Low Cycle Fatigue and Thermo-Mechanical Fatigue of Uncoated and Coated Nickel-Base Superalloys
Svjetlana Stekovic
Division of Engineering Materials, Department of Management and Engineering, Institute of Technology, Linkoping University
Abstract
High strength nickel-base superalloys have been used in turbine blades for many years because of their superior performance at high temperatures. In such en- vironments 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 se...Morervice 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 thermo- mechanical 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 set-up 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 2000h 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 thermo- mechanical 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.
keywords:Aero engine; AMDRY995; AMDRY997; CN91; CMSX-4; Coatings; crack initiation; diffusion barrier; gas turbine; IC1; IC3; IN792; innovative coatings; LCO22; long-term aging; microstructure; nickel-base superalloy; polycrystalline superalloy; RT22; SCB; single crystal
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