Superalloy Structure Research Articles

Calibrated the direct current potential drop method for fatigue crack propagation testing of nickel-based superalloy with film cooling hole

Turbine blades in aviation engines commonly feature a nickel-based superalloy structure integrated with film cooling holes to enhance inlet gas temperature. However, the presence of these film cooling holes often results in frequent occurrences of fracture failures nearby. The direct current potential drop (DCPD) method, renowned for its exceptional crack sensitivity, is frequently utilized for crack length monitoring. This study establishes a FEM model of the film cooling hole plate specimen to determine the optimal probe point location. Subsequently, a mapping relationship is derived to calibrate Johnson’s formula, accounting for the unequal crack lengths at the edges of film cooling holes. It is confirmed that the crack length measured by the DCPD method represents the cumulative crack lengths on both sides of the hole. Fatigue crack propagation experiments are then conducted, with crack length monitored using a microscope. The results affirm the successful application of the calibrated formula to the film cooling hole plate specimen, exhibiting an average error within 0.1 mm.

Thermomechanical Peridynamic Modeling for Ductile Fracture.

In this paper, we propose a modeling method based on peridynamics for ductile fracture at high temperatures. We use a thermoelastic coupling model combining peridynamics and classical continuum mechanics to limit peridynamics calculations to the failure region of a given structure, thereby reducing computational costs. Additionally, we develop a plastic constitutive model of peridynamic bonds to capture the process of ductile fracture in the structure. Furthermore, we introduce an iterative algorithm for ductile-fracture calculations. We present several numerical examples illustrating the performance of our approach. More specifically, we simulated the fracture processes of a superalloy structure in 800 ℃ and 900 ℃ environments and compared the results with experimental data. Our comparisons show that the crack modes captured by the proposed model are similar to the experimental observations, verfying the validity of the proposed model.

Features of VZhL21 Nickel-Base Superalloy Structure Formation During Selective Laser Melting, Vacuum Heat Treatment, and Hot Isostatic Compaction

This article considers features of the material structure of high-strength alloy VZhL21 obtained by selective laser melting (SLM) in different stages of its post-treatment. On the basis of analyzing the microstructure the so-called “process window” that is the interval of volumetric energy density for effective variation of (SLM) parameters during preparation of this material is determined. A study of synthesized VZhL21 alloy material is conducted by transmission microscopy with analysis of phase components and alloying element distribution within the volume of material. The efficiency of vacuum-heat and gasostatic treatment regimes is evaluated during crack healing. The structure of the material obtained is studied by light, scanning, and transmission microscopy after vacuum-heat, gasostatic, and heat treatment. The main structural components are determined by XPA.

Experimental and modeling study of compressive creep in 3D-woven Ni-based superalloys

Micro-architectured Ni-based superalloy structures, with Ni-20Cr-3Ti-2Al (wt.%) composition and γ/γ′-microstructure, are created by a multi-step process: (i) non-crimp orthogonal 3D-weaving of ductile, 202 μm diameter Ni-20%Cr wires, (ii) gas-phase alloying with Al and Ti, (iii) simultaneous transient-liquid phase (TLP) bonding between wires and homogenization within wires via interdiffusion, (iv) solutionizing to create a single-phase solid solution, and (v) aging to precipitate the γ′ phase. The creep behavior of these 3D-woven γ/γ′ nickel-based superalloys is studied under uniaxial compression via experiments at 825 °C and via finite element (FE) analysis, using a 3D model of the woven structures obtained through X-ray micro-tomography. The creep strain rate for the woven Ni-based superalloy is higher than that for the bulk superalloy due to the lower solid volume fraction of the woven structure, while the creep exponents are identical. The compressive creep behavior is sensitive to the geometry of the woven structures: fewer wires perpendicular to the load and fewer bonds between wires cause lower creep resistance of the woven structure, due to a reduction in load transfer from the longitudinal wires (which are primarily load-bearing) and the perpendicular wires. Creep buckling of longitudinal wires drastically reduces creep resistance of the woven structure, confirming the importance of maintaining longitudinal wires vertical and parallel to the uniaxial compression direction. Finally, reducing wire cross-section, e.g., via oxidation, reduces creep resistance. The oxidation kinetics of the wire structures at 750, 825, and 900 °C displayed parabolic rate constants comparable to commercial Ni-based superalloys, but indicates that up to 35% of the wire cross-section is oxidized after 7 days at 825 °C, such that oxidation-resistant coatings are needed for long-term use in oxidative environment.

Structure and preferred orientation of primary phases of Ni-Al-Co-based alloy alloyed by Rhenium after annealing and creep exposure

The present paper investigates phase composition and fine structure of Ni-Al-Co-based alloy alloyed by Rhenium (Re) (~3 at.%), as well as studies preferred orientation of the given alloy primary phases by means of scanning electron microscopy and X-ray diffraction analysis. The original state was given as the alloy which underwent a complete cycle of heat treatment: stepwise homogenization, cooling, annealing, and cooling at 100°C/min back to room temperature. The alloy was further subject to annealing at 1000°C for 105 h and 968 h. It was also exposed to creep experiments at the same temperature for 105 h (loading 320 MPa), and for 968 h (220 MPa). γ- and γ′-phases were established to be the primary phases in both states of face-centered cubic crystal lattice. γ′-phase (phase with ordered atom arrangement having L12 superstructure) was the primary phase in all states of the alloy. Investigations showed that Re being able not to dissolve in the primary phases served as a phase-forming element. Phase composition and phase morphology were studied. The influence of high-temperature annealing and creep on superalloy structure and morphology of γ′-phase were defined. The influence of creep on the structure was proven to be different from the annealing effect. First, creep generated larger dissipation of orientations than annealing. Second, anisotropy of γ′-phase cuboids could be observed at meso-level. Cuboids partially joined together. Third, spheroidized cuboids could be often met after creep.

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

ABSTRACTThis 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.

Interaction of Creep and High Cycle Fatigue of IN 713LC Superalloy

The study deals with the interaction of creep and high cycle fatigue of cast polycrystalline nickel-based superalloy IN 713LC at high temperatures. Previous works indicated that creep lifetime of superalloy structures was un-affected or even slightly increased in the cases with superimposed vibrations. The reason for this behaviour was not well described up to now. Therefore, set of fatigue tests was conducted at high mean stresses level to observe this phenomenon. The mean stress was kept constant while the stress amplitudes were selected in order to measure wide range of conditions from pure creep to pure fatigue. Fractographic analysis by scanning electron microscopy (SEM) was done with the aim to identify governing damage mechanisms for particular test conditions as a preliminary evaluation of conducted tests.

The Effect of Superalloy Structure on Ultrasonic Wave Parameters

Abstract This paper analyses the nickel based superalloy Inconel 713C casts typically used in high and low pressure turbines of aircraft engines. The ingots were manufactured in the Research and Development Laboratory for Aerospace Materials at the Rzeszów University of Technology. The superalloy structures were analysed by the following methods: X-ray diffraction orientation measurement and ultrasonic wave propagation. Ultrasonic techniques are mainly used to measure the blade wall’s thickness. Measurement accuracy is determined by the velocity of the ultrasonic wave in the material tested. This work evaluates the effect of the nickel-based superalloy microstructure on the velocity of the ultrasonic wave propagation. Three different macrostructures: equiax (EQ), directionally solidified (DS) and single crystal (SX) were analysed. The authors determined the crystal misorientation in the obtained casts as the deviation of [001] crystallographic direction from the withdrawal axis or the main axis of the ingots. The measurements performed allowed researchers to identify significant differences in the wave velocity between EQ, DS and SX structures.

An iterative approach of hot isostatic pressing tooling design for net-shape IN718 superalloy parts

Powder hot isostatic pressing is a resource-efficient approach for net-shape manufacture of high-value nickel-based superalloy structures. One of the key challenges to its application is the availability of modelling tools that can predict the geometrical changes that occur during the consolidation process in order to design the tooling required. In this work, the utility of a finite element code, based on the plastic collapse model, was assessed. The finite element model was then combined with an optimisation toolbox to design (in an iterative process) the tooling required to accommodate the powder consolidation process. The model was validated for IN718 superalloy using a demonstrator with complex features. Microstructural characterisation was also performed to assess the degree of densification. Although the finite element model did not account for creep deformation, good predictions were obtained. Nevertheless, predictions of the dimensions of consolidated samples were obtained, which were typically within 1 % of the observations, suggesting that plastic collapse accounts for 99 % of the geometrical changes due to the densification process for the range of hot isostatic pressing parameters investigated.

Influence of the High-Temperature Creep on the Phase Composition and Morphology of the γ' Phase of a Nickel Alloy

of electron microscopic study of Ni–Cr–Al based superalloy are presented. The superalloy prepared by the method of directed crystallization is investigated in the initial state and after creep for 10 h at Т = 1100°С and σ = 100 MPa. It is established that the alloy in both states is formed by γ and γ' phases based on the FCC crystal lattice; the γ' phase has ordered arrangement of atoms (the L12 superstructure) and is the main phase in all states. The phase composition and morphology are studied. The change in the phase composition and particle sizes of phases as a result of creep is analyzed. The influence of the high-temperature creep on the superalloy structure is established.

Nanoscale Cellular Structures at Phase Boundaries of Ni-Cr-Al-Ti and Ni-Cr-Mo-Al-Ti Superalloys

The microstructural evolution of Ni-20 pct Cr wires was studied during pack cementation where Al and Ti, with and without prior cementation with Mo, are deposited to the surface of the Ni-Cr wires and subsequently homogenized in their volumes. Mo deposition promotes the formation of Kirkendall pores and subsequent co-deposition of Al and Ti creates a triple-layered diffusional coating on the wire surface. Subsequent homogenization drives the alloying element to distribute evenly in the wires which upon further heat treatment exhibit the γ + γ′ superalloy structure. Unexpectedly, formation of cellular structures is observed at some of the boundaries between primary γ′ grains and γ matrix grains. Based on additional features (i.e., ordered but not perfectly periodic structure, confinement at γ + γ′ phase boundaries as a cellular film with ~100 nm width, as well as lack of topologically close-packed phases), and considering that similar, but much larger, microstructures were reported in commercial superalloys, it is concluded that the present cellular structure solidified as a thin film, composed of eutectic γ + γ′ and from which the γ′ phase was subsequently etched, which was created by incipient melting of a region near the phase boundary with high solute segregation.

Manufacture and Analysis of Directional Solidification Organization of CMSX-6 Nickel-Base Superalloy

In this paper, we use the High rate solidification to study process parameters and CMSX-6 super-alloy solidification structure. And then, it can provide more basis for the industrial directionally solidified casting of superalloys blades. The results show that: under certain process parameters, it can be obtained parallel columnar crystals directional solidification structure. The organization is composed of the matrix γ phase and the secondary precipitated γ ' phase; under an optical microscope, it can be observed the shrinkage porosity due to the volumes reduction arising solidification.

Thermal Strength Evaluation of the Super Alloy Structure with Various Thermal Insulation Performances by FEM and Stress-Rupture Experiment

The combustor chamber, diffuser and nozzle are the main components of the ramjet engine. In this study, the thermal strength of the combustion chamber of the ramjet engine was evaluated. The combustion chamber consists of an Inconel alloy 718 liner and a 17-4Ph stainless steel housing. The liner is rapidly heated to a high temperature. The heated liner is cooled with a film cooling method that forms a cold boundary layer to separate the hot gas from the surface of the liner. The thermo-structural analysis is evaluated the thermal strength of super alloy structure with various thermal insulation performances by finite element method with code MSC/Nastran. The result of the analysis is compared with accelerated stress rupture test. The experiment is performed to get safety design and estimate actually life-time for combustor chamber under high temperature. In general, the work in this paper is helpful to further improve the understanding and evaluation of thermal strength of the super alloy structure with various thermal insulation performances.

Superplastic Forming and Diffusion Bonding for Four-Layer Sheets Structure of Nickel-Base Superalloy

The superplastic forming and diffusion bonding (SPF/DB) is applied in aviation and space flight field. The SPF/DB process with gas pressure control for dissimilar superalloy structure was studied. Diffusion bonding parameters, including bonding temperature T, pressure P, time t, affect the joining mechanism. When the bonded specimen with 50&m thick nickel foil interlayer was tensile at room temperature, shear fracture of the joints with nickel foil interlayer takes place at the GH4141 superalloy part. The SPF/DB of four-layer sheets structure was investigated. The optimum parameters for the SPF/DB process are: forming temperature T=1243K, forming pressure P=1MPa, forming time t=35min. The microstructure of the bonded samples was characterized. The microstructure shows an excellent bonding at the interfaces. The distribution of thickness after SPF/DB was investigated.

Determining the site occupancy of Ru in the L1 2 phase of a Ni-base superalloy using ALCHEMI

Small additions of Ru significantly improve the high temperature creep properties of Ni-base superalloys, but the mechanism is uncertain. In order to understand the effects of adding Ru it is important to know the atomic site(s) occupied by Ru in the superalloy structure. We used the technique ALCHEMI (Atomic site Location by CHannelling Enhanced MIcroanalysis) on a Ni-base superalloy containing Ru and encountered unexpected problems. We have explained why the ALCHEMI technique cannot accurately be used with Ni-base superalloys. We then successfully applied ALCHEMI to the simpler ternary alloy Ni–19at.%Al–3at.%Ru and found that 60% of the Ru partitions to the Al sublattice in the Ni 3Al γ′ phase giving an absolute atomic site preference of 5:1. Assuming such site preference holds true for more complex alloys, this information can be used to better understand the role that this platinum group metals plays.

Ultra-high temperature intermetallics for the third millennium

Nickel-base superalloys are hugely successful materials, however they are approaching the limit of their high temperature capability. There is a present and future need for ultra-high temperature alloys which are strong at temperatures at least 200°C above the maximum operating temperature of nickel-base superalloys. We are attempting to design an ultra-high temperature alloy based on platinum. Despite its high cost and density such an alloy might have applications in static components for gas turbines, in rockets, glass manufacture and elsewhere. In order to achieve high-temperature strength we are attempting to mimic a nickel-base superalloy structure but using an f.c.c. platinum matrix strengthened by coherent platinum-base intermetallic particles with the L12 structure. Microstructural and phase diagram results are given for the Pt–Zr and Pt–Rh–Zr systems, both of which are shown to exhibit low-misfit f.c.c./L12 microstructures.

Directional Structures for Advanced Aircraft Turbine Blades

In order to achieve the performance and durability requirements of advanced gas turbine engines, highpressure turbine blade materials with directional structures will be required. Directional structures can be obtained by liquid-solid (solidification) or solid-solid (recrystallization) reactions, or by composite fabrication techniques. Currently, directional solidification is used to produce columnar-grained superalloy turbine airfoils. This production process can be modified to provide single-crystal superalloys or directionally solidified eutectic turbine blades. Directional superalloy structures also can be obtained by a solid-state recrystallization process, referred to as directional recrystallization. In addition, directional composite structures are fabricated by reinforcing a superalloy matrix with high-strength refractory metal wires. These five directional turbine blade materials are compared for use in advanced gas turbine aircraft engines. The present status of each advanced material is reviewed, and the advantages and limitations of each is assessed.