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Crack tips on microscopy of oxidation

Posted on 21. October, 2011.

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Time-dependent intergranular crack-growth in Ni-base superalloys is well known, however, there is a lack of understanding of the detailed effects of oxygen interaction in the crack tips.  The methodology of accessing and analysing this region is described in Materials at High Temperatures 28(4) which highlights some of the issues arising from  Microscopy of Oxidation 8.

Oxygen influenced intergranular crack propagation: analyzing microstructure and chemistry in the crack tip region

L. Viskaria*, S. Johanssonb and K. Stillera

aChalmers University of Technology, SE-41296, Göteborg, Sweden
bLinköping University, SE-58183, Linköping, Sweden
Doi: 10.3184/096034011X13189599518971

Ni-base superalloys have for decades been studied in the aspect of environmentally influenced intergranular crack propagation. An oxygen-rich environment promotes time-dependent crack growth. Oxygen interaction at the crack tip is pointed out as the reason for the degraded mechanical properties. While many aspects of this type of crack growth have been previously investigated there is still no consensus about the detailed mechanisms, much due to the lack of in-detail investigations of the crack-tip region.

Here, crack tip regions in the Ni-base superalloy Alloy 718 were studied. Specimens were subjected to 90 s hold-times at 550 °C and 650 °C. Crack growth was arrested before final fracture, allowing cross-sectional analyses of the crack-tip region using scanning electron microscopy (SEM). Detailed studies of the crack-tip region were performed using transmission electron microscopy (TEM) and atom probe tomography (APT). For both APT and TEM samples, site-specific focussed ion beam (FIB) sample preparation was performed in a combined FIB-SEM system. The methodology of accessing and analysing the crack tip region is shown. Initial results on oxidation, oxygen penetration and plastic deformation are shown and discussed.

The work shown describes the methodology of accessing specific regions of high-temperature crack tips. Through these methods, two types of crack tips were found: blunt and sharp. For the blunt crack tips regions plastic deformation was the main feature while for the sharp crack tips oxidation was far more pronounced. For the latter, a four-zone characteristic was deduced.  A layered oxide structure (with a Cr-rich oxide near the matrix and a Ni/Fe-rich oxide far from the matrix) was observed ahead of the open crack. Further ahead, the layered oxide zone was found to transform into a non-layered (presumably) Cr-rich oxide with islands of unknown nature. No oxygen segregation was observed to a grain boundary immediately ahead of an oxide tip.

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