National Bureau of Standards iMw^ ftfi WrUfir

Library, E-01 Admin. Bldg. ^'j/^1 "

NBS MISC. PUBL. 260-10

a a

l

Standard Reference Materials:

HOMOGENEITY CHARACTERIZATION OF NBS

SPECTROMETRIC STANDARDS II: CARTRIDGE BRASS AND LOW-ALLOY STEEL

U.S. Department of Commerce

National Bureau of Standards
THE NATIONAL BUREAU OF STANDARDS

The National Bureau of Standards is a principal focal point in the Federal Government

for assuring maximum application of the physical and engineering sciences to the advancement

of technology in industry and commerce. Its responsibilities include development and mainte- nance of the national standards of measurement, and the provisions of means for making

measurements consistent with those standards; determination of physical constants and

properties of materials; development of methods for testing materials, mechanisms, and

structures, and making such tests as may be necessary, particularly for government agencies; cooperation in the establishment of standard practices for incorporation in codes and specifi- cations ; advisory service to government agencies on scientific and technical problems ; invention and development of devices to serve special needs of the Government; assistance to industry,

business, and consumers in the development and acceptance of commercial standards and

simplified trade practice recommendations; administration of programs in cooperation with United States business groups and standards organizations for the development of international standards of practice; and maintenance of a clearinghouse for the collection and dissemination

of scientific, technical, and engineering information. The scope of the Bureau's activities is suggested in the following listing of its three Institutes and their organizatonal units. Institute for Basic Standards. Applied Mathematics. Electricity. Metrology. Mechanics.

Heat. Atomic Physics. Physical Chemistry. Laboratory Astrophysics.* Radiation Physics. Radio Standards Laboratory:* Radio Standards Physics; Radio Standards Engineering.

Office of Standard Reference Data.

Institute for Materials Research. Analytical Chemistry. Polymers. Metallurgy. Inorganic

Materials. Reactor Radiations. Cryogenics.* Materials Evaluation Laboratory. Office of Standard Reference Materials.

Institute for Applied Technology. Building Research. Information Technology. Performance Test Development. Electronic Instrumentation. Textile and Apparel Technology

Center. Technical Analysis. Office of Weights and Measures. Office of Engineering Standards. Office of Invention and Innovation. Office of Technical Resources. Clearinghouse

for Federal Scientific and Technical Information.**

•Located at Boulder, Colorado, 80301. •Located at 5285 Port Royal Road, Springfield, Virginia, 22171.
UNITED STATES DEPARTMENT OF COMMERCE • John T. Connor, Secretary NATIONAL BUREAU OF STANDARDS • A. V. Astin, Director

Standard Reference Materials:

Homogeneity Characterization of NBS

Spectrometric Standards II:

Cartridge Brass and Low-Alloy Steel

H. Yakowitz, D. L. Vieth, K. F. J. Heinrich,

and R. E. Michaelis

Institute for Materials Research

National Bureau of Standards Washington, D.C.

National Bureau of Standards Miscellaneous Publication 260-10

Issued December 14, 1965

For sale by the Superintendent of Documents, U.S. Government Printing Office Washington, D.C, 20402 - Price 30 cents
Library of Congress Catalog Card Number 66-60020
PREFACE

Within the framework of the NBS Institute for Materials

Research the area of standard reference materials is a broad

and important one 3 including the preparation, characterization and distribution of a wide variety of materials in such

diverse fields as metallurgy, polymers and inorganic

materials. In carrying out such a program there is much

interaction with representative of industry and science,

beginning with discussions as to which primary standard

materials will do most to advance technology, the furnishing

of materials and fabrication of samples, and the characterization and certification of the materials by cooperative

efforts. The many groups participating in a standards

program are very interested in detailed information on specific aspects of the program — but to date there has

been no publication outlet for such written discussions.

To meet this need, NBS Miscellaneous Publication 260

has been reserved for a series of papers in the general area

of "standard reference materials". This series will present

the results of studies and investigations undertaken within

the Institute for Materials Research with emphasis on the

preparation and characterization of standard reference

materials.. This subject-oriented series will provide a means for rapid dissemination of this detailed information

and we hope will stimulate the use of standard reference

materials in science and industry.,

W. Wayne Meinke, Chief

Office of Standard Reference Materials

iii
OTHER NBS PUBLICATIONS IN THIS SERIES

NBS Misc. Publ. 260 , Standard Reference Materials: Catalog

and Price List of Standard Materials Issued by the National

Bureau of Standards, October 1965. 45 cents. * (Supersedes

NBS Misc. Publ. 24l, March 1962.)

NBS Misc. Publ. 260-1, Standard Reference Materials:

Preparation of NBS White Cast 'Iron Spectrochemical

Standards, June 1964. 50 cents.

NBS Misc. Publ. 260-2, Standard Reference Materials:

Preparation of NBS Copper-Base Spectrochemical Standards,

October 1964. 35 cents.

NBS Misc. Publ. 260-3, Standard Reference Materials:

Metallographic Characterization of an NBS Spectrometric

Low-Alloy Steel Standard, October 1964. 20 cents.*

NBS Misc. Publ. 260-4, Standard Reference Materials:

Sources of Information on Standard Reference Materials,

February 1965. 20 cents.*

NBS Misc. Publ. 260-5, Standard Reference Materials:

Accuracy of Solution X-Ray Spectrometric Analysis of

Copper-Base Alloys, March 1965. 25 cents.*

NBS Misc. Publ. 260-6, Standard Reference Materials:

Methods for the Chemical Analysis of White Cast Iron

Standards, July 1965° 45 cents.*

NBS Misc. Publ. 260-7, Standard Reference Materials:

Methods for the Chemical Analysis of NBS Copper-Base

Spectrochemical Standards. In press.*

NBS Misc 0 Publ. 260-8, Standard Reference Materials:

Analysis of Uranium Concentrates at the National Bureau

of Standards. In press.*

NBS Misc. Publ. 260-9, Standard Reference Materials:

Half Lives of Materials Used in the Preparation of Standard

Reference Materials of Nineteen Radioactive Nuclides Issued

by the National Bureau of Standards. In press.*

* Send orders with remittance to: Superintendent of

Documents, U. S. Government Printing Office, Washington,

D. C. 20402,, Remittance from foreign countries should

include an additional one-fourth of the purchase price

for postage.

iv
TABLE OF CONTENTS

PAGE

1. INTRODUCTION 2

2. MATERIALS INVESTIGATED 3

3. PROCEDURE AND RESULTS FOR BRASS 5 3.1 Specimen Preparation 5 3.2 Inclusion Identification 11

3.3 Homogeneity of the Matrix 12 3.4 Electron Probe Microanalysis Results . . 17

4. PROCEDURE AND RESULTS FOR STEEL 20

4.1 Inclusion Identification 21

4.2 Tests for Other Elements 26

4.3 Summary and Conclusions for Steel .... 26

5. REFERENCES 28

LIST OF TABLES

TABLE No. PAGE

1. NBS spectrographic low-alloy steel standard

No. 463 provisional certified analysis ... 4

2. NBS spectrographic cartridge brass standards

C1102 and 1102 provisional certified

analysis • 5

3. Analytical results for Cu and Zn in

NBS-C1102 and 1102 cartridge brass 17

4. Electron probe microanalysis results for

NBS-C1102 and 1102 brasses 18

5. Analytical results for Fe, Cu, Ni_, and Si

in NBS-463 steel 27

v
LIST OF FIGURES

FIGURE No. PAGE

1. NBS-1102 brass (wrought X 100 8

2. NBS-C1102 brass (cast) near chill-cast

face X 50 9

3. NBS-C1102 brass (cast, about 3/4 inch above

chill-cast face X 50 9

4. NBS-C1102 brass (cast), typical blowholes in

cast copper-base alloys are shown X 200 ... 10

5. Electron beam scanning pictures of a typical

brass inclusion. All X 430 13

6. NBS-463 steel, as polished X 200 20

7. NBS-463 steel, etched structure in

longitudinal section X 1000 21

8. NBS-463 steel, three distinct types of

inclusions X 1000 22

9. Electron beam scanning pictures of steel,

inclusion 1. All X 430 . . . 23

10. Electron beam scanning pictures of steel,

inclusion 2. All X 430 24

11. Electron beam scanning -pictures of steel,

inclusion 3. All X 430 25

12. Portion of line scan for Mn in NBS-463 steel. 26

vi
STANDARD REFERENCE MATERIALS

:

HOMOGENEITY CHARACTERIZATION OF NBS SPECTROMETRIC

STANDARDS II: CARTRIDGE BRASS AND LOW-ALLOY STEEL

H. Yakowitz, D. L. Vieth, K. F. J. Heinrich and R c E. Michaelis

Institute for Materials Research

National Bureau of Standards

ABSTRACT

Most modern instrumental methods of analysis depend

on the use of known standards of composition for calibration. Newer analytical techniques such as the solids mass spectrometer , laser probe and, especially, the

electron probe microanalyzer have reduced the amount of

a sample which can be analyzed quantitatively to a range

of about 0.1 micrograms to as little as 0 o 00005 micrograms. As a corollary to these micrpanalytical advances

,

homogeneity requirements have become severe for the

analytical standards. This paper describes a continuation of the NBS effort to more fully characterize

existing standards as to suitability for the new microanalytical techniques [1], An NBS cartridge brass

sample in both the wrought (NBS-1102) and chill cast

forms (NBS-C1102), as well as an NBS low-alloy steel

sample (NBS-46;5), have been investigated by means of

electron probe microanalysis and optical metallography.

Some seventeen elements are contained in the brass

while twenty-five elements are in the steel. Results

for ten elements in the steel are presented while

results for six elements in the brass are given. In

the steel, Fe, Ni, Cu, and Si are essentially distributed homogeneously at micron levels while Mn, Ta, Nb,

Zr , S , and Cr are not. In the brass, Cu and Zn are distributed homogeneously at micron levels while Pb, S,

Al, and Si are not. Electron probe microanalyzer results

indicate that both NBS-1102 and NBS-C1102 brass are suitable for use as a calibration standard for electron probe

microanalysis as well as other microanalytical techniques

such as the solids mass spectrometer. The results for

brass have been corroborated by a number of laboratories

using the electron probe analyzer.

Key words: analytical standards, electron probe

microanalyzer, microanalytical techniques, NBS cartridge

brass, NBS low-alloy steel, optical metallography

1
1. INTRODUCTION

The modern day analyst is confronted with many difficult

problems in investigating materials to serve our rapidly

advancing technology. Increasingly he is asked either (1) to make determinations of chemical elements present in a material

at extremely low levels of concentration — parts per million

level and lower ; or (2) to provide quantitative analysis of

minute sample volumes^ a few cubic microns in size. The

solids mass spectrometer and the electron probe microanalyzer

,

respectively, are examples of modern instrumental methods of

analysis used by the analyst to aid in solving these analytical

problems. The full potential of these powerful analytical

tools normally cannot be realized, however , unless they are calibrated by the use of standard reference materials.

Analytical standards serve the analyst in several important ways: (1) calibration of analytical equipment, (2) checking methods of analysis and analytical techniques, (3) developing new or improved methods of analysis, and {k) evaluating the

accuracy of analytical techniques. The new microanalytical

techniques place severe requirements on the analytical standards

particularly with respect to homogeneity.

Although considerable research effort is being devoted to the preparation of analytical standards designed especially for

the microchemical techniques, as yet none are available.

The purpose of this report is to describe a continuation

of the NBS effort to more fully characterize existing standards

2
as to their suitability for the new microanalytical techniques

[1]. Once characterized, these standards will serve during the

interim period until more suitable standards designed especially

for microanalytical use can be prepared and certified.

2. MATERIALS INVESTIGATED

In a previous study, low-alloy steel, NBS-461, was fully

characterized and found to be sufficiently homogeneous that any

present microanalytical technique can be carried out with little

chance of inaccuracy due to inhomogeneity [1], In the light of

these encouraging results, it was decided to provide characterization of another low-alloy steel in this series.

On the basis of metallographic examination, it appeared

that low-alloy steel, NBS-463^ was the best of the remaining

seven standards insofar as microstructural homogeneity was concerned. Furthermore, NBS-463 steel gave evidence of having

even fewer inclusions than NBS-46l e For these reasons, NBS-463

steel was chosen for study. The composition of this steel is

given in table 1. To extend this work into the realm of non-ferrous materials,

a decision also was made to investigate samples of recently prepared NBS brass standards. Cartridge brass samples designated

1102 (wrought) and C1102 (chill-cast) were chosen. These

samples have the same analysis, but different metallurgical

structures, and certification has been made for some eleven

trace elements in addition to those normally specified. Both

the wrought and chill-cast forms were characterized in order

794-726 0-65-2
Table 1. NBS spectrographs low-alloy steel standard

No. 463 provisional certified analysis

(Revised May 5, 1965.

Element Wt Element Wt

W

(96.02)a 0.0012

c 0.19

1.15

0.031

(0.02)

0.41

0.47

0.39

0.26

0.10

0.12

0.013

0.010

0.10

0.10c

5 0.20

0.19

5 0.15

0.02

7 o.oi^

0.012

( 0.0002)

(0.002^)

(0.007)

(o.oo

6

)

w

Nb

Cu

Co

Pb

Mo

Sn

N

Values in parenthesis are not certified^ but are given

for information on the composition. Iron percent is

by difference.

to observe possible differences or difficulties attendant on

the use of either form.

The metallurgical procedure followed in the preparation of

the standard has been described in detail elsewhere [2]. The

composition of the brass is shown in table 2. The entire slab

from which the brass standards were taken was checked for macro- homogeneity by several methods and found to be entirely

satisfactory [2].

4
Table 2. NBS . spectrographic cartridge brass standards

C1102 and 1102 provisional certified analysis

(revision of July yi s 1964).

Element Wt Element Wt

Cu 72.8,- As 0.004

D Zn 2^- 10 Be 0.0000^

Pb 0.020 Bi 0.000^

5 Fe 0.011 Cd 0.004c0 Sn 0.006 Mn 0.004^

0 Ni 0.005 P 0.004g

Al 0.0007 Si (0.002) a Sb 0.005 Ag 0.0010

Te 0.000^

a Values in parenthesis are not certified, but are given

for information on the composition.

3. PROCEDURE AND RESULTS FOR BRASS

3.1 Specimen Preparation

It is a generally accepted precept that a polished unetched sample free from surface defects is the most desirable for

electron probe examination. Since brass is a "soft" metal,, it

was feared that smearing might occur on polishing. The procedure commonly adopted is to polish, etch to remove the smeared

layer and also to reveal small underlying scratches, repolish,

re-etch, then repeat until the desired surface is achieved.

The final surface should be free from scratches under examinanation at several magnifications when the sample undergoes a

5
full rotation while being viewed in polarized light. ji As a first step, the hardness values of the wrought and I cast brasses were compared with those of pure copper. The

Khoop hardness number (200 g load) of the wrought material was

135 while that of the cast material was 117. Pure copper regis-;

tered a Khoop value of 75. Based on these results, it was felt : : that the two brass forms could be polished with the same tech- : nique and that the smearing problem would be roughly comparable | to that encountered with copper.

The samples were polished on water lubricated SiC papers : through 600 grit. They were then finished on six micron diamond

on rayon followed by 1/4-micron diamond on microcloth. The

surfaces were scratch free, but a large number of what appeared \ to be grey inclusions were observed in the wrought and cast ; brasses, the number being larger in the cast form. No such

inclusions were observed in pure Cu carried through the same procedure.

Under microprobe examination, these "inclusions" were found to be essentially silica in both forms of brass.

They were cathodoluminescent under the beam, glowing a dull

blue. Since Si is present only at a 20 ppm level in the brass, \ it was suspected that the "inclusions" were really artifacts i due to polishing. Therefore, the samples were repolished on

AlgO-^ papers and finished on the diamond as before. Under thesei

conditions, far fewer "inclusions" were observed. However,

microprobe examination indicated some Al in the inclusions but

6
no Si. It was concluded that the abrasive was driving itself

I into the brass during rough polishing.

As a check to determine whether this disturbing phenomenon

was peculiar to only the NBS standards , some ordinary commercial

i brass was obtained. After receiving the same preparation as the

I standards., similar "inclusions" were observed. Under microprobe

examination Si and no Al was found after SiC papers were used

while the reverse was observed after Al^O-^ papers were used.

|f'hus 3 the phenomenon does not appear to be confined to the NBS

standards but appears rather more general.

In view of these results^ a different sample preparation

procedure was required. At first., microtomy followed by electropolishing was tried. Finally,, the as-received brass was given

'a deep electropolish directly. The bath was k-0% H-^PO^ in water,

running at 2 - ) v and relatively low current densities. The

result was a "hilly" surface suitable only for inclusion examination. Samples prepared in this fashion were used only for

inclusion identification.

For structural examination and microprobe examination for

matrix homogeneity a the procedure using Al^O^ papers was adopted

since only a small amount of abrasive pick-up was noted. To minimize time on the A1

2 0^ papers,, a "Norbide" (BN) abrasive

wheel can be used; this may, however, pit the sample.

The structure of the wrought brass is shown in figure 1. The grains are seen to be twinned,, reasonably equiaxed, and

variable in size. The ASTM grain size is 0.035 to 0.045 mm

7
Figure 1. NBS-1102 brass (wrought) X 100.

based on comparison with the standard charts [3].

The structure of the cast brass near the chill-cast face

is shown in figure 2 while the structure 3/4 inch above the

chill-cast face, i.e. at the top of the issued standard, is shown in figure 3. As expected, the dentritic cross-section

at the top is larger than that at the chill-cast face. For

the wrought structure, 75 to 100 grains occupy the same area

as the average dendrite cross-section at the chill-cast face.

For the wrought structure, 300 to 400 grains occupy the same area as the average dendrite cross-section at the top of the

chill-cast standard.

The cast form exhibits a porosity which is typical of

contracted gas blowholes in cast copper alloys [4].
Figure 2. NBS-C1102 brass (cast) near chill-cast

face X 50.

Figure 3. NBS-C1102 brass (cast., about 3/4 inch above chillcast face X 50. Note subdendritic structure.

9
Figure 4 shows several small gas holes near the chill cast

face. These irregularly shaped holes range in size from one

to about ten microns and are located primarily at the

dendrite boundaries.

Figure 4. NBS-C1102 brass (cast),, typical blowholes

in cast copper-base alloys are shown X 200.

Some true inclusions were observed in both the wrought and cast forms after the sample preparation problems were overcome. These are extremely small and appear to be essentially round

indicating that they are spheres in three dimensions. The size

of the inclusions ranges from submicron to about two microns in

diameter. They are a dull grey in color. In the cast form,

they are randomly distributed and are, on the average slightly

larger at the top of the sample than near the chill-cast face.

10
In the wrought samples they are usually found ^t the- grain

boundaries; several samples cut from different samples of the

standard confirmed this.

By counting inclusions in the polished and etched section,

it was determined that about 200 are present lying in the entire

cast face of the standard as issued, i.e., 1 1/4 x 1 1/4 x ~5/k inch samples. If these are assumed to be spheres of two micron

diameter, there are then about 800 such inclusions per cubic

millimeter of metal. The ratio of brass volume to inclusion

volume is over one hundred million to one. If it is assumed

that the sample for the usual microanalytical technique is a sphere 50 microns in diameter, the probability of obtaining

one inclusion in such a sample is negligible. The probability

for a random one micron probe to strike such an inclusion is

less than one in two million.

3.2. Inclusion Identification

The true inclusions were subjected to qualitative electron

probe analysis. In both the cast and wrought forms, the results

essentially were identical. The inclusions are cathodoluminescent; they glow with an orange-gold color under electron excitatation at 30 keV. Wavelength scans were taken of several inclusions in both forms of the brass.

The presence of reasonably large amounts of Pb and S was noted. Some variable amounts of Zn, Si and Al were also found.

The concentration of these elements varied considerably from

inclusion to inclusion. No other elements were indicated to

794-726 0-65-3
be present. Approximately twenty such inclusions were examined

in each form of the brass. A set of typical scanning display

pictures (figure 5) illustrates the presence of Pb 5 S 9 Si a Al

and Zn and the absence of Cu. These results indicate that Pb and Si cannot be uniformly

distributed within the brass. Although not appearing on the

certificate of analysis, chemical determinations had been made

for sulfur with an averaged result of 0.002$. The sulfur probably was introduced during the melting from the charcoal cover

[2]. Figure 5 shows that the Al and Si appear to be at the same location within the inclusions. Unf ortunately, the

presence or absence of other impurities, such as carbon; which

might have originated from the same source could not be checked with the electron probe micr oanalyzer available.

3.3. Homogeneity of the Matrix

The homogeneity of the sample in Cu and Zn was investigated

by means of the electron probe microanalyzer . Two separate

channels using LiF analyzing crystals and sealed proportional

detectors were employed. The instrument was operated at 30 keV

with probe currents of 100 nanoamps. The probe diameter was approximately one micron. The count rate for Cu was on the order

order of 13,500 c/s whereas the count rate for Zn was on the

order of 4000 c/s. Signal-to-background ratios for Zn were greater than 100 to 1.

12
13
G Figure 5. (cont'd) Electron beam scanning pictures of a typical brass inclusion. All X 4^0.

A. Specimen current. B. Pb-M„ x-ray. C. S-K

x-ray. D. Si-K x-ray. E. PA1-K x-ray.

a

F. Zn-K

a x-ray.

a

G. Cu-K

a x-ray.

a 14
A set of random mechanical line scans was run on each

brass form as a preliminary step. These were designed to show

up fairly gross inhomogenities of Cu and Zn, and also to see whether or not random striking of a blowhole or inclusion would

occur. Some three scans of about five millimeters total length

were run on both forms of the brass. No effect of striking an inclusion or hole was observed since there was never any drastic

reduction in intensity of the Cu and Zn signals simultaneously.

The scan rate was 1.6 microns per second; hence, about

8000 counts per micron were obtained for Cu and about 2500

counts per micron were obtained for Zn. Therefore the statistical fluctuation (three standard deviations) was approximately

yfo for Cu and 6% for Zn. No fluctuation greater than this was observed for either Cu or Zn in any of the' line scans which

represented 15,000 one micron samplings for each brass form.

Other sources of fluctuation were probe instability and

non-flatness of the sample. With the former, the Cu and Zn count rates would vary in the same fashion, i.e., both would

decrease gradually if the probe current decreased and vice

versa. Since Cu plus Zn comprises 99.95^ of the sample, no

real double effect should be observable.

Sample flatness problems can be solved by properly aligning

the spectrometers. If the spectrometer is properly aligned, a raising or lowering of the sample produces a decrease in count

rate. Hence, a double effect similar to that for probe instarbility would result if the sample height with respect to the

15
spectrometer were changed.

After the line scans were completed, point counts in

each form of the brass were taken. Twenty-four points were taken at random in the wrought sample s while forty-two points

were taken in the cast sample, distributed more or less evenly

along the twelve line segments of a "tic-tac 1 toe " grid

extending across the entire specimen. Each point was counted

twice for both Cu and Zn as an internal check on probe

stability. The two counts were always statistically in agreement. The sample was refocused in the light optics at each

point. Two separate operators took points on each sample.

The results for the cast form of the brass show that the

point giving the highest Zn count rate is +1.3$ higher in

apparent Zn concentration than the mean. The results for the

lowest Zn point give a value 2.2$ below the mean. The standard

deviation for counting error is 0.57$ (mean total number of Zn counts accumulated, N, was 79^224). The standard deviation

computed from the 42 determinations of Zn content is 0.79$.

If the mean Zn count is considered to correspond to the

certified value of 27.10$, the analytical range corresponding

to two standard deviation limits is 26.7$ to 27.5$. The

average sampling was about 10 cubic microns of the brass per

point; this corresponds to about 10-11

g of brass. Results

for both brass forms are given using the foregoing notation

in table 5.

16
Table 3. Analytical results for Cu and Zn in NBS-C1102

and 1102 cartridge brass.

Element Coefficient Standard Analytical range

of variation deviation (%) (two standard deviation

for counting computed from limits) of composition

experiment at three micron level

of spatial resolution

.0019

.00^6

.0027

.0036

0.72

0.79

2 0 2 1.0

26.7 to 27.5

71.8 to 73o8

71.1 to 7^.6

25o9 to 28 0 3

fjcast

DWrought

3.^. Electron Probe Microanalysis Results

As the final proof, it was deemed necessary to perform a complete electron probe microanalysis of each brass standard

for both Cu and Zn. This was done as a separate experiment

approximately ten days after all other data had been taken.

The voltage used was 30 kV while a constant incident beam

current of 50 nanoamps was employed. This resulted in a count

rate of about 10,000 c/s in pure Cu and 5500 c/s in pure Zn using LiF crystals in conjunction with sealed proportional

counters. Some five points each of Cu and Zn were recorded on both brasses. Background was counted - 0.05 S off peak for Cu

and - 0.1 S off peak for Zn. It is interesting to note that

the background count for both Cu and Zn was about the same in

17
the pure element as in the brass. The x-ray emergence angle

was 52.5°.

The lines examined were Zn-K and Cu-K . The Duncumb

a a corrected Philibert equation [5] was used for absorption

correction in Zn; the correction required was found to be less

than 0.1%. The Castaing fluorescence correction for Cu-K

a being fluoresced by Zn-K^ was used [6]. Mass absorption

coefficients for both K and KQ lines were taken from

cx p Heinrich [7]. The results are shown in table k'.

Table 4. Electron probe microanalysis results for

NBS-C1102 and 1102 brasses.

Element Form Apparent (fo) Actual {%)

Cu Cast 73A 72.5

Zn Cast 27.8 27.8

Cu Wrought 72.5

Zn Wrought 27.7 27.7

As can be seen, the results in table 4 agree very well

with the certified values of 72.85 for Cu and 27.10 for Zn. As predicted, the agreement between the cast and wrought forms

is excellent. These results have been corroborated by 5 other

electron probe laboratories who received wrought and cast pieces cut at random from different samples of the brass.

It is suggested that because of the very small difference

between apparent and true concentrations these brasses can be

18
used to calibrate electron probe microanalyzers . Thus, an operator could check to see that the instrument is giving

proper relative intensity data at various times during an experiment by merely taking five minutes to check the Cu and

Zn values in the brass.

5.5. Summary and Conclusions for Brass

The cast form of the cartridge brass designated NBS-C1102

shows no evidence of inhomogeneity greater than about 1.6$ of

the amount present for either Cu or Zn for samples of 10" 11 g. Therefore , this is the maximum degree of inhomogeneity

to be expected for any technique sampling a larger amount of

the brass.

The wrought form shows an apparently larger degree of

inhomogeneity which is 2.0% for Cu and k.kfo for Zn. Nevertheless, neither of these figures is prohibitive when micron

sized analytical samples are to be investigated.

Both forms contain small inclusions which are comprised

of variable amounts of lead, sulfur, aluminum, zinc, and

silicon but no copper. These inclusions are easily avoided

in the electron probe microanalyzer ; the probability to strike

an inclusion lying beneath the polished surface is negligible

for any take-off angle. Likewise, the probability for the

Inclusions to affect any analysis by one percent is negligible

for those elements not present in the inclusions.

Care must be taken in the surface preparation of the brass

to avoid occlusion of materials. Care also should be taken

19
to make sure that an undisturbed structure is being analyzed.

It is asserted that when these precautions are taken that

NBS cartridge brass 1102 or C1102 can be used for any present

day microanalytical technique with little chance of inaccuracy

due to inhomogeneity. Electron probe microanalysis results

confirm this assertion.

4. PROCEDURE AND RESULTS FOR STEEL

Steel samples were metallographically polished until no scratches were observed when the specimen was viewed in polarized light. The as polished surface is shown in figure 6; the inclusions are readily apparent. The etched structure in

figure 7 shows a large number of small spheroidized carbides.

The average carbide diameter appears to be one to two microns.

Figure 6. NBS-463 steel, as polished X 200.

20
Figure 7. NBS-4635 steel, etched structure in

longitudinal section X 1000.

4.1. Inclusion Identification

Since all constituents except iron are present at low

levels a it was felt that a concentration of any element within

the inclusions would preclude the possibility of that element

being homogeneous within the steel. Therefore s the inclusions

were investigated first.

Surprisingly, three distinct types of inclusions were

observed. These are shown marked in figure 8. The first is

a plain straight edged small inclusion which is yellow and

which is fairly plentiful. Such inclusions were found to contain a large proportion of Zr and some Or. Scanning electron probe pictures illustrate this (figure 9). No other

elements of atomic number greater than 11 could be found.

21
Figure 8. NBS-463 steel, three distinct types of

inclusions X 1000.

The second type of inclusion resembles a stringer. It is

a composite and is grey. These contain Zr s Ta, Nb and a trace

of S. No other detectable elements were found. Scanning

pictures are shown in figure 10 except for S which gave too weak a signal for photography.

The third type of inclusion is not plentiful. It is a small greyish composite which is cathodolumine scent, glowing

a light beryl blue under the electron beam. These inclusions

contain only Zr and S among the detectable elements. It is

surmised that they consist largely of ZrOg. The scanning

photographs are shown in figure 11. It should be mentioned

that iron is depleted in all three types of inclusions.

22
c

Figure 9. Electron beam scanning pictures of steely

inclusion 1. All X ^30.

A. Specimen current. B. Zr-L

a x-ray.

C. Cr~K x-ray.

23
C D

Figure 10. Electron beam scanning pictures of steel,

inclusion 2. All X 4^0.

A. Specimen current. B. Zr~La x-ray.

C. Ta-L x-ray. D. Nb-L x-ray.

24
A B

Figure 11. Electron beam scanning pictures of steel,

inclusion 3. All X 4^0.

A. Specimen current. B. Zr-L x-ray.

C. S-K x-ray.

a

2.5
4„2. Tests for Other Elements

A mechanical line scan of Mn concentration was run at a rate of 1.6 microns per second., The line-to-background ratio

was 6 to 1. A portion of the recording of this scan is shown

in figure 12. The Mn concentration is seen to fluctuate as much as twenty percent about the mean over relatively short

distances. Area scans confirmed the observation that Mn is

not uniformly distributed.

- ! -

i ; 100^

Mn SIGNAL !ii!ir'

•"' •'"=" y/^.I/^j.-Ih^/',,

If -\.1"\jLJ: \1. .... ! V. 'Ml .!..(.

Htohtfrpfwl BACKGROUND

' INTENSITY

Figure 12. Portion of line scan for Mn in

NBS-463 steel.

Point counts away from inclusions were made for Fe 5 Cu,

Nij and Si. Two different operators alternated in taking the

data. Each point was randomly picked and counted twice in the same fashion as the brass; the sample was refocused in the

light optics at each point. The results for each of these

elements are presented in table 5. 4.3. Summary and Conclusions for Steel

Of the ten elements in the steel investigated , five were found to be concentrated in one or more of three distinct types

26
Table 5. Analytical results for Fe 3 Cu, Ni, and

Si in NBS-463 steel.

Element Coefficient Standard Analytical range

of variation deviation {%) (two standard deviation

for counting computed from limits of composition

experiment at three micron level

of spatial resolution

.0195

.0078

.0154

.0032

4.4

1.6

2.9

6.0

92.9 to 99.2

0 o 39 to 3*4:5

0A3 to 0.51

0.35 to

^Thirty two points examined.

Thirty four points examined.

c Twenty five points examined.

of inclusions. These were Ta, Nb_, S } Cr and Zr. It is

therefore concluded that the steel is unsuitable as a microanalytical standard for these elements.

Additionally manganese was found to be non-unif ormly

distributed. This element is not concentrated in inclusions,,

However } four elements namely Fe, Cu, Ni and Si were found to exhibit uniform distribution within the matrix of the steel.

Each is depleted within the inclusions

„

Assuming roughly the same number of inclusions 'in NBS-463 steel as was determined present in NBS-461 steel,

(the actual number in NBS-463 is less) approximately the same probabilities concerning the inclusions should apply [1],

Thus } the probability for a random one micron probe to strike

an inclusion is about 1 in 350 and about one inclusion should

27
occur per fifty micron sphere of steel sample. For a sampling

procedure , which scans a large area of the surface, a number

of elements may show compositional fluctuations from one test

to another.

It is concluded that Cu, Ni and Si in steel can

be analyzed by any microanalytical technique with little

chance of error due to Inhomogeneity. Iron is a borderline

case if one is seeking absolute accuracy c The elements Ta,

Nb, S, Cr, Zr, and Mn should not be used in the steel for

microanalytical standardization.

5 . REFERENCES

[1] R. E. Michaelis, H. Yakowitz, and G. A. Moore, Metallographic Characterization of an NBS Spectrometric LowAlloy Steel Standard, J. Res. NBS 68A, 3^3 (1964),

Issued in expanded form as NBS Misc." Publ. 260-5.5 1-17 (1964).

[2] R. E. Michaelis, L. L. Wyman, and R„ Flitsch, Standard

Reference Materials: Preparation of NBS Copper-Base

Spectrochemical Standards, NBS Misc. Publ c 260-2,

1-^6 (1964).

(3] 1964 Book of ASTM Standards, "Standard Methods for

Estimating the Average Grain Size of Metals" Part 51 > 225 (Plate III) (1964).

[4] E. P. Polushkin, Defects and Failures of Metals,

Elsevier Publishing Company, New York, p # 47 ff, 1956.

[5] P. Buncumb and P. K. Shields, Effect of Critical Excitation Potential on the Absorption Correction in X-ray

Microanalysis, Tube Investments Research Laboratories

Technical Report No. l8l, 1-9 (1964).

[6] R c Castaing, Application of Electron Probes to Local

Chemical and Crystallographic Analysis, Thesis to University of Paris, p. 85 ff, 1951,

[7] K. F. J. Heinrich, X-ray Absorption Uncertainty. Proc. Symp. Elect. Probe Microanal. Wash. D. C. 1964, J. Wiley and Sons, Publishers. In press.

28

S GOVERNMENT PRINTING OFFICE : 1965 0—794-726
Announcement of New Publications on Standard Reference Materials

Superintendent of Documents

Government Printing Office

Washington, D. C. 20402

Dear Sir: Please add my name to the announcement list of new

publications to be issued in the series: NBS Miscellaneous

Publication 260 - Standard Reference Materials.

Name

Company

Address

City State Zip Code

(Notification key N-330)


U.S. DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20230 POSTAGE AND FEES PAID

U.S. DEPARTMENT OF COMMERCE OFFICIAL BUSINESS