Ni-based single crystal superalloys are widely used in the manufacture of aero engine turbine blades because of the excellent mechanical properties at high temperature. With the development of single crystal superalloys, the content of refractory elements is constantly increased (especially Re) to improve the high temperature capability, which in turn leads to the decrease in microstructural stability of alloys, such as the TCP phase precipitation. It is important to find one element which not only can maintain high temperature performance but also does not evidently promote TCP phase precipitation and is very cheap in price to replace Re partially. W is one of the most important solution strengthening elements in superalloys, its diffusion rate in Ni matrix is close to Re and far below the other alloying elements, meanwhile, the advantage of low price make it to be the most suitable substitute of Re. However, there is little work about the effect of W on microstructural stability in Re contained third generation superalloys. In this work, the effects of W on the elemental segregation, elemental partitioning ratio of γ /γ′, microstructure evolution and TCP phase precipitation during thermal exposure at 950, 1000 and 1050 ℃ have been investigated in a third generation Ni-based single crystal superalloys with varied contents of W (6%~8%, mass fraction). The results show that the addition of W has no obvious effect on segregation of the alloying elements of as-cast alloys as well as the morphology, size and volume fraction of γ′ phase after heat treatment. During the thermal exposure at 950 ℃, the connection and deformation of γ′ phase are accelerated, but its coarsening rate is decreased with increasing W content. The TCP phases precipitated in three alloys during thermal exposure are mainly μ phase and σ phase. The area fraction of TCP phases is increased slightly with the W addition during thermal exposure, which is the largest at 1000 ℃, less at 950 ℃ and the least at 1050 ℃.
Ni-based single crystal superalloys have been widely used in manufacturing the critical components of aero-engines, such as turbine blades and vanes. Improvements in phase stability on the addition of Ru are well known in the field of Ni-based superalloy development. Cr is beneficial to hot co-rrosion resistance of Ni-based superalloys. Generally, superalloys which used under easy corrosion conditions should contain high levels of Cr. Early researches about the influence of Ru on solidification microstructures in Ni-based single crystal alloys are mostly focused on low-Cr systerms (<6%). Since Cr has complex interactions with Ru, it is meanful to study the effects of Ru on solidification microstructures in high-Cr (>10%) Ni-based single crystal superalloy systems. The materials used in this work are Ni-based single crystal superalloy with high Cr content. Three superalloys by changing Ru addition (0, 1.5%, 3%, mass fraction) were designed. By observing the as-cast structure, the effect of Ru on the elements distribution and the precipitation characters of different phases in these alloys were studied. It is found that as the Ru content increases, the primary and secondary dendrite arm spacings decrease gradually; the volume fraction of (γ+γ′) eutectic increases firstly and then decreases; the γ′ size is reduced progressively. The addition of 3%Ru leads to the formation of β-NiAl phase, which contain a certain amount of Cr, Co and Ru except the basic elements Ni and Al. The typical "reverse partitioning" of other alloying elements is exhibited with the addition of Ru, while the formation of β-NiAl phase can reduce the "reverse partitioning" of other alloying elements. The addition of Ru could enhance the segregation of positive segregation elements Ta, Al and negative segregation element Re while reduce the segregation of positive segregation elements Mo and Cr.
The minor element in alloy greatly aggravate the segregation of main elements and formation of harmful phase, resulting the deterioration of mechanical properties. Low segregation technology of cast superalloy was pioneered by Prof. Shi Changxu and co-workers in the early eighties. The technology is to control the content of minor element, such as P, Si, B and Zr, to lower the solidification segregation in the super-alloy. The working temperature and mechanical properties of superalloy can be increased greatly by using the low segregation technology. A series of alloys, such as M17 and GH738 with low segregation and excellent properties, had been developed. This study extends low segregation technology to 30Cr2Ni4MoV steel of large shaft for thermal power equipment, 690 alloy for steam generator tube in nuclear power plant, and uranium alloy for nuclear fuel. The solidification and segregation behaviour in the 30Cr2Ni4MoV steel was investigated, it is found that the minor elements of O and Al are essential for the formation of serious solidification segregation in the steel. Moreover, the solidification behavior of 690 alloy has been studied. S and N increases solidification interval, and the effect of S is greater than that of N. The solidification segregation of 690 alloy can be alleviated by controlling the contents of the S and N. Finally, the solidification temperature interval of high carbon uranium is calculated. With the car bon content increasing from 0.01% to 0.03%, the solidification interval is from 40 ℃ to 75 ℃. Thus, for the radioactive uranium alloys, minor elements show segregation to some extent in the residual liquids of final solidification zone. The minor elements in U-6Nb alloy are C, N and O. For uranium with high carbon content, the minor elements are C and O.
A new directionally solidified Ni-based superalloy is developed for industrial gas turbine applications, which has high strength and excellent hot corrosion resistance at high temperatures. The high strength of the alloy is primarily derived from precipitation hardening by ordered L12 γ′ phase. To achieve a uniform distribution of precipitated γ′ particles for optimized mechanical properties, the suitable heat treatments are used. However, the heat treatment temperature in Ni-based superalloys is limited by the problem of incipient melting. Incipient melting microstructrue evolution during heat treatment has been hardly reported. Therefore, the behaviors of incipient melting and its effect on mechanical properties in the new directionally solidified superalloy DZ444 with high boron have been investigated in this work. The results show that some interdendritic regions of the as-cast DZ444 sample exhibit many of γ′/γ eutectic, MC carbides and multi-phase eutectic-like constituent which are composed of boride, Ni5Hf and η phases. During solution treatments, incipient melting does not occur in boride or Ni5Hf phase with low melting point firstly, but appears in γ matrix around multi-phase eutectic-like constituent which is affected significantly by borides. Compared to DZ444 alloy with the normal boron content, incipient melting occurs at the lower temperature in the range between 1160 ℃ and 1170 ℃. Incipient melting can occur significantly with the increase of the solid solution temperature or time. Incipient melting consists of typical γ dentrites and a lot of tiny precipitation particles after the water quenching (WQ) method following solution treatment. However, incipient melting forms multi-phase eutectic-like constituent, γ matrix and γ′/γ eutectic successively during air cooling (AC) following solution treatment, and the morphology of multi-phase eutectic-like constituent is similar to that of as-cast alloy. Firstly, a so-called incipiently melted circle (IMC) forms around multi-phase eutectic-like constituent; with the increase of the solid solution temperature or time, IMC extends inwards which makes γ matrix and multi-phase eutectic-like constituent in this circle melt successively. Finally, a incipiently melted pool forms gradually. Incipient melting is limited to the IMC below 1200 ℃ and the area of incipient melting changes with temperature or time correspondingly. However, incipiently melted region (IMR) expands outwards continuously which makes γ matrix outside the incipiently melted circle melt when the temperature is higher than 1210 ℃. Especially, IMR swallows up plenty of γ matrix, and many matrix islands, regions unmelted, exist in IMR above 1250 ℃ which destroys the continuity of the matrix significantly. The incipient melting has a minor effect on the tensile properties, nevertheless, decreases the creep-rupture properties remarkably. The degradation of mechanical properties mainly results from the increasing of the incipient melting area fraction and size.
GH3625 alloy is a wrought nickel-based superalloy mainly used in aeronautical, aerospace, chemical, nuclear, petrochemical, and marine applications industry due to its good mechanical properties, processability, weldability and resistance to high-temperature corrosion on prolonged exposure to aggressive environments. However, in medium and high temperature environment during long-term service, the γ'' is a metastable phase, easily transformed into stable δ phase, or δ phase directly formed in the γ matrix so that alloy performance was deteriorated, leading to the result of alloy failure. At the present work, mass fraction of δ phase in GH3625 superalloy hot-extruded tube cold deformed to different reductions and then aged at 800 ℃ for different times, were measured by XRD. The effect of cold deformation on the law and kinetics of δ phase precipitation was investigated by SEM, EDS and Image-Pro Plus metallographic analysis. The results show that δ phase first precipitates at the deformation twin and grain boundaries as well as deformation bands, and then precipitates in the grains. The amount of δ phase at the deformation bands increases with the increase of cold deformation. The morphologies of δ phase change gradually from needles to spheroids or rodlike with increasing cold deformation. With the extend of ageing time, the average size of δ phase increases which grows according to LSW theory. At 800 ℃, the relationship between the precipitation content of δ phase and ageing time follows Avrami equation. As cold deformation increases, the content of δ phase increases, the time index n decreases, whereas the δ phase precipitation rate increases. Cold deformation promotes the precipitation of δ phase. The solute drags of Nb in soild solution and pinning of δ phase inhibits the grain growth during ageing process of cold deformed GH3625 superalloy hot-extruded tube. The hardness of the alloy increases with the extension of the holding time at ε =35% but no obvious change at ε ≥50%.
K452 alloy is a nickel-based cast superalloy having the good tensile properties at high temperature and excellent corrosion resistance. It has been applied as a blade material of engines when environmental temperature is not above 950 ℃. It is found that the tensile properties of the alloy have become more scattered and unstable although its chemical compositions are not changed. Hence, the tensile properties of the alloy were studied in order to increase its stability at high temperature and improve its applied properties. Tensile specimens were prepared using the different re-melting processes. Tensile tests were done at 900 ℃. When the pouring temperature was 1430 ℃, tensile properties were not only lower than expected, but also had great degree of dispersion, i.e., the vales of ultimate strengths changed in the range of 410 MPa and 510 MPa, and the elongations changed in the range of 3.5% and 22.0%, the average contents of O and N were the highest among three tested conditions. The highest N content was 0.0028%. And the shrinkage area was higher than those in other two re-melting processes. When the pouring temperature was 1500 ℃, the tensile properties were improved, and their changing scopes became small, the average contents of O and N decreased, the shrinkage area decreased. When the refining temperature was 1590 ℃ and the holding time was 5 min, both average contents of O and N were decreased greatly, the shrinkage was not seen in the fracture surfaces. And the tensile properties were improved. Furthermore, their changing scopes were very small.
The low cycle fatigue (LCF) experiments of nickel-based turbine disc alloy GH4738 have been carried out at different temperatures in air to investigate the influence of temperature on fatigue crack growth (FCG) behavior of GH4738 alloy. The FCG curves (da/dN-ΔK and a-N) and their regularity have been obtained. The results show that there is a sensitive range of temperature in which the fatigue life for GH4738 decreases sharply. The microstructures and fracture surface morphologies of the GH4738 samples tested at different temperatures were observed by FE-SEM, and changes of the mechanical properties of GH4738 at high temperature were also taken into account through modifying the stress intensity factor amplitude, ΔK. The interruption experiments were carried out at 700 ℃ and room temperature, respectively, to investigate the crack growth mode and oxidation degree at the crack tip and grain boundary of the samples. And the essential reason of temperature influence on FCG behavior of GH4738 was discussed. The result showed that as the temperature increases, the fatigue crack growth rate (FCGR) of GH4738 accelerates, the fracture surface tends to coarse, and the failure mode converts from a mixed transgranular and intergranular fracture to totally intergranular fracture. The fatigue crack growth lifetime decreases remarkably at 650~700 ℃, existing a temperature-sensitive region under ΔK=40 MPam1/2 and 30~40 μm grain size conditions, which is mainly caused by the oxidation at elevated temperature, independent of the microstructure and mechanical property. Oxygen diffuses into the grain boundary through crack tip and slip band, or penetrates directly into the grain boundary, reacts with active elements (Co, Ti, Al) and generates brittle oxides. These brittle oxides result in weakening of grain boundary and significant decrease of fatigue property of GH4738.
High alloying Ni-based powder metallurgy (PM) superalloys show excellent fatigue performance and damage tolerance properties, and good creep resistance at 750 ℃, and are used for advanced gas turbine disks and other hot components. The hot-working window of high alloying PM superalloy is narrow because of its poor workability. The formation of the γ+γ′ microduplex structure during the thermomechanical processing always results in a decrease in flow stress and a promotion of hot plasticity. However, the stability of the γ+γ′ microduplex structure has not been evaluated. The high temperature flow behavior of a Ni-based superalloy FGH98 prepared by hot isostatic pressing has been examined by means of uniaxial compression testing isothermally at 1060, 1105, 1138 and 1165 ℃ and at constant true strain rates between 0.01 and 10 s-1. The microstructural evolution and instabilities during plastic flow have been studied. Under all testing conditions, the as-hipped material exhibits flow hardening, flow softening and steady-state flow sequentially with the true strain increased. The dynamic recrystallization occurs and the γ+γ′ microduplex structures are generated when steady state flow or highest strains achieved at temperatures below the γ′ solvus. The formation of the γ+γ′ microduplex structures results in a remarkable decrease in grain size and a promotion of hot plasticity. The relationships between steady-state grain sizes and steady-state stresses during deformation and the formation mechanism of the γ+γ′ microduplex structure were analyzed. The possibility of the microstructure controlling during hot working was discussed.
The structure formation of superalloys is very complicated because of their multicomponent composition and multiphase transition processing. Duo to the limitation of some pre-conditions, the structure formation can not be accurately determined by thermodynamic calculation method. Knowledge about the structure is critical for the design of the following heat treatment process. In this work, a single crystal (SC) sample of superalloy CM247LC was directional solidified in a labor Bridgman furnace with a pulling rate of 0.2 mm/min and then water quenched, to investigate the solidification sequence including MC carbide and γ/γ′-eutectic. It was observed that the γ-phase is firstly formed in the form of dendrites; it is then followed by the homogeneously precipitation of MC carbides from the liquid behind dendrite tips. Near the end of solidification the interdendritic residual liquid transits into γ/γ′-eutectics. It is interesting to found that the γ/γ′ eutectics do not nucleate on the existing γ -phase, but preferably on the MC carbides which have completely different chemical composition and crystal structure. The result of EBSD examination indicates that the γ/γ′ eutectics formed on the MC substrates have random crystal orientations compared to the SC γ -matrix, exhibiting the misoriented multi-crystal microstructure in the so called "single crystal" superalloy casting.
Recently, a new Co-Al-W-based alloy with ordered L12 structure has been attracted much attention of researchers, these alloys have higher melting point than Ni-base superalloys with morphologically identical microstructure, but grain defect formation caused by thermosolutal convection has become an important problem for its application. Magnetic field is always applied to damp the convection which reduces the formation of defects. However, there are hitherto few papers to investigate the effect of magnetic field on grain defects during Co-Al-W-based alloy directional solidification. In this work, The effect of high magnetic field on the solidification structure and macrosegregation in directionally solidified Co-Al-W-based alloy was investigated. The results showed that the application of longitudinal magnetic field can induce convection and cause deformation of the solid-liquid interface shape, forming the macrosegregation and the stray grains in the mushy zone at the pulling rate of 5 μm/s. With the increase of pulling rate, the macrosegregation and the stray grains disappeared gradually at 2 T magnetic field. While the transverse magnetic field was applied, the macrosegregation became serious and the number of the stray grains increased. The macrosegregation further became more serious and the columnar-to-equiaxed transition was induced after adding the Ta element. The main reason of undercooling nucleation and columnar-to-equiaxed transition (CET) was the microsegregation induced by thermoelectric magnetic convention.
As a result of increasing energy demands and accelerated environmental problems, there is an urgent need to improve the thermal efficiency of ultra supercritical (USC) power plants. To achieve this goal, advanced ultra-supercritical (A-USC) technologies with the main steam temperature of 700~750 ℃ and pressure of 35 MPa have been developed quickly in recent years. One of the most promising candidate Ni-based superalloys for the main steam pipe of 700 ℃ ultra-supercritical coal-fired power plants is Inconel 740H, which is a modified version of Inconel 740 developed by Special Metals Corp (SMC). Compared with IN740, the Ti/Al ratio in IN740H is lowered in order to stabilise the microstructure at long ageing times. In addition, the Nb content is lowered to improve the weldability. In this work, the microstructure evolutions, the nucleation and propagation mechanisms of microcracks in the nickel base superalloy Inconel 740H at 750 ℃ high temperature were studied by the self-developed in situ high temperature tensile stage inside a SEM. The results showed that under the uniaxial tensile stress at 22 ℃ room temperature and 750 ℃ high temperature conditions, the grain boundaries of Inconel 740H alloy are always the most primary crack sources. The strength of grain boundaries is higher than that of grains under the room temperature, and the microcracks will be nucleated at the grains as well, but the relative strength of grain boundaries will be weaken under the high temperature, which makes the microcracks tend to nucleate at grain boundaries. The experimental results also showed that the influence of high temperature on the mechanical properties is very significant, the high temperature reducing the activate energy of slip and weakening the strength of the grain boundaries, so that more slip systems activated and the grain boundaries occurring bending and sliding deformation, so further enhance the ability of plastic deformation of alloy. However, the reduction of relative strength of alloy grain boundaries leads to microcracks nucleation and propagation more easily from grain boundaries and lower the yield strength and tensile strength of alloy.
Ni-based supperalloys are widely applied in manufacturing of compressor and turbine discs and polycrystal turbine blades in the hot section of aero-engines, since they possessed excellent mechanical strength and creep resistance at high temperatures. Generally, hot working is an effective way for shaping metals and alloys. Lots of typical metallurgical behaviors occurred, which were related to the hot working parameters, including deformation temperature, strain rate and strain. And BP-ANN (artificial neural network based on the error-back propagation) as well as Arrhenius types models were the two of most acknowledged constitutive models to determine the relationship between the flow behavior and hot deformation parameters of various metals and alloys, at present. In order to investigate the relationship between deformation parameters and flow stress behavior, and precisely simulate the flow behavior during hot deformation processes of GH4720Li alloy, the hot compressive tests of GH4720Li alloy were conducted at the deformation temperature range of 1060~1140 ℃ and strain rate range of 0.001~1 s-1 on Gleeble 3500D thermal simulation testing machine in this work. The relationship between microstructure and hot deformation conditions was identified. The influence of hot processing parameters on flow stress behavior was analyzed. The temperature sensitivity of the flow stress decreased with increasing temperature at a strain rate of 0.1 s-1. The peak stress increased 23 MPa when the deformation temperature decreased from 1100 ℃ to 1080 ℃, only increased 7 MPa when decreased from 1140 ℃ to 1120 ℃. In addition, the Arrhenius model as well as BP artificial neural network model was established according to the true stress-strain curves. It shows that the established BP artificial neural network model can well exhibit the flow stress behavior of GH4720Li alloy compared with the Arrhenius model during hot deformation. The correlation coefficient between experimental findings and predicted flow stress determined by ANN model and Arrhenius model is 0.998 and 0.949, respectively. In addition, the dynamic recrystallization mechanism of the studied alloy was identified according to the deformed microstructure. Microstructure observation of the samples deformed at 1140 ℃ indicated that the discontinuous dynamic recrystallization was the main nucleation mechanism and newly grain nuclei distributed along the deformed grain boundaries. The dynamic recrystallization grain size of GH4720Li alloy decreases with the increase of strain rate when the samples deformed at 1140 ℃ and a strain of 0.8.
In the aerospace industry, due to the increasing hardness and tensile strength of nickel-based superalloys, the traditional manufacturing methods are difficult to produce, which limits the freedom of part design and process. Selective laser melting (SLM) has great potential in this field with its additive manufacturing concept and full melting during the process. Although the dense part can be easily obtained in SLM, the residual stresses and micro-cracks in the machining process still affect the dimensional accuracy and reliability of the parts. In SLM process, rapid and complex changes of temperature and stress are observed in the vicinity of the molten pool. Understanding these changes will help to improve the quality of the process. In this work, a finite element model (FEM) is established to calculate the temperature and residual stress distribution near the weld pool during the SLM of Hastelloy X superalloy, The model uses a composite Gauss heat source to consider the influence of optical penetration depth, and implements the transformation of powder, molten pool and solid metal by changing the material properties with temperature. Comparison with the test results shows that the model can simulate the distribution of temperature field and the residual stress in SLM process well. The simulation results show that with the increase of laser power, the width, length and depth of melting pool were enlarged, the cooling rate decreases; with the increase of the scanning speed, the width and depth of melting pool decreases, the length remained unchanged, the cooling rate increase. After cooling, there is a large tensile stress on the surface of the model. As the depth increases, the tensile stress decreases rapidly and eventually becomes compressive stress.
Seeding technique is a promising method for growing single crystal superalloy blade. However, sometimes stray grains nucleate in the transformation process of single crystal structure from a seed, which always cause failure of single crystal growth. In order to obtain single crystal with high perfection structure, Ni-based single crystal superalloy was prepared with low-segregated seeds by high rate solidification (HRS) method in the dual heating zone furnace. The melt-back zones of seeds were investigated systematically, and the results showed that a fusion zone without microsegregation exists in front of the melt-back equilibrium interface of seeds, in which solidification interface transited from planar to cellular. Further experiments showed that increasing the W content of seeds or the solidification rate can both accelerate the whole non-steady transition process and make fusion zone shrink. Compared with the traditional seeding method, the low segregated heterogeneous seeding technique can increase the casting yield by avoiding the nucleation of stray grains in the fusion zone, which caused by the pinched-off secondary dendrites and constitutional undercooling.
Aluminide coatings are widely employed to protect internal cooling channels of high grades blades and buckets in gas turbines have always been in severe conditions including high temperature oxidation and hot corrosion. There is a major concern for the application of aluminide coatings that refer to the inter-diffusion between aluminide coating and superalloy substrate at high temperatures. Diffusion of Al from the coating to the underlying substrate usually leads to depletion of Al in the coating, resulting in inferior oxidation resistance of the coating. Accordingly, Ni declines to diffuse counter currently from the substrate into the coating, as well as other refractory elements, such as Cr, Mo and W etc.. The inter-diffusion between aluminide coating and superalloy substrate results in degradation or various evolution behaviors of aluminide coatings, in other words, substrate composition significantly affects the properties of aluminide coatings. CoAl coating was prepared on directionally solidified superalloy DZ466 by low pressure chemical vapour deposition (LP-CVD). Oxidation behavior and microstructure evolution of CoAl coating was investigated during long term (about 5000 h) exposure at 900 ℃. Results suggested that, high concentration of aluminum did help to form Al2O3 on the surface of coating, improving oxidation resistance of DZ466 at 900 ℃. Evolution of matrix phase and precipitates in the CoAl coating during exposure was displayed, β-NiAl/CoAl phase in the coating transformed gradually to γ'-Ni3Al phase, higher transformation rate for the γ' phase closed to the substrate due to the diffusion between the coating and the sub strate superalloy. During exposure, α-Cr phase precipitated in the middle layer, which inclined to form close to carbides and grow by consuming them. Needle like TCP phase (μ phase) grew in the inner layer that arranged in order, which was due to the cubic microstructure of γ/γ'. Heredity-effect was in company with the precipitates evolution.
It has been pointed out recently that the compositions of industrial alloys are originated from cluster-plus-glue-atom structure units in solid solutions. Specifically for nickel-based superalloys, after properly grouping the alloying elements into Al, Ni-like (, including Ni, Co, Fe, Re, Ru and Ir), γ′γ′, including Ta, Ti, V, Nb), and γ-forming Cr-like (γ, including Cr, Mo and W), the optimal formula for single-crystal superalloys has been established [Al-12](Al1γ′0.5γ1.5). In this work, the first generation single-crystal superalloys were investigated on the basis of the proposed formula, by using =(Ni and Co), γ′=(Ta and Ti), and γ=(Cr, Mo and W). Two series of alloys were designed, formulated respectively as group A: [Al-Ni11Co1](Al1TaxTi0.5-xCr1W0.25Mo0.25), with x=0, 0.25 and 0.5 (the corresponding mass fractions of Ta and Ti are respectively 0Ta-2.65Ti, 4.82Ta-1.26Ti and 9.32Ta-0Ti), and group B: [Al-Ni12-yCoy](Al1Ta0.25Ti0.25Cr1W0.25Mo0.25), with y=1.5, 1.75, 2 and 2.5 (the corresponding mass fractions of Co are respectively 9.43Co, 11Co, 12.57Co and 15.71Co). The single-crystal superalloys were prepared using selector technique. And then they underwent the following tests of incipient melting, standard heat treatment and 1000 h long term ageing at 900 ℃. It is found that: (1) In group A, with increasing Ta content (decreasing Ti), all the incipient melting temperatures are increased to above 1330 ℃, and to the highest value is between 1335 ℃ and 1340 ℃ for alloy 9.32Ta-0Ti; the γ/γ′ lattice negative misfits after standard heat treatment are reduced from -0.262% (0Ta-2.65Ti) to -0.247% (9.32Ta-0Ti); the γ′ coarsening tendency after long-term ageing is deduced, and alloy 9.32Ta-0Ti has the lowest coarsening rate (K=5.6×10-5 μm3/h). (2) In group B, the Co content does not influence the incipient melting temperature (always above 1330 ℃) and the coarsening rate of γ′ after long-term ageing. The major role of Co is to increase the mean size of the γ′ precipitates to about 0.55 μm and the γ′ volume fraction to about 69% after the standard heat treatment. These two groups of alloys have their γ′ coarsening rates approaching the level of third-generation single-crystal superalloys (K≈(2.08~3.82)×10-5 μm3/h).
Industrial gas turbines (IGTs) are the key equipment to achieving energy strategy, such as energy conservation and clean power generation. When the large and complex IGT blades are fabricated by the conventional Bridgman directional solidification process, the thermal gradients at the solidification front are low and unstable, resulting in some disadvantages: the coarse dendrite structure with severe dendritic segregation, the increased occurrence of casting defects and the poor performance of mechanical properties. These disadvantages provide a good opportunity for rapid development of the directional solidification with high thermal gradient (HG), such as the liquid metal cooling (LMC). In the present work, the physical basis of HG process, the microstructure, mechanical properties, solution heat treatment, and casting defects of the superalloys processed by HG process, have been reviewed. The HG process increases the thermal gradient and the cooling rate, thus permitting microstructural improvements including a more homogeneous fine-dendrite structure with lower elemental segregation and shrinkage porosity, and refinement of carbide, γ′ phase and eutectic, reducing the volume fraction of eutectic and shrinkage porosity. During the solution heat treatment, the HG process increases the incipient melting temperature and reduces the residual segregation as well as the content of solution pore. The HG process could effectively inhibit the formation of freckle chains, increase the critical withdrawal rate of the stray grain formation, and decrease the degree of the misorientation of the <001> grain orientation from the casting axis. Moreover, the HG process could improve the mechanical properties including the stress rupture life, low-cycle fatigue (LCF), high-cycle fatigue properties and short-term strength, but the improvement might be reduced at higher temperature or under the oxidation condition.
With the increase of the alloying degree and structural complexity as well as larger size in Ni-based superalloy blades, it is essentially important to suppress the solidification defects. When the electromagnetic field is introduced into solidification process, the solidification properties of alloy can be modified without changing the alloy composition, which can well eliminate the casting defects, such as the composition segregation, and optimize the solidification microstructure and improve properties. The effect of induction coil magnetic field on solidification structure of DD90 single crystal superalloy is studied by changing the thickness of graphite sleeve. The distribution of magnetic field and flow field in alloy melt are analyzed by Ansys finite element analysis (FEM). The results show that when the thickness of the graphite sleeve is 10~30 mm, the monocrystalline remains intact and the primary dendrite arm spacing increases with increasing the thickness, while the second dendrites are the opposite rule. Moreover, the as-cast microstructures of γ′ phase size, eutectic structure and content increase significantly, and the element segregation increases simultaneously with increasing the graphite sleeve thickness. The Ansys FEM shows that the magnetic field and flow velocity in the melt decrease with the increase of the thickness of graphite sleeve. Based on the thermoelectric magnetic convection induced by the magnetic field during solidification and the effect of the convection on the microstructure, the above phenomenon is analyzed and discussed.
Microelement B is widely added into almost all commercial superalloys because B contributes to strengthening grain boundaries at high temperature during service. Generally, B is present in two different forms. Besides the solute state in matrix, B tends to react with transition elements at high temperatures, giving rise to various borides including M2B, M3B2 and M5B3 phases. An accurate knowledge of the microstructural characterizations of these borides is of great importance for a better understanding of the structure-property relationship and designing materials with improved properties. By means of various advanced techniques based on the aberration-corrected transmission electron microscopy (TEM), microstructural features of above borides have been systematically investigated. Various defect features which were controversial in the past have been clarified. In this paper, after a brief review on the studies of borides, the atomic-scale information on the microstructural features has been presented. Finally, some prospects for future studies have been proposed.
Cast superalloy is widely used in aerospace and energy industry. The research and development of these alloys is correlated with a large variety of materials and disciplines. The technology readiness level (TRL) of advanced cast superalloys is generally a mirror of the industry base of a country. China has made great progress in the field under the strong pull of demand in recent years. However, many issues are emerging in the industrial applications, reflecting a low TRL of advanced materials and a large gap between China and the developed countries. We present (1) development of directionally solidified and single crystal superalloys, (2) processing techniques of complex castings and (3) service behavior of blades as examples in this paper to explain the important role of basic research in research and development of cast superalloys.
Here some critical issues existed during forging process of Inconel 718 disks involving recrystallization mechanisms, grain growth, δ-phase morphology control and residual stress are explained. Based on the potential application prospect of selective laser melting in additive manufacture of aerocraft engine components, the specialized anisotropic microstructure and mechanical performance resulted from the rapid solidification process in selective laser melting are analyzed. Furthermore, the importance and difficulty of heat treatment in eliminating Laves-phase as well as tailoring substructure and related mechanical behavior are also discussed. The deformation mechanisms of Inconel 718 alloy at high temperature are illustrated in detail, comprising of dislocation planar slip, twinning and dislocation-shearing γ″ precipitates in complex modes. At last, a newly developed wrought nickel superalloy (Allvac 718Plus, with a increase in service temperature of 55 ℃ as compared to that of Inconel 718) is introduced, and some recent progresses aimed at modifying chemical compositions and phase compositions to improve service temperature on the basis of Inconel 718 alloy are also reviewed. The results indicate that the more stable γ″-γ' composite structure is important for the further design of next-generation wrought nickel superalloys.