Standard Reference Materials:

HOMOGENEITY CHARACTERIZATION OF NBS SPECTROMETRIC

STANDARDS IV: PREPARATION AND MICROPROBE

CHARACTERIZATION OF W-20 PERCENT Mo ALLOY FABRICATED BY

POWDER METALLURGICAL METHODS
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NATIONAL BUREAU OF STANDARDS • A. V. Astin, Director

Standard Reference Materials:

Homogeneity Characterization of NBS Spectrometric

Standards IV: Preparation and Microprobe

Characterization of W-20 Percent Mo Alloy Fabricated

Powder Metallurgical Methods

H. Yakowitz, R. E. Michaelis, and D. L. Vieth

Institute for Materials Research

National Bureau of Standards Washington, D.C. 20234

y,"^, National Bureau of Standards. Special Publication 260-16^

, , , (Formerly Miscellaneous Publication series) Issued January 1969

For sale by the Superintendent of Documents, U.S. Government Printing Office Washington, D.C. 20402 - Price 35 cents
SEP 1 2 1970
Preface Within the framework of the NBS Institute for Materials

Research the area of standard reference materials is a broad

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materials. In carrying out such a program there is much

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materials in science and industry.

W. Wayne Meinke, Chief

Office of Standard Reference Materials

iii

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vi
Contents PAGE

1. INTRODUCTION 1 2. GENERAL DESCRIPTION OF STANDARD REFERENCE

MATERIAL 480 (W-20^ Mo) 3 3. CHOICE OF MATERIAL 4 4. ALLOY PREPARATION 6 5. SUGGESTED METALLOGRAPHIC PREPARATION

FOR MICROPROBE ANALYSIS 10

6. HOMOGENEITY TESTING BY MEANS

OF THE MICROPROBE 11

7. QUANTITATIVE MICROPROBE ANALYSIS

OF Mo 15

8. CONCLUSIONS 21

9. REFERENCES 2 3 TABLES

1. The analyses of the W and Mo powder 7 2. Trace analysis, sample 10 of W-20^ Mo 9 3. Homogeneity characterization results,

SRM 480 (W-20^ Mo) 13

4. Relative 95^ confidence interval expressed in

percent of the amount of W or Mo present 14

5. Results of electron probe microanalysis

of SRM 480 (W-20$g Mo) 15

6. X-ray mass absorption coefficients for

tungsten and molybdenum (cm^/g) 17

7. Computed results for EMS-75 run at an operating voltage of 15 kV l8

8. Calculated concentrations for W-20

weight percent Mo 19

9. Values of f(x) for Mo and W in SRM 480

as a function of operating voltage 20 FIGURES

1. Micrograph of the heavily etched alloy wire ... 8 2. Overall view of SRM 480 showing the alloy

wire at center encased in pure Mo (white)

and pure W at the outer rim 9 vii
The purpose of this report is to present a detailed ac- count of the preparation for the Mo material and the

results of the critical evaluation of homogeneity by the electron microprobe. Results of studies for atomic number correc- tion and effects of operating voltage on the microprobe absorption correction also are given.

2
2. GENERAL DESCRIPTION OP

STANDARD REFERENCE MATERIAL 480 (W-2 0$g Mo)

This standard is a composite consisting of a core of

W-20^ Mo alloy wire embedded in pure Mo rod, onto which there

has been electroplated a layer of pure W. The alloy wire has

been extensively characterized by means of the electron microprobe and is satisfactory for use as a standard from the standpoint of homogeneity

Chemical analyses were performed by the Analytical Coordination Chemistry Section, Analytical Chemistry Division, NBS,

with the following recommended values for the provisional

Certificate of Analysis: Mo 21.5 wt % and W 78.5 wt %. Two

independent methods were employed, the first of which utilized

an ion exchange separation followed by gravimetric determinations of the ignited oxides. An average value of 78.5 was determined for W, with an uncertainty of 0.5 wt % (n=4) . (The

reported uncertainties are 95^ confidence intervals.) For Mo,

the average value was determined to be 21.5 with an uncertainty of 0. 3 wt ^ (n=4). A spectrophot ometric thiocyanate method

was used for the determination of Mo only. In this method Mo

+2 was reduced with Sn and the color system stabilized with

butyl cellosolve. An average value of 21.7 wt % Mo was obtained

with an uncertainty of 0.2 wt % Mo (n=6).

SRM 480 is issued as an unmounted slug of composite about

10 mm in diameter by 1 mm thick. The outer W layer is about

1 mm thick, the Mo rod is about 6 mm diameter and the W-Mo alloy wire is about 1 mm diameter. The composite can be metallurgically mounted and polished according to a procedure outlined in this report so that the standard is ready for insertion into the microprobe in about one hour.

3
3. CHOICE OF MATERIAL

Several factors contributed to the choice of Mo to be the first trial powder metallurgically prepared NBS microprobe standard. First, the Office of Standard Reference Materials at NBS believed that if the W-Mo refractory metal

combination, with its attendant difficulties such as high an- nealing temperature, high hot working temperature, and rapid

reaction rate at these temperatures, could be successfully

prepared, the way would be immediately open to prepare

several other standards by powder metallurgical techniques.

Next, General Electric 's Lamp Metals and Components De- partment, the fabrication contractor, had a great deal of ex- perience in preparing tungsten base alloys . The particular

composition 80% V!-20% Mo was chosen as a starting point since

the fabrication contractor had considerable experience with

this alloy and its behavior. Finally, the W-Mo alloy system

is interesting from the point of view of quantitative microprobe analysis. This interest arises from the fact that when

the W La lines are used for analysis, there is virtually no absorption correction [2] required for the tungsten microprobe

data. Furthermore, no characteristic fluorescence correction

is required [3]. Thus, the atomic number correction [4] is

isolated and can be studied. On the other hand, a large

absorption correction and an atomic number correction are re- quired when using the Mo La lines for analysis. Decreasing

the operating voltage decreases the absorption correction as well as the uncertainty in the absorption correction [5] without affecting the atomic number correction appreciably [6].

Thus, the Mo results can be studied as a function of operating

voltage and the superimposed effects of absorption correction

and atomic number uncertainty can be observed. Finally, if

the Mo Ka line can be used, say in conjunction with an operating voltage of 30 kV, the absorption correction for Mo is

only 2% while the atomic numbej? correction is about 10^.

Hence, the atomic number correction can be nearly isolated

4
on both elements of the system.

The choice of W-Mo was to some degree dictated by the con- stitution diagram for W-Mo alloys . This particular refractory

metal system forms a continuous series of solid solutions [?].

Therefore, by heat treating at suitable temperatures, for

suitable times, homogenization should occur. However, as will

be explained later, heat treatment alone was insufficient to produce the desired homogeneity; severe working of the material was also necessary.

As to the final standard, it was decided that any useful

microprobe specimen should contain the pure end member materials as well as the alloy. Thus, all investigators would have

the same pure metals in order to minimize confusion arising

from the possible use of impure "pure end members." Therefore,

SRM 480 (W-20^ Mo) is a composite with W-20$2 Mo alloy wire em- bedded in a Mo rod onto which pure W has been plated.

5
4. ALLOY PREPARATION

The powders chosen were one micrometer size W and two micrometer size Mo. The analyses of the powders are shown in

Table 1. Some 2800 g of W and 700 g of Mo were weighed out on a balance having a sensitivity of 1 mg and which had been

calibrated versus Class S weights . The powders were preblended in a twin shell blender equipped with an intensifier

bar. Final mixing was done by jet milling^ the powders were collected by a cyclone collector. Some 233 g of very fine

powder, collected at the upper orifice of the cyclone collector, differed in composition from the main powder and was discarded. This precluded the using of the weighed powders for

accuracy of composition.

The blended powder was hydrostat ically pressed at about

p 35,000 PSI (241 MN/m ) in rubber boots to prepare five green

ingots. These ingots were sintered for 2 hours at 2800 °C at

a pressure of 10 torr. The resulting ingots were machined

to 2.5 cm (one inch) diameter by 5 cm (two inch) long billets

and extruded in a Dynapak at 2050 °C at 10:1 extrusion ratio

to 8.9 nim (0.35 inch) diameter rod.

The rods were self-resistance heated for one hour at 2500 to 2600 °C at a pressure of 10"^ torr. One rod was swaged to 5 . 1 nm (0.20 inch) diameter and given a homogenizing anneal for one hour at 2500 °C. This rod was designated

Sample 10

Samples taken at each stage in the fabrication process were examined metallographically at NBS . The results showed

that only Sample 10 had a single phase micros tructure . Even

so. Sample 10 had large grains (ASTM 1 to 2) and porosity at grain boundaries. Furthermore, interference microscopy of

the polished-etched-polished surface revealed marked surface

relief which we attributed to very high grain boundary energy.

For these reasons. Sample 10 was annealed again for 12 hours at 2800 °C which did not noticeably improve the structure. Finally, Sample 10 was extruded from 5.1 mm (0.20 inch)

6
Table 1. The analyses of the W and Mo powders.

Spectrographic Analysis

Tun gs ten p owder Molybdenum p U W Lit: X Element ppm Element ppm Element ppm Element ppm

Al 6 Nl 27 Al 24 Ni 18

Ca 29 Cu 3 Ca 11 Cu 17 Si 7 Si 58 W 215

Mo 68 Mn 6 Mn 10 Fe 19 Mg ^ Fe 52 Mg <10

Cr 8 Sn 6 Cr 28 Sn 28

LECO Analysis

Oxygen Carbon Oxygen Carbon

1730 15

I 1780 29 Flame photometry analysis

Sodium Potassium

24 35 Apparent density by Scott-Schaef fer-White volumeter

(ASTM B-329-58)

,3 1.98 g/cm- 1.19 g/cmAverage particle diameter by Fisher sub-sieve sizer

As Lab

supplied milled

Number (ym)

Porosity

1.05

.770

1.00

.640

Lab

milled

2.1

.726 Wt % particle size distribution by photelometer

Micrometer

range

Wt % Micrometer Wt % Micrometer Wt range range

0 - 1 58.1 0 - 2 5 1 11 - 12 0.7

1 - 2 40.2 2 - 3 16 1 12 - 13 .8

2 - 3 1.6 3 - 4 22 3 13 - 14 .8

4 - 5 12 4 14 - 15

5 - 6 5 8 15 - 20 9.5

6 - 7 3 4 20 - 25 7.0

7 - 8 0 9 25 - 30 7.0

8 - 9 5 30 - 35 2.0

9 - 10 1 7 35 - 40 2.3

10 - 11 6

7
diameter to about 1.1 mm (0.045 inch) diameter in one pass

with the aid of the Dynapak. The resulting material showed no grain boundaries and no voids even when deeply etched as shown

in Figure 1. This material is the final form of the W-Mo alloy

standard. Analysis for metallic impurities in the alloy is

given in Table 2. The alloy wire was sheathed in pure Mo and the entire rod was electrodeposited from fused salt solution with high purity

tungsten to make the final composite standard which is shown

in Figure 2. The final diameter of the composite is about

6 mm (1/4 inch). The alloy yield was only 69 cm. Therefore,

the total yield of composite standards numbered only about 475.

Figure 1. Micrograph of the heavily etched alloy wire.

Note highly strained material and absence of

inclusions and voids. Interface between Mo

and the alloy wire is at upper right. XIOOO

8
Figure 2 . Overall view of SRM 480 showing the alloy

wire at center encased in pure Mo (white)

and pure W at the outer rim. XIO

9
5. SUGGESTED METALLOGRAPHI C PREPARATION

FOR MICROPROBE ANALYSIS

The following method of mechanical polishing provides a finished, satisfactory, sample in about 30 minutes. Mounting

with a stainless steel ring around the specimen minimizes

rounding

Rough grinding is carried out on wet SiC papers through

80, l60, 400 and 600 grit respectively. On the 600 grit paper,

the specimen is polished in several directions with as little

pressure as possible to completely remove coarse scratches.

Pine grinding is done with 9 ym diamond on a silk or nylon lap rotating at 250 rpm. Medium hand pressure is

applied. The lap should be nearly dry; a wet lap causes the

specimen to pit. Pine grinding should not require more than

two minutes to completely remove all scratch directionality.

The specimen is rough polished with 6 ym diamond on a canvas covered wheel rotating at 250 rpm. The canvas is again

maintained nearly dry. Light to moderate hand pressure gives

the best results.

Next, fine polishing is accomplished using 0.25 ym diamond

on a microcloth covered wheel. Again the lap is nearly dry and

only light hand pressure is used. At this point the specimen

is etched for about five seconds in Murakami's reagent [8].

Final polishing is carried out with 0.05 ym y-Al202 on Gamal cloth. Here an etch-polish-etch method is used to provide the final surface. At least three etchings are usually

required. A typical specimen is shown in Figure 2. Once prepared, the surface should need only light final polishing

from time to time.

10
6 . HOMOGENEITY TESTING BY MEANS

OP THE MICROPROBE

Pour samples were selected from the cut composite rod.

These were chosen by mixing the samples, choosing one, mixing,

choosing another and so on. Two different metallographers

prepared two specimens each following the procedure outlined

previously. Two samples were inserted into the microprobe

in the lightly etched condition while two were unetched. In all cases, a bridge of silver based paint was used to provide

a conducting path from the sample to ground. Vacuum metallized

(coated) specimens were not run. Three of the four selected samples designated EMS-72, 73,

7^ respectively, were chosen for homogeneity testing. The

fourth sample (EMS-75) was retained for independent microprobe

analysis to check the results of the other three samples

Thus, the fourth sample was treated typically as any piece of

material which might have been issued as representative of the

final standard.

Homogeneity testing by means of the microprobe was carried out using an operating voltage of 20 kV, a monitor

current (proportional to beam current) of two microamperes

corresponding to a specimen current of 0.22 microamperes.

The La lines of the W and Mo were monitored by means of LiP

(W) and EDDT (Mo) monochromat or-proportional counter channels.

The dead time in each channel was 2.3 microseconds. The x-ray

emergence angle was 52.5°.

As a preliminary check, the specimen was driven at 96 ym

per minute and ratemeter traces were obtained going all the

way across the specimen along three diameters at about 120° to each other. Except for the extreme edges, no gross inhomogeneities were observed; the specimens were deemed suitable for

final homogeneity characterization.

The method used to obtain the required homogeneity data

in a digital form was that of quantitative raster scanning

(QRS). The QRS method is a point-by-point analysis within a 11

t
given raster in which the information obtained at each point

is retained in digital form. Either the specimen or the

electron beam can be displaced to obtain the desired raster.

The device available at the National Bureau of Standards displaces the beam while the specimen remains stationary.

Heinrich has concluded that in any QRS procedure, the

most serious limitations are counting statistics and instrumental instability. In addition, if the beam is displaced,

defocusing of the x-ray optics is a distinct possibility because the x-ray source may no longer be on the focusing

(Rowland) circle [9].

To obtain satisfactory counting statistics, count rates were used corresponding to 70,000 counts/sec in pure W and

30,000 counts/sec in pure Mo for the homogeneity tests. With

a dwell time per point of 10 seconds, some 500,000 counts from

W and 50,000 counts from Mo were obtained from the W-20 Mo

alloy. With such high count rates, it is necessary to apply

appropriate deadtime corrections [10].

To reduce the effects of instrumental instability, the

monitor current was allowed to control the dwell-time per

point. This was done by means of a voltage-to-frequency con- verter which enabled the monitor current to be scaled. The

monitor current scaler was made the master controller and a fixed increment of monitor current was used to obtain counts

for points. This is equivalent to saying that the same number

of incident electrons were used to obtain data for all points.

Finally, defocusing effects were checked by observing

counts at the extreme positions of the raster. By focusing

the spectrometer with the beam in a very slow scanning mode

and observing the ratemeter output on a recorder the best re- sults were obtained. With our instrument ^ the widest raster

which showed no defocusing was a square about 48 ym on an edge.

The raster scanner was made to dwell at 100 points within this

square using a 10 x 10 matrix.

12
Five such raster scans were taken for one specimen and

four scans for the other two specimens. The data were delivered directly on punched paper tape with the values for W, Mo,

and dwell time per point being printed out. Each separate

raster was well removed on the specimen from any other raster

so that representative sampling was obtained from each sample.

These data were subjected to analyses testing for trends and

other non-random behavior. No significant departure from

randomness was observed.

Coefficients of variation were computed from these data

for both W and Mo; the results are shown in Table 3. Based on these results, it is concluded that the overall coefficient of

variation in percent can be conservatively taken to be 2.5^

for Mo and 1.^% for W. These figures then can be applied to the entire lot of alloy available.

Table 3. Homogeneity characterization results,

SRM 480 {¥-20% Mo)

.

Determinations

Specimen Condition ^CV-Mo^ ^CV-W^ (each element)

EMS-72 lightly 2.4? 1.24 400

etched

EMS-73 unetched 2.37 1.55 400

EMS-74 lightly 2.54 1.34 500

etched

a.o,rY - ^^^^ ^^^^ Mean number of counts

Approximate mean number of counts accumulated for

W = 500,000

.

Approximate mean number of counts accumulated for

Mo = 45,000

.

Since almost all microprobe quantitative analysis is

based on repetitive determinations, the expected width of a 95^ confidence interval is of interest. Table 4 shows the

expected width of the 95% confidence interval as a function of

13
the number of repeat determinations. From Table 4, it can be seen that l6 repetitions are sufficient to obtain an analysis

interval (95% confidence) of about ±1% relative. The time

required for such an analysis including the pure element and

background determinations is about one hour based on our experience in analyzing each of the four W-Mo specimens.

Table 4. Relative 95% confidence interval expressed

in percent of the amount of W or Mo present.

(t^_^) (^CV)/(n)^/2

(n = number of determinations)

n W Mo

10 ±1.1 ±1.9

16 ±0.8 ±1.3

25 ±0.6 ±1.0

As a final check, the pure W and Mo in the composites

were compared with NBS pure W and Mo. For an accumulation of

300,000 Mo counts and 900,000 W counts, differences between

the two pairs of materials were not observed.

14
7. QUANTITATIVE MICROPROBE ANALYSIS

OF Mo

From the standpoint of homogeneity, the intensity ratio

of the alloy to the respective pure metal (the k value) should

be the same within the limits predicted by Table 4 from sample

to sample at a given operating voltage. Let us call this the

k-criterion

The foregoing postulate was investigated by performing

microprobe analysis on each of three specimens as well as the

one not checked for micro-homogeneity. The operating voltages

were 15 and 10 kV respectively. The specimens concerned were

one etched and two unetched samples respectively. In addition,

one of the etched specimens was checked at 20 kV using all of

the 500 points obtained for homogeneity characterization. The

La line of W was measured for 20 and 15 kV, the Ma line at

10 kV. Count rates were adjusted to about 10,000 counts/sec

for the pure elements in determining k values. Results are given in Table 5 for k. The results show the k criterion

definitely is satisfied. The conclusion is that SRM 480 is

suitable for use as a microprobe standard from the standpoint

of sample-to-sample homogeneity.

Table 5. Results of electron probe microanalysis

of SRM 480 (W-20^ Mo)

Operating k^ k^

Specimen voltage (kV) Mo W

EMS 74 20 0.143 0.772

EMS 72 15 0.163 0.742

EMS 73 15 0.164 0.744

EMS 75 15 0.166 0.739

EMS 72 10 0.210 0.723

EMS 73 10 0.212 0.729

EMS 75 10 0.213 0.726

^Dead time and background corrected intensity ratio.

15
The computations to obtain the mass fractions by microprobe analysis were performed with the multi-access computer

following a program by Helnrlch and Rasberry [11]. The

following corrections were performed:

1. Absorption correction by the method of Phlllbert [12]

and Duncumb [13] with the Duncumb expression for electron re- tardation, a, modified by Helnrlch [6]. The mass absorption

coefficients used will be discussed later.

2. Atomic number correction by the method of Thomas [l4]

as modified by Helnrlch [11]. In addition, for this study,

the effective voltage suggested by Thomas was modified to agree

with results of numerical integration [15] for the stopping

power portion of the correction. Backscatter factors [4] and mean ionization potentials [4] used will be discussed. No fluorescence corrections were applied.

The program follows the iteration procedure of Crlss and

Birks [l6] but does not normalize the intensity ratios.

Iteration stops when variations for all elements are less than

0.001^. In our case, this required only three iterations for

all the analyses. The data were input as background- and

dead- time corrected intensity ratios . One run using the

correction procedure of Brown based on the electron transport

equation was also performed using the Naval Research Laboratory's computer through the courtesy of Dr. Brown.

To determine the uncertainty of the calculated composition

(C) value as a function of input parameters, such as mass absorption coefficients and backscatter factors, the data were corrected using various input parameter data. Mass absorption

coefficients for W and Mo listed in Table 6 were obtained from

the tables of Dewey [17], Frazer [l8], and Helnrlch [19] re- spectively. These tabulations show a range of about 25% in

the value of the mass attenuation coefficient of W for Mo La radiation.

16
Table 6. X-ray mass absorption coefficients

for tungsten and molybdenum (cm2/g).

Emitter Mo La W La Reference

Absorber

Mo

Mo

Mo

W

W

W

686

728

4049

3251

705.5 140 .4 139

17

18

1917

18

19

139.8

137.3

149

3145.2 150 .8

Using the same atomic number correction parameters and

varying the mass absorption coefficients gives a spread of

about 10^ relative for Mo for any atomic number correction at 15 kV. There is no spread for W. At 20 kV, the situation is a spread of about 15^ relative for Mo for any atomic number

correction and no spread for W. At 10 kV, W Ma and La lines were used for the respective analysis . Dewey does not give mass absorption coefficient values for W Ma. Still at 10 kV,

the spread is 7.5^ relative for Mo. For W, only Heinrich and

Prazer's data were available; there is no spread of results.

Holding the mass absorption coefficients constant and

varying the atomic number correction parameters gives the

spread due to the atomic correction. Here, backscatter

factors were used as given by Duncumb-Bishop [4], DuncumbKulenkampff [4] and Springer [20] respectively. For any set

of mass absorption coefficients, the spread at 15 kV for Mo

was 4.5^ relative for Mo and 0.6^ for W. At 10 kV, the spread

for Mo was still ^% relative but was 0.7^ for W. The atomic

number results are not altered by more than 0.3^ by the

averaging procedure of Duncumb-Reed [4] (separating the

stopping power and backscatter portions of the correction)

rather than thar of Castaing [l4] (use of a factor).
Therefore, the uncertainty in the mass absorption coefficient of W for Mo La is primarily responsible for the large

spread of results for Mo when Mo La lines are used for analysis

The major contributor to uncertainties in the atomic number

correction term is the backscatter factor; the stopping power

portion of the correction had little effect on the uncertainty.

For Mo analysis, using the La lines, both the absorption and

atomic number corrections contribute to the overall uncertainty

As an example, the results for one sample are given in Table 7.

Table 7. Computed results for EMS- 75 run at an operating voltage of 15 kV.

Input Normalized Normalized f(x) f(x)

parameters Mo W Mo W Mo W

F,D-B 0 202 0 .790 0 .204 0.796 0 .568 0.980

H,D-B 0 .197 0 .790 0 .200 0.799 0.575 0 .980

DEW,D-B 0 221 0 .790 0 .219 0 .781 0.518 0.982

DEW,D-K 0 .216 0 .791 0 .215 0.785 0.517 0 .982

DEWjS 0 211 0 .794 0 .210 0 .790 0 .516 0 .982

H,S 0 .189 0 .79^ 0 .192 0.808 0.573 0.980

H,D-K 0 193 0 .791 0 .196 0.804 0.574 0 .980

BROWN 0 225 0 768 0 .227 0.773

Note: All stopping power values calculated using an effective

voltage of 9.18 kV for Mo and 13.2 kV for W. Mean ionization potentials from reference [21]. Other combinations of input data for the stopping power portion of

the correction varied from that used by less than 0.7^.

Input parameter legend

H: Mass absorption coefficients due to Heinrich [19].

F: Mass absorption coefficients due to Frazer [I8]. DEW: Mass absorption coefficients due to Dewey [17]. D-B: Backscatter factor due to Duncumb-Bishop [4]. D-K: Backscatter factor due to Duncumb-Kulenkampff [4].

S: Backscatter factor due to Springer [20].

BROWN: Computer program due to D. B. Brown using calculated

correction parameters. Run by Dr. Brown using our

data.

18
Table 8 gives results for each sample using Heinrich's mass absorption coefficients and the Duncumb-Reed [4] data for

backscatter factors in the atomic number correction.

Table 8. Calculated concentrations for W-20 weight percent

Moa,b ,c^

Operating Molybdenum Tungsten

Specimen voltage (kV) c C norm C C norm

EMS 74 20 0 .202 0.199 0 .812 0.801

EMS 72 15 0 .194 0.197 0 .792 0 .803

EMS 73 15 0 .195 0.197 0 .794 0.803

EMS 75 15 0 .197 0 .200 0 .790 0 .800

EMS 72 10 0 ,.228 0 .232 0 .754 0 .768

EMS 73 10 0 ,.230 0.233 0 ,.758 0.767

EMS 75 10 0 ,.231 0.234 0 .757 0.766

^Mass absorption coefficients from Heinrich [19].

Backscatter factors from Duncumb-Reed [4].

^Mean ionization potentials from Caldwell [21].

The challenge for microprobe analysis represented by

SRM 480 is clearly shown by the spread in the analytical data.

At operating voltages down to 10 kV, the absolute percentage

deviation from the chemical analysis is far greater for Mo than

for W despite the fact that the concentration of W is four

times that of Mo. The large Mo range is due to the uncertainties in both the absorption correction and the atomic number

correction. As predicted [5], lowering the operating voltage

reduces the spread in the results for Mo and improves the

accuracy since the f(x) value is correspondingly raised.

Table 9 shows f(x) for Mo and W as a function of voltage. The

variation of the W results with voltage is a function of uncertainties in the atomic number correction—most likely in the

backscatter portion of the correction. SRM 480 shows that even

under the best experimental conditions, an accurate microprobe

analysis for Mo is extremely difficult to obtain. In fact, it

is probable that simultaneous determination of W and Mo using

the respective La lines is not feasible at a single operating

voltage

.

19
Table 9. Values of f(x) for Mo and W in SRM 480

as a function of operating voltage.

Analytical

line

Operating

voltage (kV) f(x)

Mo La

Mo La

Mo La

Mo La

W La

W La

W Ma

20

15

10

5 20

15

10

0.44

0.57

0.75

0 .90

0 .96 0.98

0.84

The foregoing results clearly show that more work on correction procedures and input parameters is urgently needed.

Standards such as SRM 480 will be useful in evaluating such

correction procedures . Such a study is beyond the scope of

this paper.

20
8. CONCLUSIONS

SRM 480 (W-20^ Mo) is satisfactory for use as a microanalytical standard from the standpoint of homogeneity. For

microprobe analysis, 16 determinations are sufficient to provide a 95% confidence interval of about ±1% relative.

The need for microprobe standards is clearly shown by

the very large spread of computed concentrations using the same correction method with different input parameters. As predicted [5], the spread in the Mo analysis decreases with

decreasing operating voltage since the effect of the uncertainty of the mass absorption coefficient of W for Mo La is

thereby diminished. The uncertainty due to the atomic number

correction remains remarkably constant as a function of operating voltage; this spread is largely due to uncertainties in

the backscatter factor.

SRM 480 is versatile in that the atomic number correction

can be isolated for W or for both W and Mo. The latter can be

done by using the Mo-Ka line for analysis. In such a case, the absorption correction for Mo is only 2% but high background and fluorescence by continuum will be encountered. It

is probably better to first meet these problems which include

the use of high operating voltages but low overvolt ages , and

high background rates when using a well characterized standard

rather than an unknown.

If good stability can be obtained, analysis should be

carried out at 5 kV. Then the absorption correction, even when using the Mo La line for analysis, is only 7% while the

atomic number correction is about 20^. Hence, the problems

associated with low voltage operation can be attacked while

using a well characterized standard.

SRM 480 is relatively easy to prepare for examination.

Once prepared, little further preparation work is required.

For the W-20 Mo alloy, it was found that lightly etched and

unetched specimens gave the same results. Light etching is

21
recommended as a convenience. No inclusions or inherent

voids were observed in the final micros tructure during routine

metallographic examination ranging up to a magnification of

1500 diameters

.

22
9 . REFERENCES

[1] Yakowitz, H., Vieth, D. L, and Michaells , R. E., "Homogeneity Characterization of NBS Spectrometric Standards

III: White Cast Iron and Stainless Steel Powder Compact",

NBS Misc. Publ. 260-12, (1966) 13 pp.

[2] Castaing, R., "Application of Electron Probes to Local

Chemical and Crystallographic Analysis", Thesis,

University of Paris, 1951.

[3] Duncumb, P. and Shields, P. K., "Calculation of Fluorescence Excited by Characteristic Radiation in the X-Ray

Microanaly zer" , in X-Ray Optics and X-Ray Microanalysis,

Pattee, H. H., Cosslett, V. E. and Engstrom, A. Editors,

Academic Press, New York, 1963, p. 329.

[4] Duncumb, P. and Reed, S. J. B., "Progress in the Calculation of Stopping Power and Backscatter Effects", in

Quantitative Electron Probe Microanalysis, Heinrich,

K. F. J., Editor, NBS Special Publication 298 (1968)

p. 133.

[5] Yakowitz, H. and Heinrich, K. F. J. "Quantitative Electron

Probe Microanalysis: Absorption Correction Uncertainty",

Mikrochimica Acta No. 1, 182 (1968).

[6] Heinrich, K. F. J. "Common Sources of Error in Electron

Probe Microanalysis", Advances in X-ray Analysis, Vol 11,

Plenum Press, New York, 1968, p. 40. [7] Hansen, M. , Constitution of Binary Alloys, second edition,

McGraw-Hill, New York, 1958, p. 980.

[8] Kehl, G. L., Principles of Met allographic Laboratory

Practice, third edition, McGraw-Hill, New York (1949).

[9] Heinrich, K. F. J., "Scanning Electron Probe Microanalysis", ASTM-STP 430, 315 (1968).

[10] Heinrich, K. F. J., Vieth, D. L. and Yakowitz, H. "Correction for Non-Linearity of Proportional Counter

Systems in Electron Probe X-Ray Microanalysis", Advances

in X-Ray, Vol 9, Plenum Press, New York, 1966, p. 208.

[11] Heinrich, K. F. J. and Rasberry, S, D., "MULTI6". This

is a computer program in the BASIC language which

corrects electron microprobe data. Available from

K. F. J. Heinrich, National Bureau of Standards,

Washington, D. C. 2023^.

23
[12] Philibert, J., "A Method for Calculating the Absorption

Correction in Electron-Probe Microanalysis", in X-Ray Optics and X-Ray Microanalysis, Pattee, H. H., Cosslett,

V. E., and Engstrom, A. editors. Academic Press, New York

1963, p. 379.

[13] Duncumb, P. and Shields, P. K., "Effect of Critical Excitation Potential on the Absorption Correction", in The

Electron Microprobe, McKinly, T. D., Heinrich, K. P. J. and Wittry, D. B., editors, John Wiley and Sons, New York,

1966, p. 284.

[l4] Thomas, P. M. , "A Method for Correcting for Atomic Number

Effects in Electron Probe Microanalysis", U. K. Atomic

Energy Authority Report AERE-R 4593, (1964) 16 pp. [15] Heinrich, K. F. J. and Yakowitz, H. "Quantitative

Electron Probe Microanalysis: Atomic Number Correction

Uncertainty" (to be published).

[16] Criss, J. W. and Birks, L. S., "Intensity Formulae for

Computer Solution of Multicomponent Electron Probe

Specimens", in The Electron Microprobe, McKinley, T. D.,

Heinrich, K. F. J. and Wittry, D. B., editors, John

Wiley and Sons, New York, 1966, p. 217.

[17] Dewey, R. D., "A Study of X-Ray Mass Absorption

Coefficients with Tables of Coefficients", Reynolds

Metals Co. Report 36-XMD-210, (I967) 3^ PP

.

[18] Frazer, J. Z., "A Computer Pit to Mass Absorption

Coefficient Data", Institute for the Study of Matter,

University of California, La Jolla Report S.I.O. 67-29,

(1967) 19 pp. [19] Heinrich, K. P. J., "X-Ray Absorption Uncertainty", in

The Electron Microprobe, McKinley, T. D., Heinrich,

K. P. J. and Wittry, D. B., editors, John Wiley and Sons,

New York, I966, p . 296

.

[20] Springer, G., "The Loss of X-Ray Intensity due to Backscattering of Electrons in Microanaly zer Targets", Mikrochimica Acta No. 3, 587 (I966).

[21] Caldwell, D. 0., "Range-Energy Relation and Masses of

the New Particles", Phys . Rev. 100 , 291 (1955).

24
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