Single Crystal Blades Research Articles

Effect of directional solidification on the structure and properties of Ni3Al-based alloy single crystals alloyed with W, Mo, Cr, and REM

The effect of the solidification rate (R = 2, 5, 10, 20 mm/min) at the same solidification gradient (G = 150°C/cm) on the structural parameters of single-crystal blade workpieces made of an alloy based on the γ′(Ni3Al) intermetallic compound and alloyed with cobalt and rhenium apart from chromium, molybdenum, titanium, and rare-earth metal microadditions is studied. The single crystals have a dendritic-cellular structure. Primary γ′-phase precipitates are observed in the interdendritic space of heterophase γ′ + γ dendrites. An increase in the solidification rate from 2 to 20 mm/min at a solidification gradient of 150°C/min leads to refinement of all structural constituents by a factor of 1.5–2, with the morphology and the mutual position of the structural constituents being independent of the solidification rate. In experiments with moderate additional alloying with cobalt and rhenium, the yield strength increases by 10–20% and the long-term strength increases by at least 20–25% at a temperature of 900 and 1100°C upon holding for 100 and 500 h. The VKNA-25 alloy single crystals have moderate plasticity (δ = 6–20%) over the entire temperature range (20–1200°C) and have no sharp increase in the plasticity characteristic of a VKNA-1V alloy without cobalt and rhenium. During long-term tests, local raft structure regions misoriented with respect to the tension direction form in γ′ + γ dendrites. γ′-Phase nanoparticles precipitate in the γ layers. During tests, refractorymetal-rich nanoparticles of a predominantly acicular-lamellar shape precipitate in dendrite arms.

Improvement in ductility of high strength polycrystalline Ni-rich Ni3Al alloy produced by EB-PVD

Abstract A 300 μm Ni-rich Ni 3 Al sheet was produced by electron beam physical vapor deposition (EB-PVD) and followed by different heat treatments to obtain fine γ′/γ two-phase structures with large elongation. Tensile testing was performed at room-temperature, and the corresponding mechanisms were investigated in detail. Results indicated that the as-deposited Ni 3 Al alloy exhibited non-equilibrium directional columnar crystal, and transited to equiaxed crystal with uniformly distributed tough γ phase after heat treatment. Meanwhile, the fracture mechanism transited from brittleness to a mixture of ductility and brittleness modes. With an appropriate heat treatment, high strength (ultimate tensile strength obtained 828 MPa) and high ductility (elongation obtained 14.6%) Ni 3 Al alloy has been achieved, which was due to the mesh network microstructure. A series of transmission electron microscope (TEM) characterizations confirmed that the increasing flow stress of Ni 3 Al alloy was attributed to the cubical secondary γ′ phase precipitates (25–50 nm) within the γ phase. This work provides a potential strategy for repairing the worn tip of single crystal engine blades using Ni-rich Ni 3 Al alloy by EB-PVD.

Investigations of early stage precipitation in a tungsten-rich nickel-base superalloy using SAXS and SANS

Abstract Characterization of the early stage precipitation is an important issue as it controls the emerging microstructure developed on Ni-based superalloys which are used as single crystal blades in gas turbines. The difficulty is that the gamma prime precipitation occurs at high temperatures and the reaction kinetic is very fast. Therefore, experimental methods with high time resolution are needed to monitor γ′ precipitation. By using the high flux synchrotron instrument HARWI-II set up of the Helmholtz–Zentrum Geesthacht with a fast data collection detector, it was possible to observe the creation of fine precipitates and their early growth in a tungsten-rich Ni-base superalloy. The cooling down of the superalloy from the single-phase region to lower temperatures already leads to the formation of precipitates in the size range of a few nm. The formation and growth of the precipitates at this early stage is described in detail. In addition, SANS measurements were performed at the new SANS-1 instrument at Maier–Leibnitz Zentrum (MLZ) in Garching to extend the results to larger precipitate sizes. DSC experiments were used to determine the critical temperatures of the formation of these fine precipitates, as well as the temperature at which they dissolve during heating (γ′ solvus temperature).

Development of Nickel Based Superalloys for Advanced Turbine Engines

Superalloys have been developed for specific, dedicated properties and applications. One of the main application for this material is advanced, high-performance aircraft engines elements. Turbine engine creates harsh environments for materials due to the high operating temperature and stress level. Hence, as described in this article, many alloys used in the turbine section of these engines are very complex and highly optimized. This article provides an overview of structural changes that occur during the aging process of wrought and cast alloys and provides insight into the use of precipitated particles to achieve desired structures. Example will focus on alloy Inconel 718 and CMSX-4. Functional properties of these alloys can be achieved by choosing proper heat treatment parameters to obtain required rate between secondary phases. The paper also attempts to determine structural perfection and changes of crystallographic orientation along the axis of growth of single crystal nickel superalloys cast using X-ray topography and Laue diffraction method. Single crystal bars and turbine blades were manufactured in VIM furnace using the Bridgeman method. Withdrawing rates typical for CMSX-4 superalloy were used. It has been found that with increasing withdrawing rate the nature of distribution along the axis of growth of the angle of [001] direction deviation from the axis of single crystal blades growth had changed. The change of the withdrawing rate results also in the rotation of γ’ phase in the form of cubes against the axis of single crystal blades growth.

NUMERICAL SIMULATION OF DENDRITE GRAIN GROWTH OF DD6 SUPERALLOY DURING DIRECTIONAL SOLIDIFICATION PROCESS

Modern aero and power industry needs high- performance gas turbine. Directional solidification(DS) columnar grain and single crystal(SX) blade as key parts of gas turbine serve in heavy stress and high temperature conditions. The DS and SX blade are mainly produced by high rapid solidification(HRS) method, and HRS is one of useful DS technology, which has a property that the heat dissipating ways are changing during the process and the temperature gradients will vary correspondingly. The dendrite grain arrays were the substructure of a DS or SX blade. The structure of the dendrite grain arrays influences the mechanical property of the final casting very much, but is seriously affected by the solidification parameters, such as temperature gradient. In this work, the dendrite grain growth of DD6 superalloy was studied based on cellular automaton-finite difference(CA- FD) model concerning the HRS method's macro solidification parameters. Mathematic models for dendrite grain growth controlled by temperature field and solute field were built to describe the competitive growth and morphology evolution of dendrite grains. Then the dendrite calculation model was coupled with the models of DS process calculation, and some HRS solidification parameters were included, such as withdrawal rate, pouring temperature, etc. The coupled models were used to predict the dendrite grain competitive growth of DD6 superalloy during the DS process. The variation of solute distribution and the dynamic adjustment of dendritic spacing during the process could be predicted by simulating calculation. The DS experiment was carried out with a cylinder sample, and dendrite grains' distribution in the transverse and longitude section was observed by OM and SEM. Then the simulated dendritic morphology was compared with that by experiment. The primary and secondary dendritic spacing by experiment and simulation were measured, and the compared results revealed that as the DS process going on the temperature gradient decreased gradually and the primary dendritic spacing was increasing. So simulation results of the DS dendritic competitive growth were validated by the experiment results, and the proposed models could predict the dendrite grain morphology and the adjustments of DS dendritic spacing of DD6 superalloy very well.

Fatigue life of deposited repair welds on single-crystal high-temperature nickel alloy under cyclic oxidation

In modern gas-turbine units increase of gas working temperature leads to shortening of blade service life. For this reason their repair becomes a priority. Retailoring of single-crystal blade tip by hardfacing is a rather complicated task. In order to select suitable filler material, thermal cycling tests of samples of deposited welds on CMSX-4 alloy with a single-crystal structure have been performed. Evolution of the structure under the conditions of high-temperature cyclic oxidation is considered. Selection of filler material, which ensures high temperature resistance and stability of weld metal structure, was optimized. These requirements are satisfied by Co—Ni-based material, which was earlier tried out and proved to be very good in complex technology of repair of a blade from ZMI-3U alloy. The technology includes airfoil tip retailoring by hardfacing with subsequent deposition on the item surface of a high-temperature metal coating by electron beam deposition to ensure the required service properties of the item. 18 Ref., 4 Tables, 10 Figures.

Effect of the cooling rate during heat treatment and hot isostatic pressing on the microstructure of a SX Ni-superalloy

Single crystal Ni-based gas turbine blades show a combination of large casting pores and pores from the homogenization heat treatment. Both kinds of pores can only be reduced by HIP; however, HIP not only reduces porosity but also affects the size, number and morphology of γ � particles. From the HIP parameters, pressure, temperature, holding time and cooling speed, the main effect on both, the porosity and the γ/ γ � microstructure is due to temperature and cooling rate. HIP temperatures above the γ � solvus temperature allow the fastest and most effective reduction of the porosity, because only the soft γ phase is present. The recent and novel possibility of cooling the samples from the maximum HIP temperature with a fast cooling of about 200K/min, results in a fine and homogeneous distribution of γ � particles, which requires no additional solution annealing treatment to dissolve the developed γ � particles during the extremely short cooling time. Therefore, the application of HIP at super γ � solvus temperature followed by fast cooling on homogenized samples seems to have the most promising results: no porosity and fine γ/ γ � microstructure.

Isothermal Oxidation Behaviour of NiCoCrAlYTa Coatings Produced by HVOF Spraying and Tribomet™ Process

Protective NiCoCrAlYTa coatings are used on gas turbine single crystal superalloy blades to provide environmental resistance. They can be deposited by several processes. In this study, isothermal oxidation behaviour of NiCoCrAlYTa coatings produced by HVOF spraying and Tribomet™ process and deposited on single crystal nickel-based superalloy CMSX-4 were compared between 950 and 1,150 °C for several exposure durations. Microstructure and chemical composition of both coatings were examined before and after oxidation testing and quite similar observations were made for both coating processes. The combination of phase and chemical analyses allowed the establishment of an occurrence diagram of phases for both coating processes, according to temperature and duration of exposure. The obtained diagrams seemed similar for both processes. Finally both processes appeared to be equivalent for the protection of CMSX-4 superalloy in isothermal oxidation conditions.

Structure and Phase Composition of Complex Refractory Coating and of the Reaction Zone of Interaction with Single-Crystal Alloy ZhS36-VI after High-Temperature Holds

A complex coating consisting of three layers, i.e., a gas-circulation coating and a two-layer ion-plasma coating with an internal Ni − Cr − Al − Ta − Re − Y layer and an external Al − Ni − Cr − Y layer for protecting single crystal turbine rotor blades from high-temperature oxidation is considered. The fine structure and the phase composition of the complex coating and of the reaction zone of interaction with single-crystal alloy ZhS36-VI after high-temperature holds in a range of 1050 − 1300°C for 1–1000 h are studied.

Effects of Coating on Surface Recrystallization of DD6 Single Crystal Blades

The fully heat-treated DD6 single crystal turbine blades were treated in three ways: (1) surface grinding, without coating; (2) firstly surface grinding, then surface coated with HY3(NiCrAlY) anti-oxidation coating; (3) after surface grinding, coated surface with the bond layer of NiCrAlY and top layer of YSZ(Y2O3 stabilize ZrO2) for the double-layer thermal barrier coatings. All these three experimental blades were treated with vacuum heat-treatment at 1120°C for 4h. The effects of coating on surface recrystallization of single-crystal blades have been investigated. The results indicate that: just with surface polishing blade generated a 6~9μm thick cellular recrystallization zone on the surface. The γ' presented coarse morphology and distributed discretely within cellular, and the γ' growth direction was perpendicular to the cellular interface; blade surface coated with anti-oxidation coating after polishing formed a 3~6μm thick cellular recrystallization zone in the matrix interface. But the grain boundary was fuzzy and recrystallization morphology was incomplete; the matrix interface morphology of blade coated with thermal barrier coatings after polishing changed insignificantly, but some local cellular recrystallization was found. The results indicate that the coating changes the cellular recrystallization morphology of the original matrix of the blade, which can effectively reduce the occurrence degree of recrystallization.

Influences of Holes Arrangement on Creep Characteristic of Nickel-Base Single Crystal Alloy Blade Cooling Holes

Film cooling technology is developed to enhance the temperature resistant of nickel-base single crystal alloy blade. The shape, dimension, and arrangement of cooling holes impact the blade strength and life grievously. In this paper, the influences of holes arrangement on creep characteristic of cooling holes in the plate sample are investigated. The constitutive model for creep considering both cavitation and degradation damage is developed to predict the creep behavior of cooling holes. Results show that there are stress interferences among cooling holes. The distance and radius of the cooling holes impact the creep behavior of cooling holes seriously. Decreasing horizontal distance of the holes results in creep time reducing. On the contrary, increasing the vertical distance of the holes makes the creep time reduced.

Centrifugal forces influence of statics and dynamics of gas turbine engines elements

The analysis of influence of centrifugal forces on the statics and dynamics of gas turbine engines is considered. The peculiarities of the stress-strain state of the compressor and cooled single crystal blades by centrifugal force are discussed. Three-dimensional models and the finite element method are used. The effect of centrifugal force on the natural frequencies, mode shapes and the relative intensity distribution of the stresses of compressor blades are shows. Dynamics stress-strain state of gas turbines engines cases under local impact is considered. Depending on the impact speed deformation take place in the elastic or elastic-plastic stage. Account of the considered factors can improves strength of elements of gas turbine engines.

Effect of lanthanum on the high-temperature strength of single crystals of highly refractory alloy VZhM4-VI containing rhenium and ruthenium

The effect of lanthanum on the high-temperature oxidation resistance of single crystals of alloy VZhM4-VI used for casting single-crystal blades of gas turbine engines is studied. It is shown that lanthanum affects positively the resistance of single-crystal specimens of alloy VZhM4-VI to high-temperature oxidation and raises its heat resistance by a factor of 1.5 – 2.0 at 1150°C. This is proved by the data of an x-ray diffraction phase analysis of an oxide film and of a metallographic analysis of the structure of the near-surface layer of specimens of the alloy after oxidation.

Formation Mechanism of Low Angle Boundary of DD6 Single Crystal Superalloy Blades

The specimens were machined from DD6 single crystal superalloy blades with low angle boundary. The misorientation of LAB was measured with EBSD technique in scanning electron microscope. The microstructures of specimens with LAB were examined in optical microscope and scanning electron microscope. The formation mechanism of low angle boundary of DD6 single crystal superalloy blades was investigated. The results showed that he formation of LAB which is caused by the deviating orientation from ideal [001] and the angle between the crystal orientation and shell is crystal selection process acted by dendrite competitive growth rule. Part of dendrites have changed their growth orientation a little to the decreasing [001] orientation departure angle because of solidification condition fluctuating during dendrites branching process. The LAB is the obvious interface between the deforming dendrites and their surrounding dendrites.

TEM Analyses of Microstructure Evolution in Ex-Service Single Crystal CMSX-4 Gas Turbine Blade

TEM studies of the ex-service single crystal blade were performed to analyse the microstructural changes which control the degradation of the CMSX-4 superalloy during its exposure in a gas turbine. Microstructural analyses were focused mainly on evolution of the γ-γ’ morphology and examination of dislocation configurations in particular parts of the blade. Between the bottom and tip locations of the blade a pronounced differences in γ-γ’ morphology were observed. TEM analysis of the dislocation substructure showed that at the bottom of the blade the dislocation slip is mainly confined to the horizontal γ channels. Increasing of the temperature and decreasing of stress along the blade height resulted in the γ-γ’ rafting, formation of dislocation networks and intensification of the dislocation climb in vertical γ channels.

Structural Perfection of a Single Crystal Nickel-Based CMSX-4 Superalloy

The aircraft engines turbine blades are manufactured from nickel-base superalloys and they are often in a single crystal form. This ensures the best high-temperature creep resistance as compared with blades of equiaxial grains microstructure and of columnar grains microstructure. Turbine blades were manufactured in an ALD Vacuum Technologies furnace. The study has examined structural perfection of single crystal blades obtained by Bridgeman method from CMSX-4 nickel superalloy at various withdrawal rates: 1, 2, 3, 4 and 5mm/min.

Formation of stray grain in cross section area for Ni-based superalloy during directional solidification

Stray grains which may appear during production of single-crystal blades, can severely deprave the performance of turbine blades. This investigation primarily explores the conditions of stray grains formed in DZ417G superalloy by using samples with varying cross sections. The effect of withdrawal velocities on the microstructure of DZ417G superalloy in directional solidification is investigated. It is observed that the growth direction of dendrites did not change, and no stray grain appears in the re-entrant corner at low withdrawal velocity. Nevertheless, the stray grains grow up when the withdrawal velocity is higher than 150μm/s. The temperature profiles in the region of the cross section change at different velocities are examined. The increase of undercooling in the corner of the cross section change leads to the formation of stray grains.

Heat-resistant coatings for the high-pressure turbine blades of promising GTEs

Heat-resistant coatings are considered for the external surface of high-pressure turbine (HPT) single-crystal blades for promising gas turbine engines (GTEs) made from carbon-free nickel superalloys with rhenium or rhenium plus ruthenium. Nickel superalloys covered with heat-resistant coatings consisting of heat-resistant connecting layers and an external ZrO2-(7–8 wt %)Y2O3 ceramic layer are subjected to heat resistance and high-temperature tests. The test results are used to choose the heat-resistant layer that ensures the highest properties of a composition heat-resistant coating. The use of sequential chemical and physical deposition methods for coating layers is shown to be required to protect HPT blades in promising GTEs. Medium-frequency magnetron plasmachemical deposition of ceramic layers in heat-resistant coatings with a low thermal conductivity is found to be promising.

The Influence of Crystal Orientation on the Elastic Stresses of a Single Crystal Nickel-Based Turbine Blade

Single crystal nickel-based turbine blades are directionally solidified during the casting process with the crystallographic direction [001] aligned with the blade stacking axis. This alignment is usually controlled within 10 deg, known as the Primary angle. The rotation of the single crystal about the [001] axis is generally not controlled and this is known as the Secondary angle. The variation in Primary and Secondary angles relative to the blade geometry means that the stress response from blade to blade will be different, even for the same loading conditions. This paper investigates the influence of single crystal orientation on the elastic stresses of a CMSX-4 turbine blade root attachment using finite element analysis. The results demonstrate an appreciable variation in elastic stress when analyzed over the controlled Primary angle, and are further compounded by the uncontrolled Secondary angle. The maximum stress range will have a direct impact on the fatigue resistance of the turbine blade. By optimizing the Secondary angle variation the elastic stresses can be reduced, giving the potential to enhance the fatigue resistance of the turbine blade.

A hybrid process for manufacturing surgical-grade knife blade cutting edges from bulk metallic glass

Abstract The demand for precision surgical knives is enormous. Currently, diamond knives have been the preferred choice among surgeons for use in precision surgeries, owing to the extreme hardness of diamond and the sharpness that can be achieved in single crystal diamond blades, but material and processing costs are high. Bulk metallic glass (BMG) has the potential to be an economically viable material of similar performance for use in precision surgical knives. To this end, a novel hybrid manufacturing process integrating thermally assisted micro-molding and micro-drawing has been developed for producing BMG surgical-grade knife blade cutting edges with edge radii