of both y and β dynamic recrystallization (DR) after tensile tests as characterized with electron backscatter diffraction (EBSD),the superplastic deformation mecha-nism can be explained by the combination of DR and grain boundary slipping (GBS).In the early stage of superplastic deformation,DR is an important coordination mechanism as associated with the reduced cavitation and dislocation density with increasing tensile temperature.Sufficient DR can relieve stress concentration arising from dislocation piling-up at grain boundaries through the fragmentation from the original coarse structures into the fine equiaxed ones due to recrystallization,which further effectively suppresses apparent grain growth during superplastic deformation.At the late stage of superplastic de-formation,these equiaxed grains make GBS prevalent,which can effectively avoid intergranular cracking and is conducive to the further improvement in elongation.This study advances the understanding of the superplastic deformation mechanism of intermetallic TiAl alloy.     

Cavitation Damage of the single-crystal nickel-base superalloy SRR99 during creep at 850°C

Various heat treatments of the single-crystal nickel-base superalloy SRR99 have been carried out to produce a wide range of initial γ′ phase morphology. The cavitation damage has been studied after creep fracture at 850°C and stresses between 400MPa and 650 Mpa. The crep fracture surfaces and longitudinal sections of crept specimens have been examined by image analysing system in order to determine the size distribution and the area fraction of (001)-planes cracks on fracture surface characterise the creep crack damage level. The cracking morphology in fractured material as a function of stress and γ′-phase size has also been investigated. The results show, that crack propagation occurred anisotropically on (001) crystallographic planes perpendicular to the aplied stress along the γ/γ′ interfaces. It was found, that creep cracks dependent on the shape and size of γ′-phases developing at the temperature and stress level.