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Orientation imaging using spatially resolved acoustic spectroscopy (SRAS). 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.

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Orientation imaging using spatially resolved acoustic spectroscopy (SRAS)

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

what is sras
What is SRAS?

EBSD image courtesy of University of Wales, Swansea

SRAS surface acoustic wave velocity image

  • 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
f sras frequency spectrum sras
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 = fλto get the velocity
  • The patch under the grating is the patch which is measured
example images showing the capabilities of sras scalability from large to small titanium alloy

Resolution: 25μm

700μm

10mm

Resolution: 25μm

Resolution: 400μm

Example images showing the capabilities of SRAS:Scalability from large to small (titanium alloy)

84mm

Resolution: 400μm

ms-1

108μm

what s new since last afpac
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)
1 3 rd generation sras instrument
(1) 3rd generation SRAS instrument
  • New dedicated SRAS system funded by emda (East Midlands Development Agency).
  • Completion due April 2011.
  • Smaller, faster, more capable
example images from new instrument 1
Example images from new instrument (1)
  • Ti-6Al-4V
  • 170x80mm
  • 25x250μm pixel size
  • 2.2 megapixels
  • 48 minutes scan time
  • >750 points/sec
2 from contrast to orientation measurement
(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
forward model calculating saw velocities from known orientation and known elastic constants
Forward model: calculating SAW velocities from known orientation and known elastic constants

Define propagation direction l1, l2 and velocities

Define elastic constants, and multiply by rotation matrix

l1, l2 = propagation direction

 = density

V = phase velocity

C = stiffness tensors

jk = lillcijkl

d mn = determinant of |jk-jkv2|

3 = eigenvectors of displacement

substitute into |jk-jkv2| = 0

choose the 3 lower half plane roots of l3 and its 3

plot the curve of |d mn |= |cm3klk(n)ll(n) | vs. velocities

choose the minima of |d mn | to determine velocities

calculate the out of plane displacement of velocities

propagation in multiple directions single crystal ni
Propagation in multiple directions – single crystal Ni

Fit analytic curves to data to get orientation

getting the orientation
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

orientation imaging on nickel
Orientation imaging on nickel
  • Supposedly “single crystal” nickel, actually consists of two large grains

SAW velocity left-right

sras conclusions
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
acknowledgements
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