Memoirs of the Faculty of Engineering, Okayama University, Vo1.35, Nos.1&2, pp.1-8, March 2001

Microscopic Observation of Plastic DeformationofPolycrystalline Aluminum

by Laser Scanning Microscope

Takeji ABE', SONG Hualin", Yasuo AKAGI"'and Ichiro SHIMIZU"

(Received December 14, 2000)

Free surface of polycrystalline metal becomes roughened after plastic deformation. The

surface roughening is closely related to the inhomogeneity of polycrystalline metals, that is,

to the inhomogeneous plastic deformation of respective grains. In the present study,

inhomogeneous deformation on the free surface of polycrystalline aluminum specimen during

uniaxial tension is studied. The inhomogeneous deformation of grains in the central area of

the free surface of specimen is observed by the laser scanning microscope, while the

inhomogeneous deformation perpendicular to the surface is studied by the laser scanning

microscope as well as the stylus measuring instrument.

It is shown that the surface roughness and the strain of respective grains increase with

the applied strain. Discussions are made on the change in the surface roughness, the strain

in each grain and the slip-line angles with the applied strain.

1. INTRODUCTION

Free surface of polycrystalline metal becomes roughened after plastic deformation. The surface roughening is

important for the finishing of surface during plastic working. It is known that the surface roughening is closely

related to the inhomogeneity of polycrystalline metals, that is, to the inhomogeneous plastic deformation of respective

grains [1'8]. Then, the experimental study of the surface roughening is also important for the understanding of

microscopic deformation behavior of polycrystalline metals. In the present study, inhomogeneous deformation on the

free surface of polycrystalline aluminum specimen during uniaxial tension is studied. The inhomogeneous

deformation of grains in the central area of the free surface of specimen is observed by a laser scanning microscope,

while the inhomogeneous deformation perpendicular to the surface is studied by the laser scanning microscope as well

as the stylus measuring instrument. The relation between the strain in respective grains, the shape of free surface

roughness curve and the applied strain is investigated. Discussions are also made on the relation among the

variations of strain in each grain, profile wavelength, slip'line angles and the surface roughness due to the applied

strain.

* Graduate School of Natural Science and Engineering and Cooperative Research Center,

Okayama University, Tsushima-Naka 3'1'1, Okayama, 700-8530, Japan.

** Graduate School of Natural Science and Engineering, Okayama University.

*** NEC Hiroshima, Ltd., Higashi·Hiroshima.

1

2 Takeji ABE, SONG Hualin, Yasuo AKAGI and Ichiro SHIMIZU

2. EXPERIMENTAL METHOD

2. 1 Specimen

The material used is polycrystalline pure aluminum for industrial use (99.5wt%). The shape of tensile specimen is

shown in Fig. 1. The central surface of the specimen was finished with abrasive papers and diamond paste until its

center-line average roughness becomes below Ra = O.l,urn. Then, the specimen was annealed for 1hr at 400°C for

Specimen A, while for 24hr at 600°C for Specimen B, and then cooled in the furnace. The surface of the specimen was

electropolished and chemically etched in order to distinguish the grain boundary. The averaged grain sizes are

35-40,urn and 70-80,urn, respectively for Specimen A and B.

~--> ~

"I ./

_~ Ir---l

- - -- - -- --C'J ---~l~~ --- ---- - --

/' j, '"Area Of:/ ~measurement 140

x--

Fig. 1 Tensile specimen

2. 2 Surface Roughness Measurements and Observation of Microscopic Deformation

The specimen was taken out from the tensile testing machine after a certain fixed amount of strain. And

microscopic deformation on the surface, that is, the surface roughening and the strain of grains were observed with the

laser scanning microscope (Keyence, VF-7500). The operating principle of the laser scanning microscope is shown in

Fig. 2.

The average surface roughness was also measured eight lines for the central area of 8 X 8mm2 on the specimen

surface, by the use of the stylus surface roughness measurement apparatus.

l\IJi.rror

Objective lens

x· y. z· () stage

Fig. 2 Principle of Laser Scanning Microscope

Fig. 3 Optical micrograph taken by laser scanning microscope and composed with personal computer ( &x = 0.17)

Loading direction

3::()

'"'oI/>()

o'0()

ocrI/>ro<;:;.o':J

o

::2.,~.()

oro0'...,3;:;.o:J

w

4 Takeji ABE. SO!\G Hualin, Yasuo AKAGI and Ichiro SHIMIZU

2. 3 Measurement of Strain in Grains and Angle of Slip Lines

The strains of grains in the axial and in the perpendicular direction were measured for the area 0.7 x 0.6mm2 at

the central area of Specimen B. The total number of grains used for the measurement was about 80.

The angle of slip lines was also measured on the display of the laser scanning microscope

3. EXPERIMENTAL RESULTS AND DISCUSSION

3. 1 Deformation and Surface Roughness of Respective Grains

At first, deformation and surface roughness for respective grains are observed with laser scanning microscope.

Figures 3 and 4 show examples of surface observation. Figure 3 is the micrograph taken by the CCD camera and

composed with the personal computer, which shows the grain deformation and the slip lines in respective grains.

Five different grains (Grain 1-5) were chosen respectively for Specimen B-1 and Specimen B-2 as shown in Figs. 5

(a) and (b), respectively. Figs. 4 (a) and (b) show of the photographs at the applied strain E, = 0.10 for Grain 1 and

for Grain 2, respectively. Figs. 6 (a) and (b) show the corresponding surface roughness profiles for Grain 1 and Grain

2, respectively.

Fig. 7 shows the change in the average surface roughness measured for the five grains shown in Fig. 5 with the

applied strain. Fig. 8 shows the change in the strain of grain for the five grains shown in Fig. 5 with the applied

strains. It is seen from Fig. 7 and 8 that there are large variety of the deformation mode in respective grains

(a) Grain 1 of Specimen B-1 (b) Grain 2 of Specimen B-1

Fig. 4 Examples of surface photograph taken by laser scanning microscope (x 1250)

o 100 (J,I m)L-..I

(a) Specimen B-1

o 100 (p m)~

(b) Specimen B-2

Fig. 5 Distribution of grains for Specimen B-1 and Specimens B-2

Microscopic Observation of Plastic Deformation 5

100 200Measured distance (p m)

(b) Grain 2

o100 200Measured distance (p m)

(a) Grain 1

E Ra(pm) E Ra(pm)

a 0.00 0.260 4 a 0.00 0.3554 b 0.05 0.523

b 0.05 0.416 c 0.10 0.735c 0.10 0.708 d 0.15 0.769d 0.15 0.781

E e 0.20 1.305

Ee 0.20 1.533

:::t~

~

a E.... .2> 0.s:: bCl Q)

~O:I:

Fig. 6 Observation with laser scanning microscope along center line of grains (Specimen B-1)

Grain No. eGrain No.

0 1 ~ 1.50 1 iii

EE 1.5 • 2 D :::t • 2 iii lIDa lID 3 0 lU @ 3lU

iii D a:: 0a:: 4 mI In iii 4 eIn e 5 III 1.0 e 5In

~l!! 1.0 c::.s:: iii.s:: Cl •Cl ::J

::I e f1jl ~ e •e Q) BQ) ClCl f!~ 0.5 ~

iii g! 0.5 I!~ « ~" iii iii iii

••

0 0.1 0.2 0 0.1 0.2Applied strain c x Applied strain c x

(a) Specimen B-1 (b) Specimen B-2

Fig. 7 Change in average surface roughness measured for the grains shown in Fig. 4 with applied strain.

40..------------------, 40

Grain No.

~ 300 1 @

c:: • 2.~

@ 3iii @ @c:: @ 4.n;

0 5 @a 20 0Q)

0Cle ® Q£ 010 •@

~@0

0 0.1 0.2Applied strain ex

(b) Specimen B-2

Fig. 8 Change in strain of five grains with applied strain

6 Takeji ABE, SONG Hualin, Yasuo AKAGI and Ichiro SHIMIZU

3. 2 Change in Surface Roughness of Specimen

Figure 9 shows an example of the surface roughness curve with laser scanning microscope along the same selected

line on the specimen surface, for different applied strain. Fig. 9 shows the change in the surface profiles for the fixed

different applied strain with respect to the initial length. The figure is processed and combined from several

roughness measurements in a personal computer.

The wavelength of the free surface profile increases with the applied tensile strain. The mountains and valleys

produced at the early stage remain their shape, though their heights increase. In some area of grains, the

deformation in the normal z'direction is large; while it is small in the other parts of grains.

10

E:::l.~

Q)0c: 0~

~

-10

0 200 400Measurement distance (fJ. m)

EO x = 0.088EO x = 0.207EO x = 0.288

600

Fig. 9 Example of surface profile processed and combined in personal computer

Changes in the average surface roughness measured in the axial and the perpendicular directions of Specimen A

with the stylus measurement apparatus are shown in Fig. lO(a). Similar results for Specimen B are shown in Fig.

10(b). In the figures, a is the angle between the specimen axis and the rolling direction of the original aluminum

plate. It is seen that little effect ofa is observed on the changes ofthe surface roughness.

0.2 0.4Applied strain EO x

(b) Specimen B

0.2 0.4Applied strain EO x

(a) Specimen A

E--0-- a=O° :::t ~ a=O°E ---0-- a =45° a =45°2,4 m ---0----b- a=90° lY 4

ltl III--I::r- a =90°

0::: / IIIIII /

Q)III / c:Q) / , , .c:c: / , , ,

C>.c / 0,

::::l01:;) g. /0 0e 9,.00 ....

I>. Q)

~2I>. ,19'0 0

~2~

{}: m.[] ....Q) , D'

~~, ,

,0

Fig. 10 Change in average roughness measured in loading and transverse direction with applied strain for

Specimen A and B

Microscopic Observation of Plastic Deformation 7

3. 3 Strain Distribution of Gr!lins

Figure 11 (a) and (b) show the strain of respective grains in the axial (x-) direction and in the perpendicular (y-)

direction, respectively, where the distribution of strain is shown with ratio of the strain Eg< or E gy and the applied

strain Ex • The data are obtained from the measurements for about 70 grains of Specimen B. It is seen that the

shapes of the distribution are almost the same, irrespective of the applied strain Ex, which implies that the deviation

of the strain of respective grains also increases with the applied strain.

0.4r----------------.

0.4

IIIc:.~

Cl

'0o:; 0.2a::

o

--0-- E x=0.088

---.-- E x=0.169-~-E x=0.207--~--E x=0.288

1 2Strain ratio IE gx I E xl

(a) Loading direction

IIIc:.~

Cl

'0o 0.2:aia::

-0- E x=0.088

---.-- E x=0.169

-~-E x=0.207

--6--- E x=0.288

o 1Strain ratio -E 9Y I E x

(b) Transverse direction

2

Fig. 11 Distribution of grain strain measured in loading and transverse directions

3.4 Direction of Slip-lines in Grains

Figure 12(a) shows the measured angle () between the slip-lines on the surface and x-direction for respective

grains in Specimen B. Figure 12(b) shows the change in the averaged slip-line angle if with respect to the applied

strain. It is seen that the angle if decreases with the increase of the applied strain.

Figure 13 shows the angle <iJ between two kinds of slip-lines appeared in a grain, where the angle is measured so

as to the x-direction lies in between the two slip-lines. It is seen the angle <iJ between the two slip-lines also

decreases with the applied strain.

O.4r-----------------,

--0-- E x=0.088

-.-- E x=0.169-<>-- E x=0.207~- E x=G.288

0.2 0.3

Applied straine x

0.130L............................................L..................................~

-~50~

IQ)

Q)

C>c:III"0Q)Cl40~

~

7525 50Angle B(deg)

o

IIIc:"i!Cl

'0o 0.2~a::

(a) Distribution of angle () (b) Change in averaged angle if with applied strain

Fig. 12 Distribution of angles between slip-line in grains and loading direction

8 Takeji ABE, SONG Hualin, Yasuo AKAGI and Ichiro SHIMIZU

0.3

--0-- E: x=0.088

-.-- E: x=0.169

III

.~ 0.2C!'0o:;0::

0.1

o 50 100Angle It> (deg)

150

Fig. 13 Distribution of angle ¢ between two slip-line systems in grains

4. CONCLUSIONS

Microscopic deformation behavior of polycrystalline aluminum was observed with the laser scanning microscope.

The surface roughening is also measured with the laser scanning microscope as well as the stylus surface roughness

measuring apparatus. The main conclusions are as follows.

(1) The microscopic deformation behavior of respective grains in polycrystalline metal is observed successfully with

laser scanning microscope. It is applicable to clarify the deformation behavior in the plane of the specimen surface

as well as that out of the plane of the surface.

(2) It is also possible to process and combine the measured digital data of surface profile and surface photographs by

the use of personal computer. In some are in respective grains, the surface roughness increases with the applied

strain, while in the other area the surface profile scarcely changes with the applied strain.

(3) The distribution of grain strain is similar for different applied strain, if the strain of grains is expressed with the

ratio to the applied strain.

(4) The angle between the slip lines and the tensile direction slightly decreases with the applied strain. When two

pairs of the slip lines are observed in the same grain, the angle between them also decreases with the applied

strain.

The authors are indebted to the financial support of the science promotion program of the Ministry of Education

[ Fundamental Research (B)(2) 11450045, 19991

REFERENCES

[1] T. Abe, I. Shimizu and T. Nishiyama, JSME Int. J., Ser. A, 35 (1992) 462-469.

[2] I. Shimizu and T. Abe, JSME Int. J., Ser. A, 37 (1994), 403-411.

[3] T. Abe, I. Shimizu, T. Nosho and S. Nagaki, AEPA'94 (1994), Eds. B. Xu and W. Yang, 191-198, Int. Acad. Pub.

[4] T. Nosho, I. Shimizu, T. Abe, H. Kawanami and H. Terai, IMMM'95 (1995), Eds. B. Xu, M. Tokuda and J. Song, Int.

Acad. Pub, 309-314.

[5] T. Abe, Y. Tai, I. Shimizu and T. Nosho, AEPA'96 (1996), Eds. T. Abe and T. Tsuta, Pergamon, 771-776.

[6] I. Shimizu, T. Nosho, T. Abe and H. Terai, AEPA'96 (1996), Eds. T. Abe and T. Tsuta, Pergamon, 277-282.

[7] I. Shimizu, Y-Z Tai, T. Abe, J. Kawaguchi and T. Nosho, IMMM'97 (1997), Eds. M. Senoo, B. Xu, M. Tokuda and B.

Bundara, Mie University Press, 271-278.

[8] I. Shimizu, T. Abe, T. Nosho and M. Wakayama, Metals and Materials, 4 (1998), 939-943.