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

Abstract

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 versus ~a few nm), nor is the angular resolution (~1.5 degrees versus ~0.5 degrees). 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.

Imaging Material Texture of As-Deposited Selective Laser Melted Parts Using Spatially Resolved Acoustic Spectroscopy

“…Practically, the process of directly determining the orientation (the inverse problem) is difficult to solve, so a fitting algorithm is used instead [12]. The outcome of the implemented algorithm can be used to create inverse pole figures-there have been a number of past publications comparing obtained crystallographic grain orientation information using the SRAS technique against electron backscatter diffraction (EBSD) [13][14][15][16][17]. If the interrogation area is larger than the crystallographic grain size, then the predominant velocity in the area is measured-the material texture is imaged instead [9,10].…”

“…To obtain the SAW phase velocities, the mask is rotated in order to propagate the generated surface waves in different directions-the detection laser also follows with the rotating pattern. Using the described generation and detection lasers, this setup has been used to image the microstructure of titanium, nickel, steel and aluminium based alloys [13][14][15]20], as well as non-metallic materials such as polycrystalline silicon samples (once a metallic coating is applied) [16].…”

Destructive and non-destructive testing methods for characterization and detection of machining-induced white layer: A review paper

“…SRAS scans in multiple orientations, when combined, give comparable information to EBSD scans. This is at high scan rates of 1000 points per second and without the need to section all samples due to larger size constraints, although the spatial resolution is inferior to EBSD (several nm) and currently SRAS samples must be polished smooth (Mark et al, 2017). Although the capability of the current class of equipment is limited by sample roughness, techniques to circumvent this issue are under investigation (Sharples et al, 2014).…”

How does grazing incidence ultrasonic microscopy work? A study based on grain-scale numerical simulations

“…A very attractive alternative to obtain grain structure information by acoustic waves is called spatially resolved acoustic spectroscopy (SRAS) [8][9][10][11]. Thereby a laser beam structured as a fringe grating excites Rayleigh waves which are picked up optically in the vicinity.…”