INTERACTION BETWEEN OXIDATION AND THERMO-MECHANICAL FATIGUE IN IN738LC SUPERALLOY -I (original) (raw)
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Cyclic Oxidation Behavior of IN 718 Superalloy in Air at High Temperatures
Oxidation of Metals, 2011
Ni-base superalloy IN 718 was cyclically oxidized in laboratory air at temperatures ranging from 750 to 950 °C for up to 12 cycles (14 h/cycle). The kinetic behaviour as well as the surface morphology, and the oxide phases of the scales were characterized by means of weight gain measurements, cyclic oxidation kinetics, scanning electron microscopy equipped with energy dispersive spectroscopy (SEM-EDS), and X-ray diffraction (XRD) analysis techniques. The results showed that as the oxidation temperature increased, the oxidation rate, the external scale thickness, and internal oxidation zone increased. It was suggested that the oxidation rate was controlled by the diffusion of substrate elements in the alloy and the inward diffusion of oxygen through the oxide scale. The oxidation kinetics followed a sub-parabolic rate law and, the activation energy of oxidation was 249 ± 20 kJ mol−1. The scaling process was controlled mainly by the diffusion of chromium, titanium, manganese, and oxygen ions through the chromia scale. IN 718 showed low weight gain and very slow reaction rates of substrate elements at 750 °C. At 850 °C, a continuous and very thin oxide scale was formed. At 950 °C, XRD and EDS-elemental mapping analysis revealed that a complex oxide scale had formed. It consisted of an outermost layer of TiO2–MnCr2O4 spinels, inner layer of Cr2O3, and the inner most layer composed of Ni3Nb enriched with Nb, Ti and Al oxides underneath the chromia layer. The oxide scale at this temperature seemed to be thicker layer, significant spallation and volatilization had apparently occurred, and greater internal corrosion was identified. The doping effect of titanium was observed, where it was found to be diffused through the chromia scale to form TiO2 at the oxide-gas interface as well as internally and at the oxide alloy interface. The amount of rutile (TiO2) at the oxide surface increased with temperature. In view of Mn contents in the alloy, the manganese–chromium spinel oxide was inferred to have played an important role in cyclic oxidation behaviour of IN 718, where the change in oxidation kinetic was noted. The Al contents would cause internal Al-rich oxide formation at grain boundaries.
Literature survey on oxidations and fatigue lives at elevated temperatures
1984
Nickel-base superalloys are the most complex and the most widely used for high temperature applications such as aircraft engine components. The desirable properties of nickel-base superalloys at high temperatures are tensile strength, thermomechanical fatigue resistance, low thermal expansion, as well as oxidation resistance. At elevated temperature, fatigue cracks are often initiated by grain boundary oxidation, and fatigue cracks often propagate along grain boundaries, where the oxidation rate is higher. Oxidation takes place at the interface between metal and gas. Properties of the metal substrate, the gaseous environment, as well as the oxides formed all interact to make the oxidation behavior of nickel-base superalloys extremely complicated. The important topics include general oxidation, selective oxidation, internal oxidation, grain boundary oxidation, multilayer oxide structure, accelerated oxidation under stress, stress-generation during oxidation, composition and substrate...
Effects of cyclic stress and temperature on oxidation damage of a nickel-based superalloy
Materials Science and Engineering: A, 2011
Oxidation damage, combined with fatigue, is a concern for nickel-based superalloys utilised as disc rotors in high pressure compressor and turbine of aero-engines. A study has been carried out for a nickel-based alloy RR1000, which includes cyclic experiments at selected temperatures (700°C~800°C) and microscopy examination using Focused Ion Beam (FIB). The results suggest that the major mechanism of oxidation damage consists of the formation of surface oxide scales and internal micro-voids and oxide particles beneath the oxide scales, which become more severe with the increase of temperature. Applying a cyclic stress does not change the nature of oxidation damage but tends to enhance the extent of oxidation damage for temperatures at 750°C and 800°C. The influence of cyclic stress on oxidation damage appears to be insignificant at 700°C, indicating a combined effect of cyclic stress and temperature. Further energy dispersive x-ray (EDX) analyses show the enrichment of Cr and Ti, together with lower Ni and Co levels, in the surface oxide scales, suggesting the formation of brittle Cr 2 O 3 , TiO 2 , NiO and Co 3 O 4 oxides on the specimen surface. Penetration of oxygen into the material and associated internal oxidation, which leads to further 2 material embrittlement and associated failure, are evidenced from both secondary ion imaging and EDX analyses.
Materials Science and Engineering: A, 2015
Accelerated oxidation and corrosion of aero engine components due to the presence of hot salt mixture in the operating condition is known as hot corrosion. Hot corrosion degrades life of aero engine components and is broadly divided as type I and type II based on the prevailing temperature and type of salt present in the salt mixture and accordingly the mechanisms of degradation are different. Though type I and type II hot corrosion mechanisms are established but simultaneous interaction of cyclic load and hot corrosive media on aero engine material is relatively unknown. The different micro-mechanisms of interaction of type-I and II hotcorrosion and low-cycle fatigue is studied in the present investigation by conducting fatigue tests at 700, 800 o C on bare and salt-coated specimens; studying specimen surface degradation and fracture surface through extensive microscopy. Type-I hot corrosion is induced at 800 o C by depositing 90%Na 2 SO 4 +10%NaCl and type-II HC is induced at 700 o C by depositing 88%Na 2 SO 4 +7%NaCl+5%NaVO 3 on fatigue test specimens. Fatigue life is significantly reduced in both the hot corrosive atmospheres; however, the operative mechanisms are significantly different. Type-I atmosphere leads to sporadic fluxing of protective Cr 2 O 3 layer exposing the substrate to hot corrosive media leading to grain-boundary oxidation and inter-granular failure whereas; type-II atmosphere leads to surface cracking and trans-granular failure.
Oxidation effects on the fatigue crack growth behaviour of alloy 718 at high temperature
Acta Materialia, 1997
The purpose of this study was to investigate oxidation assisted crack growth phenomena encountered in nickel-based alloys at high temperatures. Fatigue crack growth tests conducted at 650°C and under a range of oxygen partial pressures revealed the existence of a transition pressure. This pressure is in no way correlated to the loading conditions, but rather it varies with the chromium content in the alloy, and is furthermore directly linked to the oxidation mechanisms which were identified by using analytical TEM. By means of specific mechanical tests, superimposing a square wave oxygen pressure cycle to a fatigue or creep-fatigue mechanical cycle, various fundamental aspects of the local interaction between oxidation and deformation at the crack tip were investigated. Embrittlement is due partly to the nickel oxide nucleation and partly to the stress relaxation ability of the material. Chemical and microstructural modifications are recommended in order to improve the cracking resistance. Copyright @ 1997 Acta Metallurgica Inc. Rbum&Cette ttude traite de la propagation des fissures assist&e par I'environnement dans des alliages de nickel a haute temptrature. Une etude fine du couplage mCcanique-oxydation a ttC menCe. Des essais de fissuration sous diffkrentes pressions d'oxygkne montrent I'existence d'une pression de transition indtpendante des conditions de sollicitation mtcanique. Cette pression de transition dtpend de la teneur en chrome du matiriau et est directement r&&e aux mecanismes d'oxydation identifies en microscopic analytique. Des essais superposant un crCneau de pression B un cycle de chargement mCcanique en fatigue lente ou fatigue-fluage permettent de preciser le mode d'endommagement par I'oxydation. Ce dernier est IiC B la formation d'oxyde de nickel et g l'aptitude du mattriau h relaxer rapidement la contrainte. Des modifications chimiques et microstructurales de ces alliages sont prkconiskes pour amiliorer leur resistance B la fissuration.
Low Cycle Fatigue and Thermo-Mechanical Fatigue of Uncoated and Coated Nickel-Base Superalloys
2007
High strength nickel-base superalloys have been used in turbine blades for many years because of their superior performance at high temperatures. In such environments superalloys have limited oxidation and corrosion resistance and to solve this problem, protective coatings are deposited on the surface. The positive effect of coatings is based on protecting the surface zone in contact with hot gas atmosphere with a thermodynamically stable oxide layer that acts as a diffusion barrier. During service life, mechanical properties of metallic coatings can be changed due to the significant interdiffusion between substrate and coating. There are also other degradation mechanisms that affect nickel-base superalloys such as low cycle fatigue, thermo-mechanical fatigue and creep. The focus of this work is on a study of the low cycle fatigue and thermomechanical fatigue behaviour of a polycrystalline, IN792, and two single crystal nickel-base superalloys, CMSX-4 and SCB, coated with four different coatings, an overlay coating AMDRY997, a platinum aluminide modified diffusion coating RT22 and two innovative coatings with a NiW interdiffusion barrier called IC1 and IC3. An LCF and TMF device was designed and setup to simulate the service loading of turbine blades and vanes. The LCF tests were run at 500 ℃ and 900 ℃ while the TMF tests were run between 250 ℃ and 900 ℃. To simulate service life, some coated specimens were long-term aged at 1050 ℃ for 2000 h before the tests. The main conclusions are that the presence of the coatings is, in most cases, detrimental to low cycle fatigue lives of the superalloys at 500 ℃ while the coatings do improve the low cycle fatigue lives of the superalloys at 900 ℃. Under thermomechanical fatigue loading conditions, the coatings have negative effect on the lifetime of IN792. On single crystals, they are found to improve thermo-mechanical fatigue life of the superalloys, especially at lower strains. The tests also indicate that long-term aging influences the fatigue life of the coated superalloys by oxidation and diffusion mechanisms when compared to the unaged specimens. The long-term aged specimens exhibit longer life in some cases and shorter life during other test conditions. Fatigue cracks were in most cases initiated at the surface of the coatings, growing both intergranularly and transgranularly perpendicular to the load axis. vii viii This thesis has been carried out at the Division of Engineering Materials, Department of Management and Engineering at Linköping University. The research project was generously supported by grants provided by the European Community, Linköping University and the Brinell Centre at the Royal Institute of Technology in Stockholm, Sweden. I warmly thank my supervisor Professor Emeritus Torsten Ericsson, for his kind guidance throughout this research work. He has provided invaluable lessons on both technical and non-technical matters and I have spent a lot of time learning from his professionalism. I owe also my sincere thanks to Professor Sten Johansson for positive attitude towards PhD students and his tireless work in editing texts and improving English. My deepest and most thanks belong to Christian Schlauer for his help in computing and programming and his encouragement during the times when my own faith grew thin. I am very grateful to the many staff in the laboratory and
Materials
The disadvantages of widely used austenitic stainless steels are their low hardness and relatively low fatigue strength. Conventional chemical-thermal surface treatments are unsuitable for these steels since they create conditions for inter-granular corrosion. An effective alternative is a low-temperature surface treatment, creating an S-phase within the surface layer, but it has a high cost/quality ratio. Austenitic steels can increase their surface micro-hardness and fatigue strength via surface cold working. When the goal is to increase the rotating bending fatigue strength of austenitic chromium-nickel steels, and the requirements for significant wear resistance are not paramount, diamond burnishing (DB) has significant potential to increase the fatigue strength and, based on the cost/quality ratio, can successfully compete with low-temperature chemical-thermal treatments. The main objective of this study is to establish the effect of DB on the rotating fatigue strength of AISI ...
Thermomechanical fatigue behaviour of nickel base superalloy IN738LC Part 2 – Lifetime prediction
Materials Science and Technology, 2001
An experimental programme was carried out to study the thermomechanical fatigue life of the nickel base superalloy IN738LC used in gas turbines. First, out of phase and in phase thermomechanical fatigue experiments were performed on uncoated and air plasma spray coated materials. In the temperature range investigated, it was observed that deposition of a NiCrAlY coating did not affect the thermomechanical fatigue resistance. A physically based life prediction model that takes into account the contribution of different damage mechanisms was then applied. This model successfully reflected the temperature and strain rate dependences of isothermal cycling fatigue lifetimes, and the strain-temperature history effect on thermomechanical fatigue lifetimes. MST/4898
Materials and Corrosion, 2006
The effect of cycling parameter variation (i.e. oxidation temperature, upper and lower dwell time, humidity in test gas) of the oxidation/spallation kinetics on four alloys was investigated within the framework of an EC funded research project (COTEST). For this purpose, specimens of AISI 441, Alloy 800H, CM 247 and P91 were subjected to thermocyclic testing in dry or humidified air. It was found that a minimum of 300 h accumulated hot dwell time is required for meaningful test results. Detailed characterisa-tion of the corrosion products was performed using OM, SEM/EDX and XRD. The net weight change curves were evaluated with regards to the characteristic quantitative parameters describing oxide growth rate, time to onset of spallation/breakaway and weight of spalled oxide. Analysis of these values was made by the ANOVA method, which allows assessment of the significance of the roles of different test parameters and parameter variations on the oxidation/ spallation kinetics from a limited number of experiments.