Single Crystal Nickel Superalloys Research Articles

Effects of high energy laser peening followed by pre-hot corrosion on stress relaxation, microhardness and fatigue life and strength of single crystal nickel CMSX-4® superalloy

This study investigated the stress relaxation and fatigue life and strength of laser peened single crystal nickel superalloy specimens following hot corrosion exposure and then fatigue testing compared to un-peened and shot peened specimens. The specimens were treated by conventional laser peening and a new cyclic laser peening plus thermal microstructure engineering process. Stress measurements by slitting showed the plastic penetration depth of laser peening exceeded shot peening by a factor of 24. Un-peened and peened specimens were exposed to sulphate corrosives at 700°C for 300 hours and then fatigue tested. Tests of five non-laser peened specimens all failed in low cycle fatigue regime whereas three identically tested laser peened specimens all achieved multi-million-cycle runout without failure, indicating fully consistent large benefit for life by laser peening. Additional tests showed fatigue strength improvement of 2:1 by laser peening.

Computer Simulation and Optimization of the Single-Crystal Nickel Superalloy Compositions: IV. Computer Method for Optimizing the Superalloy Composition

Computer Simulation and Optimization of the Single-Crystal Nickel Superalloy Compositions: IV. Computer Method for Optimizing the Superalloy Composition

The Manufacturing and Experimental Validation of a Nickel Superalloy Double-Wall, Effusion Test Specimen

Abstract With the hot stage of a modern aeroengine operating with combustor firing temperatures well beyond the melting point of the nickel superalloys from which the turbine blades are manufactured, developments to the methods of cooling of these components are required to advance performance. Double-wall, effusion systems exhibit a quasi-transpiration like cooling effect with recent work demonstrating their exceptional cooling performance. Such systems are characterized by two walls, one with impingement holes and the other with film cooling holes, that are mechanically and thermally connected via pedestals. However, manufacturing such geometries from single-crystal nickel superalloys remains a significant barrier to entry into service. This paper presents a method of manufacturing double-wall effusion specimens from a nickel superalloy commonly used in modern commercial high-pressure turbine components. The method maintains the mechanical integrity associated with nickel superalloys. Details of the method are presented alongside X-ray and GOM laser scan data of a flat-plate test article that demonstrates the success of the manufacturing process. Aerothermal testing of the specimen in a bespoke recirculating wind-tunnel facility was undertaken in which the overall cooling effectiveness of the system is obtained. The results reaffirm the excellent cooling performance of double-wall, effusion systems and further validate the manufacturing methodology as a method by which to realize enhanced cooling effectiveness in service.

Computer Simulation and Optimization of the Single-Crystal Nickel Superalloy Compositions: III. Computer-Aided Development of Alloys Using Software Tools

Computer Simulation and Optimization of the Single-Crystal Nickel Superalloy Compositions: III. Computer-Aided Development of Alloys Using Software Tools

Effects of high-energy laser peening followed by pre-hot corrosion on stress relaxation, microhardness, and fatigue life and strength of single-crystal nickel CMSX-4® superalloy

This study investigated the stress relaxation and fatigue life and strength of laser-peened single-crystal nickel superalloy specimens compared to unpeened and shot-peened specimens following hot corrosion exposure and then fatigue testing. The specimens were treated by conventional laser peening and a new cyclic laser peening plus thermal microstructure engineering process. The latter treatment supports the benefit of a unique process involving application of layers of laser peening using high energy with large footprint spots combined with interspersed cyclic annealing. Stress measurements by slitting showed the plastic penetration depth of laser peening exceeded shot peening by a factor of 24. Unpeened and peened specimens were exposed to sulphate corrosives at 700 °C for 300 h and then fatigue tested. Tests of five non-laser-peened specimens all failed in low-cycle fatigue regime, whereas three identically tested laser-peened specimens all achieved multi-million-cycle runout without failure, indicating fully consistent large benefit for life by laser peening. Additional tests also showed fatigue strength improvement of 2:1 by laser peening. Residual stress measurements post hot-corrosion exposure and fatigue testing showed notable 5 mm depth retention of residual eigenstress in a laser-peened specimen.

Influence of Post-Bond Heat Treatment on Microstructure and Creep Behavior of the Brazed Single-Crystal Nickel Superalloy.

Post-bond heat treatment (PBHT) is an effective way to improve the bonding quality of a brazed joint. Herein, brazing of a nickel-based single crystal superalloy is carried out using a Ni-Cr-Co-B-Si-Al-Ti-W-Mo filler alloy, and the microstructure and creep property of the brazed joint are systematically investigated using scanning electron microscopy (SEM), Thermo-Calc software, an electron probe micro-analyzer (EPMA), X-ray diffractometer, confocal scanning laser microscope (CSLM), and transmission electron microscopy (TEM). The results reveal that the as-prepared joint only consists of an isothermally solidified zone (ISZ) and an athermally solidified zone (ASZ), where the cubic γ′ phase is observed in the ISZ, and skeleton-like M3B2, γ + γ′ eutectic and reticular G phases are observed in the ASZ. Furthermore, the γ + γ′ eutectic and G phases disappear and the M3B2 alters from a skeleton-like to block-like shape in the ASZ after PBHT. Meanwhile, some lath-like M3B2 phases are precipitated at the edge of the ISZ and several M3B2 phases are precipitated in the base metal, forming a new zone in the brazed joint, namely at the diffusion affected zone (DAZ). Owing to the removal of low melting point eutectics from the as-prepared joint, the creep life also increases from 188 h to 243 h after PBHT. The current work provides a method for the optimization of brazed joints based on the Ni-based single crystal superalloy.

Computer Simulation and Optimization of the Single-Crystal Nickel Superalloy Compositions: I. Modern Computer Simulation Methods in Designing Nickel Superalloys

Computer Simulation and Optimization of the Single-Crystal Nickel Superalloy Compositions: I. Modern Computer Simulation Methods in Designing Nickel Superalloys

Influence of system alloying and dimensional mismatch of crystalline lattices of g- and g'- phases on the characteristics of the strength of single-crystal nickel superalloys

Purpose of the work. Obtaining predictive regression models, by means of which, it is possible to adequately calculate the mechanical properties of monocrystalline nickel-base superalloys without conducting previous experiments.
 Methods of research. For experimental and theoretical studies of temperature performance, a working sample of alloys is formed consisting of known industrialt nickel-base superalloys for monocrystalline castings of domestic and foreign production, which due to contents of the main elements, cover a wide range of alloying. The obtained values were processed in the Microsoft Office with the least squares method, with the obtaining of the "parameter-property" correlation dependencies with the obtaining of mathematical equations of regression models (trend lines) that describe these dependencies optimally.
 Received results. The influence of alloying elements on their tendency to the formation of phases in foundry heat-resistant nickel alloys is considered. Based on the influence of elements on phase formation, the coefficient of the ratio of alloying elements Kg¢ in the alloys of this class was first developed.
 A close correlation dependence of the ratio Kg¢ with a dimensional discrepancy between the γ- and γ-phases crystalline lattice (localized) was found. It has also been shown that for multi-component nickel systems it is possible to predict with high probability misfit, which significantly affects the strength characteristics of alloys of this class.
 The regression models of correlation dependencies on dimensional inconsistency (γ / γ'-misfit) are given, which allow to predict the boundaries of short-term and long-term strength of alloys. It is shown that the magnitude of the misfit at the operating temperature should tend to zero. This will increase the structural stability by minimizing structural stresses, which has a positive effect on the strength and plastic characteristics.
 Scientific novelty. For the first time, a coefficient of the ratio of alloying elements Kg¢, which has a close correlation with the strength and dimensional discrepancy of casting nickel-base superalloys, is proposed.
 Practical value. The perspective and effective direction in solving the problem of forecasting of the main characteristics affecting the complex of service properties of alloys, both in the development of new monocrystalline nickel-base superalloys, and in the improvement of the composition of known industrial brands of this class, is shown.

Embedded Temperature Sensor Evaluations for Turbomachinery Component Health Monitoring

Current rotorcraft gas turbine engines typically use titanium alloys and steel for the compressor section and single-crystal nickel superalloys for the hot-section turbine stator vanes and rotor blades. However, these material selections are rapidly changing due to increased requirements of power-density and efficiency. Future U.S. Army gas turbine engines will be using ceramic matrix composites for many high temperature engine components due to their low density and improved durability in high temperature environments. The gas turbine industry is also actively developing adaptive concept technologies for production and assembly of modular gas turbine engine components with integrated sensing. In order to actively monitor engine components for extended seamless operation and improved reliability, it is essential to have intelligent embedded sensing to monitor the health of critical components in engines. Under this U.S. Army Foreign Technology Assessment Support (FTAS) program funded research project, embedded fiber-optic temperature sensors from U.K.-based company, Epsilon Optics Ltd (Fordingbridge, UK)., were experimentally evaluated to measure temperature responses on typical turbomachinery component material coupons. The temperature responses from this foreign technology sensor were assessed using a thermomechanical fatigue tester with a built-in furnace to conduct thermal cycling durability experiments. The experimental results obtained from the durability performance of this embedded fiber Bragg sensor are reported in this paper. This sensor technology, upon maturation to higher TRL (technology readiness level), can greatly reduce the lifecycle cost of future U.S. Army gas turbine engines.

Electron backscattered diffraction analysis of cold work in a shot peened single crystal nickel superalloy

Shot peening has traditionally been thought to improve the fatigue performance of polycrystalline nickel-based superalloys by inducing a compressive residual stress at the surface of the material. Jet engine turbine blades, manufactured from single crystal superalloys, are routinely exposed to high temperatures. This high temperature exposure can cause relaxation of residual stress arising from shot peening, while a surface layer of cold work is retained. The cold work layer therefore appears to be responsible for improved fatigue damage resistance in such components. The work reported in this study has built up a systematic understanding of how varying shot peening intensity, coverage and shot size affects the amount and depth of cold work by examining samples of a common superalloy, CMSX-4Ⓡ, peened under eighteen different conditions. Local misorientation analysis in electron backscattered diffraction was used to investigate cold work structure. Intensity was seen to increase cold work depth, with a diminishing gradient, in µm/Almen, at higher intensity. Coverage increased the amount of cold work at a given depth, the severity of slip bands and the regularity of cold work. Shot size had a weak effect on cold work, slightly increasing its depth with increasing shot size, but concurrently decreasing its magnitude. In addition, the orientation of the crystal matrix relative to the sample was found to influence the resultant cold work. Samples with shot peened face normals closer to a <110> orientation displayed lower cold work than those with normals closer to <100>.

Reduction of Silicon from a Ceramic Mold during Directional Solidification of Single-Crystal Nickel Superalloys

Silicon is one of the most harmful impurities in nickel superalloys used for casting aviation turbines: it decreases the high-temperature strength and the ductility. The reduction of silicon from a ceramic mold is experimentally studied during the directional solidification of a single-crystal nickel superalloy.

In-situ SEM study of slip-controlled short-crack growth in single-crystal nickel superalloy

Initiation and growth of short cracks in a nickel-based single crystal were studied by carrying out in-situ fatigue experiments within a scanning electron microscope (SEM). Specimens with two different crystallographic orientations, i.e., [001] and [111], were tested under load-controlled tension fatigue in vacuum. Slip-caused crack initiation was identified at room temperature while initiation of a mode-I crack was observed at 650 °C. Slip traces continuously developed ahead of the crack tip once initiated and acted as nuclei for early-stage crack growth at both room and high temperature (650 °C). These slip traces were caused by accumulated shear deformation of activated octahedral slip systems, which were specifically identified by analysing the surface slip traces and crack-propagation planes. The crack-growth rates were evaluated against stress intensity factor range, revealing the anomaly of slip-controlled short-crack growth. The effects of crystallographic orientations and temperature on fatigue crack growth were subsequently analysed and discussed, including the influence of microstructural features such as carbides and pores.

Desulfurization of Single-Crystal Nickel Superalloys with Vacuum Melting

An unfavorable effect is established for a high sulfur content on the high-temperature strength properties of single-crystal alloy ZhS36 in stress-rupture tests based on 500 h, on the average number of cycles to failure during low-cycle fatigue tests, and also on coated alloy isothermal heat resistance. The unfavorable effect of sulfur may be overcome by alloy refinement with lanthanum followed by melt filtration through a ceramic foam filter during casting. Metallographic study confirms the results obtained for the effect of alloy refinement with lanthanum.

ВЛИЯНИЕ ПОВЕРХНОСТНО-АКТИВНЫХ ПРИМЕСЕЙ И ДОБАВКИ ЛАНТАНА НА СТРУКТУРУ И СВОЙСТВА МОНОКРИСТАЛЛИЧЕСКОГО ЖАРОПРОЧНОГО НИКЕЛЕВОГО СПЛАВА ЖС36

ВЛИЯНИЕ ПОВЕРХНОСТНО-АКТИВНЫХ ПРИМЕСЕЙ И ДОБАВКИ ЛАНТАНА НА СТРУКТУРУ И СВОЙСТВА МОНОКРИСТАЛЛИЧЕСКОГО ЖАРОПРОЧНОГО НИКЕЛЕВОГО СПЛАВА ЖС36

Thermomechanical fatigue failure investigation on a single crystal nickel superalloy turbine blade

A thermomechanical fatigue (TMF) test rig was developed consisting of the loading, heating, synchronizing, cooling and monitoring systems. Moreover, the specimen was particularly designed to provide adequate load-bearing capability at the blade tip in order to simulate the actual turbine blade loads in laboratory. TMF test on a single crystal nickel superalloy turbine blade was conducted. Then the specimen was segmented in a novel way to preserve the crack surface to investigate the TMF mechanism of the turbine blade. Visual examination together with the metallographic analysis indicated that the most severe TMF damage occurred at the pin-fin fillet on the suction side near the trailing edge. Furthermore, finite element simulation results were compared to the experiment, in which a viscoplastic constitutive model was employed.

The Influence of Overaluminizing on TGO Formation on CMSX-4 Nickel Superalloy during Isothermal Oxidation

In the paper the formation of Thermally Grown Oxides on newly developed Thermal Barrier Coatings were described. The CMSX-4 Single Crystal nickel superalloy was used as a base material. The bondcoat was produced by overaluminizing of an MeCrAlY–type coating deposited by low pressure plasma spraying method (LPPS). The outer ceramic layer of yttria oxide stabilized zirconia oxide (Metco 6700) was deposited by plasma spray physical vapour deposition (PS-PVD). The isothermal oxidation test at 1100oC for 1000h shown that thickness of the TGO layer in overaluminized bondcoat was significantly thicker in comparison with conventional LPPS-sprayed MeCrAlY bondcoats in the same type of TBCs.

The Microstructure of Hafnium Modified Aluminide Coatings Deposited by CVD Method

The paper presents results of microstructural analysis of Hf-modified aluminide coatings. The coating was obtained using chemical vapour deposition (CVD) method at 1040°C using BPX-Pro 325 S equipment (Iond Bond). The deposition process time was 960 mintutes. The IN-718, IN-100 as well as CMSX-4 single-crystal nickel superalloys were the substrate material. The observation of coating was carried out using scanning electron microscopy. Chemical composition was analyzed using EDS method. The results showed that hafnium accumulates mainly on diffusion/additive layer interface and forms a „chain” of small precipitations. Hafnium was found in the additive NiAl layer of aluminide coating deposited on IN-100 superalloy. Its amount did not exceed 0.3 at %.

Temperature Dependence of the Anisotropy and Creep in a Single-Crystal Nickel Superalloy

The thermomechanical response of the second-generation single-crystal nickel superalloy (SC180) was obtained for wide range of temperatures (25–1000°C). Uniaxial tension and stress relaxation experiments were performed to study the influence of [100] and [110] crystallographic orientation on stress anisotropy and creep responses. Experiments were conducted using micromechanical testing systems and strains were measured using two-dimensional digital image correlation technique. Results were reported on coefficient of thermal expansion, Young’s modulus (E), yield strength, work hardening (n), and activation energies (Q). The stress relaxation experiments were used to calculate activation energy in [100] and [110] crystallographic directions and found to be 300 kJ/mol and 350 kJ/mol, respectively.

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.

Use of Rhenium-Containing Master Alloy in Melting Single-Crystal Nickel Superalloys

Melting technology is developed for high-temperature rhenium-containing alloys ZhS32-VI, ZhS36-VI, and ZhS32U-VI in vacuum induction furnaces using a rhenium-containing master alloy MR75. Time parameters are determined for introducing the master alloy during alloy melting. Advantages of using this master alloy are demonstrated compared with rhenium rods.