1. Steve D. Sharples, Wenqi Li, Richard Smith, Matt Clark and Mike Somekh Applied Optics Group, Electrical Systems & Optics Research Division Faculty of Engineering, University of Nottingham. AFPAC, January 2011 Orientation imaging using spatially resolved acoustic spectroscopy (SRAS)

2. What is SRAS? Spatially Resolved Acoustic Spectroscopy A Laser ultrasound technique for imaging microstructure** Uses SAW velocity as contrast - varies with grain orientation - varies with SAW direction Produces nice images of grains and tells us how they are orientated Tells us all sorts of nice information about the sample microstructure EBSD image courtesy of University of Wales, SwanseaSRAS surface acoustic wave velocity image

3. f-SRAS: frequency spectrum SRAS Excite with short (ns) laser pulses projected through optical grating. The grating generates narrowband SAWs. Only one wavelength, λ (the grating period). Detect the SAWs with a broadband optical detector. Measure the frequency on a scope. Use v = f λ to get the velocity The patch under the grating is the patch which is measured

4. f-SRAS: taking a velocity measurement

5. A few nice pictures…

6. Austenitic stainless steel weld L-R

7. Austenitic stainless steel weld U-D

8. Example images showing the capabilities of SRAS: Scalability from large to small (titanium alloy) Resolution: 400μm 10mm Resolution: 25μm Resolution: 400μm 84mm700μm Resolution: 25μm ms -1 108μm

9. What’s new since last AFPAC? 1. Instrumentation  A dedicated SRAS microscope  Smaller, much faster, cheaper, simpler  Will have ability to scan on “rough surfaces” next month!  Higher spatial resolution 2. Determination of orientation from SAW velocities  cubic crystals (e.g. nickel, aluminium)

10. (1) 3 rd generation SRAS instrument New dedicated SRAS system funded by emda (East Midlands Development Agency). Completion due April 2011. Smaller, faster, more capable

11. Example images from new instrument (1) Ti-6Al-4V 170x80mm 25x250μm pixel size 2.2 megapixels 48 minutes scan time >750 points/sec

12. Example images from new instrument (2)

13. (2) From “contrast” to orientation measurement The velocity depends on the crystallographic orientation Ok to go from orientation to velocity (forward) Trickier to invert this problem So… Solve the forward problem v=f( orientation ) Fit the data to the forward problem to find the orientation

14. Forward model: calculating SAW velocities from known orientation and known elastic constants Define elastic constants, and multiply by rotation matrix Define propagation direction l 1, l 2 and velocities substitute into |  jk -  jk  v 2 | = 0 choose the 3 lower half plane roots of l 3 and its  3 plot the curve of |d mn |= |c m3kl  k (n) l l (n) | vs. velocities choose the minima of |d mn | to determine velocities calculate the out of plane displacement of velocities l 1, l 2 = propagation direction  = density V = phase velocity C = stiffness tensors  jk = l i l l c ijkl d mn = determinant of |  jk -  jk  v 2 |  3 = eigenvectors of displacement

15. First the forward problem for cubic Nickel

16. SAW velocity as a function of orientation: cubic crystal: Nickel

17. Propagation in multiple directions – single crystal Ni Fit analytic curves to data to get orientation

18. Getting the orientation… Analytically calculated velocity as a function of orientation + Measure velocity as a function of propagation direction on surface + Simple fitting algorithm = Orientation of the crystals

19. Propagation in multiple directions – single crystal Ni

20. Orientation imaging on nickel Supposedly “single crystal” nickel, actually consists of two large grains SAW velocity left-right

21. SRAS: Conclusions SRAS is faster and fancier than ever before! We got a nice new machine thanks to EMDA It will have optically rough surface capability shortly We can go from measurement to orientation Next: More forward modelling Slicker fitting Strategies for speed vs information Higher resolution

22. Acknowledgements Steve Sharples Wenqi Li Richard Smith RCNDE EMDA RR Aeroengines EPSRC University of Wales (Swansea) For more information or if you have an interesting sample, please email: steve.sharples@nottingham.ac.uk

23. Propagation in multiple directions – single crystal Al Offset on each of the planes:  (100) plane: 80 degrees  (110) plane: 28 degrees  (111) plane: 19 degrees Pseudo-surface wave detected in preference to generalised SAW  We can account for this in our curve-fitting

24. Propagation in multiple directions – single crystal Al Offset on each of the planes:  (100) plane: 80 degrees  (110) plane: 28 degrees  (111) plane: 19 degrees Pseudo-surface wave detected in preference to generalised SAW  We can account for this in our curve-fitting