SURFACE CHARACTERIZATION OF HYDROGEN CHARGED AND UNCHARGED

ALPHA-2 AND GAMMA TITANIUM ALUMINIDE ALLOYS USING AES AND REELS

by

M. R. SHANABARGER

University of California, Santa Barbara

Santa Barbara, California 93106

ABSTRACT

The surfaces of selected uncharged and hydrogen charged

alpha-2 and gamma titanium aluminide alloys with Nb additions

have been characterized by Auger electron (AES) and reflected

electron energy loss (REELS) spectroscopy. The alloy surfaces

were cleaned before analysis at room temperature by ion

sputtering. The low energy (500 eV) ion sputtering process

preferentially sputtered the surface of all the alloys,

significantly enhancing the Nb surface concentration. The

surface concentrations were determined by comparing AES data from

the alloys with corresponding data from elemental references.

No differences were observed in the Ti or Nb Auger spectra

for the uncharged and hydrogen charged alloys, even though the

alpha-2 alloy had 33.4 atomic % dissolved hydrogen. Also, no

differences were observed in the AES spectra when hydrogen was

adsorbed from the gas phase. Bulk plasmon energy shifts were

observed in all alloys. The energy shifts were induced either by

dissolved hydrogen (alpha-2 alloy) or hydrogen adsorbed from the

gas phase (alpha-2 and gamma alloys). The adsorption induced

plasmon energy shifts were greatest for the gamma alloy and cp-Ti

metal.

* Work supported by NASA Grant NCC-2-63.

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INTRODUCTION

This report contains results from two on-going studies of the

surfaces of titanium aluminide alloys. The first study is to

assess the effect of ion sputtering on the surface composition of

selected titanium aluminide alloys with niobium (Nb) additions.

The second study is to investigate the interaction of hydrogen

with selected titanium aluminide alloys and, in particular,

to evaluate potential techniques for detecting hydrogen on the

surface or in the near surface region.

The preparation of "clean" surfaces is a prerequisite to

conducting detailed investigations of surface phenomena and

conditions, and for certain manufacturing processes. Energetic

ion sputtering is the standard technique used to produce "clean"

surfaces under high vacuum conditions. Ion sputtering is known,

however, to differentially sputter components of multi-component

material systems. This can lead to the surface composition

differing significantly compared with the bulk composition. The

ion sputtering, for the work reported herein, was done with the

sample at room temperature. The sputtered surfaces were not

given a high temperature anneal after the sputtering process was

completed. For these conditions, it will be shown that the

surface concentration of Nb in titanium aluminide alloys is

significantly enhanced over the bulk concentration.

The surface compositions were determined using Auger electron

spectroscopy (AES) and two analysis methods. In the first

method, an element's Auger "signal" amplitude was compared

directly to the corresponding amplitude of a pure elemental

reference sample. In the second method, published AES relative

elemental sensitivity factors where used to estimate the surface

composition from the relative Auger derived "signal" amplitudes

[i]. The two methods produced reasonable agreement and has

allowed a calibration of the AES relative elemental sensitivity

factors for this alloy system and the particular instrument used

in this work. This will facilitate the AES composition

determination of titanium aluminide alloy surfaces when elemental

reference standards are not present or cannot be utilized in the

experimental arrangement [2].

The second study, that of hydrogen surface interactions,

employs Auger electron spectroscopy and reflected electron energy

loss spectroscopy (REELS) to detect surface hydrogen. The direct

detection of hydrogen with Auger electron spectroscopy is not

possible, but the Auger spectrum of a metal, especially a hydride

forming metal, is often modified by dissolved hydrogen. In

addition, the presence of dissolved hydrogen often modifies the

reflected electron energy loss spectra (REELS) in the same

metals. In this work, no effect of either dissolved or absorbed

hydrogen was observed in Auger spectra taken at room temperature.

Significant shifts were observed in the surface plasmon loss peak

observed with REELS, both for dissolved hydrogen and absorbed

hydrogen.

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EXPERIMENTAL CONSIDERATIONS

Alloy Composition

Five titanium aluminide alloys were investigated in this

study. Three have primarily the alpha-2 chemistry (Ti3AI) and

two, the gamma chemistry (TiAI). The measured bulk compositions

for each of the alloys is shown in Table I. One of each alloy

chemistry was charged with hydrogen. They were charged at 600 C

for four (4) hours in about 760 torr.(l x 105 N/m 2) of gaseous

hydrogen and slow cooled in hydrogen to room temperature. The

hydrogen content for these alloys in shown in Table I.

To remove any significant pre-existing surface layer from

prior processing or machining, the surface of each alloy sample

to be analyzed and each reference material was given a

metallurgical polish to at least a 3 micron finish. This surface

was then given a light acid etch to remove any evidence of the

polishing material. After being thoroughly rinsed in de-ionized

water and dried, the sample was ready for analysis.

Ion Sputterinq Parameters

A Physical Electronics (Perkin-Elmer) model C02-300

differentially pumped ion gun was used as the sputtering source.

The ion gun was run with a gas pressure of 1.5 x 10 -4 torr.(2 x

10 -2 N/m 2) in the ionizer region and a pressure of about 1 x 10 -7

torr.(l.3 x 10 -5 N/m 2) in the analyzer chamber. Beam current to

the sample was about 1 microamp. Gross surface cleaning (the

clean surface is very reactive and quickly forms an air-oxide

coating when cleaned outside the ultra-high-vacuum environment)

was done at an ion beam energy of 4 keV. Long term (at least one

hour) sputtering of the clean surface was done with a beam energy

of 500 eV to produce a stable "steady-state" low energy sputtered

surface.

Over the course of this work, different sputtering angles

(the angle between the sample surface normal and the incident ion

beam direction) and different sputtering gases were used to clean

the surface of the materials. The majority of the sputter

cleaning was done at a grazing incidence angle of about 75 ° from

the sample normal. Some work was done at a nearly normal

incidence angle of 15 ° from the sample normal. Two different

sputtering gases were used, initially Argon (Ar) and eventually

Xenon (Xe). It was discovered that a significant amount of the

sputtering gas becomes embedded in the titanium aluminide samples

during sputtering. The Auger electron spectrum for Ar overlaps

portions of the Nb spectrum, complicating the interpretation of

the Nb spectrum. To remove this uncertainty, the Ar was replaced

with Xe. These variations in the sputtering parameters did not

appear to influence the results to be described below.

- 14 -
TABLE I

The bulk metallic concentrations and AES surface concentrations

for the titanium aluminide alloys studied in this work•

Sample

alpha-2-1

alpha-2-2

alpha-2-3

gamma-i

gamma-2

Notes:

Concentration, Atomic %

Ti A1 Nb H

bulk 66.2 20.7 12.7

surface 55.0 20.6 18.0

surface/bulk 0.83 0.99 1.42

0.92

bulk, nominal [i] 65.0

surface 58.9

surface/bulk 0.91

24.0 Ii.0

25.1 21.3

1.05 1.94

bulk 67.9 21.7 10.2

surface 64.6 20.4 15.0

surface/bulk 0.95 0.94 1.47

33.43[4]

bulk 50.5 47.0 2.2

surface [2] 46.4 49.1 4.6

surface/bulk 0.92 1.04 2.1

0.001

bulk 52.4 47.3 0.i

surface [2] 52.3 47.7 [3]

surface/bulk 0.99 1.01

1.68

. The bulk concentration for this alloy was not measured.

The values shown are the nominal design values.

. These surface concentration were estimated using the

revised AES Relative Elemental Sensitivity Factors shown

in Table II.

3. AES signal not observed, below the sensitivity limit.

• X-ray analysis of this sample shows a significant change

in the dominant crystal structure and suggests that the

sample is predominantly hydrided. Identification of the

hydride phase is, however, not complete and this is only a

tentative identification.

Electron Spectroscopy

The Auger electron spectrum (AES) and the reflected electron

energy loss spectrum (REELS) were measured with a Physical

Electronics model 10-155 cylindrical mirror analyzer (CMA) with

an integral-axis electron gun. The integral-axis electron gun

provided normal (to the sample surface) or near normal (about 15 °

from the normal) excitation of the electron processes. An

_15 -
off-axis electron gun was used for grazing incidence excitation

(about 75° from the sample normal). Although grazing incidence

excitation tends to enhance the surface sensitivity of the Auger

measurements, no significant differences between the two

excitation modes were observed in this work. The AES and REELS

data presented, herein, are for near-normal excitation.

The analyzer was mounted in a bakable ultra-high-vacuum

system. After bakeout at 250 C, the base Dressure of the vacuum

system was about 5 x 10-5 torr.(6.6 x i0 -B N/m2) with all the

electronics operating. The specimens and reference materials

were mounted on a rotating carousel supported from a precision

UHV manipulator. This allowed measurements to be made repeatedly

on the same area of each specimen or on different areas, if

desired.

When used to measure the Auger spectrum, the CMA was operated

in the ,'derivative detection mode", where the derivative of the

electron energy distribution with regard to electron energy is

detected. The modulation on the CMA in the "derivative detection

mode" was set at 4 eV peak-to-peak. The excitation electron beam

energy was fixed at 3 keV with the beam current to the sample

adjusted to 5.00 ± 0.05 microamps.

The reflected electron loss spectrum could be observed in

either the "derivative mode" or by measuring the electron energy

distribution directly (integral detection mode). All of the

REELS results quoted herein were obtained from data obtained in

the integral detection mode. As a matter of operating procedure,

the apparatus was adjusted and calibrated first in the REELS

"derivative mode" before acquiring either Auger spectrum data or

integral REELS data. The excitation electron beam energy for the

REELS measurements was i000 eV. In the "derivative mode," the

beam current to the sample was set at 5.00 _ 0.05 microamps and

the CMA modulation set at 1 eV peak-to-peak. In the "direct"

mode, the CMA is not modulated, but the electron beam current is

modulated. The average beam current to the sample was about 40%

of the constant value.

RESULTS and DISCUSSIONS

AES: Surface Concentrations

Presented in Fig. 1 are examples of the Auger electron

spectrum for sputter cleaned surfaces of a representative alpha-2

and gamma alloy which have not been hydrogen charged. The

initial, unsputtered surface Auger spectrum is dominated by

oxygen and carbon surface species. After the initial oxide

surface is removed, the specimens were ion sputtered for about

one hour with an ion beam energy of 500 eV to produce a

steady-state low-energy sputtered surface. The principal

elemental Auger transitions, Ti(LMV)-418 eV, Nb(MNV)-I67 eV, and

AI(LVV)-68 eV are indicated in the Fig. i.

-16-
The surface concentrations for the metallic components in the

alpha-2 alloys obtained after prolonged sputtering are shown in

Table II. These surface concentrations were obtained by two

methods. In the first method, the amplitude of a particular

element was compared directly to the corresponding Auger

amplitude from a pure elemental reference sample. The ratio of

the two amplitudes gives a direct measure of the surface

concentration of that particular element. These are the surface

concentrations shown in Table II as being obtained with the

"direct" analysis method. In the second method, published

relative elemental sensitivity factors [i] were used to convert

the peak-to-peak derivative Auger amplitudes into "normalized"

Auger amplitudes for each element. The concentration of a

particular element is then given by the ratio of that elements

normalized Auger amplitude to the sum of all the normalized Auger

amplitudes. The surface concentrations obtained by this

"indirect" method are shown in Table II along with the published

values of the relative elemental sensitivity factors used in the

analysis. The error bars shown in Table II represent the

statistical deviations for at least four separate random

measurements.

Fig. 1

I

(alpha-2) -i

-,-.t

-,-t

<

I-t

<

A1 (LVV) Nb (MNV) Ti (LMV)

gamma-i

NI

l l l I t

i00 200 300 400 500

ELECTRON ENERGY, eV.

AES spectra of the sputter cleaned surfaces of an

uncharged representative alpha-2 and gamma titanium

aluminide alloy.

-17-
TABLE II

The AES surface concentrations and AES relative elemental

sensitivity factors for the metallic components in the alpha-2

alloys studied.

Sample

alpha-2-1

alpha-2-2

alpha-2-3

Analysis

Method

Surface Concentration, Atomic %

Ti A1 Nb

LMM(418 eV) LVV(68 eV) MNN(167 eV)

Direct [I] 55.0±7.7 20.6±2.7 18.0_3.2

Indirect [2] 62.1/5.0 21.0±3.6 16.9_2.2

Direct [I] 58.9±8.8 25.1_3.7 21.3±3.8

Indirect [2] 59.8±4.8 20.9±3.6 19.2_2.5

Direct [i] 64.6±5.2 20.4±2.6 15.0_2.6

Indirect [2] 55.9±7.9 24.2_3.2 13.9±2.4

AES Relative Elemental Sensitivity Factors

Literature Value [ref. i]

Revised, This Work

0.44 0.235 0.26

0.51_+0.06 0.20_0.04 0.2920.04

The "indirect" analysis procedure, using relative elemental

sensitivity factors, is often utilized where it would be

difficult during the measurements to compare with elemental

reference samples [2]. The small differences obtained for the

surface concentrations using these two methods result from the

specific instrument characteristics used to measure the Auger

spectrum and matrix effects in the particular alloys. The

published sensitivity factors were corrected to agree with the

concentrations obtained with the reference samples. The

corrected sensitivity factors are shown in Table II. These

sensitivity factors were used to obtain the surface

concentrations for the metallic components in the gamma alloys.

The final corrected surface metallic elemental concentrations for

both the alpha-2 and gamma alloys are presented in Table I.

It can be anticipated that sputtering the surface of these

multi-component alloys will produce differences between the

surface concentrations and the bulk concentrations. Shown in

Table I is the ratio of the surface concentration to the bulk

concentration for the sputtered alloy surfaces. Least affected

by the sputtering process is the aluminum composition, which has

a surface-to-bulk concentration ratio of about one for all the

alloys. The titanium composition is reduced slightly in the

surface region. The reduction in the Ti surface concentration

may be correlated with the presence of Nb in the composition,

since the niobium concentration is significantly enhanced in the

-18 -
surface region by the sputtering process. The minimum niobium

surface enhancement is about 40%, while the maximum enhancement

is about 100% over the bulk concentration. Enrichment of the

surface niobium may increase the surface reactivity, especially

of the alpha-2 alloys, since niobium is very reactive with most

gases, especially hydrogen. It remains to be determined if the

niobium surface enrichment isthermally stable. High temperature

vacuum annealing may dissolve the niobium and produce a surface

matching the bulk composition.

AES: Hydrogen Interactions

Shown in more detail in Fig. 2 is the Ti(MVV) Auger spectrum

at about 27 eV for an uncharged and the hydrogen charged alpha-2

alloy. The point to be noted here is that there is no apparent

effect of the absorbed hydrogen on the Ti(MVV) Auger transition,

even though the hydrogen charged specimen contains 33.4 at.%

hydrogen. Similarly, no differences are observed in the Ti(LMM)

and Ti(LVV) Auger spectra for these two alpha-2 alloys. These

comments can be extended, also, to the gamma alloys where no

differences in the Auger spectra were observed between the

charged and uncharged alloy.

The absence of an effect of absorbed hydrogen on the Ti Auger

spectra is surprising, since there are several reports in the

literature of absorbed hydrogen affecting the Auger spectra of

pure (alpha) Ti. In particular, it is reported that the Ti(MVV)

spectrum is reduced in amplitude and distinctly split into two

spectra with a splitting of about 5 eV for hydrogen charged Ti

[3, 4]. The Ti(LMM) and the Ti(LVV) spectra are affected by

hydrogen with the splitting of some spectra and energy shifts

Fig. 2

-4

C

D

Z

D

(alpha-2) -1

0.92 at.% H

'i

Nb (MNV)

Ti (MVV)

i !

i00 2OO

ELECTRON ENERGY, eV

(alpha-2) -3

33.4 at.% H

0

AI (LW)

)

Ti (MVV)

i i

i00 200

ELECTRON ENERGY, eV

Detail of the Ti(MVV) Auger spectrum for an uncharged and

a hydrogen charged alpha-2 titanium aluminide alloy. The

hydrogen charged alloy has a hydrogen concentration of

33.4 at.% H.

-19 -
in others [4]. Hydrogen interaction with pure niobium produces,

also, measurable energy shifts in the Nb(MNV) spectrum [5]. No

comparable-magnitude Auger spectra energy shifts or splittings

were observed with the titanium aluminide alloys studied in this

work.

To evaluate whether the absence of a hydrogen effect on the

Auger spectra is an intrinsic effect or the result of hydrogen

loss in the surface region due to the sputtering process, gaseous

hydrogen was adsorbed onto the surface while observing the Auger

spectra. The hydrogen gas pressure was 5 x 10 -5 torr.(6.6 x 10 -3

N/m 2) [6]. The Auger signal amplitudes were slightly reduced,

but no significant changes in the Auger spectra of either the

alpha-2 or gamma alloys were observed. This does not, however,

conclusively establish that surface hydrogen does not have an

effect on the Auger spectra of the titanium aluminides, since the

hydrogen surface coverage is unknown. As will be presented in

the next section, however, absorbed and adsorbed hydrogen does

affect the REELS. This suggests that the hydrogen surface

concentration is not insignificant at the absorption pressure

used. It suggests, also, that the surface hydrogen concentration

was not significantly reduced by the Auger (or REELS) electron

excitation beam.

REELS: Hydroqen Interactions

Presented in Fig. 3 are representative examples of the REELS

spectra obtained from uncharged alpha-2, uncharged gamma, and a

chemically pure (cp) Ti reference sample. The dominant peak is

the elastically scattered exciting electron beam. The first

major loss peak in the inelastically scattered electrons has been

identified as the bulk plasmon peak [4]. Measurements of the

energy of the bulk plasmon energy with respect to the elastic

peak (at i000 eV) are shown in Table III. These measurements

were made on uncharged and hydrogen charged alloys as well as

after having hydrogen adsorbed onto their surfaces.

For the alpha-2 alloy charged with hydrogen (alpha-2-3), the

bulk plasmon occurs at about 1 eV higher energy than for the

uncharged alloy (alpha-2-2). The fact that the large amount of

absorbed hydrogen in the alpha-2-3 alloy (33.4 at.% H) is

affecting the bulk plasmon energy is consistent with published

studies on other hydrogen charged hydride forming metals. It was

shown, for example, that the bulk plasmon energy can be

correlated with the degree of hydriding of titanium [7]. The

maximum shift for TiH 2 is about 3 eV [4]. The lack of a shift in

the gamma alloy may be related to the relatively small amount of

hydrogen absorbed by this alloy (1.7 at.% H).

To determine the affect of adsorbed hydrogen on the bulk

plasmon energy, gaseous hydrogen was adsorbed onto the sputter

cleaned surface and maintained on the surface by a constant

background hydrogen pressure of 5 x 10 -5 torr.(6.6 x 10 -3 N/m 2)

[6] while acquiring REELS data. Shifts were observed in the bulk

-20 -
TABLE III

Bulk plasmon energies for hydrogen absorbed in the bulk and

adsorbed on the surface of selected titanium aluminide alloys.

Bulk Plasmon Energy (eV)

Clean Surface Absorbed Adsorbed H-Induced

Sample -No Hydrogen- Hydrogen Hydrogen Shift[l]

alpha-2-2 16.7 17.7 +i.0

alpha-2-3 17.7 17.7 +i.0

gamma-i 15.6 18.0 +2.4

gamma-2 15.6 17.9 +2.3

cp-Ti 15.7 18.0 +2.3

plasmon energies of the uncharged alloys only. These results are

shown in Table III. No shifts were observed for either the

hydrogen charged alpha-2 or gamma alloys. The shift (about 1 eV)

i i •

Fig. 3

4_

n_

Z

H

Elastic Peak

Bulk

(alpha-2) -2

gamma-i

cp-Ti

[ I I I I I

0 I0 20 30 40 50

ELECTRON LOSS ENERGY, eV

The reflected electron energy loss spectrum for an

uncharged alpha-2 alloy, gamma alloy, and cp-Ti. The

first major peak measured from the main elastic peak is

the bulk plasmon loss peak.

-21 -
observed for hydrogen adsorbed on the alpha-2 alloy corresponds

to the shift obtained for the hydrogen charged alpha-2 alloy.

The largest shifts were observed for hydrogen adsorption onto the

uncharged gamma alloy and the cp-Ti reference sample. These

shifts are about 2.3 eV.

There is at present no understanding for the magnitude of the

bulk plasmon energy shifts. It is interesting to note that the

largest adsorption shift occurs with the gamma alloy which has a

negligible absorption shift. The gamma alloy has, also, less

apparent solubility for hydrogen and less surface Ti (and Nb) to

react with, compared with the alpha-2 alloys.

SUMMARY

Measurements have been made of the surface composition of ion

sputtered alpha-2 and gamma titanium aluminide alloys containing

Nb. For low energy "steady-state" sputtering conditions, the Nb

surface concentration is significantly enhanced (40-100%)

compared to small changes in the A1 and Ti surface

concentrations. A calibration of the AES relative elemental

sensitivity factors for Ti, AI, and Nb was made for the titanium

aluminide alloys.

No hydrogen effects were observed in the Ti or Nb Auger

spectra of either alloy. This observation is for absorbed

hydrogen (up to 33.4 at.% H in alpha-2) and surface hydrogen

adsorbed from the gas phase.

Hydrogen effects were observed in the reflected electron

energy loss spectrum. Here hydrogen interactions shifted the

bulk plasmon to higher energies; not only for absorbed hydrogen

(alpha-2), but, also, for hydrogen adsorbed onto the surface from

the gas phase (alpha-2 and gamma). The magnitude of the shifts

were distinctly different, +i.0 eV for the alpha-2 alloy and +2.3

eV for the gamma.

ACKNOWLEDGEMENTS

The author would like to thank the following individuals:

Lawrence N. Hjelm of the Air Force Wright Aeronautical

Laboratories for his assistance in obtaining the alloys used in

this study, and Dr. Abdus Kahn of United Technologies

Corporation, Pratt & Whitney Aircraft Division for providing

samples of the hydrogen charged alloys and for his interest and

many active discussions related to this work.

-22-
REFERENCES

•

•

•

4.

5.

6.

7.

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Physical Electronics Division, Perkin-Elmer Corporation, Eden

Praire, Minnesota (1978).

R.A. Outlaw, W.S. Lee, S.N. Sankaran, and R.K. Clark:

"Variation of Ti-14AI-21Nb Surface Composition with

Temperature," Proceedinqs of the 3rd NASP Workshop:

Hydroqen-Material Interactions, May 31 - June 2, 1989,

Scottsdale, Arizona, to be published.

F. Pellerin and J.P. Langeron, Surface Sci. 126(1983)444.

P. Bracconi and R. Lasser, Appl. Surface Sci. 28(1987)204.

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Uncorrected gauge reading•

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