Gas Turbine Engine Materials Research Articles

Investigation Mechanical and Microstructural Behavior of Operated and Unoperated Turbine Rotor Discs

In gas turbine engines, turbine sections are exposed to high temperature and loads during operation and therefore turbine inlet boundary is considered the most challenging for turbomachines. Ni-based super-alloys are used for turbine materials in gas turbine engines because of their superior mechanical characteristics at high temperatures. In this research, two Mar M-247 polycrystalline cast Ni-based turbine rotors, which were operated and non-operated in turbojet engine tests, had the same design and applied the same two-step aging heat treatment, were examined microstructurally and mechanically. As a result of the mechanical analysis, it was observed that the tensile strength (Rm (P): 537Mpa; Rm (G): 590 Mpa) ductility, and hardness (429 HV (P), 440 HV (G)) of the operated turbine rotor increased. In the microstructure analysis, blocky and Chinese script-like carbides and gamma prime eutectic islands were found in both turbine structures. EDS analysis showed that the carbides in the matrix are rich in Hf, Ta, and Ti, and the grain boundary carbides are rich in Cr and W.

Machine-learning-based intelligent framework for discovering refractory high-entropy alloys with improved high-temperature yield strength

Refractory high-entropy alloys (RHEAs) show significant elevated-temperature yield strengths and have potential to use as high-performance materials in gas turbine engines. Exploring the vast RHEA compositional space experimentally is challenging, and a small fraction of this space has been explored to date. This work demonstrates the development of a state-of-the-art machine learning framework coupled with optimization methods to intelligently explore the vast compositional space and drive the search in a direction that improves high-temperature yield strengths. Our yield strength model is shown to have a significantly improved predictive accuracy relative to the state-of-the-art approach, and also provides inherent uncertainty quantification through the use of repeated k-fold cross-validation. Upon developing and validating a robust yield strength prediction model, the coupled framework is used to discover RHEAs with superior high temperature yield strength. We have shown that RHEA compositions can be customized to have maximum yield strength at a specific temperature.

Analysis on the Suitability of Powder Metallurgy Technique for Making Nickel Based Superalloys

Nickel based superalloys are of a typical class of materials with an exceptional capability of retaining strength and toughness at high temperatures. Also, they have high resistance to degradation in corrosive environments. These properties of superalloys make them ideal materials for gas turbine engines used in power generation, marine engineering and aircraft applications. Earlier, researchers studied about the suitability of cast and wrought superalloys for their applicability in gas turbines. Superalloys are developed from polycrystalline to directionally solidified alloys and finally single crystal alloys by investment casting process for making turbine blade. In this work, the powder metallurgy technique is analysed for determining its appropriateness for making nickel based superalloys in gas turbines. This technique includes powder blending, compaction and sintering with or without post heat treatment for making a product. This process can offer high performance applications, where unique microstructural and mechanical characteristics are required. Refractory material can be easily alloyed by this technique, to enhance heat bearing capabilities. In this work, the development of nickel based superalloys by powder metallurgy technique, their composition as well as the influence of various elements on mechanical properties is analysed. Major defects like prior particle boundary (PPB) precipitation, thermally induced porosity (TIP), and non-metallic inclusion have to be taken care, in order to see that the mechanical properties are not deteriorated. It is observed that alloys like IN100, Rene95, and N18, Rene 88DT, ME3 and R1000 developed by powder metallurgy technique have sufficient temperature bearing capability for 1000 hr service life at 630 MPa. Alloys like U720 Li, IN 718 plus and U 720, made by casting technique and alloys like ME3, IN 100, and U720, made by powder metallurgy technique are compared on the basis of stress and temperature bearing capability. It is found that ME3 alloy prepared by powder metallurgy technique is the efficient nickel based superalloy with maximum heat bearing capability.

Materials for Gas Turbine Engines: Present Status, Future Trends and Indigenous Efforts

Materials for Gas Turbine Engines: Present Status, Future Trends and Indigenous Efforts

Study of the Effect of Alloying and Hot Deformation on Properties of Small Stampings of Alloys Based on TiAl Intermetallic

Intermetallic alloys based on TiAl compound are most promising high-temperature materials for gas turbine engines. The effect of alloying TiAl with active elements together with hot deformation and heat treatment on the structure of stamped components is studied. Alloy composition is selected and hot pressure treatment and heat treatment parameters are selected providing an improved set of mechanical properties in stamped components.

Investigation of Microstructure and Mechanical Performance of IN738LC Superalloy Thin Wall Produced by Pulsed Plasma Arc Additive Manufacturing.

The IN738LC Ni-based superalloy strengthened by the coherent γ′-Ni3(Al,Ti) intermetallic compound is one of the most employed blade materials in gas turbine engines and IN738LC thin wall components without macro-cracks were fabricated by pulsed plasma arc additive manufacturing (PPAAM), which is more competitive when considering convenience and cost in comparison with other high-energy beam additive manufacturing technologies. The as-fabricated sample exhibited epitaxial growth columnar dendrites along the building direction with discrepant secondary arm spacing due to heat accumulation. A lot of fine γ′ particles with an average size of 81 nm and MC carbides were observed in the interdendritic region. Elemental segregation and γ–γ′ eutectic reaction were analyzed in detail and some MC carbides were confirmed in the reaction L + MC→γ + γ′. After standard heat treatment, bimodal distribution of γ′ phases, including coarse γ′ particles (385 nm, 42 vol.%) and fine γ′ particles (42 nm, 25 vol.%), was observed. The mechanism of microstructural evolution, phase formation, as well as cracking mechanisms were discussed. Microhardness and tensile tests were carried out to investigate the mechanical performance. The results show that both the as-fabricated and heat-treated samples exhibited a higher tensile strength but a slightly lower ductility compared with cast parts.

Fabrication of the Nb–16Si Alloy Powder for Additive Technologies by Mechanical Alloying and Spheroidization in Electric-Arc Discharge Thermal Plasma

The development of new, more refractory heat-resistant materials for gas-turbine engines is one of most important problems of modern materials science. This is associated with the fact that nickel superalloys currently used for this purpose have a lower melting point of ~1400°C, which limits their own maximal working temperature by a range of 1100–1150°C. The Ni alloys can be replaced by natural composites, in which refractory metals are a matrix, while their silicides are intermetallic hardeners. Only three “refractory metal–silicon” binary systems manifest stability to the Me5Si3 silicide, notably, Nb5Si3, Re5Si3, and W5Si3. From the viewpoint of a combination of a high melting point and a low density, the Nb5Si3 compound is optimal among other silicides. The use of alloys of the Nb–Si system in additive manufacturing machines is of considerable interest. This work presents the results of experimental investigations into the treatment of the Nb–16 at % Si powder fabricated using mechanical alloying of elemental Nb and Si powders in the thermal plasma flux. The Nb–16Si alloy powder is fabricated by the mechanical alloying of powders of pure elements in a Fritsch Pulverisette 4 planetary mill. The powder spheroidization is performed in a plasma installation based on a discharge vortex-stabilized electric-arc thermal plasma generator. Based on the results of experimental investigations, the principal possibility to perform the plasma spheroidization of particles of the Nb–16Si alloy prepared by mechanical alloying is shown. It is shown that the surface of spheroidized particles is rough and reflects the cast material structure. Three phase components Nb5Si3, Nb3Si, and Nbss having different optical contrast are revealed in microslices, which is confirmed by X-ray phase analysis.

Crack formation and microstructure-sensitive propagation in low cycle fatigue of a polycrystalline nickel-based superalloy with different heat treatments

Gas turbine engine materials demand high performance of fatigue life under alternative loading. In this paper, crack initiation and propagation in the low cycle fatigue (LCF) of a polycrystalline nickel based superalloys was studied via experimental and numerical methods. LCF experiments were conducted with the specimens subjected to different heat treatment regimes, including of standard heat treatment (SHT), heat isostatic pressing (HIP), and combined heat treatment (HIP+SHT). Simulations based on finite element method (FEM) were implemented with the 3D digital model obtained from synchrotron radiation X-ray computerized tomography (CT) to investigate the effect of microporosity. Results indicated that microporosity was a dominant factor of fatigue crack and the parameters of microporosity including of porosity, effective diameter, and distribution co-contributed to affect fatigue life. The δ stack and carbide were observed to be fatigue crack sites. The HIP specimen had long fatigue cycle life since casting pore and acicular δ were eliminated, while further SHT can reintroduce δ into matrix resulting in high strength and low fatigue life. LCF crack behaviors in HIP+SHT superalloy depended on surface quality, stress concentration, and the quantity of carbide.

PREPARATION OF SPHERICAL NB-16SI ALLOY POWDERS FOR ADDITIVE TECHNOLOGIES BY MECHANICAL ALLOYING AND SPHEROIDIZATION IN ELECTRIC ARC THERMAL PLASMA

The creation of new higher melting temperature materials for gas turbine engines is one of the most important tasks of modern materials. This is due to the fact that nickel superalloys currently used for these purposes have a low melting point about 1400 °C which limits their own maximum working temperature to 1100–1150 °C. Ni alloys can be replaced by natural composites with refractory metals as a matrix and their silicides as intermetallic hardeners. Only three of refractory metal – silicon binary systems exhibit stability to the Me5Si3 silicide, namely Nb5Si3, Re5Si3 and W5Si3, Nb5Si3 is the best compound among other silicides with regard to the combination of high melting point and low density. The use of Nb–Si alloys in additive manufacturing machines is of considerable interest. The paper presents the results of experimental studies on the thermal plasma processing of Nb–16Si alloy powder prepared by mechanical alloying of Nb and Si elemental powders. Nb–16Si (at.%) alloy powder was prepared by mechanical alloying of pure element powders using the Fritsch Pulverisette 4 planetary mill. Spheroidization was carried out on a plasma unit based on vortex-stabilized arc thermal plasma generator. The results of experimental studies conducted confirmed the possibility to perform plasma spheroidization of Nb–16Si alloy powder particles obtained by mechanical alloying. It is shown that the particle surface after spheroidization is rough and reflects the cast structure of the material. Three phase components having different optical contrast are revealed on microsections: Nb5Si3, Nb3Si and Nbss, which is confirmed by X-ray diffraction.

Effects of combustion and salt-fog exposure on fatigue behavior of two ceramic matrix composites and a superalloy

Tension–tension fatigue behavior of two ceramic matrix composites (CMCs), SiC/SiC (Nicalon™/SiC) & oxide/oxide (Nextel™720/AS), and a superalloy (Rene 41) was characterized under combustion environment as well as under alternate salt-fog exposure for 4 h and combustion fatigue for 6 h. The run-out limit was 25 h (90,000 cycles) of fatigue. Test temperatures were 900, 1050, and 1050 °C for Rene 41, SiC/SiC, and oxide/oxide CMCs, respectively, and the applied maximum fatigue stress was 62 MPa. All three materials survived up to 25 h under combustion fatigue without any exposure to salt. Rene 41 and Nicalon™/SiC survived up to 25 h (90,000 cycles) under the alternate exposure of salt-fog and combustion fatigue. However, exposure to salt-fog exposure (5 wt%) had very detrimental effect on the combustion fatigue resistance of Nextel™720/AS. The reduction in salt concentration to 0.5 wt% had less detrimental effect. These tests were supplemented with more tests, where applied stress level, test temperature, and salt concentration were changed. Out of these three gas turbine engine materials, Nicalon™/SiC exhibited better fatigue performance in terms of applied temperature and stress level under combustion condition as well as when exposed to alternate salt-fog and combustion.

A Probabilistic Framework for Gas Turbine Engine Materials With Multiple Types of Anomalies

Some rotor-grade gas turbine engine materials may contain multiple types of anomalies such as voids and inclusions that can be introduced during the manufacturing process. The number and size of anomalies can be very different for the various anomaly types, each of which may lead to premature fracture. The probability of failure of a component with multiple anomaly types can be predicted using established system reliability methods provided that the failure probabilities associated with individual anomaly types are known. Unfortunately, these failure probabilities are often difficult to obtain in practice. In this paper, an approach is presented that provides treatment for engine materials with multiple anomalies of multiple types. It is based on a previous work that has been extended to address the overlap among anomaly type failure modes using the method of Kaplan–Meier and is illustrated for risk prediction of a nickel-based superalloy. The results can be used to predict the risk of general materials with multiple types of anomalies.

Probabilistic Treatment of Crack Nucleation and Growth for Gas Turbine Engine Materials

The empirical models commonly used for probabilistic life prediction do not provide adequate treatment of the physical parameters that characterize fatigue damage development. For these models, probabilistic treatment is limited to statistical analysis of strain-life regression fit parameters. In this paper, a model is proposed for life prediction that is based on separate nucleation and growth phases of total fatigue life. The model was calibrated using existing smooth specimen strain-life data, and it has been validated for other geometries. Crack nucleation scatter is estimated based on the variability associated with smooth specimen and fatigue crack growth data, including the influences of correlation among crack nucleation and growth phases. The influences of crack nucleation and growth variability on life and probability of fracture are illustrated for a representative gas turbine engine disk geometry.

Stress exponent and primary creep parameters using single specimen and strain relaxation and recovery test

Strain relaxation and recovery test (SRRT), requiring one specimen and viscous (permanent) strain, ɛ v (≤0.001 per test) on full unloading during primary creep, is presented with results on gas-turbine engine materials: Ti-6246 at 600 °C, Discaloy at 500 °C, IN-738LC at 850 °C and Waspaloy at 732 °C. It is shown that a ‘steady-state’ in irreversible viscous flow develops during primary creep; the shape of the creep curve is controlled by time-dependent reversible delayed elastic (anelastic) response. The average viscous strain rate during primary-creep, ε ˙ v ( av ) ( = ε v / t SR ) for load duration, t SR and corresponding ɛ v can be used for the determination of the stress exponent, n v for viscous flow. It is shown that the value of n v for primary-creep is comparable to the stress exponent, n min for minimum creep rate. Using a single specimen, SRRTs also allow determinations of Young's modulus, stress exponent, s for delayed-elasticity (anelasticity), about one-third to fourth of n v, and other parameters for the constitutive equation for primary creep—strictly before creep enhancement due to the onset of microcracking activities. Short-term and long-term SRRT data on Waspaloy indicated that the creep strain at minimum creep rate consists of a significant amount of recoverable strain (32% at 450 MPa and 38% at 650 MPa).

An Energy-Based Uniaxial Fatigue Life Prediction Method for Commonly Used Gas Turbine Engine Materials

AbstractA new energy-based life prediction framework for calculation of axial and bending fatigue results at various stress ratios has been developed. The purpose of the life prediction framework is to assess the behavior of materials used in gas turbine engines, such as Titanium 6Al-4V (Ti 6Al-4V) and Aluminum 6061-T6 (Al 6061-T6). The work conducted to develop this energy-based framework consists of the following entities: (1) a new life prediction criterion for axial and bending fatigue at various stress ratios for Al 6061-T6, (2) the use of the previously developed improved uniaxial energy-based method to acquire fatigue life prior to endurance limit region (Scott-Emuakpor et al., 2007, “Development of an Improved High Cycle Fatigue Criterion,” ASME J. Eng. Gas Turbines Power, 129, pp. 162–169), (3) and the incorporation of a probabilistic energy-based fatigue life calculation scheme to the general uniaxial life criterion (the first entity of the framework), which is capable of constructing prediction intervals based on a specified percent confidence level. The precision of this work was verified by comparison between theoretical approximations and experimental results from recently acquired Al 606-T6 and Ti 6Al-4V data. The comparison shows very good agreement, thus validating the capability of the framework to produce accurate uniaxial fatigue life predictions for commonly used gas turbine engine materials.

Wear properties of functionally gradient layers on stainless steel substrates for high temperature applications

Metal substrate components have been coated with functionally gradient coatings (FGCs) to improve thermal barrier properties and heat efficiency of high temperature materials in gas turbine engines, nuclear fusion equipment, diesel engines, jet engines and space shuttles. In this study, functionally graded materials (FGMs), which contain NiCrAl, 75% NiCrAl+25% MgZrO 3, 50% NiCrAl+50% MgZrO 3, 25% NiCrAl+75% MgZrO 3 and 100% MgZrO 3 coating layers, have been produced onto stainless steel substrates via an atmospheric plasma-spray technique. The fabricated samples were characterized by means of an optical microscope, scanning electron microscope (SEM), X-ray diffraction and microhardness tester. The wear behaviour of the FGCs was evaluated under dry conditions using a pin-on-plate configuration. The surface morphologies of the FGM samples after wear experiments were examined by SEM. The wear mechanism of the FGCs is discussed based on SEM observation of the worn surface morphologies.

Creep life prediction for udimet 720 material using the initial strain method (ISM)

Despite of considerable research results of uniaxial tension creep available for superalloys, few studies have been made on high temperature creep using the Initial Strain Method (ISM). In this paper, the real-time prediction of high temperature creep strength and creep life for the nickel-based superalloy Udimet 720 (high-temperature and high-pressure gas turbine engine materials) was performed on round-bar type specimens under pure static load at the temperatures of 538°C, 649°C, and 704°C. The predictive equation derived from the ISM in creep tests showed better reliability than those from LMP(Larson-Miller Parameter) and LMPISM (Larson Miller Parameter-Initial Strain Method) specially for long time creep prediction (103-105h)

An assessment of the role of near-threshold crack growth in high-cycle-fatigue life prediction of aerospace titanium alloys under turbine engine spectra

Increasingly accurate life prediction models are required to utilize the full capability of current and future advanced materials in gas turbine engines. Of particular recent interest are predictions of the lifetimes of engine airfoil materials that experience significant intervals of high-frequency, high-cycle fatigue (HCF). Conventional life management practices for HCF in the turbine engine industry have been based principally on a total-life approach. There is a growing need to develop damage tolerance methods capable of predicting the evolution and growth of HCF damage in the presence of foreign object damage (FOD), low cycle fatigue (LCF), and surface fretting fatigue. To help identify key aspects of the HCF life prediction problem for turbine engine components, a review is pressented of the extensive results of an Air Force research contract with Pratt & Whitney on the high strength titanium alloy Ti-8Al-1Mo-1V. Data from this representative turbine-airfoil material are used to examine the applicability of linear elastic fracture mechanics methods for prediction of service lifetimes under load spectra that include high cycle fatigue. The roles of fatigue crack initiation and growth are examined for materials that are nominally-defect-free, as well for materials that have experienced significant prior structural damage. An assessment is presented of the potential utility of the conventional threshold stress intensity factor range, ΔK th, defined by testing specimens containing large cracks. Although the general utility of a large-crack-ΔK th approach is questionable due to the potentially rapid growth of small fatigue cracks, the low allowable stresses involved in turbine engine high cycle fatigue appear to limit and simplify the small-crack problem. An examination is also presented of the potential effects of high-cycle fatigue and low-cycle fatigue (HCF/LCF) interactions.

Application opportunities for nanostructured materials and coatings

Nanostructured materials have the potential to change materials science as we know it today significantly, as well as to provide a new generation of materials with a quantum improvement in properties. While many interesting properties have been generated in the laboratory, there is still much work to be done before there are production applications for nanostructured materials and coatings in gas turbine engines and similar demanding strength- and temperature-limited applications. This paper (1) describes the need for improved materials in gas turbine engines, (2) summarizes the improved physical and mechanical properties that have been reported for nanostructured materials, (3) discusses a research and development methodology that has the potential for accelerating technology implementation, and (4) describes high pay-off applications.

The potential for nanostructured materials in gas turbine engines

There is a continuing need for gas turbine engines with improved durability and performance. Of all the technologies contributing to future gas turbines, advanced materials, such as nanostructured materials, are expected to provide more than 5096 of the required improvements. Nanostructured materials are required with improved strength, toughness and temperature cgpabditks. The enhanced plasticity and dfusivity of nanostructured materials can also serve to facilitate the fabrication of complex materials and structures. This paper describes a procedure for the aggressive research and development of nanostructured materials.

The oxidation resistance of MoSi2 composites

Recent work at the Naval Air Development Center demonstrates that the oxidation resistance of MoSi2 is not significantly reduced by XD™ compositing. The MoSi2 + SiC material exhibited an oxidation resistance equal to or better than that of base MoSi2. This excellent oxidation resistance, in conjunction with the enhanced high-temperature strength provided by XD particles, makes these materials highly attractive for use as future gas turbine engine materials. The problem of low room-temperature ductility and fracture toughness will be approached by incorporating other types of reinforcement phases.