Overlay copy technique to provide high-contrast electron micrographs for automatically determining quantitative microstructure of multiphase specimen

Cremens, W. S.

Norris, L. F.

Weeton, J. W.

English

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N A S A  TECHNICAL NOTE 
00 - 
N A S A  .__-. - TN- D-3658 
LOAN COPY: RETU' 
KIRTLAND AFB, N 
AFWL [VVLiL-2 
OVERLAY COPY TECHNIQUE TO 
MICROGRAPHS FOR AUTOMATIC 
METALLOGRAPHIC ANALYSIS 
PROVIDE HIGH-CONTRAST ELECTRON 
by L. Fredrick Norris, Wulter S. Cremens, 
undJohn W. Weeton 
Lewis Reseurch Center 
CZeveZund, Ohio 
! 
J 
I 
N A T I O N A L  AERONAUTICS A N D  SPACE A D M I N I S T R A T I O N  WASHINGTON,  D. C. 0 OCTOBER 1966  
https://ntrs.nasa.gov/search.jsp?R=19660028391 2020-03-24T01:46:39+00:00Z
I 
TECH LIBRARY KAFB, NM 
OVERLAY COPY TECHNIQUE T O  PROVIDE HIGH-CONTRAST ELECTRON 
MICROGRAPHS F O R  AUTOMATIC METALLOGRAPHIC ANALYSIS 
By L. Fredrick N o r r i s ,  W a l t e r  S. C r e m e n s ,  and John W. Weeton 
Lewis R e s e a r c h  C e n t e r  
Cleveland,  Ohio 
N A T I O N A L  AERONAUTICS AND SPACE ADMINISTRATION 
For sale by the Clearinghouse for Federal Scientific and Technical Information 
Springfield, Virginia 22151 - Price $1.00 
OVERLAY COPY TECHNIQUE TO PROVIDE HIGH-CONTRAST ELECTRON 
MICROGRAPHS FOR AUTOMATIC METALLOGRAPHIC ANALYSIS 
by L. Fredrick Norris, Walter S. Cremens, and John  W. Weeton 
Lewis Research Center 
SUMMARY 
The use of automatic film scanning instruments for determining quantitative micro- 
structural  parameters of multiphase specimens examined by electron microscopy has been 
severely handicapped by the difficulty in obtaining an image of the microstructure with 
sufficient contrast between phases. An overlay copy technique with an automatic film 
scanner to obtain the required contrast is described. The technique is illustrated by 
determining particle-size distribution and volume-percent dispersoid in a specimen of 
nickel containing thoria particles. Results a r e  in good agreement with those from a 
commercial semiautomatic particle-size analyzer. 
e ra l  application to metallographic analysis of multiphase microstructures with various 
measurement methods. 
The overlay copy technique has gen- 
INTRODUCTION 
Automatic film scanning instruments are of interest  as a means of rapidly determin- 
ing quantitative microstructural parameters in metallic and ceramic multiphase specimens 
(ref. 1). The use of such instruments for  analyzing electron micrographs prepared from 
replicas, however, has been severely handicapped by the difficulty in obtaining a photo- 
graphic image of the microstructure with sufficient contrast between phases to give ac- 
curate results. Refinements in shadowing techniques for  electron microscopy have im- 
proved definition of interphase boundaries only. Improvements in replica preparation and 
in photographic techniques have not, as yet, provided sufficient contrast between phases, 
for example, between the particles and the matrix in a dispersion microstructure. In 
addition, direct  imaging of the microstructure does not, in general, permit selection of 
the phases to be measured or exclusion of pores, unwanted phases, or artifacts. A 
method for obtaining the necessary phase contrast in electron micrographs made with 
replication techniques is described herein. This method permits measurement of phases 
with an automatic film scanner and the preselection of the described phases o r  structures 
to be  measured. The technique is illustrated by determining particle- size distribution 
and volume-percent dispersoid in  a sample of TD-Nickel (thoria particles in a nickel ma- 
trix). Measurements with an automatic film scanner were compared with those made 
with a commercial semiautomatic particle-size analyzer, which previously had given 
reasonably accurate results. 
MATERIALS, APPARATUS, AND PROCEDURE 
The TD-Nickel examined, nominally Nickel + 2-weight-percent thoria, is an early 
version of that product and shows a coarser  dispersion than is currently being produced. 
A specimen was cut from a bar  and annealed at llOOo C for 1 hour in vacuum. A trans- 
verse  section was examined. 
Apparatus 
The automatic film scanner was a Flying Spot Particle Quantizer and Lineal Analyzer 
manufactured by the Airborne Instrument Laboratory of Cutler-Hammer (ref. 2) for the 
Lewis Research Center. 
of dispersion microstructures. In the instrument a cathode-ray tube scans a ras te r  
(linear scan pattern) on the 3 5-millimeter film being examined. A photomultiplier tube, 
which detects variations in intensity of the light spot after i t  has passed through the film 
and a high-resolution optical system, feeds logic circuits and a monitor oscilloscope. 
The output from the logic circuits is fed to a digital display counter. In this study, the 
instrument measured total scan line across  the matrix and total scan line across  the 
particles. It a lso counted individual particles and sized them in accordance with their 
maximum intercept length. 
comparison purposes. In this semiautomatic instrument an  iris diaphragm is used to 
control the diameter of an illuminated circular spot, which is projected onto a photo- 
graphic print. The a r e a  of the lighted spot is matched visually by the operator to the a rea  
of a particle. When the particle is matched, the operator closes a switch; this procedure 
automatically classifies that particle into one of 48 size ranges and records it as a unit 
count on the display counter corresponding to that size range. Particles were therefore 
counted and sized according to their number and area in the micrograph. The use of this 
apparatus for the measurement of particle size is reported in reference 3. 
This instrument was specifically designed to obtain parameters 
A Zeiss Particle Size Analyzer was used to count and to size dispersoid particles for  
2 
P roced u re 
The specimen of TD-Nickel was mounted, polished, etched, and replicated according 
to standard procedures for electron microscopy. One photographic print at a magnifica- 
tion of 45 000 was used for analysis. 
automatic film scanner was used and consisted of the following four steps: 
An overlay copy technique for preparing a high-contrast 35-millimeter film for the 
( a )  Prepared by conventional replication techniques of electron 
microscopy. 
(b) Printed from f i lm prepared by overlay copy technique. 
Figure 1. - Micrographs of TD-Nickel prepared by conventional 
electron microscopy and by overlay technique. Original mag- 
nification, 45 000; reduced 49 percent in print ing. 
(1) A photographic print of the micro- 
structure (fig. l(a)) was stapled below a 
transparent plastic sheet. 
(2) The thoria particle images to be 
measured were blackened on the plastic 
sheet with either a crayon designed for  writ-  
ing on glazed surfaces or  a pen containing an  
ink o r  paint which did not spread on the plas- 
tic. The limits of the micrograph field were 
also marked to maintain the correct  field 
area.  
(3) After marking, the plastic sheet 
was trimmed to the size of the field in the 
micrograph and taped to a sheet of white 
paper. 
(4) The black-on-white subject was r e -  
photographed to give a high-contrast 35- 
millimeter film negative suitable for use in 
the film scanner. The film negative was then 
positioned in the film scanner, and particle- 
size distribution and particle and matrix 
traversed lengths were measured. 
automatic film scanner were compared with 
similar values for the same area  of the 
specimen made with a Zeiss semiautomatic 
particle -size analyzer. To permit examina- 
tion of the same area ,  a photographic print 
(at a magnification of 45 000) was made from 
the same 35-millimeter film prepared for 
the film scanner. This print (fig. l(b)) was 
used with the particle-size analyzer so  that 
The values derived from the use of the 
3 
the same enhanced image was measured in both instruments. Exploring the absolute ac- 
curacies of e i ther  method of analysis was beyond the scope of this investigation, and only 
a relative comparison of the two methods was made. 
particle diameters on the film o r  the micrograph by the appropriate magnification factors. 
The volume percentage of dispersoid was obtained in the film scanner from the usual 
lineal analysis relation for volume fraction, namely, the quotient La/(La + Lm), where 
La is the total scan line across  the particles and Lm is the total scan line across  the 
matrix. For the Zeiss particle-size analyzer, the sum of the individual particle areas  
was divided by the total micrograph area. 
The particle -size distributions were determined with both instruments by correcting 
RESULTS AND DISCUSSION 
The results of this study have demonstrated the feasibility of using a transparent- 
overlay copy technique for producing high-contrast images of dispersion microstructures 
suitable for analysis in an automatic film scanner. 
between the matrix and the dispersoid is illustrated in figure 1.  
The achievement of high contrast 
Figure l(a) is the micro- 
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part icle-size analyzer 
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Part icle size, p 
Figure 2. - Measurements of particle-size d is t r ibut ions in TD-Nickel 
by two methods of analysis. 
graph of the TD-Nickel studied, show- 
ing the fine thoria particles to be 
counted and sized, while figure l(b) is 
a print made from the 35-millimeter 
film prepared by the overlay technique. 
The results of determinations of 
particle -size distribution and volume- 
percent dispersoid for the same area  of 
the specimen by the two methods of 
analysis a r e  given in figure 2 and 
table I. Figure 2 shows the cumulative 
distribution of thoria particle sizes. 
The curve obtained with the automatic 
film scanner indicates a somewhat 
coarser particle distribution than that 
obtained with the commercial particle- 
size analyzer. The displacement aver - 
ages approximately 10 percent of the 
(larger) value obtained with the auto- 
matic film scanner and is less at the 
extremes of the particle-size range. 
4 
TABLE I. - MEASUREMENT OF AMOUNT OF 
DISPERSOID I N  TD-NICKEL BY TWO 
METHODS OF ANALYSIS 
[Number of particles counted, 161. ] 
Method of Analysis 
~ 
Automatic film scanner 
Commercial particle-size 
analyzer 
Measured amount 
of dispersoid, 
volume percent 
1.92 
1. 88 
This discrepancy is believed to be due to 
differences in the definition of particle 
size by the two methods. In the automatic 
film scanner, particle size is the maxi- 
mum intercept along the scan direction, 
while with the Zeiss instrument it is the 
diameter of a n  equivalent area circle. 
These values would not be expected to be 
identical for irregular particles. 
Values for the measured volume- 
percent dispersoid a r e  given in table I. 
The value obtained with the automatic 
film scanner (1.92 volume percent) is about 2 percent larger than that obtained with the 
particle-size analyzer (1.88 volume percent). The differences between these measure- 
ments by the two methods a r e  within the data spread usually encountered in quantitative 
metallography. 
In comparison with the semiautomatic commercial particle-size analyzer, which 
itself is much faster than pencil-and-ruler methods, use of the overlay technique with an 
automatic film scanner reduced the time for analysis of dispersion microstructures by 
90 to 95 percent. Such time saving is considerable since several  thousand particles and 
several  a reas  of the specimen should be counted in order  to characterize the microstruc- 
ture accurately. 
Use of the overlay copy technique is not restricted to dispersion microstructures. 
It can be applied to any multiphase microstructure where phase boundaries o r  other 
microstructural features can be clearly defined. 
applications a r e  measurements of the fraction recrystallized or  the fraction transformed. 
An important advantage is that this technique allows preselection of the phases to be mea- 
sured. Microstructural features, such as pores, unwanted phases, o r  false structures 
(artifacts) from improper etching o r  replication, can be eliminated or treated separately 
during the preparation of the overlay. It is believed that the overlay technique could be 
used advantageously with optical micrographs as well as with electron micrographs, and 
also with other measurement methods (such as the microdensitometer method of Allio 
and Randall (ref. 4)). 
Examples of potential metallographic 
CONCLU S IONS 
An overlay copy technique for preparing high-contrast film negatives of dispersion 
microstructures was applied to automatic quantitative metallography. The technique was 
5 
demonstrated with a replication electron micrograph of TD -Nickel, which was analyzed 
for  particle -size distribution and volume -percent dispersoid with an  automatic film 
scanner. The following conclusions were drawn: 
1. The results were in good agreement with measurements made with the use of a 
commercial semiautomatic particle -size analyzer. 
2. Microstructural parameters of dispersion alloys can be determined extremely 
rapidly compared with conventional methods of lineal analysis and particle sizing and 
counting. a 
scanner has been made possible by the overlay technique. Preselection of phases o r  
microstructural features to be measured can be made when the overlay is prepared. 
3. Quantitative metallography of multiphase microstructures with an automatic film 
I 
Lewis Research Center, 
National Aeronautics and Space Administration, 
Cleveland, Ohio, August 9, 1966, 
129 -03-01 -05-22. 
REFERENCES 
1. Moore, G .  A. : Direct Quantitative Analysis of Photomicrographs by a Digital Com- 
puter. PhotographicSci. Eng., vol. 8, no. 3, May-June 1964, pp. 152-161. 
2. Mansberg, H. P. : Photo-Optical Field Measurement by Flying Spot Scanners for  
Biomedical Applications. SOC. Photographic Instrumentation Engr. J. , vol. 2, 
no. 3, Feb. -Mar. 1964, pp. 83-88. 
3. Montgomery, D. W. : New Techniques of Par t ic leSize Analysis. Rubber Age, 
vol. 94, no. 5, Feb. 1964, pp. 759-761. 
4. Allio, R. 6. ; and Randall, C. H. : The Quantitative Analysis of Microstructure with 
Densitometer Data. AIME Trans. ,  vol. 224, no. 2, Apr. 1962, pp. 221-227. 
6 NASA-Langley, 1966 E -3537 
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Overlay copy technique to provide high-contrast electron micrographs for automatic metallographic analysis

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