MATEC Web of Conferences
Volume 14, 2014EUROSUPERALLOYS 2014 – 2nd European Symposium on Superalloys and their Applications
|Number of page(s)||6|
|Section||Session 10: Mechanical Behavior III: Polycrystalline Alloys|
|Published online||29 August 2014|
Thresholds of time dependent intergranular crack growth in a nickel disc alloy Alloy 720Li
1 School of Metallurgy and Materials, The University of Birmingham, Birmingham B15 2TT, UK
2 Rolls-Royce plc., PO box 31, Derby DE24 8BJ, UK
a Corresponding author: email@example.com
At high temperatures in air, introducing a dwell period at the peak stress of fatigue cycles promotes time dependent intergranular crack growth which can increase crack growth rates by upto a few orders of magnitude from the rates of transgranular fatigue crack growth in superalloys. It is expected that time dependent intergranular crack growth in nickel-based superalloys may not occur below a critical mechanical driving force, ΔKth−IG, analogous to a fatigue threshold (ΔKth) and a critical temperature, Tth. In this study, dwell fatigue crack growth tests have been carefully designed and conducted on Alloy 720Li to examine such thresholds. Unlike a fatigue threshold, the threshold stress intensity factor range for intergranular crack growth is observed to be highly sensitive to microstructure, dwell time and test procedure. The near threshold crack growth behaviour is made complex by the interactions between grain boundary oxidation embrittlement and crack tip stress relaxation. In general, lower ΔKth−IG values are associated with finer grain size and/or shorter dwell times. Often a load increasing procedure promotes stress relaxation and tends to lead to higher ΔKth−IG. When there is limited stress relaxation at the crack tip, similar ΔKth−IG values are measured with load increasing and load shedding procedures. They are generally higher than the fatigue threshold (ΔKth) despite faster crack growth rates (da/dN) in the stable crack growth regime. Time dependent intergranular crack growth cannot be activated below a temperature of 500 ∘C.
© Owned by the authors, published by EDP Sciences, 2014
This is an Open Access article distributed under the terms of the Creative Commons Attribution License 4.0, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
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