Local Deformation in Welded Superalloys with Microstructural Gradients

The next generation of high-efficiency power plants requires superalloy components capable of withstanding 750°C / 100 MPa for 100,000 hours. Although polycrystalline Inconel 740H base metal exhibits the necessary creep strength, welded metal fails prematurely at precipitate-free zones along grain boundaries. This results from local strain accumulation due to small-scale variations in the structure and chemistry of the material. Despite the critical role of these heterogeneities in determining the behavior of a part under loading, their local effects are not captured by traditional, average measures of strain across the sample during testing. Measurement of local deformation is essential to accurately modeling and predicting the long-term creep behavior of heterogeneous materials at high temperatures. 

In this work, local strain is measured by digital image correlation (DIC). 200 nm hafnia speckles are applied to the gauge section of base and weld metal tensile creep samples via e-beam lithography. By acquiring images of these speckle patterns in a scanning electron microscope (SEM) during deformation, digital image correlation can be employed to map the local strain response of a material with spatial resolution of ~1 μm. This produces full-field strain maps that are correlated with the orientation of the underlying crystal via electron backscattered diffraction. Regions of high strain localization are also examined by removing foils via focused ion beam for site-specific microstructure analysis. By examining foils with transmission electron microscopy, the active creep deformation mechanisms in the fasteststraining regions are identified. In contrast to previous high-temperature SEM-DIC testing, these experiments also demonstrate the development of appropriate ex situ testing procedures that enable longer tests spanning weeks or months. This is accomplished by using a calibration speckle pattern to correct for day-to-day variations in the SEM, and by testing samples in a reducing atmosphere to prevent surface oxidation. Three custom variants of IN740H are used to separately examine the effects of gradients in grain size/morphology, chemical composition, and precipitation. 

Custom software was used to simulate pseudo-random speckle patterns for DIC optimization. For an image resolution of 60 nm/pixel, the optimal hafnia speckle size is 200 nm with an average spacing of 600 nm. The optimized patterns were subsequently fabricated using e-beam lithography and subjected to high temperature testing in an inert atmosphere. Custom software was also successfully implemented to correct day-to-day SEM image variations that impede accurate image correlation. Characterization of IN740H and the three variant alloys with electron backscatter diffraction and energy dispersive x-ray spectroscopy confirmed the presence of the desired microstructural gradients at the welds. Creep data, including full-field strain maps, are forthcoming.

These experiments directly link microstructure to local deformation behavior and elucidate the rate dependencies of active creep mechanisms. This information will directly inform strategies for creep mitigation and modeling. The ex situ techniques developed here are also applicable to performing high-temperature, long-term DIC in other materials such as superalloys or ceramic matrix composites.

Industry Sponsor: NSF

Faculty: Michael Mills (OSU)

Graduate Student: Connor Slone