ACOUSTIC MICROSCOPY TECHNIQUES FOR OBSERVING DISLOCATION DAMPING

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ACOUSTIC MICROSCOPY TECHNIQUES FOR

OBSERVING DISLOCATION DAMPING

J. Weaver, G. Briggs

To cite this version:

JOURNAL DE PHYSIQUE

Colloque C10, suppldment au n012, Tome 46, ddcembre 1985 page C10-743

ACOUSTIC MICROSCOPY TECHNIQUES FOR OBSERVING DISLOCATION DAMPING

J.M.R. WEAVER AND G.A.D. BRIGGS

University of Oxford, Department of Metallurgy and Science of Materials, Oxford OX1 3PH, U.K.

Abstract

-

A unique advantage of scanning acoustic microscopy lies in the ability to image the interaction of acoustic waves with a specimen This can be made quantitative, so that the velocity and attenuation of surface waves can be measured over a few microns. This technique can be applied to the detection of dislocation damping in the acoustic microscope.

The principle of acoustic microscopy is that, since the wavelength of any radiation is inversely proportional to its wavelength, by using a sufficiently high frequency one can obta'n an arbitrarily short wavelength [I]. The velocity of sound in water

t

is 1500 m s-

,

and so at 3 GHz the wavelength is comparable with the wavelength of light. Since the resolution of a microsoope is determined by the wavelength of the radiation used, it is thus possible to obtain acoustic images with resolution comparable to those of a good light microscope. This is done by using a plane ZnO transducer

to

generate the acoustic waves

on

one surface of a sapphire disk, and a spherical surface ground in the other side to bring the acoustic waves to a focus on the axis of the lens in a coupling medium such as water. The refractive index is very high in this situation, and so the aberrations are small, generally less than a tenth of a wavelength. In the reflection acoustic microscope, pulsed acoustic waves are used, they are reflected by the specimen back through the lens to the transducer, and seperated from the tranmitted pulse by time-gating. To obtain an image, the lens is moved parallel to the specimen surface in a raster,

and

in this way a S C ~ Mimage is ~ built up. For a further introduction to scanning acoustic microscopy see reference [ 2 ] .

The scanning stage of the acoustic microscope at M o r d University is shown in figure 1. The lens itself is housed in the brass cylinder in the centre of the photograph This is mounted on a pair of leaf springs so that it can vibrate from left to right to provide the fast scan, and it can move fore and aft on a linear track, driven by the micrometer at the left. There is provision for levelling the specimen and for scanning it vertically, and the acoustic lens

can

be moved aside

to

allow an optical lens turret to be inserted.

Acoustic microscopy is unique in its ability to image directly the interaction of acoustic waves with the elastic properties of a specimen. This is illustrated by

C10-744 JOURNAL DE PHYSIQUE

Figure 1

- The scanning stage of an acoustic microscope,

images of g r a i n s t r u c t u r e i n s t a i n l e s s s t e e l i n f i g u r e 2. This specimen was polished but not etched, and t h e c o n t r a s t a r i s e s from t h e v a r i a t i o n s of e l a s t i c properties from grain to grain. This contrast must be interpreted carefully; one o n n o t simply say that a bright region i n an acoustic micrograph corresponds t o a region of high o r low modulus or density. Rather, it is necessary to understand how t h e c o n t r a s t v a r i e s with defocus, i.e. a s t h e specimen 'is moved towards t h e l e n s r e l a t i v e t o t h e f o c a l plane. The images i n f i g u r e 1 were taken a t two d i f f e r e n t values of defocus, and a s a r e s u l t t h e c o n t r a s t i s q u i t e d i f f e r e n t i n t h e two p i c t u r e s although they a r e of t h e same area of t h e specimen; indeed complete r e v e r s a l of c o n t r a s t has occured. This v a r i a t i o n of t h e transducer voltage V

responsible for the image brightness with the specimen defocus z has become known as V[zl, and has been extensively studied both t h e o r e t i c a l l y and experimentally [3,4,5]. A dominant r o l e i n t h e behaviour of V[z] a t negative defocus [i.e. with the specimen closer to the

lens

than the focal plane] is played by surface acoustic waves i n t h e specimen, of which t h e b e s t known i s t h e Rayleigh wave, which can propagate i n t h e surf ace of an i s o t r o p i c half-space. The a c o u s t i c microscope i s s e n s i t i v e t o anything t h a t a f f e c t s t h e propagation of t h e s e surface a c o u s t i c acoustic waves, and t h i s can include factors affecting the velocity, such a s changes i n t h e m a t e r i a l p r o p e r t i e s o r t h e a d d i t i o n of a surface coating, o r f e a t u r e s t h a t scatter them, such as surface cracks, o r attenuation mechanisms. This is discussed

in some d e t a i l in reference [ 2 1.

I11

-

'I¶iE EsFEa! OF

lnsmcmIcN

DAMPING cN V[z]

The attenuation mechanisms t h a t affect V[z] may be either e l a s t i c mechanisms, such as grain boundary scattering [6], o r loss mechanisms, such a s dislocation damping

JOURNAL DE PHYSIQUE (30-745

rgpb-

Figure 2

-

Acoustic micrographs of a s t a i n l e s s s t e e l specimen, obtained with a d e f m s of [ a ] 4 m i c r o n s , [bl 10 microns. The t o t a l width of each image corresponds to 0.23 m.

later. All these measurements w e r e made a t 40°C The effect of the deformation was to change the form of the V[z] curve, but a f t e r ten minutes o r so t h i s e f f e c t had to a large extent relaxed. The change in V[z] is a t t r i h t e d t o the attenuation due t o dislocation damping. This is confirmed by the subsequent relaxation of the effect, which is a t t r i b u t e d t o p o i n t d e f e c t s migrating t o t h e d i s l o c a t i o n s , and thereby quenching t h e i r vibrations [8]; it is hard t o see how any gross deformation of the surface geometry could relax i n this way. Comparison of figures 3[al and 3[bl shows how an image of a zone of plastic deformation, f o r example a t the t i p of a stressed crack, could be obtained i n refelction in the acoustic microscope. I f t h e specimen were a t focus l i t t l e c o n t r a s t would be seen; t h i s is t r u e of most p r o p e r t i e s of i n t e r e s t i n scanning a c o u s t i c microscopy. I f it were defocused t o t h e f i r s t shoulder on t h e V[zl curve, a t about -5 microns, then a region of p l a s t i c deformation would show up a s bright i n an acoustic image. On the other hand i f the specimen were moved t o z = -1 2 microns, corresponding t o t h e next peak, then t h e same region would appear dark.

C10-746 JOURNAL DE PHYSIQUE

Considerable progress has been made i n the quantitative analysis of V[zl [5]. This

has been developed most accurately using a c y l i d r i c a l , o r line-focus, lens, which has t h e advantage t h a t t h e velocities of surface acoustic waves can be measured in d i f f e r e n t progagation directions on anisotropic surfaces, but t h e same methcd may be

agplied t o Vlz] measured with a spherical imaging lens. The experimental technique i s t o measure V[zl extremely a c c u r a t e l y on a surface, and record t h e r e s u l t s d i g i t a l l y . The measured V[z] is then c o r r e c t e d f o r n o n - l i n e a r i t i e s i n t h e electronic circuitry, and interference due t o reverberations i n the lens i s f i l t e r e d out. The e f f e c t s of l e n s c h a r a c t e r i s t i c s a r e then removed by s u b t r a c t i n g a V[z] curve measured from a surface t h a t approximates to an ideal reflector; lead is often used f o r t h i s purpose. A f t e r some f u r t h e r f i l t e r i n g , a curve i s l e f t t h a t correspOnas to interference between the surface acoustic waves and the remainder of t h e reflected acoustic power. kom t h i s curve two very important parameters can be deduced. The f i r s t is t h e v e l o c i t y of t h e s u r f a c e a c o u s t i c waves, which can be

determined from t h e periodicity z' of t h e oscillations i n V[z] using t h e formula:

where vo is t h e v e l o c i t y i n t h e coupling f l u i d , and f i s t h e frequency. Second, from the rate of decay of t h e oscillations the attenuation of the surface waves can be determined. I f t h e amplitude of t h e o s c i l l a t i o n s decays a s exp(a'z), t h e n t h e a t t e n u a t i o n a of t h e s u r f a c e waves per u n i t l e n g t h can be determined from t h e formula:

where s i n e o = v & , b y S n e l l ' s law [ a f u r t h e r term can b e added t o c o r r e c t f o r attenuation i n t h e coupling fluid]. This is a composite attenuation, and includes the decay due to energy from t h e surface wave radiating i n t o the fluid. This can be allowed f o r e i t h e r by calculation or by measurement on a surface w i t h no internal attenuation. I n many cases, however, it may be t h e change i n atten&tion that is of interest, and this .can be measured directly.

The use of t h e method to measure change i n attenuation can be i l l u s t r a t e d from two V[z] curves f o r zirconia toughened alumina ceramics. These curves, which w e r e measured with a c y l i n d r i c a l l e n s a t 21 5 MHz, a r e presented i n f i g u r e 4. The two ceramics were of similar composition, but by controlling t h e aglomeration of the zirconia phase i n the second specimen, it was possible to increase t h e measured hend s t r e n g t h considerably. The f i r s t specimen c o n t a i n s a much l a r g e r d e n s i t y of microcracks, and these would be expxted t o give a substantial attenuation i n any a c o u s t i c measurement. This expectation i s r e f l e c t e d i n t h e V[z] curves. Figure 4[a] shows a steady decrease i n t h e amplitude of t h e oscillations i n V[z]. _{F i y} 5 [ a l is t h e same d a t a w i t h t h e corresponding curve f o r l e a d s u b t r a c t e d , and t h i s makes t h e exponential decay in t h e amplitude of t h e oscillations even more apparent [ n o t e t h a t t h e v e r t i c a l s c a l e i s d i f e r e n t from f i g u r e 41. From t h i s urve a Rayleigh wave velocity of 5200 *50 m s' and an attenuation of 96 t 6 dB nun-' can be deduced. Figwe 4[bl shows V[zl f o r the second specimen with less micrccracks, and

here t h e attenuation is much lower. Again this is clearer a f t e r V[zl f o r lead

9

been subtracted, i n f i g u r e 5[bl. There is a change i n v e l o c i t y t o 5909 t 5 0 m s-

but a much larger change i n the attenuation, which is now 25 t 6 dB mm-

.

It w o u l i

[a1 [bl

Figure 4

- V[z] f o r two specimens of zirconia toughened alumina.

Figure 5

- V[z] curves of figure

4 with V[z] f o r lead subtracted t o show the r o l e of the surface wave interference.

A special in-situ straining stage has been b u i l t for the acoustic microscope; t h i s is shown i n figure 6[a], with a close up i n figure 6Ibl. The straining i s achieved

by two motors, whic can be operated differentailly i n order t o ensure that a region being imaged remains i n the f i e l d of view. The stage i s f i t t e d with a load cell and a transducer t o measure extension, s o t h a t t h e behaviour of t h e specimen can be

monitored.

A specimen of 99.999% pure aluminium was prepared a s follows. The material w a s cut, rolled t o reduce the diameter by 50%, and milled t o shape. The surface t o be

measured was

then

mechanically ground and polished t o a 1 micron finish,

a d

then

electropolished i n a solution of 20% perchloric acid i n ethanol a t -20°C with 10 V

(30-748 JOURNAL DE PHYSIQUE

c - 1

Figure 6

- The straining stage f o r the a m t i c microscope.

the graph should be regarded with caution; neverless some trends are apparent. The attenuation at the higher frequency continues to relax for a longer time, which may correspond to response at shorter loop lengths. The total change in attenuation is greater for the higher frequency, though by less than f

'.

Figure 7

-

V[z] measured [a] at 600 MHz, 64, 15 and 28 minutes respectively after deformation; [b] at 875 MHz, 8, 24 and 37 minutes after deformation The total scan in z is 42 microns.

time after loading /minutes

normalized attenuat'on

/ d ~ us-

t

CIO-750 JOURNAL DE PHYSIQUE

The acoustic microscope is able t o image and measure variations in both t h e velocity and t h e a t t e n u a t i o n of Rayleigh waves. This enables d i s l o c a t i o n damping t o be

measured i n r e f l e c t i o n , a t higher frequencies than have been achieved i n bulk measurements. It i s not d i f f i c u l t t o s e e what should be done next. The next experiment must be t o take a specimen that has a s t r e s s concentrator machined i n it, apply a load, and image t h e zone of p l a s t i c deformation using t h e c o n t r a s t due t o dislocation damping.

J.M.R.W. i s supported by an SERC CASE award sponsored by AERE Harwell, under t h e supervision of Dr J.A. Hudson The straining stage was b u i l t by NPL. Dr Kushibiki e s t a b l i s h e d t h e q u a n t i t a t i v e f a c i l i t i e s a t Oxford, while on an SERC V i s i t i n g Fellowship. The analysis of the ceramic V[zl was made by M r J.M. Rowe. We wish to express our thanks t o a l l these.

Lemons, R.A. and Quate, C.F. (1979). Acoustic microscopy. I n Physical a c o u s t i c s (ed. W.P. Mason and R.N. Thurston) Vol. 14, pp 1-92. Academic press, London.

Briggs, G.A.D. (1 985). An i n t r d u c t i o n t o scanning a c o u s t i c microscopy. Oxford University Press and the Royal Microscopical Society, Oxford.

Quate, C.F.,, Atalar, A. and Wickramasinghe, H.K. (1 979). Acoustic microscopy with mechancal scanning

- a review.

Proc. IEEE 67, 1092-1 14.

Somekh, M.G., Briggs, G.A.D. and I l e t t , C. (1 984). The e f f e c t of anisotropy on contrast i n the scanning acoustic microscope. Phil. Mag. 49, 179-204.

Kushibiki, J. and Chubachi, N. (1 985). Material c h a r a c t e r i z a t i o n by l i n e - focus-beam acoustic microscope. IEEE-SU Trans. Sonics and Ultrasonics, special issue on acoustic microscopy (ed. S.D. Benett).

Yamanaka, K. (1 983 1. Surf a c e a c o u s t i c wave measurements u s i n s an i m ~ u l s i v e convergin&

be&.

J. Appl. Phys. 54, 4323-9.

-

Weaver, J.M.R., Briggs;G.A.D. and Somekh, M.G. (1983). Acoustic microscopy of ultrasonic attenuation. J. Ph s. 12 (C9), 371 -6.