Comparison of grain to grain orientation and stiffness mapping by spatially resolved acoustic spectroscopy and EBSD

What is it about?

Our aim was to establish the capability of spatially resolved acoustic spectroscopy (SRAS) to map grain orientations and the anisotropy in stiffness at the sub-mm to micron scale by comparing the method with electron backscatter diffraction (EBSD) undertaken within a scanning electron microscope. In the former the grain orientations are deduced by measuring the spatial variation in elastic modulus; conversely, in EBSD the elastic anisotropy is deduced from direct measurements of the crystal orientations. The two test-cases comprise mapping the fusion zones for large TIG and MMA welds in thick power plant austenitic and ferritic steels, respectively; these are technologically important because, among other things, elastic anisotropy can cause ultrasonic weld inspection methods to become inaccurate because it causes bending in the paths of sound waves. The spatial resolution of SRAS is not as good as that for EBSD (∼100 μm vs. ∼a few nm), nor is the angular resolution (∼1.5° vs. ∼0.5°). However the method can be applied to much larger areas (currently on the order of 300 mm square), is much faster (∼5 times), is cheaper and easier to perform, and it could be undertaken on the manufacturing floor. Given these advantages, particularly to industrial users, and the on-going improvements to the method, SRAS has the potential to become a standard method for orientation mapping, particularly in cases where the elastic anisotropy is important over macroscopic/component length scales. Although for many engineering applications it is acceptable to assume that materials are homogeneous at the macroscale, in reality materials are highly anisotropic at the micron scale. Metallic materials generally comprise many small crystalline regions, called grains, each differently oriented and having different properties (such as stiffness) in different directions. In this paper we compare two methods for determining the polycrystalline microstructure of materials; one highly novel (SRAS) and the other a benchmark standard (EBSD). The current standard method for determining the orientation of the grains in a material is electron backscatter diffraction (EBSD). In EBSD a beam of electrons is scanned across a material surface. This interacts with the material and the diffracted beam is detected. The characteristics of the diffracted beam reveal details of the orientation of the crystal beneath the beam. In the new method, spatially resolved acoustic spectroscopy (SRAS), a laser directed through a grating at the material excites elastic waves in the surface, which are detected nearby. The characteristics of the waves are related to the stiffness of the material in different directions. In both methods the beam/laser is scanned across the material surface to collect information at hundreds of points, which make up a map of the material orientation across the surface. One reason to collect such maps is for the correction of ultrasonic flaw detection signals from inspection of large welds, for example. In ultrasonic inspection, sound waves are sent through the weld and, by measuring their time-of-flight and intensity upon exit, variations in their path caused by flaws can be detected. Models of the paths are created and fit to the data to accurately locate the flaws. Problems arise when inconsistencies in the ‘solid’ (unflawed) material cause path changes that confuse the location of the flaw. Variations in the stiffness of the material in the path direction due to the variations in orientation of the structure, coupled with different stiffnesses for different directions in the structure, cause such problems. In these cases, stiffness maps of the weld cross-sections, determined from orientation maps and structure stiffness data, are incorporated into the fitting models. The two samples we characterized were sections cut through thick, multipass welds in power-plant steels. We determined that SRAS is a good method for measuring the maps required to correct ultrasonic flaw detection signals. EBSD has better spatial and angular resolution, but SRAS is faster, requires less preparation of the material surface and can accept larger and more irregularly shaped samples; also, the resolution of SRAS is easily sufficient. There is excellent potential for SRAS to become a standard inspection technique.

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