Fatigue Behavior of Precipitation Hardening Alloys in the LCF and VHCF Regime

Abstract:

LCF/HCF strength of precipitation hardening alloys is primarily controlled by its heat treatment condition. However, for a nickel-based superalloy and a wrought aluminium alloy it will be shown, that the VHCF behavior cannot solely be explained by the precipitation morphology. Damage accumulation is dominated by microstructure related slip localization, grain morphology and microstructural flaws. In contrast to the LCF behavior, the prediction of cyclic strength in the VHCF regime requires a detailed analysis of the competing microstructural crack initiating characteristics. Hence, new fatigue life prediction models have to be developed, which consider a statistical analysis of the failure-relevant inhomogeneities. In the case of the two materials studied, VHCF behavior is dominated by isolated and inhomogeneously distributed irreversible slip accompanied by a low dislocation density. The formation of single slip bands in favorably oriented grains in Nimonic 80A results in a decrease of the VHCF strength for the peak-aged condition. The overaged condition shows better VHCF strength due to a more homogeneous distribution of slip bands, as dislocations pile up at the overaged precipitates due to the very low strain amplitudes, while in the LCF regime the Orowan mechanism results in a weaker cyclic strength compared to the peak-aged condition. Crack initiation of the aluminium alloy EN AW-6082 on the one hand depends on the size and distribution of primary intermetallic particles of the Al-Fe-type acting as local stress raiser embedded in a strong matrix in case of the peak-aged condition. In contrast, such stress peaks are less failure-relevant due to the softer solid solution depleted matrix. Hence, the heat treatment alone does not define VHCF behavior.

• Periodical: Advanced Materials Research (Volumes 891-892)
• Pages: 476-481
• DOI: https://doi.org/10.4028/www.scientific.net/AMR.891-892.476
• Online since: March 2014
• Authors: Martina Zimmermann, Christian Stoecker, Martin Cremer, Anton Kolyshkin, Hans-Jürgen Christ
• Keywords: Aluminium Alloy, Damage Mechanism, Nickel-Based Alloys, Precipitation Hardening, Very High Cycle Fatigue (VHCF)

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