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Rational Bureau of Standards

Library, E-01 Admin. Bldg. MAR 1 8 1971

National Bureau of Standards

Library, E-01 Admin. Bldg, OCT 1 1981 131005

NBS SPECIAL PUBLICATION 260-22*1

U.S. ARTMENT

OF

3MMERCE

National Bureau

of Standards

Standard Reference Materials:

HOMOGENEITY CHARACTERIZATION

OF Fe-3Si ALLOY

UNITED STATES DEPARTMENT OF COMMERCE • Maurice H. Stans, Secretary NATIONAL BUREAU OF STANDARDS • Lewis M. Branscomb, Director

Standard Reference Materials:

Homogeneity Characterization of Fe-3Si Alloy

H. Yakowitz, C. E. Fiori and R. E. Michaelis

Institute for Materials Research

National Bureau of Standards Washington, D.C. 20234

\J - £ .National Bureau of Standards Special Publication 260-22

Nat. Bur. Stand. (U.S.), Spec. Publ. 260-22, 30 pages (Feb. 1971) CODENs XNBSA

Issued February 197 1

For sale by the Superintendent of Documents, U.S. Government Printing Office, Washington. D.C 20402 (Order by SD Catalog No. C13.10:260-22). Price 35 cents.

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vi
TABLE OF CONTENTS

PAGE

Introduction 2 General Description of the Proposed New

Standard 3 Homogeneity Characterization of the Fe-3Si

Alloys 3 Results of Homogeneity Testing 8 Preliminary Homogeneity Characterization of

SRM 1134, Silicon Steel 14 Quantitative Electron Probe Microanalysis of

the Pe-3Si 14 Conclusions 19 Acknowledgements 20 References 21

LIST OF TABLES

PAGE

Table No.

1. Statistical Profile of the Samples

Characterized 9

LIST OF FIGURES

PAGE

Figure No.

1. Structure of the Fe-3Si sheet stock 4 2. Choice of samples for the homogeneity

study 5

vii
3. Plots of data values taken directly from

TOPO program 10

4. Relative topography plotted directly by

TOPO program 12

5. Topographs plotted directly by the TOPO

program showing absolute deviations from

the mean concentrations for Si and Pe . , . . 13

6. Calibration curves used for quantitative

electron probe microanalysis of silicon and

iron - . 18

viii
HOMOGENEITY CHARACTERIZATION OF Fe-3Si ALLOY

H. Yakowitz, C. E. Piori and R . E. Michaelis

Institute for Materials Research

National Bureau of Standards

Washington, D.C. 20234

An alloy of iron-3-22 wt . pet. silicon (Fe-3Si) was characterized with regard to chemical homogeneity of iron

and silicon at the micrometer level of spatial resolution.

This alloy is satisfactory for use as a homogeneous standard for electron probe microanalysis. The samples were

cut from coarse-grained sheet stock to a final size of about

3mmx3mmx0.28mm thick. Homogeneity was checked by means of quantitative raster scanning in which a square

matrix (1.1 mm x 1.1 mm) of individual points is analyzed

by the microprobe. Each matrix represents 400 separate

analyses. Usually, the same matrix was rerun so that each

point was sampled twice. The coefficient of variation for

both the iron and silicon is less than one percent. Quantitative microprobe analysis was also carried out on this alloy

giving a silicon content of 1 and an iron content of 9 6 - 9 % •

Key words: Fe-3Si alloy; homogeneity testing; metallography;

microprobe analysis; spectrometric analysis;

standard reference materials.

1
INTRODUCTION

This study forms part of the continuing program at the

National Bureau of Standards for homogeneity characterization

of new and existing standard reference materials, [1-4]. The

purpose of this program is to provide standards suitable for

the new microanalyt ical methods, especially electron probe

microanalysis. The work described in this paper concerns an iron-3-22 wt . pet. silicon alloy (Fe-3Si). This alloy is

coarse-grained sheet stock having the so-called Goss orientation in which a (110) grain is parallel to the surface and

[001] is the rolling direction. This material is widely used

commercially in power transformers.

This report describes the methods and results of homogeneity characterization of this alloy. The methods and

results of quantitative electron probe microanalysis of the

sheet stock are also presented. The Fe-3Si alloy is interesting from this point of view because of the very strong

absorption of silicon x-rays by the iron. An appreciable

atomic number effect is also present, caused by the difference in atomic number of the pure silicon standard (Z=l4)

and the average atomic number of the Fe-3Si alloy (Z=25.6).

In addition to the sheet stock, one specimen of an ex- isting NBS standard, designated SRM-1134 Silicon Steel, was also tested for homogeneity. This material contains 2.89%

silicon

2
GENERAL DESCRIPTION OF THE PROPOSED NEW STANDARD

The sheet stock was cut into pieces approximately

3 mm x 3 mm. The thickness of the sheet is about 0.28 mm. The material is coarse-grained as revealed by a direct HF

etch (figure la). Each new standard is supplied as a single

piece, HF etched and ready for met allographic mounting and

polishing

The standard was easily prepared for microprobe examination. The mounted specimens were ground on water-lubricated

SiC papers in the usual progression of 80, 220, H00 and 600

grit size. Rough polishing was accomplished on a wheel cov- ered with 6 ym diamond impregnated nylon cloth rotating at 125 RPM. Fine polishing was done on a microcloth impregnated

with 1/4 ym diamond rotating at 125 RPM. The specimen was finished on an 0.05 ym A^O^ impregnated microcloth rotating

at 150 RPM. Total polishing time was about 30 min. The

specimens were lightly etched in freshly prepared 3% nital.

Etching revealed the grain boundaries and produced some pits

(figure lb). The specimens were then ready for microprobe

examination

HOMOGENEITY CHARACTERIZATION OF THE Fe-3Si ALLOYS

The original sheet stock was a rectangular piece about

55 mm by 250 mm. Samples for homogeneity testing were taken

from positions in the sheet as shown in figure 2. All six of

these samples were tested for homogeneity by means of the

microprobe
s

Figure 1. Structure of the Pe-3Si sheet stock.

b. After HF etch showing typical grain structure x25.

After metallographic preparation (see text)

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The basic method used for homogeneity testing was that

of quantitative area scanning [5, 6]. In this method, the

electron beam is made to move discretely some fixed increment

of distance, to dwell for a predetermined period while data

are taken and then to move to the next point. A square matrix

of points (20 x 20) was examined; the spacing between points

was about 54 um. Hence, 400 separate points were examined

in an area of slightly over one square millimeter. Usually,

this matrix was repeated so that each point was investigated

twice

In order to examine such a large matrix, non-dispersive

x-ray analysis was used. The use of curved crystal spectrometers would lead to serious defocusing of the x-rays if the

matrix is 50 um x 50 ym or larger. The x-ray output from the

specimen was analyzed by a lithium-drifted silicon detector.

The spectrum contains Si Ka,3 and Fe Ka and Fe K3 peaks.

Single channel analyzers with wide enough windows to enclose

the entire Si and Fe peaks were set. A third single channel

analyzer was set between as an indicator of background levels.

Certain precautions must be followed when using the procedure outlined above. Total count rate cannot be raised to high levels or the Si (Li) detector will be overloaded. Furthermore, line to background ratios are low in comparison to those obtained with spectrometers. Therefore, long counting

times per point may be required.

6
The experimental conditions chosen were a 15kV operating

voltage coupled with a monitor current of about 0.26 uamp

The specimen current was approximately 0.09 uamp. The Si

signal-to-noise ratio was 1.2 to 1 and that of the iron was

4 to 1. A forty-second fixed counting time resulted in about

50,000 counts to be accumulated in the Si channel and 170,000

counts in the Fe channel.

Runs were made overnight; the data were punched directly

onto paper tape in computer-compatible format. Stability of

the instrument was checked by directing the monitor current,

which is proportional to the actual beam current [7], through

a voltage-to-frequency converter and scaling the output. Monitor current drift then is a direct measure of instrumental

drift. For a usual run, drift of monitor current was about

0.3% per hour.

After completion of a run, the paper tape was read onto

a magnetic tape. Statistical and spatial analysis of the data

were performed with the aid of a program called TOPO [8]. This

program does statistical analysis of the raw intensities. The

first fifty individual matrix points and each matrix row of

points (twenty) are then plotted. A Poisson plot, percent

occurrence in terms of sample standard deviation(s) versus

square root of intervals about the mean, is displayed. All

points deviating more than four sigma from the mean are labeled. Next, apparent concentrations for each point are printed

out. The program assumes that the mean value is equal to the

7
concentration of the element in question and does a linear

comparison to obtain the apparent concentrations. Concentration deviations are also printed.

If data are available from reruns of the same matrix of

points, the program then looks for trends and makes statistical drift and overlap corrections. All of the information

listed above is printed out for this corrected data except

the concentration information. The program prepares relative

topographs for each element in terms of standard deviations

and absolute topographs in terms of absolute percent increments . RESULTS OP HOMOGENEITY TESTING

Statistical data for each Pe-3Si sample tested are shown

in Table I, together with data from the existing SRM (to be

discussed later). Prom these data a conservative estimate of

the coefficient of variation, equivalent to 100s divided by

the mean x-ray count, is one percent in the entire lot of

standards

The matrices used in homogeneity testing were set around

grain boundaries and triple-points of grains to emphasize any

possible concentration variations associated with these structural details. A portion of the TOPO program results for Sample 3 is shown in figures 3-5. Figure 3 shows the raw intensity data from two runs over the same matrix. The maximum

range of groups of twenty points, corresponding to one row,

is about 300 counts for silicon and 800 counts for iron.

8
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PLOT OF OATA VALUES AVERAGED IN SUBGROUP"; OF 20 16/6UU 168100 luSoOO 169100 169600 170100 170600 17)100 171600 172100 172600

(b)

Figure 3. Plots of data values taken directly from TOPO

program averaged in row groups of twenty for

(a) Si and (b) Fe versus position in the matrix.

Total time for two runs of the matrix (800

positions) is 10.5 hours.

10
Table I shows that is 252 counts and s pg

is 48l counts

for Sample 3. Relative topographs in terms of these s values

are shown in figure 4. These topographs show that four points

in 800 are greater than 4s from the mean in iron. No points

in 800 fall into this category in silicon. There is some cor- relation of positive deviations in iron with negative deviations

in silicon. However, figure 5 shows that in terms of absolute

percent, all deviations are small. All 800 determinations of

silicon were in the range 2.94 to 3-34 percent silicon. For

iron, all 800 determinations were in the range 95-9 to 97-9

percent. For the iron, 667 points of the 800 total - over Q0% - were in the range 96.7 to 97-1 percent.

From these and similar data on all specimens tested, we conclude that the Fe-3Si sheet stock is entirely satisfactory

for use as a homogeneity standard in electron probe microanalysis. There appears to be some correlation of positive

and negative deviations for iron and silicon in the samples

studied. But in terms of absolute percent, this correlation

may be ignored for the intended application. Structural details such as grain boundaries and triple-points do not play

a significant role. These conclusions are based on analysis

at more than 3700 individual points in the samples investigated .

11
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PRELIMINARY HOMOGENEITY CHARACTERIZATION OP SRM 1134,

SILICON STEEL

These tests of SRM 1134 comprising 800 and 648 point

determinations indicated that this material appears to be

homogeneous at micrometer levels of spatial resolution. However, we emphasize that no extensive tests of this material

were performed. The results were derived from two separate

matrices run on a single specimen cut from the SRM 1134 disk

which had been selected at random from stock.

QUANTITATIVE ELECTRON PROBE MICROANALYSIS OP THE Fe-3Si

In addition to the homogeneity studies just described,

quant it itative microanalysis was also carried out on sample

#6 from the middle of the original Fe-3Si sheet. For this

purpose, data were collected by using curved crystal spectrometers. For the silicon analysis, an ADP crystal, sealed

proportional detector system was used; for iron analysis, a LiF crystal sealed proportional detector system was used.

Monitor current was set at 0.1 yamp . An opera.ting voltage

of lOkV was chosen. Count rates for the iron and silicon in

the specimen were 5300 cts/sec and 600 cts/sec, respectively.

One hundred and four separate points were analyzed representing a systematic traverse of several grains and including

analyses at triple-points and grain boundaries.

The 10 kV operating voltage was chosen in order to reduce

the effect of uncertainties in the microprobe absorption cor- rection for silicon [9]. Ten kilovolts was the lowest oper14
ating voltage to provide satisfactory iron intensity. Pure

iron and silicon were used as reference standards.

The required corrections for the data were: (1) background subtractions for the sample and standards, (2) a detector dead-time correction for the iron, (3) absorption,

and, (4) atomic number.

(1) Ziebold's method for background correction was used

[10]. The spectrometers were maintained on the iron and

silicon peaks and adjacent elements used to determine the

background level. For the Pe, background was determined

on Mn and for Si, on Al . A weight-fraction average of

the background measured from each element individually can

be used to obtain the specimen background level [10].

(2) Dead-time correction was made according to the methods

described by Heinrich, et al [7]. The value of the deadtime, t, was 2.3 usee.

(3) For the absorption correction, Heinrich's modification [11] of Philibert's equation [12] was employed to calculate the absorption factor, fix)- Tne value of the

electron stopping power, a, is given as k . 5x10^/ (

1 • 6

- E

c 1,67

) with E and E

c being the operating and critical

excitation potentials for the x-ray line being used for

analysis

Mass absorption coefficients were taken from Heinrich's tables [13]. The value of fix) computed under
these conditions and with a 10 kV operating voltage was

0.76 for Si and 0.996 for Pe in the specimen. These val

ues of £(x) meet the criteria suggested for minimizing

absorption correction uncertainties [ 9 ]

(4) The atomic number effect is determined by both

electron backseat tering and electron retardation, which

depend upon the average atomic number of the target.

Therefore, if there is a difference between the average

atomic number of the specimen given by (Z = EC^Z^) and

that of the standard, an atomic number correction is required. For Fe-3 Si, the value of Z is 25-64, therefore

a somewhat larger effect would be expected for Si (Z=l4)

analysis than for Fe (Z=26) analysis. For S: analysis

in Fe-Si the magnitude of the atomic number correction

is in the opposite direction to that of the absorption

correction

The atomic number correction formulation given by

Duncumb and Reed [14] was used. Electron backscatter

factors, R, given by these authors were chosen since

they best approximate the few experimental data available [15]. Bethe's non-relat ivistic relation for

electron stopping power, S, was used with the ionization potential, J, given by Berger and Seltzer [16].

Heinrich and Yakowitz have investigated error propagation in the atomic number correction term [15]. In general, the magnitude of this term decreases as U = E/E

16
increases - but very slowly (five percent for a tenfold

increase in U) . But the uncertainty in the atomic number correction term remains remarkably constant as a function of U. This is because the R and S uncertainties

tend to counterbalance one another [15]. Thus 3 no in- crease in the error due to the atomic number correction

is to be expected at low U values and hence the choice

of 10 kV as operating voltage is still valid for obtaining the highest accuracy.

No characteristic fluorescence correction is required for

the iron analysis. Secondary characteristic fluorescence of

Si Ka by Pe Ka is negligible [17]. No correction for fluorescence effects caused by the x-ray continuum was made. This

correction is negligible in the analysis of Pe-3Si alloys [18].

All of the relations for converting measured data to con- centration values have been programmed for computer reduction.

The measured intensity ratio (k) was computed as C, the con- centration, was incremented by 0.005 concentration units in

the regions 0 - C gl

- 0.15 and 0.85 - C pe- 1.0. The results

are shown in figure 6 and served as calibration curves for the

Pe-Si analyses.

The results for iron and silicon may be summarized as follows

1. Silicon content :3.1^# (104 determinations)

2. Observed range of silicon content :2.89$ to 3.29%

17
0.14

Figure 6. Calibration curves used for quantitative electron

probe microanalysis of (a) silicon and (b) iron.

C is the calculated concentration and k is the

background and deadtime corrected intensity ratio

of silicon or iron in the alloy to pure silicon

or iron. Valid at 10 kV operating voltage and

x-ray take-off angle of 52.5° • 18
3. We may be 99% confident that the interval 3-12 and

3.16% would bracket the mean silicon concentration

given by the microprobe.

4. Iron content : 9 6 . 9 % (104 determinations)

5. Observed range of iron content : 9 ^ • ^ to 99-4%

6. We may be 99% confident that the interval 96.6 and

91.2% would bracket the mean iron concentration

given by the microprobe.

These values may be compared with the values of 3-22 percent silicon and 96.7-96.8 percent iron (by difference) determined by chemical analysis. The value of 3-22% for silicon is

estimated accurate to within plus or minus 0.02 percent by the

analyst. Agreement between chemical analysis and electron probe

microanalysis is considered satisfactory.

CONCLUSIONS

The sheet stock containing 3-22% Si and 96.7-96.8$ Fe is

entirely suitable for use as a microprobe analysis standard.

A conservative estimate of the coefficient of variation for

both iron and silicon is ±1% . The results of quantitative electron probe microanalysis

gave 3.1^$ Si and 96.9$ Fe as compared to 3-22$ Si and 96.7

to 96.8$ Fe (by difference) given by chemical analysis. Correction of microprobe data was carried out using only the set

of input parameters described in the text.

The existing standard, SRM 113*1, Silicon Steel, was subjected to preliminary homogeneity testing. The results indi19
cate that this material is homogeneous at micrometer levels

of spatial resolution.

ACKNOWLEDGEMENTS

Special thanks are due to Dr. K. F. J. Heinrich and

R. L. Myklebust for assistance with the microprobe work

and to R. Paulson who performed the wet chemical analysis

of the sheet stock. Helpful discussions with Dr. A. W. Ruff, Jr. are also acknowledged.

20
REFERENCES

[I] Michaelis, R. E. , Yakowitz, H. 3 and Moore, G. A.,

J. Res. Nat. Bur. Stand. (U.S.), 68A, 343(1964).

[2] Yakowitz, H., Vieth, D. L . , Heinrich, K. F. J., and

Michaelis, R. E., Advances in X-ray Analysis, 9_, 289(1966)

[3] Yakowitz, H., Vieth, D. L., and Michaelis, R. E.,

Nat. Bur. Stand. (U.S.), Spec. Publ. 260-12 (1966).

[4] Yakowitz, H., Michaelis, R. E., and Vieth, D. L.,

Advances in X-ray Analysis, 12, 418(1969).

[5] Heinrich, K. F. J., ASTM Spec. Tech. Publ. 430 , 315(1968).

[6] Yakowitz, H., and Heinrich, K. F. J., Metallography 1(1),

55(1968).

[7] Heinrich, K. F. J., Vieth, D. L . , and Yakowitz, H.,

Advances in X-ray Analysis, £, 208(1966).

[8] Rasberry, S. D., and Heinrich, K. F. J., TOPO Program

for Microprobe Analysis. Information available from

S. D. Rasberry, Nat. Bur. Stand., Washington, D.C. 20234.

[9] Yakowitz, H., and Heinrich, K. F. J., Mikrochimica Acta

1968 (1) , 182.

[10] Ziebold, T. 0., The Electron Microanaly zer and Its Applications, Notes of a special course taught at MIT,

(1965), pp. 5-6.

[II] Heinrich, K. F. J., Advances in X-ray Analysis, 11 , 40(1968)
[12] Philibert, J., X-ray Optics and X-ray Microanalysis,

Pattee, H. H., Cosslett, V. E., and Engstrom (ed.),

p. 379 (Academic Press, New York, 1963).

[13] Heinrich, K. F. J., The Electron Microprobe, McKinley,

T. D., Heinrich, K. F. J., and Wit try", D. B., (ed.),

p. 296 (Wiley, New York, 1966).

[14] Duncumb, P., and Reed, S. J. B., Nat. Bur. Stand. (U.S.)

Spec. Publ. 298, 133(1968).

[15] Heinrich, K. F. J., and Yakowitz, H., Mikrochimica Acta

1970 , 123.

[16] Berger, M. J., and Seltzer, S. M., Nat. Acad. Sci.,

Nat. Res. Council Publ. 1133 , 205(1964).

[17] Heinrich, K. F. J., and Yakowitz, H., Mikrochimica Acta

1968 , 905.

[18] Myklebust, R. L., Yakowitz, H., and Heinrich, K. F. J.,

to be published.

22
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