The evolution of grain-boundary cracking evaluated through in situ tensile-creep testing of Udimet alloy 188
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M. Nowell and S. Wright EDAX-TSL, Inc., Draper, Utah 84020 (Received 27 May 2007; accepted 6 November 2007)
In situ scanning electron microscopy was performed during elevated-temperature (艋760 °C) tensile-creep deformation of a face-centered-cubic cobalt-based Udimet 188 alloy to characterize the deformation evolution and, in particular, the grain boundary-cracking evolution. In situ electron backscatter diffraction observations combined with in situ secondary electron imaging indicated that general high-angle grain boundaries were more susceptible to cracking than low-angle grain boundaries and coincident site-lattice boundaries. The extent of general high-angle grain-boundary cracking increased with increasing creep time. Grain-boundary cracking was also observed throughout subsurface locations as observed for postdeformed samples. The electron backscattered diffraction orientation mapping performed during in situ tensile-creep deformation proved to be a powerful means for characterizing the surface deformation evolution and in particular for quantifying the types of grain boundaries that preferentially cracked.
I. INTRODUCTION
Recent progress in the development of in situ scanning electron microscopy (SEM) techniques for understanding the deformation behavior of materials, combined with the high-beam current and stability of a hot Schottkey field emission gun (FEG) and the latest camera technology for rapidly obtaining high-quality electron backscattered diffraction Kikuchi patterns (EBSPs), now allows for a unique capability for imaging surface deformation. It is possible to make unprecedented submicron resolved measurements of the local two-dimensional crystalstructure distribution in materials during high-temperature tensile-creep deformation. This article describes the technique that has been developed to acquire secondary electron (SE) detector images and electron backscattered diffraction (EBSD) orientation maps in situ during tensilecreep deformation at temperatures as high as 760 °C. Udimet 188 (also known as Haynes Alloy No. 188) is a cobalt–nickel–chromium–tungsten commercially available alloy with good creep strength and oxidation resistance up to 1093 °C. It has exhibited good fabricability, tensile strength and elongation-to-failure, weld-
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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2008.0058 500
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J. Mater. Res., Vol. 23, No. 2, Feb 2008 Downloaded: 16 Mar 2015
ability, low-cycle fatigue resistance, and corrosion resistance.1–6 It has useful applications in gas turbines, combustors, flame holders, liners, and transition ducts. A complete understanding of the physical mechanisms responsible for the elevated-temperature creep behavior and associated microstructure–property relationships of Udimet 188 is lacking. Previous data have suggested that a high fraction of low-angle boundaries (LABs) and coincident site lattice boundaries (CSLBs) significantly enhances resistance to creep and grain-boundary sliding at elevated
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