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Strain controlled fatigue and fracture behaviour of uncoated and coated polycrystalline and single crystal nickel-base superalloys at elevated temperature
Linköping University, Department of Mechanical Engineering, Engineering Materials. Linköping University, The Institute of Technology.
(English)Manuscript (preprint) (Other academic)
Abstract [en]

In this paper, the low-cycle fatigue life and mechanisms governing the fracture behaviour of coated nickel-base superalloys are presented and discussed. Cylindrical solid specimens were cyclically deformed with fully reversed tension-compression loading total strain amplitude control at two elevated temperatures and a constant strain rate of 10-4 s-1 (6%/ min) in air atmosphere without any dwell time. Three nickel-base superalloys, IN792, CMSX-4 and SCB, were coated with three different coatings: an overlay coating AMDRY997, a diffusion coating RT22 and an innovative coating ICl. The cyclic stress response, low-cycle fatigue (LCF) life and final fracture behaviour at the two temperatures are observed and compared.

At 500oC the coatings reduced fatigue life relative to the uncoated specimens while at 900oC the coated specimens showed longer life at all strain ranges than the uncoated specimens except RT22 under certain test conditions. The decrease in the fatigue life was caused by brittle coating cracking under their ductile to brittle transition temperature (DBTT). Over DBTT, lower yield strength of the coated superalloys with subsequent increase in ductility could cause the improvement of the fatigue life. These cracks could be also slowed by oxidation on front of the crack tip.

All uncoated and coated superalloys exhibit hardening and higher stress levels at higher applied strain amplitudes and at 500°C. At 900oC softening occurred together with lower stress response level. The coatings lowered the stress level response of the superalloys from about 12% to 31 %. Higher hardening was observed for polycrystalline IN792 caused by dislocation pileups at the the grain boundaries.

Most of the observed cracks initiated at the coating surface and majority was arrested in the transition zone except for IN792 where internal pores served as initiation sites for most cracks. Some improvement in the fatigue life have also been seen in coated IN792. No cracks found initiated from TCP phases were found. Cracks initially grew more or less perpendicular to the load axis in Stage II manner. Crack propagation path in IN792 is governed by grain or dendrite boundaries while in single crystals crack growth path is determined by concentration of deformation and damage in γ and γ' phases. Surface roughness or rumpling was found in the overlay coating AMDRY997 with some cracks initiated from the rumples maybe due to cyclic straining and not thermal cycling.

National Category
Engineering and Technology
URN: urn:nbn:se:liu:diva-100230OAI: diva2:661041
Available from: 2013-10-31 Created: 2013-10-31 Last updated: 2013-10-31
In thesis
1. Isothermal low cycle fatigue of uncoated and coated nickel-base superalloys
Open this publication in new window or tab >>Isothermal low cycle fatigue of uncoated and coated nickel-base superalloys
2004 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

High strength nickel-base superalloys have been used in turbine blades for many years because of their superior performance at high temperatures. However, the superalloys have limited oxidation and corrosion resistance and to solve this problem, protective coatings are deposited on the surface of the superalloys.

The positive effect of coatings is based on protecting the surface zone in contact with hot gas atmosphere with elements like aluminium, chromium, which form a thermodynamically stable oxide layer that acts as a diffusion barrier to slow down the react ion between the substrate material and the aggressive environment. There are also other degradation mechanisms that affect nickel-base superalloys such as aging of microstructure, fatigue and creep. Long-term aging in metallic coating results in the changes of mechanical properties due to the significant interdiffusion of the main alloying elements between substrate and coatings. However, application of the coatings has mechanical side effects, the significance of which is not yet fully investigated.

This work covers a study on the fatigue behaviour of a polycrystalline, IN792, and two single crystal nickel-base superalloys, CMSX-4 and SCB, coated with three different coatings, an overlay coating AMDRY997, a platinum aluminide modified diffusion coating RT22 and an innovative coating with an interdiffusion barrier of NiW called ICl , under low cycle fatigue loading conditions. Both low cycle fatigue properties, cyclic strain and stress response and fracture behaviour of the uncoated, coated and long-term aged coated specimens are presented.

The main conclusions are that at 500oC the presence of the coatings have, in most cases, reduced the fatigue lives of the nickel-base substrates while at 900oC the coatings do improve the fatigue lives of the superalloys except RT22 coated on some superalloys and under certain test conditions. The reduction of the fatigue life at 500oC can be related to early cracking of the coatings below their ductile to brittle transition temperature (DBTT), where their surface roughness can act as notches affecting fatigue crack initiation. The beneficial effect of the coating at 900oC may be due to slower crack propagation caused by oxidation at the front of the crack tip. The tests also indicate that long-term aging influences the fatigue and fracture behaviour of coated superalloys by oxidation and diffusion mechanisms when compared to non-aged and uncoated samples. The aged samples 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 intergranularly perpendicular to the load axis.

30 p.
Linköping Studies in Science and Technology. Thesis, ISSN 0280-7971 ; 1120
National Category
Engineering and Technology
urn:nbn:se:liu:diva-22795 (URN)2128 (Local ID)91-85295-58-2 (ISBN)2128 (Archive number)2128 (OAI)
Available from: 2009-10-07 Created: 2009-10-07 Last updated: 2013-10-31

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