Steve D. Sharples, Wenqi Li, Richard Smith, Matt Clark and Mike Somekh

Applied Optics Group, Electrical Systems & Optics Research

DivisionFaculty of Engineering, University of Nottingham.

AFPAC, January 2011

Orientation imaging using spatially resolved acoustic spectroscopy (SRAS)

What is SRAS?EBSD image courtesy of University of Wales, Swansea SRAS surface acoustic wave velocity image

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

f-SRAS: taking a velocity measurement

A few nice pictures…

Austenitic stainless steel weld L-R

Austenitic stainless steel weld U-D

Example images showing the capabilities of SRAS:

Scalability from large to small (titanium alloy)

Resolution: 400μmResolution: 400μm

10mm

Resolution: 25μmResolution: 25μmResolution: 400μmResolution: 400μm

84mm 700μm

Resolution: 25μmResolution: 25μm

ms-1

108μm

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) 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)

Ti-6Al-4V

170x80mm

25x250μm pixel size

2.2 megapixels

48 minutes scan time

>750 points/sec

Example images from new instrument (2)

(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

Define elastic constants, and multiply

by rotation matrix

Define propagation direction l1, l2 and

velocities

substitute into |jk-jkv2| = 0

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

plot the curve of |d mn |= |cm3klk(n)ll(n) | vs.

velocities

choose the minima of |d mn | to determine velocities

calculate the out of plane displacement of velocities

l1, l2 = propagation direction

= density

V = phase velocity

C = stiffness tensors

jk = lillcijkl

d mn = determinant of |jk-jkv2|

3 = eigenvectors of displacement

First the forward problem for cubic Nickel

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

Propagation in multiple directions – single crystal Ni

Fit analytic curves to data to get 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

Propagation in multiple directions – single crystal Ni

Orientation imaging on nickel

Supposedly “single crystal” nickel, actually consists of two large grains

SAW velocity left-right

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

Steve SharplesWenqi LiRichard Smith

RCNDEEMDARR AeroenginesEPSRC

University of Wales (Swansea)

For more information or if you have an interesting sample, please email:steve.sharples@nottingham.ac.uk