ANNEALING STUDIES OF PURE AND ALLOYED TANTALUM … · The annealing response of forged tantalum, reported by O™Brien et al. [1], demonstrated that significant differences in grain

ANNEALING STUDIES OF PURE AND ALLOYED TANTALUM EMPLOYING ROCKING CURVES

David W. Richards1, Michael P. Kramer1, Joel W. House1

and Robert J. De Angelis2 1 Air Force Research Laboratory, Eglin AFB, Florida, 32542

2 University of Florida/GERC, Shalimar, Florida, 32579 Abstract

To control the mechanical response of pure and alloyed tantalum requires tailoring the microstructure by carefully controlling thermo-mechanical processing. In this investigation X-ray rocking curves were employed to characterize the microstructure and further the understanding of the mechanisms of annealing. Plate material produced from the alloyed tantalum demonstrated annealing stages 300ºC higher than the pure tantalum. Activation energies determined from the rocking curve data were one-half to one third of the self-diffusion activation energy.

Introduction

Controlling texture and grain structure in unalloyed tantalum has been difficult [1,2]. Sources of the inhomogeneous structure are ingot chemistry, ingot grain size and mechanical processing. Introduction of high levels of plastic strain by rolling and forging prior to annealing only partially homogenize the grain structure. Heterogeneous grain structures in components generate local variations in mechanical properties and are potential sources for unanticipated failures. The annealing response of forged tantalum, reported by O�Brien et al. [1], demonstrated that significant differences in grain size, size distribution, and morphology existed in microstructures obtained after the annealing of forged tantalum. In a recent study the effects of an intermediate annealing cycle (before forging) on texture and the microstructure formation in forged and annealed tantalum plates were quantified [3]. The current investigation examines the benefits of X-ray rocking curves in studying the processes of annealing in tantalum and tantalum 2.5% tungsten.

Material and Thermo-Mechanical Processing

Chemical composition of the pure tantalum studied is listed in Table I. The material was purchased from H.C. Starck Inc., Newton, MA. The composition of the tantalum 2.5% tungsten alloy was the same except for the additional tungsten. Two 165 mm (6.5 inch) diameter ingots were produced at Starck utilizing a process consisting of vacuum arc remelting of a dual electron beam processed electrode ingots. Table I. Composition of the tantalum. (PPM by weight)

O N C H Fe Ni Cr Cu Si Ti Mo W Nb 72 11 13 1 24 29 11 1 8 5 12 117 43

The cast ingots were turned and cold swaged to 63 mm (2.5 inch) diameter bars. The swaged bars

were sectioned into 86 mm (3.4 inch) long billets. The billets were upset forged in multiple steps with the application of lubrication between each step to 228 mm (9-inch) diameter plates by 6.35 mm (0.25 inch) thick.

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The post-forge annealing study was performed on samples from two plates, one of each composition. Employing a water-jet cutter, twenty specimens approximately two by two centimeters were sectioned from each plate. These specimens were cut from a circle of material centered on a 64 mm (2.5 inch) radius of both plates. Hardness measurements were made on the surface of the plate. X-ray measurements and optical observations were made on the mid-plane of specimens that were annealed for one hour in vacuum at intervals of 50ûC at temperatures between 750 and 1300ûC for tantalum and 900 and 1500ûC for tantalum 2.5% tungsten. The specimens were wrapped in tantalum foil and annealed in a furnace under a vacuum of <10-5 torr.

Experimental

After heat treatment, hardness data were collected from the annealed specimens of tantalum and tantalum 2.5% tungsten. Rockwell B hardness numbers as a function of annealing temperature are shown in Fig. 1. Rapid softening of pure tantalum set in at 900ûC and annealing was complete at 1150ûC. For the alloyed tantalum softening started at temperatures above 1050ûC, was most rapid in the 1150ûC temperature range and was complete at 1350ûC. The hardness increase observed in specimens annealed at temperatures above 1350ûC indicated oxygen contamination occurred at these higher temperatures. The annealed specimens were sectioned on their mid-plane using a diamond saw. The split specimens were mounted and metallographically polished using the technique described by Kelly et al. [4]. All of the X-ray data was collected from the mid-plane surface of the annealed specimens employing copper Kα radiation. The X-ray data were collected on a Siemens diffractometer outfitted with a split cradle and a graphite diffracted beam monochromator. The X-ray beam geometry on the specimen at sixty degrees two theta was one centimeter by one millimeter. Rocking curve data were collected in steps of 0.02 degrees and two second counting times. Texture data were collected in five degree steps of phi and chi to describe the {110}, {200} and {211} pole figures. The pole figure data was processed employing the analysis software package popLA [5] to calculate the inverse pole figure. Inverse pole figure data were analyzed using the techniques reported in [6,7] to give the fraction of grains in the plane of the plate that have normals with orientations within 12.5 degrees of the <111>, <110> and <100> directions. These quantitative texture results

20

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750 850 950 1050 1150 1250 1350 1450

Annealing Temperature, C

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dnes

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ockw

ell B

Ta-WTa

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700 800 900 1000 1100 1200 1300


Den

sity

{h

kl},

Tim

es

Ran

do

m {100}

{111}

Fig. 1. Rockwell B Hardiness of Tantalum and Tantalum 2.5% Tungsten Versus Annealing Temperature.

Fig. 2. Densities of Planes within 12.5 Degrees of <111> and <100> in Tantalum Versus Annealing Temperature.


0

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900 1000 1100 1200 1300 1400 1500


Den

sit

y {

hkl}

, T

imes R

an

do

m

{100}

{111}

0

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200

300

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600

30 35 40 45 50 55 60 65 70 75 80Omega, Degrees

Inte

nsit

y,

CP

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As Forged

850 C

Fig.3. Densities of Planes within 12.5 Degrees of <111> and <100> in Tantalum 2.5% Tungsten Versus Annealing Temperature.

Fig. 4. (222) Rocking Curves of Tantalum in the �As Forged� Condition and After Annealing at 850ºC.

from the pure tantalum and alloy plates were plotted versus annealing temperature and are shown in Figs. 2 and 3 respectively. Texture strengths of the <110> components were less than 0.01 times random and were not included in these figures. During the initial stage of annealing of the pure tantalum (see Fig. 2) in the temperature range of 750°C to 900°C, where the softening rate is at its maximum, there is an increase in the {100) component. In this same temperature range the {111} texture strength decreases by about a factor of four. Annealing at temperatures of 900°C to 1100°C decreases the fraction of grains with {100} orientation and increased the {111} component more than a factor of four. At annealing temperatures above 1100°C preferred growth of the {111} grains occurs producing texture strengths greater than eight times random and the magnitudes of the {100} components are below 0.3 times random. The texture changes demonstrated by the alloyed tantalum (see Fig.3) during annealing were in general similar to those observed in pure tantalum. The strength of the <111> component increases at higher annealing temperatures and the <100> component decreases. However the magnitude of the changes in texture strength during annealing were lower in the alloyed material.

0

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Omega, Degrees

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PS

950 C

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2500

30 35 40 45 50 55 60 65 70 75 80Omega, Degrees

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ns

ity

, C

PS

1000 C

Fig. 5. (222) Rocking Curve of Tantalum After Annealing at 950ºC.

Fig. 6. (222) Rocking Curve of Tantalum After Annealing at 1000ºC.


Rocking curves of the (222) obtained from the pure tantalum in the �as forged� and after annealing at 850ûC are shown in Fig. 4. The (222) rocking curves for the tantalum after annealing at 950ûC and 1000ûC are shown in Figs. 5 and 6. Intensity spikes were observed to form on the rocking curve obtained from the material annealed at 850ûC. These spikes are from poligonized regions of the structure. These occurred in an annealing temperature range in which the hardness was just starting to decrease. After annealing at 950ûC and 1000ûC the spikes increased in intensity, as the scale of the ordinate in Figs. 5 and 6 indicate, signifying the formation of larger diffracting crystallites. (222) Rocking curves from the tantalum 2.5% tungsten alloy in the �as forged� condition and after annealing at 1050ûC are shown in Fig. 7. The (222) rocking curves for the alloyed material after annealing at 1150ûC and 1500ûC are shown in Figs. 8 and 9. Intensity spikes of about one-tenth of those at 1150ûC were observed to form on the rocking curve obtained from the alloyed material annealed at 1050ûC. These spikes indicated that poligonization was occurring in the alloyed material at a temperature 300ûC higher than in the pure tantalum.

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1150 C

Fig. 7. (222) Rocking Curves of Tantalum 2.5% Tungsten in the �As Forged� Condition and After Annealing at 1050ºC.

Fig. 8. (222) Rocking Curve of Tantalum 2.5% Tungsten After Annealing at 1150ºC.

The averages of the intensity spikes present in the (222) rocking curves from the pure and alloyed tantalum were determined at each of the annealing temperature. The averaging was accomplished by subtracting the background intensity from a pattern, performing a peak search to identify seventy to one hundred peaks, summing the intensity values at the peak maximums and dividing by the number of peaks. Plots of the logarithms of the magnitude of the intensity spike averages versus the reciprocal of the annealing temperature are shown in Figs. 10 and 11. Activation energies for the mechanism controlling poligonization were calculated from the slopes of these plots. The activation energy was determined to be 29,200 calories per mole for pure tantalum and 55,800 calories per mole for tantalum 2.5% tungsten. The energy value for pure tantalum, about one-third of the activation energy of self-diffusion, is in the range of activation energy values for boundary diffusion [8]. The much greater activation energy for boundary migration observed for the tantalum 2.5% tungsten indicates the additional tungsten solute atoms reduce the mobility of the migrating boundaries significantly.


0

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30 35 40 45 50 55 60 65 70 75 80Omega, Degrees

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nsit

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1500 C

Fig. 9. (222) Rocking Curve of Tantalum 2.5% Tungsten After Annealing at 1500ºC.

Fig. 10. Logarithm of the Average Magnitude of the Intensity Spikes on the (222) Rocking Curve of Tantalum Versus the Inverse of the Annealing Temperature.

Fig. 11. Logarithm of the Average Magnitude of the Intensity Spikes on the (222) Rocking Curve of Tantalum 2.5% Tungsten Versus the Inverse of the Annealing Temperature.

Discussion

There are two advantages in employing rocking curves to investigate the annealing behavior of tantalum and its alloys. The most apparent advantage was the appearance of intensity spikes in the rocking curve patterns. The lowest annealing temperature that was associated with the appearance of the intensity spikes defined the annealing temperature that produced significant recovery of the deformed substructure. Annealing at higher temperatures provided a method to study the crystallite size growth kinetics. The second and less obvious advantage of the rocking curve technique is the data it provides on the grain orientation relationships between the deformed structure and the recovered or recrystallized material. Nucleation of the recrystallized grains in both tantalum materials investigated took place within ten degrees of the center of the intensity distribution of the rocking curves. The magnitude of the intensity spikes in this angular range is proportional to the strength of the <111> component of texture. This is easily seen by comparing the intensities of the intensity scales on the plots of the rocking curves with the texture strengths of the <111> shown in Figs. 2 and 3.

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0.6 0.65 0.7 0.75 0.8 0.85 0.9 0.95 1

1000 / Annealing Temperature, K

Ln o

f Ave

rage

Inte

nsity

of R

ocki

ngC

urve

Spi

kes 29,200 Cal / Mole

0

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6

8

0.6 0.65 0.7 0.75 0.8 0.85 0.9 0.95 1

1000 / Annealing Temperature, K

Ln o

f Ave

rage

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nsity

of

Roc

king

Cur

ve S

pike

s

55,800 Cal / Mole


Acknowledgements

The financial support from the United States Air Force for RJD under contract FO8635-98-D-0016 is acknowledged.

References

[1] O�Brien, J. M., Hosford, W.F. and Landrum, R.L., Manufacturing and Metallurgical Evaluation of Cold Forged Tantalum and Tantalum Tungsten Alloy, Technical Report, AFRL-MN-EG-TR-2001-7031, Eglin AFB, Florida, May 2001.

[2] Bingert, J.F., Desch, P.B. Bingert, S.R., Maudlin, P.J. and Tome, C.N., Proc. Fourth Intl. Conf. Tungsten, Refractory Metals and Alloys, eds. Bose, A. and Dowding, R.J., MPIF, 1998, 169-178.

[3] House, Joel W., Bingert, John F., De Angelis Robert J. and O�Brien, James M, Materials Science Form, 2002, 408-412, 553-558.

[4] Kelly, A.M. Bingert, S.R. and Reiswig, R.D., Microstructural Science, 1996, 23, 185-195. [5] Kallend, J.S., Kocks, U.F., Rollett, R.D. and Wenk, H.-R, Mater. Sci. and Eng., 1991, A132, 1-11. [6] De Angelis, R., Snyder, T., House, J. and Hosford; W., Adv. X-Ray Anal., 2000, 42, 510-520. [7] Hosford, W., O�Brien, J., House, J. and De Angelis; R., Adv. X-Ray Anal., 2000, 42, 502-509. [8] Shewmon, Paul G., Transformations in Metals, McGraw Hill Book Co., New York, 1969, 63-65.


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ANNEALING STUDIES OF PURE AND ALLOYED TANTALUM … · The annealing response of forged tantalum, reported by O™Brien et al. [1], demonstrated that significant differences in grain