improving tantalum's oxidation resistance by al+ ion implantation

Improving Tantalum's Oxidation Resistance by AI + Ion Implantation

M. SAQIB, J .M. HAMPIKIAN, and D.I. POTTER

Tantalum was implanted with 180 keV AI § ions to fluences up to 3 • 10 ~8 A l+ / cm 2. Subsequent microchemical and microstructural observations showed that an amorphous layer covered the surface and extended to depths near 3000 A for fluences above 2.4 • 1018 Al+ /cm 2. The layer, comprised of --70 at. pct A1 and - 3 0 at. pct Ta, crystallized at temperatures above 500 ~ Oxidation measurements, performed in one atmosphere of air and at temperatures below 600 ~ showed that the layer stopped oxidation of the implanted tantalum, while unimplanted tantalum oxidized rapidly. The protection provided by the implantation deteriorated somewhat by temperatures near 735 ~ but still reduced the oxidation rate by a factor of 5. The deterioration is caused by localized rupturing of the implanted layer and the resulting oxidation of the underlying tantalum. At 910 ~ the implanted tantalum oxidized almost as rapidly as unimplanted tantalum.

I . I N T R O D U C T I O N

T A N T A L U M and its alloys possess many properties which make them prime candidates for use in aerospace and other industries. They exhibit high melting temperatures and, as a result, retain several desirable mechanical properties over a wide temperature range that extends to at least 2200 ~ In addition, tantalum is easily formed or worked at room temperature. Such attributes make these materials excellent choices for fabrication of parts intended for use at elevated temperatures. Unfortu- nately, tantalum and many of its alloys oxidize rapidly at temperatures above 300 ~ t2[ Protective coatings are usually required to achieve useful service lives and dependability under such circumstances. Conventional coatings often suffer from exfoliation or lack of adher- ence at the coating/substrate interface. Such problems can be circumvented if the coating can be made an in- tegral part of the substrate, so that no abrupt coating/ substrate interface is present. As demonstrated in this paper, ion implantation is useful in creating such coatings, and aluminum ion implantation is particularly help- ful in retarding oxidation of tantalum, at least in the intermediate temperature range up to - 8 0 0 ~ Ion implantation has the further advantage of altering surface properties such as oxidation, while leaving desirable bulk properties unaltered.

Kaufman et al . pj have investigated the response of tantalum to oxidation after implanting it with selected elements, including aluminum. Their findings, in the case of aluminum implantation, were that implantation reduced oxidation by about a factor of 2 at 500 ~ but did not cause a reduction at 1000 ~ the second of the two temperatures investigated. It was suggested that the implantation fluence of 2 • 1017 A l + / c m 2 used in that work,

M. SAQIB, formerly Graduate Student, Metallurgy Department, University of Connecticut, is Research Associate with the Department of Mechanical Systems and Engineering, Wright State University, Dayton, OH 45435, and Visiting Scientist, Wright-Patterson Air Force Base, Dayton, OH 45433. J.M. HAMPIKIAN, Graduate Student, and D.I. POTTER, Professor of Metallurgy, are with the Metallurgy Department, School of Engineering and Institute of Materials Science, University of Connecticut, Room 111,97 North Eagleville Road, Storrs, CT 06269-3136.

Manuscript submitted December 20, 1988.

and the resulting implanted concentrations that ap- proached 20 at. pct A1, was near that where saturation due to sputtering occurs. However, our previous work [41 and that presented here shows that much higher aluminum concentrations are obtained at higher fluences. These higher fluences, and the resulting oxidation barriers they produce, have effectively stopped oxidation at temperatures where the layers are stable, as will be described later in this paper.

The work that follows was based on the premise that high fluence A1 § implantation of tantalum, and other refractory metals as well, would yield intermetallic compounds of aluminum at the surface of the refractory metal. Improved oxidation resistance was then expected, based on the low oxidation rates reported for these inter- metallics when they were investigated in bulk form. [5,6] Both the premise and the expectation proved true for temperatures up to - 7 0 0 ~ with large reductions in oxidation rates measured for oxidation times often lasting several days. The results of the oxidation measurements are presented first, and these are followed by an ex- amination of the microstructures observed in the as- implanted specimens and in implanted-then-annealed specimens. The latter leads to an appreciation of the changing structures that prevail as a result of metal atom diffusion during oxidation. Incipient failure of the implanted layers was noted for oxidation temperatures above - 8 0 0 ~ The surfaces of the oxidized specimens were observed, and possible mechanisms governing the fail- ures are presented in Section IV.

II. E X P E R I M E N T A L P R O C E D U R E

Sheets of 99.98 pct pure tantalum, containing 25 ppm each of niobium and tungsten and 70 ppm of oxygen by weight as major impurities, were cold rolled to reduce their thickness to 0.25 mm. This stock was cut into 4-mm-wide strips and annealed for one hour at 1550 ~ achieved by passing an electrical current through each strip. The pressure during annealing in an ion-pumped vacuum chamber was maintained below 10 -6 Pa. Spec- imens intended for microstructural characterization were obtained by punching 3 mm discs from the annealed strips,

METALLURGICAL TRANSACTIONS A VOLUME 20A, OCTOBER 1989-- 2101

while 4 m m x 6 mm specimens were cut for oxidation experiments. All the specimens were then electro- polished to remove any deformed materials at the edges. Some of the resulting material was used in this form, while the remainder was implanted with 180 keV AI + ions to fluences as high as 3.0 x 1018 ions /cm 2 and at fluxes n e a r 1014 ions/cm 2 s. The pressure during implantation was always below 10 -6 Pa. The specimens were attached to a water-cooled copper heat sink in order to minimize any heating due to the ion beam. All specimen temperatures remained below 100 ~ the minimum temperature detectable with our infrared pyrometer. All the specimens intended for oxidation measurements were implanted with 2.4 x 10 TM Al + / cm 2, first on one side, then the other, leaving the edges unimplanted.

Selected 3-ram specimens were annealed after implantation at temperatures from 450 ~ to 800 ~ The composition vs depth profiles in the as-implanted and implanted-and-annealed states were determined with Rutherford backscattering spectrometry (RBS) using 1.4 MeV 4He + ions. Specimens for microstructural analysis in the analytical transmission electron microscope were prepared by electropolishing from the back side to the implanted surface. Thin, electron-transparent areas near the holes were used for electron imaging, electron diffraction, and energy dispersive X-ray (EDX) analysis.

The kinetics of thermal oxidation was measured with a thermogravimetric microbalance exhibiting a sensitiv- ity of ---0.04 mg. The specimens were suspended from a quartz fiber inside a 19-mm-ID quartz reaction tube. The tube, heated from the outside, was flushed with argon prior to heating. The desired temperature was reached within ten minutes, whereupon bottled air was intro- duced at a flow rate of about 40 ml /min . The surfaces of selected oxidized specimens were examined with scanning electron microscopy (SEM) and optical microscopy.

I I I . R E S U L T S

A. Oxidat ion Measurements o f Tantalum and AI + Implan ted Tantalum

The kinetics of tantalum oxidation varies from linear to parabolic, depending on temperature and oxygen pressure, [7,81 tending to be linear, at least at shorter times, at temperatures below 800 ~ and pressures near one atmosphere. The oxidation kinetics measured for unimplanted tantalum in the present work was also linear. The results of these measurements are presented so they can be compared with A1 + implanted tantalum and provide a comparison with the work of other researchers, thus testing our measurement capabilities. The response of unimplanted tantalum to oxidation at different temperatures, expressed as weight gain per unit area vs time, is presented in Figures 1 through 6. The plots are linear, and the rate constants are the slopes of these plots. The oxidation rates increase rapidly with oxidation temperature, as will be discussed later in reference to Figure 13. Note the changes in scale on both axes between the various plots in Figures 1 through 6. The unimplanted specimens were completely oxidized through the thickness in times of an hour or less at temperatures above 735 ~ Thus, the 910 deg specimen in Figure 6 shows a plateau

in weight gain after about 0.5 hour. The weight gain of - 7 milligrams is as expected for total conversion of the tantalum to Ta205. Some of the specimens failed when the hole by which they were suspended broke through the sample perimeter, as signaled by the abrupt end of data points and an "X" on the plots, as in Figures 2 through 5.

Implantation provided considerable improvement in the oxidation resistance at 735 ~ and greatly reduced the oxidation rates at temperatures below this, as portrayed by the curves marked AI+/Ta in Figures 1 through 5. The oxidation rate at 735 ~ the initial slope of the curve in Figure 5, is reduced by a factor of 4 for A1 + implanted tantalum as compared to unimplanted tantalum. Some of the contribution to the oxidation of the implanted specimens must be attributed to the oxidation of the edges of the specimens. Since these edges were not implanted, they oxidize at the rate of unimplanted tantalum. This contribution to the oxidation of the implanted specimens is indicated by the dashed line in each of Figures 1 through 6 and was readily calculated by multiplying the weight gain for unimplanted tantalum at each time by the specimen edge area to total area ratio. The "edge effect" has not been subtracted yet in Figures 1 through 6. After ap- plying this correction to the 735 ~ data in Figure 5, the A1 + implantation reduces the oxidation rate by a factor of 6.

In some cases, the initial linear oxidation kinetics, observed at short oxidation times, did not continue for longer oxidation times. This effect was particularly noticeable for the implanted specimens in Figures 3 through 6 and is due to exfoliation or spalling of the oxide that forms on the unimplanted edges of the specimens. The spalling is apparent in the figures as a sharp decrease in weight gain. The spallation weight loss can be accounted for by adding these sudden weight losses to the weight gain vs time curves. The curves then remain linear, as shown, for example, by the dot-dashed lines in Figures 4 and 5. Vertical arrows on the various curves are used to indicate the onset of the spalling.

The beneficial effects of the A1 + implantation became more evident with decreasing oxidation temperature. The implantation yielded a fifteenfold decrease in oxidation rate, relative to unimplanted tantalum, at 640 ~ In the case of all three temperatures investigated below 640 ~ namely, 450 ~ 500 ~ and 550 ~ shown in Figures 1, 2, and 3, respectively, we note the measured weight gain vs time curves for the AI + implanted tantalum super- impose on the "edge effect" lines. Deviations below this can be accounted for, as above, by spalling only at the specimen edges. This was confirmed by optical and scanning electron microscopy, as described below. Thus, the oxidation is essentially stopped at these temperatures as a result of the implantation. In contrast, this beneficial effect of implantation is essentially lost at 910~ (Figure 6), where the slope of the curve for implanted tantalum, when corrected for edge effects, is only slightly less than that for unimplanted tantalum.

B. A luminum Concentra t ions and Micros tructures o f Al + Implanted Tantalum

As mentioned in Section I, the intent of implanting aluminum into tantalum was to produce surface layers

2 1 0 2 - - V O L U M E 20A, OCTOBER 1989 METALLURGICAL TRANSACTIONS A

r-

t~

1.0

0.8

0.6

m

o- E

t-- 0.4

0.2

0.0 0

Ta

25 50 75

Time (hr)

"Fig. l - -Oxidat ion at 450 ~

0 0

/ . . . . _ . . . .

20 40 60 80

Time (hr)

Fig. 2--Oxidation at 500 ~

10

iN"

8

Ta

6

4

2

o r - . - . Y - - T . . - . . . . .

0 2 4

Time (hr) Fig. 3 - -Oxida t ion at 550 ~

'15

/ i . I ,,+k;

O , . . . . , . . . . , . . . .

0 1 2 3

Time (hr) Fig. 4- -Oxidat ion at 640 ~

10 T ~ 10

I 6 / �9 6

4 / " AI+/Ta / " 4

o o . . . . . - , - . - . = - _ _ _ _ _ _ _ _ _ _ - - - - - - - - - - 0 1 2 3 4 5 0.0 0.2 0.4 0.6 0.8

"time (hi') Time (hr)

Fig. 5 - -Oxida t ion at 735 ~ Fig. 6- -Oxidat ion at 910 ~

Figs. l - 6 - - T h e solid curves describe specific weight gain v s oxidation time in one atmosphere of bottled air at temperatures indicatcd. The dashed lines indicate the corrections that must be applied to correct for oxidation at the unimplanted edges of the specimens. The dot-dashed lines in Figures 4 and 5 portray oxidation in the absence of oxide spallation. Vertical arrows indicate the onset of spallation.

METALLURGICAL TRANSACTIONS A VOLUME 20A, OCTOBER 1989. 2103

of intermetallic compounds that would resist oxidation. In particular, we wanted to produce TaA13, a compound known to have a particularly low oxidation rate. I6] To this end, 180 keV AI § ions were implanted into tantalum at several fluences up to 3 x 1018 A l r / c m 2. Though the implanted aluminum concentration profiles can be calculated, [9~ these calculations rely on accurate values of such parameters as the sputtering yield of 180 keV A1 r on tantalum and tantalum-aluminum alloys. Since these parameters were not known a p r i o r i , the concentration profiles were measured by RBS.

Figure 7 shows the aluminum concentration profiles observed after implanting tantalum to three different AI r fluences. The lowest fluence shown, 6 • 1017 Al+/cm 2, results in an aluminum concentration near 30 at. pct at the surface. This concentration rises to about 37 at. pct at depths near 800 ,&, then steadily decreases at greater depths, reaching zero near 5000 A. The depth over which the peak concentration remains nearly constant broadens and extends to the implanted surface at the two higher fluences shown in Figure 7, 2.4 and 3.0 • 1018 A l r / c m z. The relatively constant aluminum concentration in these two cases, about 70 at. pct, extends to a depth of about 2000 A before decreasing. The concentrations measured by RBS were confirmed by thin foil microanalysis in the analytical transmission electron microscope, using TaA13 to determine the Cliff-Lorimer constant, tl~ Figure 8 compares the compositions measured by the two methods. As can be noted from this figure, as well as from Figure 7, A1 § implantation to fluences beyond 2.4 x 1017 Alr / cm 2 produces little, if any, change in the concentration profile. At this fluence, the aluminum concentration has reached such a level that one aluminum atom is sputtered from the surface with the arrival of an A1 r ion in the ion beam.

The microstructures in the implanted tantalum were observed by transmission electron microscopy (TEM). Such images show the material's surface and material extending beneath this surface to depths ranging from about 400 A to about 1000 A, depending on foil thickness. Implantation to fluences up to 1.6 x 1018 ions/cm 2

�9 6 . 0 x I017 80 A 2 . 4 x 1018

z ~ 0 3 . 0 x 1018

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'tO u

20

0 I I I i 0 8 0 0 1600 2400 3 2 0 0 4 0 0 0 4800

DE PT H ( .,[ )

Fig. 7 - - Implanted aluminum concentration vs depth beneath the surface, as measured by RBS. Fluences in Al§ 2 are as indicated.

/

7 0

6 0

50

4 0

3 0

2 0

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0 0

g b.

co

r

z

I I I m I 1.0 2 .0 3 .0

F L U E N C E ( 1 0 1 8 i o n s / c m 2 )

Fig. 8--Aluminum concentrations implanted into tantalum and measured in TEM foils with EDX spectroscopy (A) compared with concentrations measured by RBS and averaged within 700 ,& depth of the surface ([~).

produced dislocations but no phase changes, t4] The body- centered cubic tantalum retained its structure with the addition of as much as 66 at. pct A1, corresponding to a fluence of 1.65 x 1018 Al§ 2. Beginning at this fluence, the BCC phase transforms to an amorphous phase, and the transformation is complete by the fluence of 2.4 x 10 TM A l r / c m 2. Figure 9(a) shows a diffraction pattern from the amorphous phase. This pattern exhibits the characteristic diffuse central intensity and higher-angle diffuse peaks expected from an amorphous materialJ ~1 The amorphous phase and the aluminum concentration of about 70 at. pct were present at the start of oxidation, since these specimens were all implanted with 2.4 • 10 TM Al+/cm 2.

The stability of the amorphous phase at elevated temperatures was investigated by studying the effect of annealing on microstructures that contained the phase. No phase changes were observed during one hour anneals at temperatures below 450 ~ An amorphous-to-crystalline transformation was noted after a half hour anneal at 600 ~ (Figure 9(b)). Dendrites are evident in this micrograph. Selected area diffraction patterns from these dendrites, as in the upper left of Figure 9(b), showed that each den- drite is a single crystal. Comparisons of the d spacings with those from Ta-A1 compounds showed conclusively that the dendrites were composed of the compound TaA13 .E~2] The diffraction pattern from the matrix, lower right inset in Figure 9(b), shows broad diffraction rings in addition to the diffuse intensity from the amorphous phase. These rings also match those from TaA13, sug- gesting that the matrix is composed of nanocrystalline TaA13 and amorphous phase.

Figure 9(c) shows the effect of 600 ~ annealing on the aluminum concentration profile. A profile measured after 60 minutes of annealing is compared with the as- implanted profile, both from specimens implanted to

(a) (a)

(c) (d)

Fig. 9--Electron microscope observations of tantalum after implanting with 2.4 • 10 's AI +/cm2: (a) diffraction pattern from as-implanted amorphous phase; (b) dendrites of TaAI~ in amorphous phase, 600 ~ for 30 rain; (c) aluminum concentration v s depth, comparing profile after 60 rain at 600 ~ with as-implanted profile; and (d) aluminum plate/ets in TaAI3 after 30 rain at 800 ~

2.4 x 10 TM Al+/cm 2. The profile sharpens with annealing, and a relatively uniform aluminum concentration near 75 at. pct extends to a depth of about 3000 ,~. Figure 9(d) shows the microstructure observed after an 800 ~ anneal. Selected area diffraction shows the matrix is crystalline T a m l 3. The dark rod-shaped phase is face-centered cubic aluminum, and its presence may play a role in oxidation, as discussed later.

C. Microstructural Effects of Oxidation on the Surface orA l + Implanted Tantalum

The surfaces of tantalum and AI § implanted specimens were examined, following oxidation, with optical and scanning electron microscopy. Thick yellow-white layers of Ta205 covered unimplanted specimens oxidized 90 hours at 500 ~ The implanted specimens, however, remained shiny with no signs of oxide, i .e. , no discol- oration. This was the case for the implanted regions, while the edges showed severe oxidation to Ta2Os. This con- firms our previous contention that the major contribution to the weight gains and losses of the implanted specimens in Figures 1 through 3 is the "edge effect." Figure 10 is an optical micrograph recorded from an oxidized implanted specimen. The white features observed in the micrograph are the amorphous phase that has crystallized to TaA13.

At temperatures above 500 ~ the unimplanted tantalum surfaces were covered by oxide in times of an hour or less. The implanted tantalum surfaces were much more resistant to this oxide formation. Oxidation at 640 ~ yielded surfaces similar to those observed after 500 ~ oxidation: In addition, small cracks that were not present at lower temperatures were seen in the oxide (Figure 11). Here, in the right side of Figure 11 (a), one can note small grains of Taml3 surrounded by amorphous material. This

(a)

(b)

Fig. I 1 - - S c a n n i n g electron micrographs showing surface cracking of implanted tantalum after oxidation for 3.2 h at 640 ~ at (a) low magnification and (b) high magnification. Left and right photos in each case were recorded in the secondary and backscattered electron modes, respectively.

Fig. 10 - -Op t i ca l micrograph of AI + implanted tantalum after oxidation for 90 h at 500 ~

crystallization is occurring on top of a single tantalum substrate grain. The underlying substrate grain boundaries are visible in the left side of Figure 11 (a). The cracks form at these grain boundaries, as well as at the crystalline/amorphous interfaces (Figure l l(b)). The cracks most likely form during cooling from the oxidation temperature, since no oxide forms on the tantalum exposed by the cracks (compare with Figure 12) and oxidation proceeds rather slowly at this temperature (Figure 4).

The oxidation rates of implanted tantalum are appre- ciable, relative to unimplanted tantalum, at temperatures near and above 735 ~ (Figure 5). The implanted surfaces show localized oxidative attack at these temperatures. After four hours of oxidation at 735 ~

2106 VOLUME 20A, OCTOBER 1989 METALLURGICAL TRANSACTIONS A

(a)

(b)

Fig. 1 2 - - S c a n n i n g electron micrographs from implanted tantalum showing surface blisters that formed during 4 h of oxidation at 635 ~ at (a) low magnification and (b) higher magnification.

(Figure 12(a)), the protective aluminide layer is dis- rupted by blister-like features. Needles of TazO5 form within these blisters during oxidation (Figure 12(b)). Slip lines are also evident in Figure 12(a), most likely re- suiting from plastic deformation during cooling of the aluminide layer. Specimens oxidized at 910 ~ were coated with Ta2Os, whether or not the specimens were implanted prior to oxidation.

IV. D I S C U S S I O N

The oxidation of unimplanted tantalum has been described by other researchers. 17,8,~3-~51 Initially, oxygen dissolves in the metal without nucleation of oxide phases. Suboxides of tantalum then form with further increase of oxygen in solution, and these act as precursors to the equilibrium oxide Ta2Os. Oxide platelets forming at the oxide-metal interface often penetrate into the metal, causing excessive stresses and eventual rupture of the

surface. Fast transport of oxygen, due to mechanical failure of the oxide layer aided by short circuit diffusion via dislocations and grain boundaries, results in the re- placement of parabolic oxidation kinetics by the linear kinetics observed here.

The oxidation rates measured for tantalum in the present work are compared with those measured by Schmidt e t a l . tlsl in Figure 13. The rate constants for A1 + implanted tantalum are also shown in this figure when possible; those not shown were zero or below the detectability limit of our measurements. We note from Figure 13 that the protection provided by A1 + implantation is lost at temperatures above about 800 ~ Significant decreases in the oxidation rates of tantalum result f rom A1 + implantation for oxidation temperatures at least up to 735 ~ These decreases can be explained, to some degree, by considering the oxidation of TaA13 and the closely re- lated intermetallic compound of NbA13. Both compounds exhibit low oxidation rates at high temperatures, where tantalum oxidizes very rapidly. Paine et a l Y 61 find total specific weight gains of about 2.5 m g / c m 2 and 7.0 m g / c m 2 during exposure of NbA13 and TaAI3, respectively, to air for one hour at 1260 ~ Assuming linear oxidation kinetics, the rate constant for TaAh is only

I 0 0 0

T E M P E R A T U R E ( ~

1400 I000 8 0 0 6 0 0 500

I m - AI+ / Ta ,~ ~ - - 0 - - unimp. Ta

~E I00 - ( . I

I0

. I -

.01 4

OD E

l - Z <[ I-- or) Z 0 (..1

W I- <C IZ:

<C w Z ._1

Schmldt et. al.

B

Bulk ToAI 3

I I 6 8

10 4

T ( K )

I0 12 14

Fig. 1 3 - - R a t e constants plotted vs reciprocal temperature. Data obtained in present work is compared with that of Schmidt e t a l . ~5~ The circles and squares, describing unimplanted and AI ' implanted tantalum, respectively, portray the present work.

METALLURGICAL TRANSACTIONS A VOLUME 20A, OCTOBER 1989 2107

0.07 m g / c m 2 / h at 1260 ~ and this rate is indicated in Figure 13 for comparison with the other rates. One might anticipate that the oxidation rates of Taml3 would be even lower than this at lower temperatures. Thus, the presence of TaA13 on the surface of the AI § implanted tantalum provides an explanation for the reduced oxidation rates at temperatures where we have observed its presence, in particular, at temperatures from 500 ~ to 730 ~ At lower temperatures, the implanted layer remained amorphous during the oxidation times used here. Thus, the amorphous Ta-A1 phase, like TaA13, is highly oxidation-resistant compared to tantalum, and it also prevents oxidation of the underlying tantalum.

The oxidation rates of the implanted tantalum might be expected to remain much less than those of unimplanted tantalum to temperatures well above 800 ~ provided the TaAI3 film maintained its integrity. This integrity is clearly lost, however, due to the formation of blisters and exposure of the underlying tantalum to oxidation. The blisters may signal the onset of spalling of the TaAI3. It is interesting to note in this regard that NbA13 is susceptible to low-temperature disintegration in the temperature range of 550 ~ to 850 ~

What then causes the onset of blister formation, the loss of oxidation protection, and, perhaps more impor- tantly, what can be done to prevent it? Several possible causes of blistering remain to be tested in the hope of preventing it, and these form the basis for our future work. The possibilities to be tested include the following:

(1) that oxygen diffuses through the implanted layer, clusters to form suboxides, and, subsequently, that blisters form, rupture, and trigger spallation; and (2) that phase separation occurs, leading to the presence of free aluminum metal along with TaA13 (Figure 9(d)), and that the melting of the free aluminum near 660 ~ activates blister formation and local oxidation.

V. SUMMARY AND C O N C L U S I O N S

Aluminum, in the form of 180 keV AI + ions, was implanted into tantalum to fluences up to 3 x 10 ~8 Al+ /cm 2. Oxidation measurements were performed in one atmosphere of bottled air at temperatures between 450 ~ and 910 ~ using the tantalum implanted with 2.4 x 10 t8 A l+ /cm 2 as well as unimplanted tantalum. The results of these measurements and subsequent microstructural examinations are as follows:

1. Oxidation was not detected in the implanted areas during oxidation at temperatures below 600 ~ though unimplanted tantalum oxidized rapidly.

2. The protective layer resulting from implantation was composed of approximately 70 at. pct A1 and 30 at. pct Ta. This layer was amorphous in the as- implanted condition and crystallized to form TaAI3 when subjected to temperatures above --500 ~

3. Both amorphous Ta-AI and TaA13 provided good protection to oxidation at temperatures less than 800 ~ The protective quality of the implanted layer deteriorated at temperatures above this. By 910 ~ the oxidation rate of A1 § implanted tantalum was nearly equal to that of unimplanted tantalum.

It is concluded that the protection afforded by the implantation is a direct result of the very low oxidation rate of TaA13 and amorphous Ta-A1 of similar composition. The loss of this protection above 800 ~ is a conse- quence of local rupturing of the implanted aluminide layer, or blister formation, and the subsequent oxidation of the underlying tantalum at such locations.

ACKNOWLEDGMENTS

We thank the following people at the University of Connecticut who contributed to this research: C. Koch for assisting us in the ion implantations; L. McCurdy for TEM assistance; O. Devereux for the use of his oxidation equipment; Q. Kessel and J. Gianoupolis for assistance with the van de Graaff accelerator; and J. Soracchi and T. Swol of the instrument laboratory. We are grateful to James Steele, in particular, for help with the SEM imaging. This material is based on work supported by the National Science Foundation under Grant No. DMR 8507641. The analytical electron microscope was pur- chased with support from the National Science Foundation Grant No. DMR 8207266 and the State of Connecticut. The authors are grateful for the continuing support of these people and agencies.

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2. F.S. Shuker: Chem. Eng., 1983, vol. 90 (9), pp. 81-84. 3. E.N. Kaufmann, R.G. Musket, J.J. Truhan, K.S. Grabowski,

C.R. Gossett, and I.L. Singer: Nucl. Instrum. and Methods in Phys. Res., 1983, vols. 209-210, pp. 953-61.

4. M. Saqib and D.I. Potter: Mater. Sci. Eng., 1987, vol. 90, pp. 81-89.

5. W.W. Beaver, A.J. Stonehouse, and R.M. Paine: Proc. 5th Plansee Seminar, Metallwerk Plansee, Reutte, Austria, 1965, pp. 682-700.

6. E.A. Aitken: in lntermetallic Compounds, J.H. Westbrook, ed., Krieger Publishing Co., New York, NY, 1977, pp. 491-516.

7. Per Kofstad: J. Electrochem. Soc., 1963, vol. 110, pp. 493-501. 8. J. Stringer: High Temperature Corrosion of Aerospace Alloys,

AGARDograph #200, NATO, Tech. Editing and Reproduction Ltd., London, 1975, pp. 551-66.

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10. G. Cliff and G.W. Lorimer: J. Microsc., 1975, vol. 103, pp. 203-07.

11. B.E. Warren: X-ray Diffraction, Addison-Wesley Publishing Co., Reading, MA, 1969, pp. 116-50.

12. W.B. Pearson: A Handbook of Lattices Spacings and Structures of Metals and Alloys, vol. 2, Pergamon Press, Long Island, NY, 1967, p. 132.

13. F.E. Bacon and P.M. Moanfeldt: in Columbium and Tantalum, F.T. Sisco and E. Epremian, eds., John Wiley & Sons, New York, NY, 1963, ch. 9, pp. 347-74.

14. J. Stringer: Reviews on High Temperature Materials, 1973, vol. 1 (3), pp. 256-84.

15. F.F. Schmidt, W.D. Klopp, W.M. Albrecht, F.C. Holden, H.R. Ogdon, and R.I. Jaffee: U.S. Air Force Technical Report WADD-TR-59-13, 1959, pp. 152-60.

16. R.M. Paine, A.J. Stonehouse, and W.W. Beaver: WADC TR 59-29, Parts I and II, PB Report 161,683, U.S. Department of Commerce OTS, 1960.

17. J.J. Rausch: ARF 2981-4, Armour Research Foundation, Chicago, IL, August 31, 1961, NSA 15-31171.

2108--VOLUME 20A, OCTOBER 1989 METALLURGICAL TRANSACTIONS A

improving tantalum's oxidation resistance by al+ ion implantation

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